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Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions (2024)

Chapter: 3 Sources and Composition of Indoor Particulate Matter

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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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3
Sources and Composition of Indoor Particulate Matter

This chapter discusses recent scientific literature on the sources of indoor fine particulate matter (PM2.5) and ultrafine particles (UFPs) in order to provide background information for subsequent chapters in this report and to identify persistent knowledge gaps on this topic. The research reviewed was mostly limited to publications in the past decade and references therein, focusing primarily on sources of importance to residential and school environments in the United States. In some cases, where there were particularly relevant earlier papers or the publication record was relatively spare, it was necessary to reach back further in time. There are numerous sources of PM2.5 and UFP, particularly in residences, which vary in relative importance from residence to residence. As such, the focus of this report is on the most extensively studied sources which have the greatest relevance in terms of health risks associated with exposure.

The chapter first presents an overview of indoor PM2.5 and its components. Outdoor sources of indoor PM2.5 are then discussed, followed by the individual indoor sources of PM2.5, which are grouped based on the mechanisms that generate or produce these particles. Five main generation mechanisms are covered: combustion processes, other non-combustion heating processes, mechanical particle resuspension, residual particles from liquid aerosol evaporation, and secondary particles formed through chemical reactions. Traditional and under-explored sources are included, with a greater focus placed on potentially new and unknown sources. When available in the literature, source-specific information on chemical composition and health effects are presented, with a more comprehensive review of health effects set forth in Chapter 6 of this report. The chapter concludes with a summary of the findings and conclusions and with the recommendations that the committee developed on the basis of the findings and conclusions.

The standard definition of PM2.5 is the mass concentration of particles with aerodynamic diameters less than or equal to 2.5 µm. While this standard EPA definition is based on aerodynamic diameter, some studies report other size metrics including number concentrations which are more sensitive to smaller particles including ultrafine particles. Corrections can be applied to convert between different size metrics including particle shape, density, and optical properties. For a more thorough discussion, the reader is referred to the original publications for details. Finally, it is important to note that ultrafine and submicron particles often dominate particle number concentrations, whereas particles larger than 0.1 microns dominate in terms of mass concentrations.

INTRODUCTION AND BACKGROUND

Particulate matter (PM) found indoors originates from outdoor air that penetrates into buildings as well as from a wide variety of indoor sources. Indoor particulate matter reflects the great diversity of indoor environments that exist and the activities performed in each of them.

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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This diversity is a major challenge for understanding the impact of indoor sources on indoor PM2.5 concentrations and compositions. While ambient PM concentrations and other characteristics vary regionally and, to some extent, locally, the contributions of indoor sources to indoor PM can vary from building to building, unit to unit, and even among different microenvironments within the same building or unit (e.g., kitchen versus bedroom in a home or cafeteria versus classroom in a school). Humans themselves are important generators of particulate matter, with the relevant activities ranging from resuspending settled dust to shedding skin flakes to exhaling respiratory particles that may contain infectious pathogens. Adding to this complexity, cultural differences and beliefs as well as socioeconomic disparities can lead to differences in both ambient and indoor PM2.5 concentrations (Adamkiewicz et al., 2011). Understanding sources can help indoor occupants make informed decisions about limiting their exposure to fine particulate matter (Klepeis et al., 2013).

Indoor sources have been found to account for approximately half of total indoor PM2.5 concentrations in homes, on average, with the remainder originating from outdoors (Bi et al., 2021; Meng, et al., 2005; Wallace et al., 2022). The type and nature of PM sources determine important particle characteristics, such as emission strength, particle size distribution, and chemical composition, as well as other important parameters for human exposure and health (further discussed in chapters 5 and 6 of this report), such as source duration, frequency, time of day, and source location relative to the receptor, and indoor ventilation or mitigation strategy, among others. And while the global burden of disease related to ambient air pollution has been estimated (Cohen et al., 2017), a question remains as to how to understand the implications of such estimates for outdoor exposure given the fact that people in the United States spend the majority of their time indoors (Klepeis et al., 2001).

Building-related factors contribute to indoor PM2.5 concentrations and composition by affecting how outdoor particles penetrate from outdoors and how indoor particles are diluted and exfiltrated from indoors, as discussed in Chapter 4. Socioeconomic status also plays a role in indoor concentrations, potentially due to a combination of indoor sources and the presence of higher-leakage areas that allow greater penetration of outdoor PM (Adamkiewicz et al., 2011; Mendell et al., 2022). For example, indoor PM concentrations were found to be two times higher in social (subsidized) housing than in single-family homes in Toronto, Canada (Mendell et al., 2022). Building-related factors affecting indoor particle penetration and dynamics are also discussed in greater detail in Chapter 4.

INDOOR PM2.5 CONCENTRATIONS AND COMPOSITION

Indoor PM2.5 of Outdoor Origin

Ambient fine particulate matter (PM2.5) has been widely reported as an important cause of mortality, both worldwide and in the United States (Cohen et al., 2017; Di et al., 2017; Fann et al., 2012). Although epidemiological studies rely on ambient PM2.5 concentrations to determine health impacts, people report spending nearly 90 percent of their time indoors, on average, including nearly 70 percent in residences (Klepeis et al., 2001). Canadian estimates were found to be very similar to these U.S.-based numbers (Leech et al., 2002). It is important to note that these studies were completed two to three decades ago and have not been updated based on newer activity pattern surveys which could reflect some changes, including as a result of the COVID-19 pandemic. As further discussed in Chapter 4, because outdoor PM2.5 infiltrates and

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

persists indoors, the bulk of human exposure to PM of outdoor origin is likely to take place indoors. In fact, indoor exposure to PM2.5 of outdoor origin has been estimated to account for perhaps 40–60 percent of the mortality burden of PM2.5 exposure in the United States (Azimi and Stephens, 2018).

Ambient sources of PM2.5 have been extensively investigated. Major outdoor contributors of PM2.5 comprise both natural and anthropogenic sources. Ambient air quality standards refer to PM2.5 and PM10; however, the size distributions for different PM sources have the potential for myriad particle size distributions, some of which may not be properly represented through mass-based measurements in those size cutoffs (Morawska et al., 2008). Essentially, ambient particles fall into several PM size ranges determined by their formation process, sources, and sinks: ultrafine, submicron, and supermicron particles. Windblown dust and sea spray aerosols are mechanically generated and thus have larger supermicron sizes. They represent the dominant atmospheric aerosols by mass. They are composed of salts, metals, minerals, and bioparticles. In addition to salts, sea spray aerosol has been shown to contain a significant amount of organic components, particularly at the smallest submicron sizes. Ambient PM also includes bioparticles, which can penetrate into indoor areas and contribute to the complexity of the indoor microbiome. Anthropogenic PM2.5 sources include industry, power generation, transportation, and domestic burning activities. Fuel combustion, including the burning of heating oil and wood (including wildfires), contributes significantly to ambient PM2.5. Most combustion-derived aerosols occur in the submicron size range. The vast majority of particles (by number) occur in the ultrafine size range (<100 nm). Sources of ultrafine particles include new particle formation and direct emissions from combustion processes.

The amount that outdoor pollution contributes to indoor air depends on a number of factors, including the proximity of a building to point and mobile sources, factors associated with boundary layer meteorology, urban and regional air pollution, and a number of building-related factors such as ventilation and infiltration rates, as well as location of air intakes (for nearby sources). A significant number of schools are located near roadways (15 percent of schools are located within 820 ft of a major roadway) and thus are heavily affected by vehicular pollution (Kingsley et al., 2014). As wildfires become more common due to changing climate, a number of studies have found an impact of wildfires on indoor air quality. In California, an analysis of networks of consumer-grade particle sensors showed that indoor PM2.5 concentrations nearly tripled during wildfires (Liang et al., 2021). A 2019 study found that wildfires and vehicle emissions significantly increased the indoor air PM2.5 concentrations due to natural ventilation and infiltration in economically disadvantaged homes in Denver (Shrestha et al., 2019). During the wildfire season, these homes were heavily affected by long-range transported wildfire plumes, which led to indoor PM2.5 levels that were nearly 5 times higher than outdoor levels. During transport from outdoor into indoor environments, particulate matter can undergo multiple physical and chemical transformations, leading to changes in particle concentrations and size distribution, due to transport through the building envelope and physical and chemical differences between outdoors and indoors (Abt et al., 2000). Chapter 4 further elaborates on these and other processes.

Indoor Sources of PM2.5

Numerous everyday indoor activities produce PM, with new potential sources emerging over time as novel consumer products, activities, and habits appear. While traditional and relatively well understood indoor PM sources, such as combustion processes, still play an

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

important role in indoor exposure, novel indoor sources, such as 3D printers and electronic cigarettes, add complexity to indoor environments. Indoor sources are complex not only because of inherent differences in source strengths, particle size distributions, and composition, but also in terms of timescale, which may even vary considerably across the same type of source. As a generalization, some sources emit particles more continuously (e.g., humidifiers, pilot lights, wood burning stoves or fireplaces), while others generally emit over shorter periods (e.g., cooking, burning incense or candles, operating printers). Table 3-1 shows some common examples of indoor particle sources and some of their typical characteristics.

Table 3-1 makes the case for the variability and complexity of indoor sources, which can be very short-lived (e.g., spray products) or longer-duration (e.g., fireplaces) and can emit very small particles (e.g., gas combustion) or very large particles (e.g., dust resuspension) with a wide range of compositions. In the next section, some major indoor sources of fine PM are presented grouped under their main underlying PM generation mechanism. Both traditional and underexplored sources are included, with more focus placed on potentially new and unknown or emerging sources. For several of these sources, the present body of knowledge ranges from very low to high, depending on the parameter of interest. Many of the studies presented in the sections below report indoor particle concentrations resulting from these sources, while others measure particle emissions rates (sometimes in terms of particle mass, sometimes particle number). Fewer studies report source-specific particle size distributions, particle composition, or, in some cases, source-specific health effects.

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

TABLE 3-1 Examples of Indoor Sources of PM2.5 That Are Relevant in Many U.S. Homes and Schools, With Some of Their Characteristics

Type of source Dynamics/Duration
Intermittent1 Continuous1 Primary particle size2
Combustion
  • Gas stoves
  • Cigarettes and cigars3
  • Fireplaces
  • Wood stoves
  • Pilot lights
  • Candles
  • Incense
  • Ultrafine and fine
Heating Processes
  • Laser printing
  • Cooking
  • Clothes ironing
  • Hair styling tools
  • 3D printing
  • Essential oil vaporizers
  • Hair dryers and hand dryers
  • Electronic cigarettes
  • Ultrafine for most sources; fine and supermicron for cooking
Water Droplet Evaporation
  • Spray products
  • Respiratory emissions
  • Humidifiers
  • Ultrasonic essential oil diffusers
  • Artificial fog machines
  • Ultrafine for humidifiers, fine and supermicron for respiratory emissions
Mechanical Dust Resuspension
  • Walking and physical activity
  • Cleaning
  • Vacuuming
  • Motors and other machinery
  • Supermicron
Chemical Processes
  • Secondary aerosol formation
  • Additive, oxidizing air cleaners
  • Ultrafine

1 For this table, sources that occur on the order of seconds to minutes were considered intermittent, and sources that may last for hours to days were considered continuous.

2 The following particle size cutoff definitions are used in this report: Ultrafine: <100 nm, Fine: <2.5 µm, Supermicron: particles >1 µm.

3 Although tobacco smoke products are not covered in this report, they are well-known indoor combustion sources.

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

Indoor PM From Combustion-Related Sources

Indoor combustion sources emit products of incomplete combustion and thermal nitrogen oxides (NOx). Most of these sources are intermittent with emission periods of minutes to hours, while some emit more continuously. The extent and nature of particle emissions from combustion-related sources in the indoor environment are defined by the type of fuel (composition), fuel consumption rates, extent of oxygen supply, flame temperature, and degree of emissions containment. A summary of several important indoor combustion sources is provided below, grouped according to fuel type: natural gas, wood, other fuels used for heating (such as propane), and candles and incense. Each source in this section is relevant to some percentage of U.S. residential buildings, and a few are relevant to school buildings. Second-hand tobacco and marijuana smoke can be major sources of PM2.5 in homes but are excluded from this discussion.

Combustion of Natural Gas or Propane

Natural gas and propane are combusted indoors for purposes of cooking (via stovetops and oven burners) and heating (via fireplaces, furnaces for space heating, hot water heating, and clothes drying). Natural gas is also combusted indoors by pilot lights that serve combustion appliances though pilot lights have been decreasing in prevalence as older equipment is retired. In each case, particle emissions to the indoor environment are dominated by ultrafine particles. Most combustion appliances are required by code to utilize exhaust vents to direct products of incomplete combustion to the outdoors. These exhaust systems often capture most but not all fine PM and, in the case of range hood exhaust, are often missing in older homes or not used by occupants while cooking.

Previous studies on emissions from natural gas cooking have focused on residential as opposed to school environments, although some of what has been gleaned from these studies has some relevance to the use of natural gas for cooking in school cafeterias. As of 2020, 38 percent of Americans used natural gas for indoor cooking, including ovens and stove top burners (EIA, 2022). States with households most likely to use natural gas for cooking are California (70 percent) and New Jersey (69 percent), with Illinois and New York both exceeding 60 percent. Other than Georgia, residences in southeastern states have relatively low (<20 percent) usage rates of natural gas for cooking.

During the combustion of natural gas or propane, a number of pollutants are emitted, including particulate matter and gasses such as carbon monoxide, carbon dioxide, nitric oxide, nitrogen dioxide, formaldehyde, acetaldehyde, and more (Lebel et al., 2022; Mullen et al., 2016; Singer et al., 2017). Cooking with natural gas or propane generates a substantial amount of UFPs predominantly associated with the combustion of the gas and, to a much smaller extent, desorption of SVOCs from cooking utensils followed by condensation in room air (Wallace et al., 2008; 2017). Details related to the combustion of natural gas are reasonably well understood. For example, for stovetop burners the mixture of air and natural gas flows into the bottom of the burner cap and issues from holes situated around the circumference of the cap. Primary aeration is mostly in the range of 40–60 percent of the stoichiometric air requirement, a level of aeration that results in flames that prevent soot formation and yields the characteristic blue flame of natural gas burners (Wagner et al., 2010). Moreover, unvented pilot lights that serve natural gas appliances also contribute significantly to UFP concentrations, often operating continuously (Bhangar et al., 2011).

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

Inhalation exposure studies specifically associated with cooking with natural gas have largely been complicated by emissions from cooking oils and foods (see section on cooking below), i.e., as opposed to just combustion of natural gas, or have focused on nitrogen dioxide and other gasses as opposed to UFPs or PM2.5 (e.g., Logue et al., 2014; Paulin et al., 2014). Few studies have definitively resolved the health effects of such exposures, particularly as related to particle emissions.

Kile et al. (2014) identified children in the United States aged 2–16 years who lived in homes where gas stoves were used. After adjusting for other risk factors, children in homes for which an exhaust fan was not used during gas stove operation had lower lung function and higher odds of asthma, wheeze, and bronchitis compared with children in homes where an exhaust fan was used while operating a stove. The authors speculate that agents such as nitrogen dioxide and particle-bound polycyclic aromatic hydrocarbons (PAHs) may have played a role in these results, but these were not measured.

Wagner et al. (2010) studied particle emissions from a single gas burner consistent with a stovetop burner. Mean emitted particle diameters were observed to be approximately 7 nm for partially premixed flames and approximately 10 nm for non-premixed flames. These values are consistent with geometric mean diameter (GMD) ranges of 4 nm to 8 nm reported by others (Patel et al., 2021; Rim et al., 2016; Wallace et al., 2008). The percentage of primary air had a particularly strong impact on particle emissions; emissions during combustion of natural gas were at a minimum at a primary aeration level of 60–65 percent.

Pilot lights, small gas flames used to ignite a larger burner flame, were once extensively used in home stoves and ovens but have largely been replaced in favor of electric ignition systems. However, they are still used for water heaters, central heating systems, and some fireplaces. Where used, pilot lights are a continuous source of ultrafine particle emissions to indoor air. Patel et al. (2021) used measurements from the HOMEChem study to estimate ultrafine particle number and mass concentrations associated with a number of sources, including the pilot light on a propane stove. They estimated a mass emission rate of ultrafine particles to be 0.9 ± 0.2 (standard deviation [SD]) µg/min and a number emission rate of sub-10 nm particles to be 1.6 ± 0.6 × 1012 particles/min with a GMD of ~5 nm. Bhangar et al. (2011) reported similar emission rates, of 0.58 × 1012 and 1.6 × 1012 particles/min for pilot lights in two California homes.

Wallace et al. (2008) completed 42 experiments in a National Institutes of Standards and Technology (NIST) test house to study UFP emission rates and size distributions produced by the burner flame and gas oven alone, e.g., without pots or food. For experiments with a single burner, the peak particle concentrations across all sizes were 4 to 8 times higher in the kitchen than in a bedroom, with peak values exceeding 106 particles/cm3 in the kitchen. While stovetop burners produced size distributions with GMDs in the range of 4 to 7 nm, the gas oven and broiler generally produced larger particles with GMDs of up to 24 nm. Peak emission rates of UFPs were estimated to be 2.8×1014 – 7.8×1014 particles/hr for a single gas burner with high gas flow and 1.8×1013 – 3.1×1014particles/hr for oven bake/broil experiments. The addition of food being cooked or water boiled did not appreciably change the geometric mean diameter of particles and generally reduced UFP emissions to some extent.

Singer et al. (2017) completed field measurements in nine homes with natural gas for cooking and measured particles with diameters of 6 nm or larger in the kitchen and bedroom area of each home. The ratio of the kitchen-to-bedroom 1-hour integrated particle count concentration ranged from approximately 1 to nearly 10, with differences associated with the residence floor

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

plan, e.g., the location of the bedroom relative to the kitchen. Boiling and simmering events were completed using the stovetop and oven in the absence and presence of range hood exhaust or air mixing by a forced air system. Particle number emission factors ranged from less than 5×106 to over 20×106 per joule of gas burned.

Studies for which ultrafine particle number emissions are back-calculated based on measurements in chamber or home air may significantly underestimate emission source strength if they do not account for coagulation and surface deposition of particles. Rim et al. (2016) completed experiments in a full-scale test house to determine UFP emissions (2–100 nm) for a number of sources, including unobstructed (no cooking) natural gas burners, with consideration of coagulation and deposition processes. Their measurements reflected a shift in particle sizes from smaller to larger over time, well predicted using a dynamic coagulation model. The UFP source strengths ranged from 1.7×1014/hr to 4.6×1015/hr, a higher upper bound than reported by others and values that are considerably higher than would have been the case if coagulation had not been accounted for when back-calculating emissions from indoor air measurements.

Past natural gas cooking studies have focused on concentrations or emissions of ultrafine particles, and few have reported particle mass concentrations or emissions. Hubbard et al. (2005) activated two gas burners for 15 minutes on a stovetop and observed a rapid 17 µg/m3 increase in PM1 concentration measured approximately 7 m from the stove, with subsequent decay shortly after switching off burners.

While the literature on natural gas combustion and its effects on indoor air quality is dominated by cooking appliances, several other sources exist, including unvented natural gas space heaters, unvented natural gas fireplaces, gas water heaters, and clothes dryers. Weichenthal et al. (2007) measured ultrafine particle concentrations outdoors and inside 36 homes in Canada and found that forced-air gas heating systems (n = 10 homes) were not an important predictor of indoor UFPs, particularly when compared with cooking and also cigarette smoking (n = 3 homes). Wallace and Ott (2011) observed significant increases above median background concentrations (multiplier of approximately 2.5 to 6) for UFP greater than 10 nm in diameter (instrument detection limit) when a vented gas space heater was used in the basement of a townhouse. Dutton et al. (2001) studied unvented gas fireplaces

in two homes and observed eight different PAHs, all with four or five rings, in PM2.5. Wallace (2005) studied a gas clothes dryer vented to the outdoors over an 18-month period in an occupied townhouse. Ultrafine particle concentrations increased during dryer use by a factor of 10, with short-term peak concentrations exceeding 100,000 particles/cm3 and a bimodal size distribution with peaks at less than 9.8 nm and 30 nm. The emission rate was estimated to be 6×1012 particles per drying episode, with emissions likely much higher given that measurements were limited to particle diameters of 9.8 nm and above. Wallace hypothesized that emissions were likely from the combustion chamber below the tumbler.

The chemical composition of ultrafine particles emitted from combustion of natural gas has not been widely explored, particularly in the United States. Murr et al. (2004) described the importance of natural gas combustion as a source of carbon nanotubes and other nanoform UFPs. A greater crystalline appearance of nanoparticles was evident for combustion of propane than for natural gas, perhaps an important difference for those who employ propane in rural areas and manufactured homes. However, the health significance of the chemical composition of ultrafine particles emitted from combustion of natural gas has not been widely explored, particularly in the United States. Murr et al. (2004) described the importance of natural gas combustion as a source of nanotubes and other nanoforms. A greater crystalline appearance of nanoparticles was

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

evident for combustion of propane than for natural gas, perhaps an important difference for those who employ propane in rural areas and manufactured homes. However, the health significance of nanostructures was not described, and the results were qualitative as opposed to quantitative. See and Balasubramanian (2008) analyzed the composition of UFPs emitted from the combustion of natural “town” gas in an apartment in Singapore. For steaming and boiling water to cook tofu they observed that approximately 50 percent of total PM2.5 emissions were in the form of organic carbon (OC). Sub-ng/m3 concentrations of individual PAHs were measured, summing to less than 0.05 percent of the total organic carbon (OC). A large number of metals were associated with UFPs. The source of these metals was not identified.

Combustion of Wood

Major devices used to combust wood indoors include traditional masonry fireplaces, wood stoves, pellet stoves, and masonry heaters. Wood combustion in these devices is the primary heat source for approximately 1.7 million U.S. homes and provides for some energy needs in another 10 million homes (EIA, 2023). It is also a major source of outdoor air pollution. In all but eight states, residential wood burning is one of the three largest contributors to ambient PM2.5 (Marin et al., 2022). In 2017, combustion of wood provided 2.2 percent of residential energy but was responsible for 98 percent of total PM2.5 emissions associated with residential fuel combustion (EPA, 2017).

There are two pathways for indoor exposure to fine particulate matter emitted by the residential combustion of wood. The first is direct emissions of smoke that escape from a device housed indoors, e.g., a wood stove. The second involves exhaust to outdoors by a chimney with penetration back into homes (becoming outdoor pollution of indoor origin) (Pierson et al., 1989). The remainder of this section is intended to summarize knowledge of fine particle emissions from the two major types of devices used for residential wood combustion: fireplaces and wood stoves.

The health effects of short-term exposures to wood combustion have been documented, but the evidence generally does not allow for a separation of the effects of PM2.5 and gaseous pollutants. Furthermore, health effects are often associated with outdoor pollutant concentrations. Residential wood combustion is estimated to be responsible for approximately 10,000 American deaths each year (Penn et al., 2017) as well as 44 percent of total stationary and mobile source polycyclic organic matter emissions and 25 percent of all air toxic cancer risks (EPA, 2015). Pollutants associated with wood combustion can increase susceptibility to respiratory infections, cause asthma symptoms and acute bronchitis, increase the risk of developing chronic obstructive pulmonary disease (COPD), particularly among women and smokers, elevate the risk of heart attacks, and lead to greater risk of hypertensive pregnancy disorders (Assibey-Mensah et al., 2019; Hopke et al., 2020; Marin et al., 2022; Naeher et al., 2007; Unosson et al., 2013). Furthermore, wood smoke has a disproportionate impact on rural and some low-income communities. Of rural homes, 27 percent use wood combustion for heating, while the same is only true for 6 percent of urban households (Marin et al., 2022). The amount of residential wood combustion is highest in households with annual incomes less than $40,000 per year (Marin et al., 2022). Up to 89 percent of homes in the largest sovereign Native American nation within the U.S. (Navajo nation) use wood stoves for heating (Environmental Law Institute, 2021).

Fireplaces are important sources of fine particulate matter emissions. The Environmental Protection Agency (EPA) estimated that there were over 17.5 million fireplaces in the United States in 2016 (EPA, 2016). Most are used to combust wood or synthetic logs as the fuel source.

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

For traditional wood-burning fireplaces, major factors that affect indoor pollutant emissions include the ventilation conditions of the fireplace at the time of combustion; the species of wood used as fuel and its moisture content; and combustion conditions, e.g., how wood is split and interacts with oxygen during combustion (Castro et al., 2018; Stabile et al., 2018). Most studies of wood-burning fireplaces have focused on indoor emissions of carbon monoxide, nitrogen dioxide, and coarse and fine particulate matter. A few have addressed the composition of particulate matter.

The inhalation dose of fine particulate matter emitted from fireplaces to indoor air can be significant. Stabile et al. (2018) completed sampling in 30 residences and observed an order of magnitude increase in estimated lung-deposited surface area of particles when an open fireplace was used as opposed to one with a closed opening. Buonnano et al. (2012a) studied 8- to 11-year-old children for 5 months and observed elevated values of inhalation dose for those who lived in homes with fireplaces. Dacunto et al. (2013) completed experiments to determine PM2.5 emissions from three open fireplaces using cherry wood and a commercial synthetic log. They observed steady PM2.5 emission rates of approximately 16 to 18 mg/min into the indoor spaces. A spike in emissions occurred when combustion was extinguished with water.

The composition of particle emissions from fireplaces is primarily organic carbon (OC) in nature with a small fraction of elemental carbon (Castro et al., 2018). Castro et al. (2018) studied an open fireplace with combustion of oak in a single home. They observed a 15.7 (mean) +/- 0.6 µg/m3 (standard deviation) increase in organic carbon above pre-burn baseline in the home. The count-median diameter was approximately 0.2 µm with most particles in the coarse mode in the range of 2 to 3 µm. The organic carbon fraction includes a range of polycyclic aromatic hydrocarbons (PAHs) (De Gennaro et al., 2016). A large number of metals have also been observed in particle emissions associated with wood smoke from fireplaces (Castro et al., 2018; Stabile et al., 2018).

Wood stoves are also major contributors to fine particulate matter in both outdoor and indoor air. Fleisch et al. (2020) measured indoor PM2.5 and its components in 137 homes occupied by pregnant women in northern New England. Moderately higher PM2.5 and much higher black and elemental carbon concentrations were observed in homes with wood stoves in operation. Non-EPA-certified stoves, older stoves, and wood that was not properly dried were associated with higher particulate matter concentrations, particularly black carbon. Black carbon (BC) is composed almost entirely of elemental carbon (EC) and is often found in fine PM as a result of incomplete combustion of fossil fuels and biofuels, and biomass, and therefore reflects the contribution of combustion sources to fine PM. By contrast, brown carbon (BrC) is defined as light-absorbing organic carbon and is emitted primarily by biomass burning.

Semmens et al. (2015) studied PM2.5 in 96 northwestern and Alaskan homes that used wood stoves as their primary heat source. They observed relatively high average indoor particulate matter concentrations in homes with wood stoves and an inverse association between household income and both PM2.5 and smaller size fraction particle number concentrations. Weichenthal et al. (2007) studied PM4 and UFP concentrations associated with wood stoves in seven Canadian homes. Homes using wood stoves had significantly higher overnight baseline UFP concentrations than homes in the same study that employed forced-air natural gas furnaces for heating. Median, 75th-percentile, and maximum UFP concentrations were higher in homes with wood stoves than in those using electric, natural gas, or oil heating systems. Salthammer et al. (2014) measured UFP and PM2.5 concentrations in seven German homes before, during, and after operation of wood stoves. They observed significant increases in UFP concentrations in

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

each home during wood stove use. Siponen et al. (2019) measured personal concentrations of PM2.5 and a BC surrogate for 37 elderly subjects over a 6-month period in Finland. They observed average increases in personal concentrations of 20 percent and 9 percent of PM2.5 and BC, respectively, when a wood stove was used for room heating. Frasca et al. (2018) studied the impacts of wood stoves in two residences and observed that the major source of exposure to particulate matter was during the removal of ashes from the stoves. This process was observed to release both fine and coarse particulate matter, with relatively high amounts of copper and manganese emitted to indoor air during ash removal from a pellet stove.

A number of factors are associated with lower fine and ultrafine particle emissions from wood stoves. In general, more efficient (typically newer) devices, pellet stoves (as opposed to stacked wood stoves), dry wood (<20 percent moisture content), wood seasoned for at least 6 months, natural fire starters, and small and hot (as opposed to smoldering) combustion all lead to lower emissions (Environmental Law Institute, 2021). The type of wood also affects particle emissions. Champion et al. (2017) completed experiments on a residential wood stove and observed higher PM2.5 and organic carbon emission factors (grams emitted per kg of wood burned) for ponderosa pine relative to Utah juniper, but a lower elemental carbon emission factor for ponderosa pine. Li et al. (2018) observed ponderosa pine to have consistently stronger oxidative stress and inflammatory effects relative to Utah juniper and even coal burned in the same stove, with low volatility organic compounds, elemental carbon, and several metals (Cu, Ni, K) all positively correlated with adverse cellular responses. Nystrom et al. (2017) observed that the burn rate of ponderosa pine affects the degree of soot particles and organic content, with metals in residual ash defined by the wood content.

Combustion of Oil, Coal, and Other Fuels for Residential Space Heating and Combustion Source Contributions in Schools

Emissions stemming from the combustion of several other fuels used for indoor heating have been reported in the published literature, but to a much lesser extent than for natural gas and wood. Champion et al. (2017) reported emissions from the combustion of two high-volatile bituminous coals used in wood stoves for heating in the Navajo nation. Average emission factors (g of pollutant / kg of fuel burned) for PM2.5 and organic carbon were approximately an order of magnitude greater for coal than for two types of wood fuel. Schripp et al. (2014) studied four different unvented fireplaces with eight different ethanol-based fuels (liquid, gel, and paste) in a laboratory chamber and observed elevated number concentrations of UFPs relative to background chamber air. Similar results have been observed in homes in Chile when unvented kerosene space heaters were used for heating, with elevated levels of PM2.5, organic carbon, elemental carbon, metals, and PAHs (Ruiz et al., 2010). Indoor UFP concentrations associated with forced air oil furnaces in 10 Canadian homes were not statistically different when compared against homes using electric baseboards, natural gas, or wood stoves for heating (Weichenthal et al., 2007).

Few studies have focused on particle emissions associated with combustion sources in schools. Matthaios et al. (2022) used a least absolute shrinkage and selection operator (LASSO) mixed-effects model to study factors influencing fine particulate matter, BC, and nitrogen dioxide in 309 classrooms in 74 inner city schools in a large northeastern U.S. city. Factors that were positively associated with PM2.5 in classrooms included proximity to a school cafeteria and classrooms with windows facing a bus loading area. Time since furnace cleaning was positively

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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associated with BC concentrations in classrooms, accounting for 19 percent (absolute) of the 23 percent of school-related factors associated with BC.

Combustion of Candles

Candles are generally used for several hours after ignition (Wallace et al., 2019), with extensive use around holidays (Andersen et al., 2021). Primary locations in U.S. homes, in descending order, include living rooms, kitchens, and bedrooms (Wallace et al., 2019).

Studies on the health effects of candles are sparse. Lim et al. (2022) cited several studies with conflicting results related to the effects of candle emissions on reduced lung function. Lim et al. (2022) also studied inflammatory markers and lung function for pollutants of both indoor and outdoor origin and women and men between the ages of 49 and 63 years old in Copenhagen suggested no adverse effects of candles on lung function. Loft et al. (2022) found no statistically significant associations between candle use and risk of cardiovascular and respiratory events based on a cohort of 6,757 participants in Copenhagen, Denmark. Shehab and Pope (2019) exposed human subjects to candle smoke in a room, and their tests indicated a statistically robust decline in cognitive function after exposure.

Candles vary by type of wax (fuel), fragrance ingredients and load, type and composition of wick, colorants, and shape (e.g., filled container versus open pillar) (Andersen et al., 2021; Salthammer et al., 2021). The primary waxes used in candles consist of C20 to C40 hydrocarbons, long-chain fatty acids, and their esters (Salthammer et al., 2021). Common waxes include paraffin, stearin, beeswax, soy, palm oil, and associated mixtures (Andersen et al., 2021; Salthammer et al., 2021). The typical contribution of fragrances varies from 0 percent (unscented) to approximately 5 percent of overall candle weight and are generally essential oils and their mixtures (Derudi et al., 2012; Salthammer et al., 2021). The National Candle Association estimates that more than 10,000 different candle scents are available in the United States (National Candle Association, n.d.). Candle wicks are generally cotton and sometimes paper and vary by length, thickness, and the additives used as flame retardants (to control flame). Wick additives may vary for different fuels and are generally inorganic, e.g., phosphates and nitrates (Andersen et al. 2021; Salthammer et al. 2021).

Burn rates for candles are generally in the range of approximately 3 to 7 g/hr (Andersen et al., 2021; Salthammer et al., 2021). Burn modes include steady burn (generally not disturbed by air flow), sooting burn (when a flame is flow-disturbed), and smoldering (immediately after extinction). These modes greatly influence the nature of emissions from candles, particularly particle size and composition (Pagels et al., 2009).

The composition of UFPs emitted by candles is dominated by water-soluble inorganic compounds associated with the burning wick (Andersen et al., 2021). UFPs contain little elemental carbon (EC) or black carbon (BC) (Andersen et al., 2021). Emissions during unsteady burning (flickering flame) can lead to larger particles with the potential for significant emissions of BC (Andersen et al., 2021; Hu et al., 2012). Candles also emit PAHs in both the particulate and gaseous phases, and a wide range of organic and inorganic gases (Andersen et al., 2021; Derudi et al., 2012, 2014; Salthammer et al., 2021). Andersen et al. (2021) reported PAH emissions of 25–578 ng/hr. Salthammer et al. (2021) reported a similar range for summed PAHs of 79–1,286 ng/hr and noted that PAH emissions for unscented candles were much less than those for scented candles. In both of these studies, gas-phase and particle phase PAHs were not separated.

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Significant variations in reported particle emissions exist in the published literature. Variations occur due to different burn rates, type of wax, type of wick, the extent and nature of fragrances, and burn conditions. Reported emissions of UFPs vary across approximately four orders of magnitude. Accordingly, candles can be major or even dominant sources of particles by number concentration in homes. For example, they were observed to be a major source of particle number concentrations in half of 56 non-smoking homes studied in Copenhagen, contributing 60 percent of exposure to particles in the diameter range of 10 to 300 nm (Bekö et al., 2013). J. Zhao et al. (2020) measured number size distributions (10–800 nm) in 40 German households over 500 days and approximately 800 indoor source events. They observed the highest emission rates from burning candles (5.3×1013 particles/hr). Burning candles and opening windows lead to seasonal differences in the contributions of indoor sources to residential exposures.

Emission rates for ultrafine particles have been reported by several research teams. Salthammer et al. (2021) reported a range of UFP emissions from 5.9×1010 particles/hr to 3.2×1012 particles/hr with a median of 9.4×1011 particles/hr across 24 experiments with a wide range of wax and fragrance conditions under steady burn conditions. They observed a particle size range of 6–60 nm with a count median diameter of 19 nm initially, dropping to 12 nm after 4 hours of steady burn. Soy-based candles had the highest emission rates across scented candles, and paraffin and palm-based candles had the lowest emission rates. Among candles without fragrances, palm and stearin-based candles had the highest emissions, and paraffin and soy-based candles had the lowest emissions. Andersen et al. (2021) reported an emissions range of 1.5×1013 particles/hr to 9.3×1013 particles/hr for five pillar candles with varying wax and wick compositions and steady vs. unsteady burn conditions. The particle diameter mode ranged from 5 to 8 nm for four candles, with a bimodal distribution with peaks at 6 nm and 200 nm for a fifth candle. Wallace et al. (2019) observed a mean UFP emission rate of 4.3×1014 particles/hr with a standard deviation of 4.6×1014 particles/hr. Some types of candles exhibited steady burn conditions while others exhibited sooting burn conditions. They suggested that the higher UFP emission rate than others had reported was due to the inclusion of a smaller particle size bin (2.33 to 2.5 nm) that dominated particle counts. They also accounted for coagulation and particle decay by deposition to back-calculate emissions; other studies used concentration measurements in chamber air without consideration of particle growth.

Reported emission rates for PM2.5 also vary considerably. Andersen et al. (2021) reported a range of 283–3,038 μg/hr, with EC emissions in the range of 30–3,132 μg/hr and OC emissions of 46–232 μg/hr, under sooting burn conditions. Derudi et al (2014) observed a range of 5.8–270 μg/hr with particles less than 250 nm dominating overall emissions. The authors did not provide specific burn conditions. Salthammer et al. (2021) reported a range of 16–379 μg/hr with palm and stearin-based candle emissions lower than soy and paraffin-based candles under steady burn conditions. On average, they observed candles without fragrance addition to have lower PM2.5 emissions than those with fragrances.

Several studies have reported particle size ranges for different burn modes. Manoukian et al. (2013) reported a particle number mode of less than 11 nm for steady burn and a bimodal distribution with peaks at less than 11 nm and at 92 nm following flame extinction (smoldering candle). Pagels et al. (2009) reported a particle number mode between 20 and 30 nm for steady burn and geometric mean diameters of 270 nm for sooting (unsteady burn) and 335 nm for a smoldering candle. An in-room particle diameter mode of 11–26 nm with a close-to-source range of 7–18 nm was reported by the Danish Environmental Protection Agency (2017).

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Combustion of Incense

Incense is burned inside homes and in some public places, e.g., retail stores, places of worship. It is used for sacred purposes by various religions across the world. In some cultures, incense is burned in the home daily in conjunction with religious rituals (Jetter et al., 2002). Among some indigenous communities of North America, the practice of smudging involves burning sacred natural medicines (e.g., sage, cedar, sweet grass) to pray and purify oneself or a specific physical space (Ko, 2020). Incense comes in various forms, including sticks (common in the United States), Joss sticks, cones, coils, rope, powders, and smudge. In addition to religious services and rituals, it is used broadly for purposes of aromatherapy and odor masking.

Incense use in the United States is increasing. In 2018, the U.S. incense market size was $128 million and is forecast to reach $281 million in 2025, growing at an expected compound annual growth rate of approximately 12 percent from 2018 to 2025 (Francis, 2020). The United States is the top importer of incense in the world, with most of its incense products coming from India, China, and Vietnam; annual import shipments to the U.S. currently stand at 71,500, imported by 2,504 U.S. importers from 1,143 suppliers (Volza, n.d.).

Incense commonly has two main ingredients: an aromatic material that is usually plant-based and a combustible base that holds the aromatic material together. The aromatic (fragrance) is released during the burning of the combustible base. Aromatic materials include wood and bark, herbs, seeds, spices, essential oils, and synthetic substitute chemicals (Jetter et al., 2002). The combustible base is often wood powder, bamboo, mucilage, and sometimes charcoal (T.-C. Lin et al., 2008; Live Smoke Free, n.d.). Incense sticks typically employ bamboo or wood for the actual “stick” onto which the incense powder is held.

The health impacts associated with exposure to incense smoke have been studied to a much greater extent than for candles. As with candles, effects on cognition have also been reported. Greater than weekly incense use by older adults in Hong Kong have been associated with poorer cognitive performance over 3 years (A. Wong et al., 2020). Mutagenic and genotoxic effects of incense smoke have also been studied. Chen and Lee (1996) reported incense smoke condensates to be mutagenic or genotoxic or both. The genotoxicity of certain incense smoke condensates in mammalian cells was observed to be higher than from tobacco smoke condensates. Friborg et al. (2008) studied over 61,000 individuals (ages 45–74) from the Singapore Chinese Health Study completed between 1993 and 1998. The individuals were initially diagnosed as being cancer free and were followed to 2005. A strong association was observed between use of incense over the subsequent 7 to 12 year period and increased risk of squamous cell carcinoma of the respiratory tract. Similarly, Geng et al. (2019) used the same initial cohort coupled to a Singapore renal registry in 2015. They observed a likely increase in end-stage renal disease for long-term daily exposure to domestic incense smoke.

Tse et al. (2011) observed an association between lung cancer and incense exposure among male smokers in China but did not find an association with non-smokers. Jetter et al. (2002) reported on three earlier epidemiological studies where no association was observed between incense smoke and lung cancer. However, in these studies incense burning was associated with greater affluence, a healthier lifestyle, and better diet that may have affected study outcomes.

The effects of incense burning on children have also been studied; the reader is directed to cited papers for details of study design and greater insights related to outcomes. Lowengart et al. (1987) used a case–control study of children of ages 10 years and under in Los Angeles County to investigate causes of leukemia. An increased risk was found for children whose

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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parents burned incense in the home; the risk was greater for frequent use. Wei et al. (2018) studied 15,310 infants in Taiwan and found that household incense burning was associated with a delay in gross motor neurodevelopmental milestones. Incense burning showed effects on coughing symptoms in primary school children in Taiwan (C.-Y. Yang et al., 1997).

Numerous researchers have studied particles and a wide range of speciated gas emissions from the burning of incense. Past studies have largely involved the measurement of dynamic or approximately steady-state pollutant concentrations in controlled laboratory chambers while burning one or more of the same type of incense, e.g., stick or cone. Measured emissions of PM2.5 are generally much greater than those reported for candles during steady or soot burn conditions, with particle size distributions shifted to larger particles compared with candles. Incense sticks burned in a test house led to concentrations in some locations of hundreds of μg/m3 (Ji et al., 2010).

Jetter et al. (2002) reported PM2.5 emissions from 23 different types of incense. Emission rates ranged from 7 to 202 mg/hr, with emission factors of 5–56 mg/g of incense burned. Smudge bundles and cone incense exhibited the highest emission rates. Emissions for incense sticks ranged from 7 to 108 mg/hr. The authors concluded that “incense emits fine particulate matter in large quantities compared to other indoor sources” and completed model simulations for a small room with predicted concentrations of PM2.5 that exceeded several thousand μg/m3.

Lee and Wang (2004) measured PM2.5 emissions from 10 different incense products (eight sticks, one bar, and one rock) purchased from around the world. Incense sticks had emission rates that varied from 10 to 301 mg/hr, reasonably consistent with the findings of Jetter et al. (2002). The rock emitted nearly 2,200 mg/hr. Emission factors ranged from 7.7 to 99.7 mg/g of incense burned for sticks and 205 mg/g burned for the rock incense.

See and Balasubramanian (2011) measured PM2.5 emissions and composition for six different brands of incense sticks procured in Singapore. The mean emission rates for PM2.5 varied from 18.5 to 60.9 mg/hr for five of the six incense sticks (with mean emission factors of 18.3–44.5 mg/g of incense burned) and only 0.6 mg/hr for the sixth (with an emission factor of 0.4 mg/g of incense burned). The sixth incense stick was marketed as “smokeless” but additional information was not provided by the authors. Emission factors for elemental and organic carbon ranged from 0.02 to 4.36 mg/hr and 0.04 to 44.4 mg/hr, respectively, with organic carbon to elemental carbon ratios (OC/ECs) varying between 0.09 and 0.56. Wang et al. (2006) tested 10 different types of incense and observed OC/EC ratios reasonably consistent with those reported by See and Balasubramanian (2011). Specifically, they observed a range of OC/EC of 0.07–0.39 for traditional incense, with an average of 0.22. For aromatic incense the OC/EC ratios were lower, with a range of 0.032–0.12 and an average of 0.077.

Median diameters of emitted particle size distributions from the burning of incense are generally larger than those for candles. See et al. (2007) completed real-time characterizations of the size distribution and number concentration of sub-micrometer particles emitted from incense smoke for four different brands of sandalwood and aloeswood incense sticks. Particle emission rates varied from 5.1×1012 to 1.42×1013/hr. Peak diameters ranged from 93.1 to 143.3 nm.

Aside from OC/EC ratios, a few authors have also provided insights into the composition of incense smoke. See and Balasubramanian (2011) observed Al and Fe to be the most abundant metals associated with incense particles. B. Wang et al. (2006) observed significant variability in the composition of different brands of incense, but in general Na, Cl, and K dominated. On average, inorganic ion concentrations were such that traditional incense > church incense > aromatic incense. Li et al. (2022) studied seven different types of incense from China and

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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characterized emission factors and composition of PAHs associated with emitted particles. PAHs constituted the largest proportion (41.5–63.7 percent) of the total quantified organics.

C. R. Yang et al. (2012) carried out laboratory experiments to explore source reduction of particulate matter (assumed to be PM2.5 but not stated in paper) and PAHs by the addition of calcium carbonate (CaCO3) to 10 different types of incense sticks. They observed a significant reduction for both as a function of the amount of CaCO3 added (mean particulate matter reductions of up to 41 percent for 30 percent CaCO3 addition).

Indoor PM from Other Heating Processes

Other indoor heating processes beyond combustion can also contribute significant amounts of PM to indoor environments. Examples include meal cooking (Y. Chen et al., 2016; Katz et al., 2021; Patel et al., 2020; Torkmahalleh et al., 2017), heating cooking utensils (Wallace et al., 2015), heating surfaces such as hot-water or electric radiators for indoor heating (Afshari et al., 2005), and even the operation of office and consumer products such as printers (He et al., 2007; Schripp et al., 2008; Scungio et al., 2017), electronic cigarettes (Fernández et al., 2015; Fuoco et al., 2014; Nguyen et al., 2019), and heated scent diffusers (Su et al., 2007).

Indoor Cooking Activities

Cooking is a major heating process that takes place daily in most residential environments and many schools. Thus, cooking is a significant source of indoor PM2.5, particularly in homes but also in school cafeterias. Cooking activities have been characterized as emitters of particles from two distinct sources: the heating source and heating the food itself. The process of heating up food, regardless of the fuel or heating source that is used, leads to the evaporation of food constituents, which then recondense as particles once they reach ambient temperature. A previous section of this chapter included emissions specific to natural gas combustion as part of a broader discussion of natural gas appliances in homes. That section did not include emissions from cooking oils or food itself, but rather just the fuel. The current section also focuses on cooking with heat sources other than natural gas or propane.

Several factors can influence the emission of cooking aerosols. Foods with higher fat content have been found to have higher particle emission rates than those with less fat (Buonanno et al., 2009). Additionally, the total exposed surface area, the smoke point of the oil used, the presence of salts, and cooking temperature have also been found to affect PM emissions from cooking (Sankhyan et al., 2022; Torkmahalleh et al., 2017). High-temperature processes such as frying, grilling, broiling, and roasting have been shown to lead to high number and mass PM concentrations (Abdullahi et al., 2013; Buonanno et al., 2011). Once cooking aerosols are emitted, they can be removed from the air via a variety of processes, described in more detail in Chapter 4 and Chapter 7 of this report. Cooking activities performed on countertop appliances (e.g., toasters and toaster ovens, countertop induction cooktops, electric pots, etc.) are of particular interest for indoor PM emissions from food and heated surfaces because their emissions are less likely to be vented using range hoods, unlike many stove tops and ovens.

The composition of indoor cooking organic aerosol (COA) was found to encompass a majority of compounds with molecular formulas containing carbon, hydrogen, and oxygen atoms only, followed by nitrogen-containing organic compounds (Masoud et al., 2022). A review by Abdullahi et al. (2013) reported the following major groups to characterize COA: alkanes, fatty acids, dicarboxylic acids, lactones, polycyclic aromatic hydrocarbons, alkanones and sterols.

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Particle-phase amides from cooking protein-rich foods have also been reported, with concentrations ranging from 45 to 218 μg/g (Ditto et al., 2022). Multiple studies have shown, through field and laboratory measurements, that cooking emissions, particularly cooking oils, generate aerosols with a range of volatilities, including a lower-volatility component not traditionally included in ambient COA models; this component has also been described as a “nonvolatile core” in some studies (Buonanno et al., 2011; Pothier et al., 2023; Sankhyan et al., 2022). Sankhyan et al. (2021) reported enhancements in BC and brown carbon (BrC) aerosol concentrations during indoor cooking activities and observed varying BC/BrC ratios. While breakfast (pan-fried sausage, fried eggs, fried tomato, toast, and coffee) emitted more BC than BrC, a traditional Thanksgiving meal (oven-roasted turkey, bread stuffing, brussels sprouts, sweet potato casserole, pies, cranberry sauce, and gravy) emitted more BrC than BC, and cooking a vegetable stir fry and a beef chili both emitted similar concentrations of BC and BrC. Despite significant growth in knowledge on the composition of cooking aerosol, some gaps in the literature remain in terms of the ultrafine and single-nanometer components of these emissions.

Furthermore, food preparation and cooking may release allergen-containing particles into the air (Kumar et al., 2021; Shale and Lues, 2007). In fact, exposure to some food ingredients via paths other than ingestion, e.g., skin contact and inhalation when associated with fine PM, is probably an underrecognized and underreported route for adverse reactions in highly sensitive individuals, as described by Ramirez and Bahna (2009). In residential and school settings, commonly reported food allergens include wheat flour, seafood, soy, peanuts, nuts, eggs, and cow’s milk. Common manifestations of allergic reactions by inhalation include respiratory and ocular symptoms as well as skin manifestations (Ramirez and Bahna, 2009). According to a review by Caffarelli et al. (2016), inhalation of food allergens has been reported to lead to asthmatic symptoms in children.|

Studies performed in residential environments have shown that cooking is a major source of indoor PM2.5, often leading to short-term, but intense increases in indoor PM concentrations. Wallace and Ott performed over 300 measurements in several homes and documented sharp bursts of particulate matter during several cooking-related activities, such as stovetop cooking, baking or broiling in the oven, using a toaster oven, and even popping corn in an air popper (Wallace and Ott, 2011). Another study in 15 homes in Australia reported that indoor frying and grilling elevated indoor PM2.5 concentrations by 30- and 90-fold, respectively, compared with background levels (He et al., 2004). The HOMEChem study found that cooking emissions led to indoor PM2.5 concentrations exceeding 250 μg/m3 (Farmer et al., 2019; Patel et al., 2020). Emissions in terms of particle (>10 nm in size) number from cooking activities have been estimated to be in the order of 1013 particles per hour of cooking (Patel et al., 2021; J. Zhao et al., 2020).

Even the mass concentration of ultrafine particles (PM <100 nm in diameter) has been found to exceed 100 μg/m3 during intense cooking events, such as the preparation of a Thanksgiving holiday meal (Patel et al., 2020). As previously noted, J. Zhao et al. (2020) performed a study in 40 German homes and found that several cooking activities, including baking, frying, using a toaster, and others, all led to particle size distributions peaking at <100 nm in size. Buonanno et al. (2009) also showed cooking emissions consisting mostly of ultrafine aerosols in terms of particle number, with particle mass peaks extending into coarse mode. Torkmahalleh et al. (2012) also reported a majority (up to 99 percent) of particle numbers in the

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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10- to 100-nm size range from heating cooking oils, with PM2.5 emission fluxes ranging from 3×105 to 6×106 μg/min/m2, normalized by the exposed surface area of oil.

In school environments, the majority of recently published studies report results obtained in other countries, particularly in Asia (Jung and Su, 2020; I.-J. Lee et al., 2022; Q. Xie et al., 2022). Published PM2.5 measurements in cafeterias or other cooking facilities in U.S. schools are sparse (Majd et al., 2019; Zhang and Zhu, 2012). A study performed in 25 public schools in the Republic of Korea found an average PM2.5 concentration of about 25 μg/m3 during cooking of “oily” foods, while outdoor concentrations were about 18 μg/m3. A study performed in different microenvironments in a university in Beijing found that the dining hall had slightly higher PM2.5 concentrations than the other indoor areas (e.g., classroom, dormitory, laboratory, etc.). In the dining hall, cooking was performed using gas-fueled appliances. Nevertheless, the majority of indoor PM2.5 exposure of students in this study was due to PM infiltration from outdoors (Q. Xie et al., 2022). PM2.5 concentrations were reported in the 80–100 μg/m3 range in two university buffets in Greece (Kogianni et al., 2021), much higher than the averages reported in other studies for indoor cafeterias, likely due to cooking activities and potentially due to indoor smoking as well. This demonstrates that indoor concentrations are likely to vary greatly depending on the amount and type of cooking activity that takes place in each cafeteria and whether there is indoor smoking.

Cookware and Other Appliances

The process of heating cooking utensils and instruments themselves, even without food, has been found to lead to the formation of particles attributed to desorption of semi-volatile organic compounds (SVOCs) present on their surface, which then recondense in the indoor air and form ultrafine particles. Wallace et al. (2015) provided evidence that organic compounds that continually deposit on indoor surfaces lead to an organic film reservoir that forms particles in air after surface heating. Dishwashing soap residue was also shown to lead to the production of large amounts of particles after surface heating. Metal objects such as pots, pans, griddles, stovetop burners, and toaster ovens all led to ultrafine particle emissions once heated (Wallace et al., 2015, 2017). This phenomenon was also observed in previous studies, although many of these studies investigated the effects of indoor dust deposition onto surfaces (Afshari et al., 2005; Ciuzas et al., 2015; Dennekamp et al., 2001; Glytsos et al., 2010; Pedersen et al., 2001). Pedersen et al. (2001) found that particle emissions occurred when the dust-laden surface was heated to at least 100 °C and that major emissions took place at >200 °C. Torkmahalleh et al. (2018) demonstrated that heating an empty pan led to similar particle number concentrations in an indoor environment as heating meat on the same pan and that particle number emissions (>10 nm in diameter) from the electric-coil stove top used for heating were negligible by comparison. This phenomenon is not exclusive to cooking surfaces; it has also been described for heating processes such as hand dryers, hair dryers, and irons, as described later in this chapter. Silberstein et al. also documented new particle formation events during mechanical heating, ventilation, and air conditioning system (HVAC) use overnight in winter following cleaning activities--evidence of SVOC desorption from HVAC surfaces by the furnace system (Silberstein et al., 2023).

Office Equipment

There are many office products whose use involve heating processes, which can lead to the evaporation of a wide variety of compounds, thus generating ultrafine aerosol upon

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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condensation in indoor air. Office products such as photocopiers, printers, and even 3D printers can be important sources of PM2.5 (mainly as UFPs) in schools and in some residential microenvironments.

Laser printers are known to emit significant amounts of particles, mostly in the submicrometer and ultrafine size ranges, according to multiple studies in office environments and in controlled chambers (C. He et al., 2007; McGarry et al., 2011; Schripp et al., 2008; Setyawati et al., 2020; Shi et al., 2015). Inkjet printers, on the other hand, have shown negligible particle emissions (Shi et al., 2015). Kogianni et al. (2021) measured PM2.5 concentrations in 20 different work environments and found average PM2.5 concentrations in the 11–15 μg/m3 range for photocopying centers and printing shops. The printing shop exhibited the highest concentration of zinc-containing aerosols of all investigated locations. McGarry et al. (2011) showed that the peak exposure to particles from laser printers can be greater than 2 orders of magnitude higher than background levels. But not all laser printers are strong particle emitters; there is great variability among printers in terms of PM emissions and their effects in indoor spaces. A study in 62 office rooms in Germany showed that PM2.5 concentrations increased in 70 percent of offices while printing a 500-page document using a laser printer (Tang et al., 2012). Shi et al. (2015) classified approximately 67 percent of the 55 laser printers they investigated in a controlled chamber study as “high particle emitters.” C. He et al. (2007) investigated 62 printers and reported that 60 percent of those did not emit submicrometer particles.

Laser printer and photocopier toner formulations include organic and elemental carbon as well as a variety of metals and metal oxides, which can all become airborne during printing (Pirela et al., 2015). Morawaka et al. (2009) demonstrated that the particles are formed during printing when the fuser unit heats the paper and the toner, volatilizing compounds that then recondense in the indoor air. This work also showed that unstable temperature conditions were the main driving factor for particle emission in the high-emitting printers. Follow-up work a decade later by the same group showed a reduction in emissions for large, commercial printers but not for desktop printers (Moraska et al., 2019). Laser printers and photocopiers contain engineered nanomaterials; inhalation exposure to these particles may lead to oxidative stress and respiratory tract inflammation, causing a variety of respiratory symptoms (Pirela et al., 2017).

Beyond laser printers and photocopiers, there is now a large body of knowledge on PM emissions from three-dimensional (3D) printers, particularly fused filament fabrication (FFF) printers. Emissions from 3D printers that employ other types of technologies have also been identified, but at a significantly lower scale compared with FFF printers (Afshar-Mohajer et al., 2015; Hayes et al., 2021b). Powder-binder jetting printers have been shown to emit coarse-mode particles, with emissions varying according to the type of powder material used (Hayes et al., 2021a).

FFF 3D printers have been shown to emit copious amounts of ultrafine PM in office spaces (Stephens et al., 2013), homes (Khaki et al., 2021), and a classroom (Vance et al., 2017) as well as in controlled laboratory studies (Azimi et al., 2016; Jeon et al., 2020; Majd et al., 2019; Mendes et al., 2017; Vance et al., 2017; Yi et al., 2016; Zhang et al., 2018). As with other sources described in this section, FFF 3D printer emissions occur when the printer nozzle heats and vaporizes a variety of semivolatile compounds, which then condense to form particles in the air (Zhang et al., 2018). Due to their small size, over 60 percent of inhaled particles from 3D printer emissions are estimated to deposit in the respiratory system, primarily in the alveolar region (J. Park et al., 2021). A comparison across different ages showed that the total PM mass deposition of particles from 3D printer operation is highest for people in the 9- to 18-year-old

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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age group, of particular interest to school environments (Byrley et al., 2021). PM emissions vary greatly with filament type (Gu et al., 2019; Vance et al., 2017; Zhang et al., 2019) and extrusion nozzle temperature (Jeon et al., 2020; Zhang et al., 2019). Among the two most popular types of FFF printing filaments, acrylonitrile butadiene styrene (ABS) leads to much higher emissions than polylactic acid (PLA) (Gu et al., 2019; Vance et al., 2017). However, particles emitted from 3D printing with PLA were shown to be more toxic than ABS-emitted particles at comparable mass doses in both in vitro and in vivo studies (Zhang et al., 2019).

Other Consumer Products and Hygiene/Personal Care Products that Involve Heating

Several commonplace personal products in the consumer market employ heating during use for a variety of purposes, from vaporizing fragrances to modifying hair or drying hands.

A variety of scenting products employ heat to vaporize a mixture of fragrances or essential oils into the indoor air. These consumer products are commonly used to mask odors and to promote psychological well-being (e.g., aromatherapy) in homes and in some school microenvironments such as bathrooms. Indoor scenting products include scented candles (discussed earlier in this chapter), wax warmers, plug-in or spray air fresheners, and essential oil diffusers, among others. Essential oil diffusers include a wide variety of products that employ different mechanisms to aerosolize or vaporize oils into the air, including heat (e.g., from a candle or electricity), ultrasonic vibrations, nebulizing actions, and capillary (i.e., wicking) action. Some of these products use heat to vaporize fragrances, whereas others emit sprays or mists directly into the air. The latter is discussed later in this chapter. Studies looking at direct PM2.5 emissions or indoor concentrations from the use of heated fragrances are scarce. Demanega et al. (2021) reported modest increases in indoor PM2.5 concentrations from heating essential oils using a candle in a chamber; concentrations peaked at 10 μg/m3 in cool and dry conditions and 31 μg/m3 in warm and humid conditions.

There is a paucity of research in the scientific literature on PM emissions from heating hair styling tools (e.g., hair dryers, hair straighteners, and curling irons). However, these products are expected to lead to indoor particle emissions due to their high surface temperatures, reported in the news media to be up to 232 °C for flat irons, and to the likely presence of oils from hair and scalp as well as a variety of hairstyling products (Kaplan, 2020; Leon, 2012). High PM2.5 concentrations observed in field measurements in hair salons have been attributed to the use of hair dryers and flat irons (Shao et al., 2021). Glytsos et al. (2010) and Ciuzas et al. (2015) operated hair dryers in laboratory settings, and both studies reported large enhancements in ultrafine PM concentrations. Meanwhile, Hussein et al. (2006) showed that operating a relatively new hair dryer in a residence had negligible effect on PM concentrations. Chamber studies by Sysoltseva et al. (2018) and by Schripp et al. (2011) found large variability among the PM emissions from different hair dryers, with the commonality that the emissions were ultrafine. One study (Taylor et al.,, 2017) included examination of two ionic hair dryers marketed as emitting silver nanoparticles to promote hair growth. It concluded that the mass of these particles was below the limit of detection in the studied models; no other PM measurements were reported. A laser hair removal procedure has been found to emit high concentrations of ultrafine aerosols, leading to an eight-fold increase in particle number concentrations above background (Chuang et al., 2016).

A clothes iron is another indoor appliance that, similarly to some hair tools, employs a surface that is heated to high temperatures (commonly 180–200 °C), and can emit ultrafine aerosols when heated (Wallace et al., 2015). Vicente et al. (2021) reported very high particle

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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number emission rates for ironing clothes, on the order of 1012 particles/min or approximately 2-8 μg/s1 of PM2.5.

Electric hand dryers are common in public bathrooms and may be used in many schools. These products may emit particulate matter during operation owing to surface heating; however, there is limited evidence published in the scientific literature. Bae et al. (2013) reported minor enhancements in particle counts (from 1 to 20 particles/cm3) while testing a nano-coated hand dryer in a chamber.

Electronic Cigarettes

Electronic cigarettes (e-cigarettes) are battery-powered devices that heat a liquid solution (commonly called “e-liquid”) and convert it into a vapor that can be inhaled. The liquid typically contains nicotine, flavorings, and other components. E-cigarettes are often marketed as safer alternatives to traditional cigarettes because they do not produce smoke byproducts. However, there is still some debate over their long-term health effects and potential risks, particularly in non-smokers and young people, who may be more likely to start using nicotine products as a result of e-cigarette marketing (Giovenco et al., 2016; Huang et al., 2019).

Although e-cigarettes do not involve combustion, they release aerosols that can remain in the air and on surfaces of indoor environments for extended periods after use. Multiple studies have investigated e-cigarette emissions in chambers (Schripp et al., 2013) and real indoor environments such as homes (Fernández et al., 2015; Loupa et al., 2019; Shearston et al., 2023), offices (Saffari et al., 2014), and vape shops (L. Li et al., 2021; Son et al., 2020), demonstrating that PM2.5 emissions are significant, although much lower than conventional cigarettes. There is significant evidence that e-cigarette-emitted aerosols contain a range of VOCs, particularly formaldehyde and acetaldehyde, in addition to ultrafine and fine particulate matter. Some common components of e-cigarette particles include nicotine, propylene glycol and glycerin (used to create a visible aerosol mist), flavorings, and trace amounts of metals such as nickel, zinc, lead, and chromium from the cartridge and heating the metal coils (Fernández et al., 2015; Li et al., 2020; Saffari et al., 2014; Salamanca et al., 2018; Son et al., 2020; Talih et al., 2016; Zhao et al., 2017). E-cigarettes have also been shown to emit particle-associated reactive oxygen species and environmentally persistent free radicals (Hasan et al., 2020).

E-cigarette emissions and their effects on indoor air quality depend on a variety of factors, including the type of device, the temperature and power settings used, the type of e-liquid or cartridge used, and the frequency and duration of use. Protano et al. (2018) showed that, over time, e-cigarette products have employed progressively lower electrical resistance and higher power conditions, leading to increasing PM2.5 emissions. Several studies and literature reviews have reported particle emissions to consist mostly of ultrafine and submicron particles (L. Li et al., 2020; Protano et al., 2018; Saffari et al., 2014; Volesky et al., 2018; T. Zhao et al., 2016).

E-cigarette use has been widely reported to lead to secondhand exposure (also referred to as “passive vaping”) to exhaled emissions by nearby users (Islam et al., 2022; Protano et al., 2018; Schripp et al., 2013; Volesky et al., 2018; M. P. Wang et al., 2016; Zhao et al., 2017). In addition, e-cigarette aerosols can leave residue on indoor surfaces and particles, which can accumulate over time and potentially affect indoor air quality, commonly referred to as thirdhand exposure (Acuff et al., 2016; Goniewicz and Lee, 2015). Colby et al. (2023) found that residual emissions from an electronic cigarette partitioned from surfaces onto other airborne particles in a manner similar to compounds from conventional cigarettes (DeCarlo et al., 2018).

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

While e-cigarettes may be considered by users a less harmful alternative to traditional cigarettes, the health effects of inhaling e-cigarette particles are not yet fully understood, and their use is not risk-free. In vivo and in vitro studies have shown that exposure to e-cigarette emissions can lead to potential cytotoxicity and genotoxicity, probably due to exposure to reactive oxygen species and aldehydes (Ma et al., 2021; Merecz-Sadowska et al., 2020). E-cigarette use has been associated with a variety of respiratory and cardiovascular effects, particularly in children and adolescents (Islam et al., 2022; M. P. Wang et al., 2016). Specifically, secondhand nicotine vaping has been associated with bronchitic symptoms and shortness of breath in young adults (Islam et al., 2022). Hypersensitivity pneumonitis has been reported as arising from firsthand and, in rare cases, secondhand exposure to e-cigarette emissions (Galiatsatos et al., 2020). These and other health effects may become important, particularly for people with underlying respiratory conditions.

Indoor Particle Resuspension and Shedding

The movement of people or equipment indoors can detach and lift particulate matter previously deposited on surfaces. This phenomenon is called particle resuspension, and it can increase indoor particle concentrations significantly (Thatcher and Layton, 1995). While resuspension is more likely to occur in large (>1 μm, especially >10 μm) particles, it can also affect fine PM. In fact, everyday activities performed in a home, such as walking, dancing, cleaning, and organizing, have been estimated to resuspend up to 0.5 mg/min of PM2.5 (Ferro et al., 2004a). Moreover, human movement, such as walking, dancing, etc., is known to generate a “personal cloud” of particulate matter, initially reported in detail during the Particle TEAM study, the first large-scale study of personal exposure to particles in the 1990s (Özkaynak et al., 1996). As the name suggests, the person generating the cloud is most likely to be exposed to it, with 1.4× concentration enhancements reported by Ferro et al. (2004b). Licina et al. estimated that 90±14 million particles/hour in the 0.3-10 µm size range are emitted during walking. Concentrations of resuspended particles typically increase closer to the ground (Khare and Marr, 2015).

A significant body of knowledge has been published on the subject of particle resuspension over the past decade, including experimental studies in houses and apartments (Ferro et al., 2004a,b; S. Park et al., 2021; Tian et al., 2018, 2021; Vicente et al., 2020) and schools, including classrooms (Bhangar et al., 2014; Leppänen et al., 2020; B. Wang et al., 2021) and gyms (Buonanno et al., 2012b), as well as in controlled laboratory chambers (Bhangar et al., 2016; Boor et al., 2015; Lai et al., 2017; Qian and Ferro, 2008; Tian et al., 2014; S. Yang et al., 2021a). Particle resuspension depends on several factors, including the surface type and surface loading (i.e., the amount of particles present on those surfaces), the type and intensity of activity, and indoor environmental conditions such as relative humidity and airflow characteristics (Mukai et al., 2009; Qian et al., 2014; Zheng et al., 2019).

Activities that were found to lead to high exposure to resuspended and shed particles were those that involved vigorous movement and those that disturbed dust reservoirs present on furniture and textiles, such as walking, dancing, dusting, folding clothes, making a bed, jumping on the bed, etc. Qian et al. (2014) performed a comprehensive review on walking-induced particle resuspension indoors and reported that particle resuspension increases with particle size, especially in the 0.7–10 μm range. B. Wang et al. (2021) found that walking activities lead to a PM2.5 resuspension fraction (i.e., mass resuspended relative to mass of settled PM2.5) of 2.2×10-8 per footstep and that this fraction did not vary with particle loading on the surface. Lai et al.

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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(2017) investigated the role of shoe type on particle resuspension and found that flat shoes induced more particle resuspension than heels and, among flat shoes, soles with no grooves were associated with more resuspension than soles with grooves. Indoor surface materials have been found to play an important role in dust resuspension. Qian and Ferro (2008) found that new, level-loop carpet led to significantly higher particle resuspension rates compared to vinyl tire flooring for particles 1–10 μm in size. Tian et al. (2014) found no significant difference in dust resuspension between carpet and hard floorings for particles in the 0.4–3.0 μm size range, but found that carpets resuspended more particles in the 3–10 μm size range. Clothing and its particle loading also influences particle emissions during human movement. McDonagh and Byrne (2014) found that up to 67 percent of contamination on clothing can be resuspended during physical activity.

Studies in real indoor environments have quantified the emissions of particulate matter from resuspension and shedding from a variety of activities. Bhangar et al. (2014) found that emissions of resuspended biological particles in a university classroom were 2×106 particles/h/person during lectures and peaked during class transitions, at 0.8×106 particles per transition, due to increased movement among students. Cleaning activities such as vacuuming, dusting, and sweeping are known to resuspend large amounts of particulate matter, including fine PM. Vacuuming can generate particles from two distinct mechanical processes: dust resuspension from surfaces and mechanical movement of the motor (S. Park et al., 2021; Vicente et al., 2020). Corsi et al. (2008) investigated the effects of vacuuming on dust resuspension in 12 different apartments and observed very small increases in indoor PM2.5 concentrations above background levels. Ferro et al. (2004a) quantified PM2.5 emissions from vacuuming in a home to be ~0.45 mg/min.

The chemical composition of resuspended particles has also been found to be largely influenced by the sources and surfaces to which they were previously attached. While in contact with indoor surfaces, resuspended particles take up a variety of semivolatile organic compounds (SVOCs) of health concern, including PAHs, PFAs, pesticides, flame retardants, and phthalates (Eichler et al., 2021). Liagkouridis et al. (2014) states that models might underestimate the release of low-volatility brominated flame retardants from products and onto indoor particles. Shi and Zhao (2015) gathered published concentrations of 38 different SVOCs associated with dust in residences in seven countries for a model evaluation and reported concentrations in the range of 10-1 to 105 ng SVOC/ng dust (<10 μm in size). Other toxicants, such as lead from painted surfaces, can also be present in resuspended particles from the breakdown of painted surfaces (Grinshpun et al., 2002; Thatcher and Layton, 1995).

Resuspended and shed particles have been found to include human skin flakes and a variety of biological pollutants and allergens of health concern, including animal dander, dust mites, bacteria and fungi, viruses, and a variety of allergens, endotoxins and mycotoxins (Khare and Marr, 2015; Kumar et al., 2021; Nazaroff, 2016; Qian et al., 2014; Yen et al., 2019b). Although the plurality of these pollutants is expected to exist in particles greater than PM2.5, some of these may also be present as a component of fine PM. Yen et al. (2019b) measured significant increases in the concentrations of particulate matter (including PM2.5), bacteria, fungi, and endotoxin from making the bed and jumping on the bed. Kvasnicka et al. (2022) developed a model showing that contaminated clothing could theoretically resuspend viable SARS-CoV-2 viruses. Indoor particle resuspension has been linked to asthma and other respiratory health conditions. Kumar et al. (2021) performed a review of biological contaminants in the indoor air environment and stated that there is a “lack of awareness about biological contamination in the

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

indoor environment and their potential sources for the spreading of various infections.” Raja et al. (2010) found that biological markers of lung inflammation in asthmatic children were associated with the concentrations of dust mite allergen and cat dander. The same study determined that the resuspension rates for cat dander and dust mite allergen were higher than those for dog dander and bacterial endotoxins.

Residual PM from Liquid Droplet Evaporation

There are several indoor processes, such as cooking, showering, spray cleaning and personal care products, and using humidifiers or nebulizers, that, by accident or by design, emit liquid droplets into the indoor air. These droplets may contain trace soluble or insoluble constituents such as minerals, salts, proteins, or microorganisms. When these droplets dry under indoor environmental conditions, they can leave behind these constituents in the form of aerosol particles that can remain suspended in air.

Respiratory Particles

Respiratory aerosols are formed by fluid film bursting and shearing forces of air passing through the respiratory system. Small aerosols which can remain suspended and build up in poorly ventilated indoor space are continuously produced by people breathing, talking, singing, coughing, and sneezing (Bake et al., 2019; Fritzsche et al., 2022; Niazi et al., 2021). The released aerosols are primarily composed of respiratory fluid, which includes a combination of water and salts, a variety of organic compounds, and microorganisms including bacteria and viruses. Prussin et al. (2023) investigated 35,000 individual respiratory particles from three healthy human subjects and found that roughly half of the emitted particles were carbonaceous (mostly organic) in nature and the remaining half were primarily made up of salt-rich particles. Notable microorganisms and viruses, primarily those responsible for a variety of respiratory diseases, have been identified in respiratory aerosol. These include Mycobacterium tuberculosis (Fennelly et al., 2012; Patterson and Wood, 2019), influenza virus (Yan et al., 2018), respiratory syncytial virus (Kulkarni et al., 2016), and SARS-CoV-2 (Coleman et al., 2022;) , as well as other respiratory bioparticles (C. C. Wang et al., 2021). It is important to note that infectious SARSCoV-2 has been detected in aerosols in indoor air, including in air samples collected in residences occupied by individuals with COVID-19 (Lednicky et al., 2020; Vass et al., 2023).

Aerosols can be generated from multiple regions of the respiratory system: the upper respiratory tract, including the oral cavity, which involves activities such as speaking and coughing, larynx region, which is active during speaking and coughing, and the lower respiratory tract, including the bronchiolar region, which can produce aerosol particles during normal breathing (Fritzsche et al., 2022; Johnson and Morawska, 2009; Johnson et al., 2011; Pöhlker et al., 2023). Once released into the air, the physical behavior of these exhaled particles, including distance they travel and how long they remain suspended, will depend on particle size, shape, and density. These exhaled aerosols can undergo evaporation the rate and extent of which depend on ambient environmental conditions, particularly the relative humidity, temperature, and air flow conditions (L. Liu et al., 2017; L. Morawska et al., 2009b; Yang and Marr, 2011), which has been shown to affect the viability of pathogens in respiratory aerosol droplets. This relative humidity–dependent viability has been demonstrated for enveloped viruses, such as influenza (W. Yang et al., 2012), as well as SARS-CoV-2 (Oswin et al., 2022).

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

Multiple studies have investigated the size distributions of respiratory droplets and aerosol particles during a variety of breathing conditions and activity levels. Multi-modal size distributions, with at least one mode below 1 μm in size and several more modes up to 100 μm being reported (Chao et al., 2009; Firle et al., 2022; Johnson et al., 2011; L. Morawska et al., 2009b; Xie et al., 2009). Johnson et al. (2011) identified size distributions of expired aerosols during a variety of activities (e.g., breathing, speaking, coughing, etc.) and found that tri-modal, lognormal size distributions of particles were commonly emitted. For speaking and coughing, two of the three identified aerosol size distribution modes fell under 2.5 μm in size. Moraswka et al. (2009b) investigated several respiratory activities, including different types of breathing, vocalizations, and coughing, and identified up to four size distribution modes, and all respiratory activities produced particles <0.8 μm in size.

The COVID-19 pandemic brought great attention to the study of aerosol emissions from a variety of respiratory activities, including speaking, coughing, singing, and playing musical instruments. As a result, several recent studies have been published on these topics. In general, breathing has been found to emit fewer aerosol particles than speaking (Alsved et al., 2020). Aerosol emissions from human speech have been found to be proportional to voice loudness and phonation frequency (pitch), and independent of language spoken, with emissions varying greatly from study to study: Asadi et al. (2019) found emissions ranging from 1 to 50 particles per second, and Alsved et al. (2020) measured emissions of up to ~1400 particles per second. Even with this large range in emission rates, loud environments where people raise their voices (e.g., school cafeterias) are likely to contain higher concentrations of respiratory aerosols compared with quiet spaces with comparable building characteristics (e.g., school libraries). Multiple studies reported higher emissions for louder vocalizations (e.g., shouting and singing) compared to speaking or breathing, with breathing leading to the lowest emissions (Archer et al., 2022; Bagheri et al., 2023; Gregson et al., 2021). Ahmed et al. (2022) reported higher emissions increasing with phonation frequency. Age may also play a role in respiratory particle emission rates, with emissions increasing with age (Archer, 2022; Bagheri, 2023). Moreover, a study on exhaled aerosols from children showed no statistical difference in respiratory aerosol emissions between SARS-CoV-2 PCR-positive negative children and adolescents (Schuchmann et al., 2023).

Playing wind instruments also leads to respiratory aerosol emissions and may be particularly relevant in school music classrooms. Firle et al. (2022) identified the size distribution of respiratory particles emitted from playing wind instruments and found that the plurality of particles were in sizes ranging from 0.25 to 0.8 μm. Aerosol emissions from playing wind instruments vary widely according to instrument, ranging from ~1 to 2,500 particles per second. The clarinet, trombone, oboe, and trumpet have been generally reported as high emitters (Firle et al., 2022; R. He et al., 2021; L. Wang et al., 2022). Firle et al. (2022) also found that emissions were generally higher for playing wind instruments compared to speaking and breathing, and Stockman et al. (2021) found that particle number concentrations at the bell of a clarinet were comparable to singing.

Many studies have demonstrated that respiratory aerosol (Asadi et al., 2020; Leith et al., 2021; Pan et al., 2022; Stockman et al., 2021) and respiratory pathogen (Leung et al., 2020; Milton et al., 2013) emissions from speaking and singing are greatly reduced when masks or respirators are worn over the speaker’s mouth. In terms of playing wind instruments, evidence has been shown that covering the instrument’s bell can reduce respiratory aerosol emissions. While Firle et al. (2022) found that covering the instrument’s bell with a surgical mask did not

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

reduce emissions, Stockman et al. (2021) and Abraham et al. (2021) found that bell coverings reduced aerosol concentrations measured in front of the instrument’s bell.

Spray Products

Many products commonly used indoors employ spraying action for a variety of reasons. These include cleaning products (e.g., disinfectant sprays, all-purpose cleaners, and glass cleaners), air fresheners (e.g., room sprays, fabric refreshers, and plug-in air fresheners), personal care products (e.g., hairspray, deodorant sprays, and perfume sprays), and art and craft sprays (e.g., spray paints and adhesive sprays). Some of these categories of products have been well characterized in terms of their air pollutant emissions, while the characteristics of others remain largely unknown.

Particle emissions from spray products depend on factors such as nozzle type, pressure inside the container (i.e., pressurized can versus manual spray pump), and contents in the liquid phase. The liquid contents may include dissolved or particle species which can both lead to the formation of dry or wet particles in indoor air, depending on the indoor environmental conditions (particularly relative humidity). Because there is a wide variety within each of these parameters, wide variability can also be expected in terms of emissions, ranging from negligible to substantial. A study by S. Park et al. (2021) showed that air freshener spraying for 1 min in a bedroom led to significant, but short-lived increases in PM2.5 concentrations in that microenvironment. Similarly, a study by Uhde and Schulz (2015) showed that automatic air freshener spray units released a mist of short-lived ultrafine particles. Kogianni et al. (2021) reported high PM2.5 concentrations (~160–170 μg/m3) in hair salons, likely due to the intense use of hair sprays. Bertholon (2015) investigated aerosol emissions by three indoor air freshener sprays from pressurized canisters in a ventilated test chamber and found that >90 percent of particles emitted were <0.3 μm in size. A number of studies have investigated particle emissions from the use of consumer spray products that contain engineered nanoparticles as part of their formulation, with silver particles and ions as well as titanium dioxide particles as common ingredients. These have been shown to emit particles <2.5 μm in diameter (B. T. Chen et al., 2010; Laycock et al., 2020). The products tested by Quadros and Marr (2011) emitted 107 to 108 particles per individual spray action. Lorenz et al. (2011) investigated emissions from spray products containing nanoparticles and found that pressurized canisters emitted particles <0.3 μm in size, while a manual spray pump had negligible emissions.

Humidifiers and Nebulizers

A variety of products have the purpose of creating a fine mist of liquid droplets, which can then be inhaled or used for other purposes. One common method to create this aerosol employs a nozzle or orifice through which high-pressure liquids or air are passed, generating aerosol (e.g., nebulizers). Ultrasonic devices use sound waves to create liquid aerosols indoors and are also commonly used in many consumer products (e.g., some humidifiers and essential oil diffusers).

A variety of products have the purpose of creating a fine mist of liquid droplets, which can then be inhaled or used for other purposes. One common method to create this aerosol employs a nozzle or orifice through which high-pressure liquids or air are passed, generating aerosol (e.g., nebulizers). Ultrasonic devices use sound waves to create liquid aerosols indoors and are also commonly used in many consumer products (e.g., some humidifiers and essential oil diffusers).

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

Highsmith et al. (1988) measured particle emissions from three types of humidifiers (ultrasonic, impeller, and steam) operated with tap water with varying levels of dissolved solids content. Ultrasonic humidifiers emitted large amounts of PM2.5, with impeller-type humidifiers emitting approximately two-thirds less and steam humidifiers emitting none. The amount of PM2.5 emitted by ultrasonic humidifiers increased with increasing suspended solid content in fill water. More recent studies have found that the composition of aerosol emissions from ultrasonic humidifiers closely resembles their fill water composition for many types of tap water sources and that most emitted particles were in the 20–40 nm size range (Sain and Dietrich, 2015). Lau et al. (2021) demonstrated that ultrasonic humidifiers elevated indoor PM2.5 concentrations in a house up to 100s of µg/m3 and that humidifiers could be indoor sources of sulfate, which may complicate tracer-based techniques for estimating ambient particle infiltration (as described in Chapter 4). Sain et al. (2018) found that higher mineral content fill water increased PM emissions from ultrasonic humidifiers and Yao et al. (2020) subsequently showed that using tap water that meets water quality standards in ultrasonic humidifiers can result in substandard indoor air quality. Dietrich et al. (2023) recently demonstrated that inhalation exposures to metals emitted from ultrasonic humidifiers using tap water as fill water greatly exceed ingestion exposures from tap water alone.

Ultrasonic essential oil diffusers can emit large amounts of particles into the air, as this is their primary purpose. Schwartz-Narbonne et al. (2021) found that PM emissions from these products varied according to oil type, but three of the four tested oils released mostly ultrafine particles, with one tested oil (grapeseed) releasing particles that were dominantly >200 nm in diameter. A follow-up study by Du et al. (2022) found that exposure to both a scented and a non-scented essential oil affected the cognitive performance of the tested human subjects, specifically leading to more impulsive decision making.

Washing Machines, Dishwashers, and Showers

Common household appliances such as clothes washing machines and dishwashers might generate aerosols during their operation due to the mechanical action of water jets, sprays, and agitation. One can infer that the size, concentration, and chemical composition of aerosols generated from these appliances may vary depending on the type of washing detergent used, the mechanical movement of the machine, and the type and level of soiling on the material to be cleaned as well as the appliance itself. There is very little information published in the scientific literature on aerosol emissions from these appliances. One study found a peak aerosol concentration in a Swedish residence of 2.5×104 particles per cm3 from laundry activities (Isaxon et al., 2015). Bekö et al. (2013), however, observed no changes in indoor particle number concentrations during washing machine operation in a study in 56 Danish homes.

The plurality of published works focuses on the potential of these appliances to harbor and release microorganisms of interest to human health. Dishwashers are known for harboring thermophilic fungi (Gümral et al., 2016). Zupančič et al. (2016) identified over 500 fungal strains in 30 residential dishwashers. Kulesza et al. (2021) identified microfungi inside 7 of 10 tested dishwashers and hypothesized that microbial aerosols can be emitted when opening these appliances before the cooling period is complete. Döğen et al. (2017) found abundant presence of the opportunistic pathogen Candida parapsilosis in a study involving 99 laundry machines in Turkey. Showers and hot faucets, on the other hand, are well-known for their potential to aerosolize microorganisms, notably respiratory pathogens such as Legionella pneumophila (Bollin et al., 1985; Niculita-Hirzel et al., 2022) and Nontuberculous mycobacteria (Shen et al.,

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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2022). Washing machines have also been found to harbor Legionella pneumophila (Kuroki et al., 2017). Flushing toilets have been shown to present a potential for aerosolizing microorganisms, some of them pathogenic (Barker and Jones, 2005; Johnson et al., 2013; Lin and Marr, 2017; Schreck et al., 2021).

Secondary Particles from Indoor Chemical Reactions

There are a variety of chemical transformations that take place in indoor environments involving chemical species that can originate from the indoor environment or that can be transported indoors from outdoor air. Some of these transformations involve the chemical oxidation of volatile organic compounds (VOCs), which can lead to the formation of secondary aerosol particles in indoor air. The National Academies report Why Indoor Chemistry Matters describes indoor chemical transformations in great detail in its Chapter 4 (NASEM, 2022). Here, the focus is on specific scenarios that can lead to the formation of indoor particles in home and school environments via chemical transformations.

Many personal care and other consumer products release VOCs by design, such as perfumes, body lotions, air fresheners, and essential oil diffusers as well as a wide variety of cleaning products. The use of these products has been linked to increases in concentrations of terpenes, aldehydes, esters, and many other VOCs (Angulo-Milhem et al., 2021; Kim et al., 2015; Nematollahi et al., 2018; Sarwar et al., 2004; Schwartz-Narbonne et al., 2021; Su et al., 2007). If chemical oxidants, primarily ozone (O3), hydroxyl radicals (OH), reactive chlorine species, and nitrate radicals (NO3), are present in indoor air, they can react with these VOCs and form particles called secondary organic aerosols (SOAs). Waring and Wells (2015) modeled this process and found that VOC oxidation indoors is likely driven primarily by reactions with O3 and OH. Nitrate radical concentrations are generally modeled to be low (or negligible) in most indoor conditions (Young et al., 2019). Ozone is considered the most prevalent indoor air oxidant and is generally brought inside from outdoors unless a device or appliance that generates indoor O3 is operating inside the building (Nazaroff and Weschler, 2022). Nazaroff and Weschler (2022) reported average indoor O3 concentrations of 4-6 ppb in homes, and Salonen et al. (2018) reported 4 and 5 ppb in schools and offices, respectively. Such low O3 concentrations exist indoors because of heterogeneous and homogeneous reactions, which reduce O3 concentrations but increase concentrations of byproducts. Hydroxyl radicals can be relevant indoors when sources of nitrous acid (HONO) or formaldehyde (HCHO) are present (Wang et al., 2020; Waring and Wells, 2015; Young et al., 2019). Nitrogen dioxide from indoor combustion sources can be hydrolyzed into HONO and nitric acid (HNO3) (Finlayson-Pitts et al., 2003). HONO may then build up on indoor surface reservoirs and slowly release over a timescale of days, depending on indoor environmental conditions (Wang et al., 2020).

A robust body of knowledge has demonstrated the potential for new particle formation indoors from the reaction of O3 and indoor-generated VOCs, particularly from terpene-containing household products, such as air fresheners, cleaning products, and personal care products (Nazaroff et al., 2006; Rosales et al., 2022; Singer et al., 2006). Coleman et al. (2008) performed a series of chamber experiments demonstrating the formation of particles from exposing cleaning products and an air freshener to O3. Uhde and Schulz (2015) released a variety of fragrance products into a test chamber and then injected O3 into the chamber, demonstrating that large amounts of SOA can be formed as a result. A study by Yen et al. (2019a) in 60 Taiwanese homes found that 30 percent of observed households made use of essential oils and

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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that the concentration of O3 was negatively associated with their use, indicating that their VOC emissions potentially reacted with indoor O3.

Human skin oils have been recently identified as a potentially rich source of O3-reactive compounds (Weschler and Nazaroff, 2023; Wisthaler and Weschler, 2010). Ozone can react rapidly with some components of skin oils that are present in human skin, hair, clothes, and other indoor surfaces. The O3-driven oxidation of skin lipids, particularly squalene, has been shown to generate single-nanometer particles in indoor air. This has been shown in an experiment with human subjects (Yang et al., 2021b), in a chamber study using soiled T shirts (Rai et al., 2013), and in bench-scale reaction chambers using pure squalene (Coffaro and Weisel, 2022; Wang and Waring, 2014). Coffaro and Weisel (2022) showed that particle emissions decrease at high relative humidities (i.e., >50 percent), likely due to a shift in the formation of higher volatility products from the O3-squalene reactions.

A relatively less explored oxidation pathway indoors involves the chlorine radical and chlorine-containing molecules. Chlorine-containing compounds are also present in indoor environments and can spur oxidation chemistry, particularly after the use of cleaning products that contain bleach (Mattila et al., 2020; Wong et al., 2017). The presence of these reactants can spur a variety of chemical reactions, including the formation of indoor particles. For example, Patel et al. (2020) showed that emissions from indoor cooking and bleach mopping reacted together to spur new particle formation indoors, likely using chlorine-containing compounds as the primary oxidant. Schwartz-Narbonne et al. (2018) demonstrated that hypochlorous acid in commercial bleach solutions can react with squalene and oleic acid, two common components of skin oil.

Indoor particles can also be formed as an unintended consequence of using some devices that are nominally intended to clean the air. Indoor air cleaning devices encompass a broad category of products used to reduce the concentration of particles, VOCs, odors, pathogens, etc., in indoor air. Indoor air cleaning techniques range from air filtration, commonly used in modern buildings and well researched for decades (G. Liu et al., 2017), to a variety of technologies that employ chemical or physical processes such as ozone generation, photocatalytic oxidation, ultraviolet irradiation, ionization, and more (Collins and Farmer, 2021; EPA, 2018; Siegel, 2016; Stephens et al., 2022). Filtration and air cleaning are topics further explored in Chapter 7 of this report. Chemical reactions between the constituents introduced to indoor air by some additive air cleaning technologies have been shown to generate gases and particles as byproducts. For example, some indoor air cleaners intentionally or unintentionally produce O3 during use (Guo et al., 2019; Morrison et al., 2014), which can lead to new particle formation through the processes described above. Worth noting, O3 generators are also commonly used for odor removal in indoor environments during remediation efforts, which should be conducted without occupants present (Tang et al., 2021). Similarly, hydroxyl radical generators and other oxidizing technologies, which have gained popularity during the COVID-19 pandemic for indoor air cleaning, may also lead to the formation of indoor particles (Collins and Farmer, 2021; Joo et al., 2021). Ionization-type air cleaners use ions generated by a corona discharge to remove particles and can, in some cases, generate O3 and spur new particle formation indoors (Collins and Farmer, 2021; Hyun et al., 2017; Niu et al., 2001; Waring et al., 2008). The use of short-wavelength ultraviolet irradiation (UVC) for germicidal purposes can also spur the generation of indoor particles via the photolysis and photooxidation of indoor VOCs (Collins and Farmer, 2021; Graeffe et al., 2023; Kang et al., 2018).

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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REGIONAL, AREA, AND LOCAL DIFFERENCES AND CULTURAL/SOCIOECONOMIC DISPARITIES IN THE SOURCES AND COMPOSITION OF INDOOR PM

There are relatively well understood local and regional spatial differences in ambient PM2.5 sources and composition across the U.S., which also often intersect with socioeconomic disparities in the population. For example, both the absolute magnitude of ambient PM2.5 concentrations and the relative proportion of major constituents vary across North America, with nitrate being more abundant on the West Coast and sulfate being more abundant on the East Coast of the United States (Samet et al., 2005). And while ambient PM2.5 concentrations have decreased in the United States in the last few decades, racial and socioeconomic disparities in ambient PM2.5 concentrations have persisted (Colmer et al., 2020; Liu et al., 2021), with inequities driven by disproportionately high consumption of goods and services by non-Hispanic white populations that result in disproportionate exposures to Black and Hispanic minorities (Tessum et al., 2021, 2019). For example, areas of historical redlining—the result of a U.S. mortgage appraisal policy from the 1930s that was racially discriminatory—were found to be associated with higher ambient PM2.5 concentrations in present times (Lane et al., 2022).

Such differences in ambient PM2.5 presumably manifest in differences in indoor PM2.5 concentrations and compositions, holding other factors constant. While studies of indoor PM2.5 composition are more limited, some studies have demonstrated that the magnitude of some constituents of indoor PM2.5 closely tracked outdoor levels (e.g., elemental carbon and sulfate), while others (e.g., organic matter) are affected more so by the presence of indoor sources (Turpin et al., 2007). Carrion-Matta et al. (2019) used a positive matrix factorization (PMF) model to estimate the major sources of indoor PM2.5 in 32 inner-city school classrooms in the northeastern United States, finding that the major contributors to indoor PM2.5 concentrations were secondary air pollution and motor vehicles, both infiltrating from outdoors. The infiltration of ambient PM2.5 is also influenced by a number of building-related factors, some of which also likely vary with socioeconomic dimensions, as explained in more detail in Chapter 4.

Moreover, variability in the types of indoor sources present in buildings is broadly expected to contribute to variability in indoor PM2.5 concentrations and compositions across spatial, temporal, socioeconomic, and even cultural dimensions, even if robust characterizations of the presence, types, and frequency of indoor emission sources for specific populations do not yet exist. For example, it is understood that there are obvious differences across regions, buildings, and populations in factors such as the predominant heating and cooking fuel types that can affect combustion emissions, the presence and use of appliances and activities that contribute to emissions from combustion and heating processes (e.g., cooking, burning incense or candles) or water droplet evaporation (e.g., humidifiers), and the frequency and intensity of cleaning activities and the types of building materials that affect resuspension from settled dust. However, beyond the fairly detailed understanding of regional differences in fuel types addressed earlier in the chapter, there is not a robust accounting to date for how such contributors to variability in indoor PM2.5 sources, concentrations, and compositions vary regionally, locally, or across socioeconomic or cultural dimensions.

To date, efforts to use such building, cultural, and behavioral characteristics to increase understanding of the variability in indoor PM levels have generally focused on integrating information on building and housing characteristics with occupant surveys of activities. For example, Baxter et al. (2007a,b) demonstrated that information from a combination of public

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×

databases (e.g., central site ambient monitoring data, location, and housing information) and occupant questionnaires used to assess housing factors and occupant behaviors (e.g., cooking times, gas stove usage, occupant density, window opening frequency, and use of humidifiers) were significant predictors of indoor PM2.5 concentrations in lower socioeconomic households in Boston. Similarly, Klepeis et al. (2017) demonstrated that indoor PM concentrations in economically disadvantaged households in San Diego (each with at least one smoking occupant) were associated with information obtained from retrospective interviews of occupants, including reports of indoor smoking of cigarettes or marijuana and non-smoking events including frying food, using candles or incense, and house cleaning. Higher particle concentrations were also associated with smaller-volume homes, and there were no associations between particle concentrations and reports of opening windows, using kitchen exhaust fans, or other ventilation activities. Meng et al. (2009) found that incorporating information on personal activities, in addition to simultaneous outdoor PM2.5 concentrations, improved the accuracy of a predictive model for personal indoor PM2.5 concentrations and even some chemical constituents in the RIOPA study.

Such information on relevant housing and behavioral factors are relatively straightforward to obtain and likely aligns with socioeconomic and cultural differences in various populations. However, such factors are not extensively documented in resources such as the EPA’s Exposure Factors Handbook (EPA, 2022), nor are they routinely incorporated into epidemiology studies. What limited information on household activities that does exist in the Exposure Factors Handbook suggests that older populations spend more time cooking (Table 16-72 in the reference) and that women, individuals with lower levels of education, and married individuals spend more time doing “household activities,” which includes housework, cooking, and several other activities that may contribute to disproportionate exposures to indoor PM2.5 (Table 16-99). However, while the impacts that such behavioral differences have on indoor PM exposures are presumably apparent and likely logical, they remain largely unquantified at scale. Accounting for such differences may be a worthwhile endeavor to (1) identify disparities in exposures to indoor PM, (2) ensure that policies or actions to reduce exposure such as eliminating particular sources or adopting specific interventions would address disparities that exist, and (3) inform estimates of the societal benefits of mitigation, or, alternatively, the societal costs of not mitigating. A more detailed discussion on PM2.5 exposure is presented in Chapter 5 of this report.

FINDINGS AND CONCLUSIONS

This chapter describes in detail numerous indoor sources of fine and ultrafine particulate matter and their indoor concentrations and compositions. Although particles from indoor sources may account for approximately half of people’s exposure to fine PM mass concentrations—and even more in terms of particle number—indoor sources and their health effects are relatively unexplored when compared with historic ambient sources. Major gaps in knowledge remain, especially related to source strengths, source characteristics that dictate emissions, pollutant composition, and source-specific health effects.

Given the literature reviewed in this chapter, the committee finds:

Ultrafine particles are an important component of many indoor sources of PM2.5. Ultrafine particles (<100 nm in diameter), a subset of PM2.5, usually contribute a small portion of the total PM2.5 mass but represent a large portion in terms of particle number concentrations. A

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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plurality of indoor sources emit primary aerosol particles through processes that involve combustion, evaporation and condensation, or chemical reactions, and all of these lead to the formation of very small clusters of primary particles which can grow into larger sizes or be lost to indoor surfaces, given sufficient time (Chapter 4). In the case of continuous sources, a background of ultrafine particles can be expected to persist indoors for prolonged periods of time. Information on indoor ultrafine particles, especially their composition and health effects, is currently limited.

Many indoor sources are intermittent and lead to localized, short-lived, and high concentrations of UFPs and PM2.5. Indoor sources of particles such as cooking, personal care products, and some office products can emit copious amounts of UFPs and PM2.5 for the duration of the emitting activity, leading to high, sometimes short-lived, PM concentrations in their vicinity. This can lead to high exposure to the people performing the activity—and potentially lower exposure to other people who may be located farther away but still in the same indoor environment. This spatiotemporal behavior of PM2.5 occurs more strongly indoors than outdoors, where the air mixing volume and timescale for particle dynamics are much larger.

Indoor sources of PM2.5 change continually with the development of new products and activities. The indoor environment changes as society and the consumer market change over time. New products are always entering our lives, homes, and schools, creating the need for a continuous reevaluation of indoor PM2.5 sources and their associated exposures. Examples at the time of writing include electronic cigarettes, air fryers, and an abundance of air cleaning devices created or reintroduced during the COVID-19 pandemic that did not exist or were not as prevalent in years prior.

Respiratory aerosol has a PM2.5 component. Discoveries related to the production of infectious aerosols during the COVID-19 pandemic indicate that aerosol particles are emitted from humans doing natural activities such as speaking and singing. These discoveries have shifted thinking on appropriate types of personal respiratory protection as well as highlighted the importance of indoor air quality in all settings.

Socioeconomic and cultural disparities in exposure to indoor PM2.5 from different sources exist but remain underexplored. While there are documented socioeconomic and cultural disparities in ambient PM2.5 sources, concentrations, and composition, less is known about how such differences manifest in differences in indoor PM2.5 exposure. Moreover, while it is expected that there is high variability in the types and magnitudes of indoor PM2.5 sources attributable to such differences, robust characterizations of the presence, types, and frequency of indoor emission sources—as well as technologies to mitigate exposures—for specific populations do not readily exist.

RECOMMENDATIONS

The committee’s review identified a number of gaps in the literature addressing the sources and composition of indoor particulate matter that limit the confidence with which it and others can offer guidance on the health effects of indoor PM and the mitigation steps that might limit adverse consequences. These gaps lead the committee to recommend that EPA, in collaboration with other governmental entities, private funders, and standards and professional organizations, should foster additional research on:

Ultrafine particles (UFP) from indoor sources. Relative number, surface area, and mass emission rates for a range of different UFP sources would help to prioritize source removal,

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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reduction, and control. Further knowledge of chemical composition may also be valuable for purposes of both source attribution and better understanding potential health effects. With this knowledge, researchers and the public could prioritize action where there is greater potential for impact. Mitigation strategies could be developed along with education initiatives to minimize people’s exposure to those indoor sources that lead to worse health outcomes. There is an opportunity to educate the general public on indoor sources of fine particulate matter, including UFP, to enable more informed decision making when choosing indoor products and activities to minimize exposure.

Ambient air pollution as a source of indoor particles. The penetration of outdoor air pollutants into the indoor environments is relatively well understood and on average particles of outdoor origin contribute approximately one-half of indoor fine PM by mass. Evidence is clear that increases in outdoor PM2.5 concentrations lead to a wide range of health effects. Knowledge of the extent to which those health effects are due to exposures to fine PM of outdoor origin while indoors remains an area for more research. Furthermore, the relative importance of particles of indoor fine PM of outdoor origin versus those emitted directly from indoor sources also remains unknown and could shed significantly light on practical mitigation strategies that maximize health benefits for building occupants.

Spatiotemporal PM2.5 variability indoors. The concentration of fine PM varies both spatially—whether a measurement is taken near or far away from a particular source—and temporally—as air movement and mixing dilutes near-source concentrations and distributes PM through a space. This variability, which results from indoor sources in indoor environments—particularly residences and schools—may significantly affect the exposure of indoor occupants. Questions remain on how acute exposures (high concentrations, short time periods) cause health effects and can be influenced by practical mitigation choices. New knowledge could help inform the type and location of mitigation strategies contextually. Simply put: not all mitigation strategies may work for all indoor PM2.5 sources, but if there is an understanding of which sources play a larger influence in the exposure of indoor occupants, decisions can be made to optimize mitigation strategies.

Establishing uniform criteria for the information needed on indoor sources to inform the assessment of exposure, health effects, and mitigation. It is impractical to address all indoor sources of PM2.5, including sources of UFPs, because they continually evolve and change along with the consumer market. If uniform criteria existed for characterizing indoor sources, it could provide a pathway to harmonize future studies in indoor particle physics and chemistry as well as the development mitigation strategies and associated communication to the public. As an initial step in this process, compiling a comprehensive indoor emissions inventory (including outdoor sources) across a wide range of particle sizes, mass and number concentrations, and compositions would help researchers and policy makers to better compare different source categories and their resulting exposures.

Methodological advances for measuring PM in the indoor environment. Many of the studies reported in this chapter evaluated different indoor sources of PM in controlled laboratory chambers or real indoor environments, and both types of studies have their own inherent challenges and limitations. One of the greatest challenges is the deployment of large, research-grade instrumentation into real, occupied indoor environments, thanks to several reasons (noise, space requirements, safety limitations, transportation, etc.). Recent advances in lower-cost, consumer-grade sensors have shown the potential for investigations in a wide variety of indoor environments and sources, but continued evolution of these sensors to measure particle counts at

Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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much smaller sizes than existing optical scattering instruments would be valuable. To capture the diversity of indoor sources and indoor environments, advances must be made in miniaturized research-grade instrumentation to characterize PM in terms of size, concentration, chemical composition, etc., and to do so at the large scales needed to advance our understanding of health effects of indoor PM2.5.

How the indoor PM knowledge gaps and research needs vary across different socioeconomic and cultural contexts. While there is a fairly detailed understanding of regional differences in ambient PM2.5 sources and its infiltration into buildings, the same does not currently exist for indoor sources of PM2.5. This knowledge would provide the tools for better public education and for the application of context-aware mitigation strategies that are sensitive to the target population and culture.

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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 27
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 28
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 29
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 30
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 31
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 32
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 33
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×
Page 34
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 35
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×
Page 36
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×
Page 37
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×
Page 38
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×
Page 39
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 40
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
×
Page 41
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 42
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 43
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 44
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 45
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 46
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 47
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 48
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 49
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 50
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 51
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 52
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 53
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 54
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 55
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 56
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 58
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 59
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 60
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 64
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 65
Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Suggested Citation:"3 Sources and Composition of Indoor Particulate Matter." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Schools, workplaces, businesses, and even homes are places where someone could be subjected to particulate matter (PM) – a mixture of solid particles and liquid droplets found in the air. PM is a ubiquitous pollutant comprising a complex and ever-changing combination of chemicals, dust, and biologic materials such as allergens. Of special concern is fine particulate matter (PM2.5), PM with a diameter of 2.5 microns (<0.0001 inch) or smaller. Fine PM is small enough to penetrate deep into the respiratory system, and the smallest fraction of it, ultrafine particles (UFPs), or particles with diameters less than 0.1 micron, can exert neurotoxic effects on the brain. Overwhelming evidence exists that exposure to PM2.5 of outdoor origin is associated with a range of adverse health effects, including cardiovascular, pulmonary, neurological and psychiatric, and endocrine disorders as well as poor birth outcomes, with the burden of these effects falling more heavily on underserved and marginalized communities.

Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions explores the state-of the-science on the health risks of exposure to fine particulate matter indoors along with engineering solutions and interventions to reduce risks of exposure to it, including practical mitigation strategies. This report offers recommendations to reduce population exposure to PM2.5, to reduce health impacts on susceptible populations including the elderly, young children, and those with pre-existing conditions, and to address important knowledge gaps.

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