National Academies Press: OpenBook
« Previous: Summary
Suggested Citation:"1 Introduction." 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.
×

1
Introduction

This chapter provides basic information about the motivation of this report and the conduct of the study, beginning with a definition of fine particulate matter and an overview of why the health effects of indoor exposure to fine particulate matter is an important issue. It then presents the statement of task for the committee responsible for this report, followed by the committee’s approach to its task. The text addresses some of the methodologic considerations that informed the committee’s evaluation of the literature and concludes with a description of the report’s organization.

WHAT IS FINE PARTICULATE MATTER?

Fine particulate matter (fine PM) is defined in this report as inhalable particles with diameters that are less than or equal to 2.5 μm. The terms fine PM and PM2.5 are used interchangeably to refer to fine particulate matter. An important subset of fine PM is ultrafine particles (UFP), particles with diameters of less than or equal to 0.1 μm, generally extending to as small as a few nm. Ultrafine particles usually account for a small fraction of fine PM mass but dominate by number and can also be important in terms of total particle surface area. Ultrafine particles are largely emitted indoors by combustion sources and condensation of hot vapors, but also results as a product of gas-phase chemistry.

Fine PM in the range of 0.1 to 2.5 μm generally exists due to coagulation of UFP, condensation of vapors onto existing particles, emissions from indoor sources, for example, soot from combustion processes, and penetration of particles of this size range from outdoors to indoors. Particles with diameters of greater than 2.5 μm are generally referred to as coarse particulate matter. Coarse PM sources are dominated by mechanically generated processes such as resuspended dust (skin flakes, tracked in soil, clothing fabrics, pet dander, insect dung, and the like) and processes leading to emissions of mold spores, pollen, and other biologic agents. Some sources emit both fine and coarse particulate matter and the impacts of the fine and coarse modes are difficult to separate.3 This is particularly true for many bioaerosols that lead to allergic reactions and airborne infectious disease transmissions, as well as the resuspension of dust from indoor surfaces. In this report the committee assumes that there is some contribution, albeit to an unknown extent, of fine PM to health impacts associated with these sources.

___________________

3 The literature and this report uses PM to refer to particulate matter without any size qualification in circumstances where this was either not measured or specified.

Suggested Citation:"1 Introduction." 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.
×

WHY IS IT IMPORTANT TO STUDY INDOOR EXPOSURE TO FINE PARTICULATE MATTER?

Indoor environments—homes, schools, offices, businesses, and shopping, entertainment, and other enclosed public and private spaces—affect our health, comfort, and productivity. People in developed countries spend most of their time indoors, so most of the adverse exposures that they encounter most likely take place indoors. Some of these exposures are generated indoors from indoor sources, while others originate outdoors and enter the indoor environment through mechanical or natural ventilation, or infiltration, or are generated by chemical processes that take place when pollutants emitted from various sources interact.

Airborne particulate matter is among the most concerning of these exposures, and fine particulate matter is especially problematic. These particles can easily penetrate deep into the respiratory system when inhaled, and prolonged exposure to them has been linked to a wide range of adverse health effects, including respiratory and cardiovascular diseases, exacerbation of asthma, lung cancer, and reproductive and cognitive effects. Certain groups are more susceptible to such adverse effects, including infants and children, the elderly, pregnant people, and individuals with pre-existing respiratory or cardiovascular conditions.

The National Academies’ Why Indoor Chemistry Matters (NASEM, 2022) report highlights the growing awareness of the importance of ultrafine particles. Specifically, UFPs with diameters between 10 and 100 nm have a much higher surface area to mass ratio than larger particles and are thus efficient at transporting chemicals to surfaces in the alveolar region of the respiratory system. UFPs can cross through alveolar epithelial tissue, thus effectively reaching other organs and the smallest UFPs can be transported along the olfactory nerve to the olfactory bulb, which raises concerns regarding potential neurological effects.

The U.S. Environmental Protection Agency (EPA) considers PM2.5 harmful to public health and the environment and regulates its concentration in outdoor air under the Clean Air Act (EPA, 2023). Other countries and governmental bodies such as the European Union have their own standards (European Commission, 2023). There are, however, no comparable regulations concerning the indoor environment as of the summer of 2023.

THE STUDY’S STATEMENT OF TASK

Against this backdrop, the EPA approached the National Academies with a request to consider the state of the science on the health risks of exposure to fine particulate matter indoors as well as engineering solutions and interventions to reduce risks of exposure to it indoors, including practical mitigation solutions to reduce exposure in residential settings. An expert committee was formed to respond to that request.

EPA charged the committee to focus on:

  • synthesizing and summarizing recent scientific literature to assess the health risks of indoor exposure to PM2.5; and
  • identifying and analyzing practical intervention approaches for PM2.5 indoors.

It was further directed to develop findings and recommendations regarding the key implications of the scientific research for public health, including potential near-term opportunities for incorporating what is known into public health practice, and to identify where additional research will be most critical to understanding indoor exposure to PM2.5 and the effectiveness of interventions. Opportunities for advancing such research by addressing

Suggested Citation:"1 Introduction." 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.
×

methodological or technological barriers or enhancing coordination or collaboration among governmental bodies and organizations were also to be noted. The committee was asked to limit their consideration to non-industrial exposure within buildings, as occupational exposures differ in their magnitude, duration, and composition and fall under the purview of a separate federal agency in the United States (the Occupational Safety and Health Administration). EPA requested that the committee focus on residential settings but also consider schools and other non-industrial indoor environments where appropriate. Environmental tobacco smoke (ETS) is a major component of PM2.5 in the indoor spaces where the source is present. ETS exposure, health effects, and mitigation are dealt with in detail in other National Academies reports and were considered outside the scope of this effort, although they are noted where appropriate.

THE COMMITTEE’S APPROACH TO ITS TASK

The committee’s approach to responding to the questions posed by EPA was informed by the work of earlier National Academies efforts related to the indoor environment, and the text in this section and the methodologic considerations section below reflect that antecedent work.

The committee undertook a wide-ranging evaluation of relevant research on particulate matter, sources of fine PM, building characteristics, exposure assessment, human health effects associated with indoor environments, and the effectiveness of exposure mitigation methods. EPA requested that the committee focus on residential settings but also consider schools and other non-industrial indoor environments where appropriate.

Although the committee did not review all such literature—an undertaking beyond the scope of this report—it did attempt to cover the work that it believed to have been influential in shaping scientific understanding at the time it completed its task in summer 2023.

Several sources of information were consulted in the effort. On health outcomes, the primary source was epidemiologic studies and, specifically, those related to indoor exposures if available. In situations where studies on health effects related to indoor exposures were not available, emerging evidence from ambient exposures was included. Most of the studies reviewed focused on exposures occurring in homes, reflecting the focus of researchers working in the field. The committee also examined the smaller literature addressing multifamily residences and schools. Clinical and toxicologic research efforts were considered as appropriate. The literature review was limited to English-language publications.

The literature of engineering, architecture, and the physical sciences informed the committee’s discussions of building characteristics, sources of fine PM, exposure assessment and characterization, pollutant transport, and related topics, and public-health and behavioral-sciences research was consulted for the discussion of public-health implications. Those disciplines have different practices regarding the publication of research results. For example, relatively few papers in the peer-reviewed literature address building construction or maintenance issues. The committee endeavored in all cases to identify, review, and consider fairly the literature most relevant to the topics that it was charged to address. As such, it focused on studies that examined exposures, buildings, and populations in the U.S., but drew on research conducted elsewhere in circumstances where it deemed comparable and informed the issue under consideration.

Papers and reports reviewed in this volume were identified through extensive searches of relevant databases. Most of the databases were bibliographic and provided citations of peer-reviewed scientific literature. The committee staff examined the reference lists of major papers,

Suggested Citation:"1 Introduction." 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.
×

books, and reports for relevant citations, and committee members independently compiled lists of potential citations based on their expertise. Input received from participants in a 2021 workshop series conducted by a predecessor committee served as a valuable source of additional information. The committee was charged to focus on practical intervention approaches for PM2.5 indoors—Chapter 7 details the methodology used in the literature search conducted on that topic.

METHODOLOGIC CONSIDERATIONS

This section presents some of the considerations that informed the committee’s approach to evaluating the scientific literature. It discusses, in general terms, the major issues involved in evaluating indoor PM levels and their effects on health and how building characteristics, occupant behavior, and the outdoor environment may affect them.

General Considerations

As noted throughout this report, little in the literature considers together in one place all of the elements in the committee’s charge. However, substantial research has been published on many key questions. For example, there is extensive literature on outdoor particulate matter and the health risks associated with fine PM in outdoor air. Although less studied, published research also documents indoor PM levels and the relationship between indoor and outdoor PM levels. And there is a large body of work reporting on how indoor sources influence indoor air quality and human health, including several National Academies reports.4 In contrast, little published research links the effect of specific exposure mitigation efforts to changes in health outcomes.

The nexus among sources of fine PM, PM exposure, building characteristics, health outcomes, and mitigation effects has not been well studied. However, the elements of this nexus are sufficiently well understood to permit the committee to conduct a scientific examination of issues, come to findings, draw conclusions, and offer recommendations. The approach taken in the report was to identify exposures and exposure circumstances believed to affect the health, safety, or productivity of building occupants; to describe the factors that influence source strength or exposure; and to explore how exposure mitigation efforts might influence these factors.

Issues Regarding Indoor Exposure to PM

Fundamentally, exposures occur when people and pollutants intersect in space and time. The magnitude of an exposure depends on its level while a subject is present. Three classes of factors govern conditions in occupied indoor environments. The first pertains to the adverse exposures themselves and includes such factors as the outdoor concentration, indoor sources and emission rates, and the physical properties of the agent. The second category pertains to buildings and includes the air exchange rate5 and other characteristics related to indoor environmental controls, as well as the presence and effectiveness of deliberate air-cleaning processes. The third category of factors pertains to characteristics of the occupants and includes the timing of their presence indoors, occupant density, and activities that may influence sources,

___________________

4 Salient conclusions from these reports are summarized in Chapter 2.

5 The terms “air exchange rate” and “air change rate” are used interchangeably in the literature and in this report.

Suggested Citation:"1 Introduction." 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.
×

intake, and exposure. Subsequent chapters of the report detail how the numerous, diverse interactions between exposures, buildings, people, and interventions influence the outcomes of interest. Cultural and socioeconomic factors affect who is exposed, how and to what extent they are exposed, how the exposure impinges on their health, and the extent to which they may be able to mitigate adverse effects, and these are also addressed.

In analyzing indoor exposure to fine PM, it is convenient to consider two components: exposure due to outdoor sources and exposure due to indoor sources. The ventilation or air-exchange rate of a building or of a room in a building—which varies from building to building and within buildings over time—can substantially influence indoor air-pollutant concentrations and other environmental conditions. For particles of outdoor origin, ventilation is effective only if intake air is properly filtered prior to entering the occupied zone, something that is infrequently done in residences and only done in schools with adequately maintained central heating, ventilation, and air conditioning (HVAC) systems.

In general, higher ventilation rates cause indoor environmental quality to become more like local outdoor environmental quality. Conversely, as ventilation rates are reduced, the indoor environment is progressively less influenced by pollutants of outdoor origin and outdoor environmental conditions and more strongly influenced by indoor sources and conditions.

The consequences of fine PM exposure depend in part on how long people spend in different types of indoor environments and on differences in the populations that occupy various building types. People spend most of their time in their own residences. Children also spend a high proportion of their time in school, and they are considered more susceptible than adults to adverse health effects of air pollution. Similarly, indoor environments occupied by the elderly or individuals with chronic health conditions are of special concern because those who are in fragile health are more susceptible to further stresses than those who are healthy.

Differentiating among building types is important for reasons that extend beyond the populations that inhabit them. Different classes of buildings may be designed, operated, and maintained differently in ways that affect how their occupants are exposed to PM. Most commercial buildings in the United States are commonly ventilated mechanically, whereas the existing stock of residential buildings is ventilated by some combination of an HVAC system (where present), air leakage across the building enclosure, and natural ventilation through open windows or doors. Buildings also differ in the types of sources present. For example, cooking is a dominant activity in restaurants, common in residences, and rare in offices. Candle use primarily takes place in residences and in buildings used for religious or cultural purposes. The intensity of cleaning activities may be higher in health-care facilities than in other types of buildings. Occupant densities and the amount of time that people spend in particular kinds of buildings also vary. Schools are both high occupant density environments and ones that children spend a substantial fraction of their time in. The number of people occupying homes, how closely they are grouped together, and how much time is spent in them differs by socioeconomic status and in some cases culture. Finally, it is important to recognize that the responsibility for environmental conditions in buildings varies markedly among building classes and that this variability influences the appropriateness of policy options to address the exposure mitigation alternatives discussed in the report.

Another important characteristic of indoor environments is their broadly distributed nature. More than half of the population of the United States lives in the 52 most populous

Suggested Citation:"1 Introduction." 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.
×

metropolitan statistical areas.6 In total, there are over 140 million housing units in the country (U.S. Census Bureau, 2023), along with tens of millions of other occupied buildings. Taking the diversity in this building stock into account is important when working to understand the public-health significance of PM exposures.

Keeping in mind that broad diversity, what factors affect indoor fine PM concentrations? According to the principle of material balance (that is, that mass is conserved), the level of a given pollutant in a particular building can be determined by accounting for the net effect of the source terms and collective removal processes. Sources include outdoor air and direct indoor emissions. Ventilation is a dilution and removal process that must always be considered when indoor sources dominate. Other removal processes can be important, such as the deposition of particles onto indoor surfaces or active filtration.

Again, generally speaking, the primary elements that can be used to ensure good indoor air quality are source control, ventilation, the proper management of indoor environmental conditions, and the appropriate use of filtration. The central principle is to remove pollutants where they are more highly concentrated, to supply clean air where and when people need it, and to maintain healthy and comfortable environmental conditions for building occupants. Chapters 37 of the report go into far greater detail on all of these points.

REPORT ORGANIZATION AND FRAMEWORK

The remainder of this report is divided into seven additional chapters and supporting appendices. Chapter 2 provides background information on two topics related to the consideration of indoor exposure to fine PM: EPA’s interests, responsibilities, and work on the topic and brief summaries of previous National Academies reports related to the indoor environment and health.

Sources of indoor fine particulate matter are addressed in Chapter 3. Indoor PM2.5 concentrations and composition are described along with the important sources of indoor fine particulate matter of both indoor and outdoor origin. The latter includes PM2.5 that penetrates through the building enclosure (its walls, windows, and the like) to the indoors. Major indoor source categories include those based on combustion processes, heating processes, resuspension from surfaces, liquid droplet evaporation, and secondary particles from indoor chemical reactions. Sources of ultrafine particles are also discussed.

The fate, transport, and transformation of indoor fine particulate matter are discussed in Chapter 4. Mechanisms that define particle dynamics inside buildings are described, and mathematical models that incorporate these mechanisms to predict the combined effects of indoor fine PM sources, sinks, and transformations are discussed. Approaches to measure particle fate, transport, and transformation are also reviewed.

Building occupant exposures to fine particulate matter are examined in Chapter 5. Inhalation exposure metrics are defined, along with a discussion of the numerous challenges associated with estimating such exposure. Exposure assessment approaches, including direct measurements, exposure reconstruction based on biomarkers and multi-pathway analysis, and indirect estimates based on deterministic or empirical models are also discussed in Chapter 5.

___________________

6https://www.census.gov/data/tables/time-series/demo/popest/2020s-total-metro-and-micro-statisticalareas.html#v2022.

Suggested Citation:"1 Introduction." 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.
×

Past studies involving applications of exposure assessments are reviewed, along with exposure trends and disparities.

The health effects associated with exposure to indoor fine particulate matter of both indoor and outdoor origin are reviewed in Chapter 6. Primary health effects include cardiovascular, pulmonary, neurological, and psychiatric diseases, endocrine disorders and adverse birth outcomes. The chapter also examines the physiological mechanisms that are hypothesized to link exposure to cellular changes and the factors that influence an individual’s susceptibility to developing clinical symptoms associated with exposure.

Practical mitigation approaches for reducing exposure to, and health effects of, indoor fine particulate matter are described in Chapter 7. The mitigation measures that are considered include source control, ventilation, central filtration and standalone air cleaning, and personal protective equipment. Factors that influence the effectiveness of these mitigation measures in reducing exposure to PM2.5 as well as its adverse health effects (which do not necessarily coincide) are discussed, including decision making and human behavior related to the measures.

The final chapter of the report, Chapter 8, builds on the preceding text, identifying the report’s major themes and highlighting the committee’s key findings, conclusions, and recommendations.

Appendix A reproduces the agendas for a 2021 (virtual) workshop series on indoor exposure to fine particulate matter and practical mitigation approaches. This workshop series, which is summarized in proceedings published in 2022 (NASEM, 2022), provided valuable information and insights to the committee. Biographic information on the committee members and staff responsible for this report are listed in Appendix B.

REFERENCES

EPA (U.S. Environmental Protection Agency). 2023. National Ambient Air Quality Standards (NAAQs) for PM. https://www.epa.gov/pm-pollution/national-ambient-air-quality-standards-naaqs-pm (accessed July 26, 2023).

European Commission. 2023. EU air quality standards. https://environment.ec.europa.eu/topics/air/air-quality/eu-air-quality-standards_en (accessed July 26, 2023).

NASEM (National Academies of Sciences, Engineering, and Medicine). 2022. Indoor exposure to fine particulate matter and practical mitigation approaches: Proceedings of a workshop. Washington, DC: The National Academies Press.

U.S. Census Bureau. 2023. Quick facts: United States. https://www.census.gov/quickfacts/fact/table/US/VET605221 (accessed July 27, 2023).

Suggested Citation:"1 Introduction." 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 11
Suggested Citation:"1 Introduction." 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 12
Suggested Citation:"1 Introduction." 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 13
Suggested Citation:"1 Introduction." 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 14
Suggested Citation:"1 Introduction." 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 15
Suggested Citation:"1 Introduction." 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 16
Suggested Citation:"1 Introduction." 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 17
Next: 2 Background »
Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions Get This Book
×
 Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions
Buy Paperback | $26.00 Buy Ebook | $20.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

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.

READ FREE ONLINE

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!