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

Chapter: 7 Practical Mitigation Solutions for Indoor PM

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Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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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7
Practical Mitigation Solutions for Indoor PM

Given the importance of fine particulate matter (PM) to human health (Chapter 6), and the exposures that occur in homes and schools (Chapter 5), there is a compelling need to develop approaches to reduce that exposure. Such mitigation must recognize the multitude of indoor and outdoor sources of fine PM (Chapter 3) and the building and occupants and other factors that affect the transport and removal of fine PM in indoor environments (Chapter 4). It must consider exposure disparities to fine PM and the resulting criticality of targeting mitigation to communities and individuals that are exposed to high levels of indoor fine PM or that have disproportionate negative health impacts from these exposures. And, important questions about availability, accessibility, and sustainability of fine PM mitigation measures for communities must be addressed.

This culminating chapter presents the results of the committee’s consideration of these complex issues. It does not provide a how-to guide for practical mitigation, nor does it call for specific interventions to be implemented. Instead, it offers a critical review of the existing literature on the effects and effectiveness of various strategies, benchmarking what is and is not known today and offering recommendations regarding the way forward. The review is focused on indoor PM exposures commonly found in U.S. and does not address the mitigation of such sources as unvented biomass fuel burning for cooking or heating, which is a major source for some people living in low- and middle-income countries (EPA, 2023; WHO, 2022).

INTRODUCTION

Mitigation—as the term is used in this report—is not just the removal of fine PM from indoor air or the limiting or elimination of exposure to that PM. The committee’s definition is broader, including considerations about the practicality of the mitigation measure such as its cost, feasibility, persistence, availability, co-benefits, negative secondary consequences and side-effects, barriers to implementation, and opportunities to address equity. The committee is critically concerned with questions of effectiveness that go beyond questions of concentration reduction to necessarily include those related to health effects.

The intent is to provide as complete a view as possible on the evidence basis for practical fine PM mitigation approaches. The specific process for locating and categorizing evidence is set forth below; the standard of evidence was high-quality investigations published in peer-reviewed journals. To be sure, there are limitations to this approach, including (1) the many practical fine PM mitigation approaches that have not been evaluated or were not part of a peer-reviewed journal article or else that have been evaluated for their impact on emissions, concentration, or exposure reduction but not health effects, (2) the large differences in the quantities of published papers on different practical mitigation approaches, (3) the large variation in the quality of

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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.
×

investigations in the peer-reviewed literature, and (4) disparities in the populations and communities that have been part of mitigation investigations. These limitations are part of a much broader limitation: the overall small number of high-quality investigations of practical mitigation measures and the absence of any large-sample-size longitudinal investigations.

A key subtlety is inherent in this definition of mitigation. There is overwhelming evidence in the literature that a wide variety of mitigation approaches can reduce indoor concentrations of fine PM. There is also a clear logical chain that goes from reduced indoor concentrations as a result of mitigation (Chapter 4) to reduced exposure (Chapter 5) and to improved health outcomes (Chapter 6). There are two primary reasons why reduced concentrations are not used as the standard in this chapter. The first is that there are incomplete data that specifically link reduced indoor concentrations to specific health outcomes. The committee has a high degree of confidence when stating that reducing indoor fine PM concentrations is beneficial for a wide variety of health outcomes. It has a much lower degree of confidence in explicitly quantifying this effect. There are important nuances concerning particle size distribution and composition that are addressed elsewhere in this report (Chapter 5 in particular) that are almost always not characterized in the literature included in this chapter. The second reason is that the context for mitigation is often as important as the mitigation measure itself. The context here means the details of the environment or system in which the measure is used (Chapter 4 has considerable detail here), the population or community that uses that measure, and the practical details of implementation. As is the case with the first reason, the literature often incompletely characterizes this context. It is for these reasons that the major recommendations of this chapter are both (1) more complete research is needed that addresses contextual factors and (2) the need for this research should not prevent the application of fine PM mitigation measures.

This chapter addresses building- and individual-level fine PM mitigation measures. It explicitly excludes from its scope any measures to address outdoor fine PM sources (e.g., reducing traffic, mitigating industrial sources). Voluntary labeling programs and interventions based on building code changes are not addressed. While programs and codes that specified the installation of high-efficiency air filtering would help to signal the importance of this intervention, there is little evidence in the published literature regarding the effectiveness of such measures for particle reduction or the generation of health benefits, along with the challenges associated with their enforcement beyond their initial application at the time of construction or certification. Although some mitigation literature suggests that the primary health benefit of any mitigation measure comes from reducing indoor exposure to outdoor fine PM (e.g., Fisk, 2013), this may arise from the vast imbalance between the amount of literature and attention on outdoor versus indoor fine PM.

Furthermore, mitigation of outdoor PM tends to occur at a political and economic scale that is much larger than the scale considered here; this chapter focuses on mitigation at the building and personal level. It needs to be pointed out that although approaches to reducing outdoor PM should be part of any overall strategy to reduce indoor PM exposure, the cost and benefits of such approaches should always be considered in the context of the building- and individual-level mitigation measures discussed in this chapter. Although the committee is not aware of such analyses, a reasonable hypothesis is that smaller-scale measures are more likely to result in health benefits at a lower cost and may allow more targeting of measures for equity and other purposes. Further, the mitigation measures discussed here are practical in the sense that they are achievable by individuals or communities.

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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.
×

FRAMEWORK

Four specific practical mitigation measure categories are considered in this chapter, organized according to the hierarchy of controls:

  1. Source control
  2. Ventilation
  3. Filtration and air cleaning
  4. Personal protective equipment (PPE)

Source control measures either eliminate indoor sources (e.g., avoiding the use of candles, incense, fireplaces, etc.) or reduce the emissions of indoor sources or their toxicity (e.g., replacing unvented combustion with electrical heat sources). Ventilation measures include measures that dilute indoor air with outdoor air (e.g., natural or mechanical ventilation) and localized exhausts (e.g., kitchen range hood) that remove fine PM and other indoor air pollutants to outside. Air cleaning measures are those that remove fine PM from recirculated indoor air (e.g., central or portable filtration). PPE, in the context of airborne exposure, refers to clothing, equipment, or devices designed to be worn and reduce the intake of pollutants, such as masks and respirators. These categories are not always mutually exclusive (e.g., kitchen range hood ventilation primarily removes particles from a specific source, and ventilation systems often have integrated filtration, while mask or respirator use can also have an infectious disease source control benefit). So, although useful as a framework for considering the evidence for different mitigation approaches, a meaningful approach to practical mitigation will likely involve multiple intervention measures.

Each of these categories has a combination of inherent, building, and behavioral factors that influence their impact as a mitigation on fine PM (Table 7-1). Inherent factors are those that are intrinsic to the particular measure itself in terms of effectiveness for reducing PM2.5 exposure. Building factors are those contextual factors from the indoor environment that influence the effectiveness of the measure. Behavioral factors arise from individual and building operator decisions about the use of such measures as well as from broad economic and social contexts. A central challenge that arises from much of the literature on fine PM mitigation interventions is that these contextual factors are often not assessed, and thus findings cannot always be generalized to other environments with different contexts.

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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 7-1 Mitigation Measures and the Contextual Factors that Influence Their Impact

Measure Inherent Factors Building Factors Behavioral Factors
Source Control
  • Emission rate
  • Emissions profile
  • Energy availability and need
  • Indoor location of source
  • Selection of emitting appliances
  • Activity/use
Ventilation
  • Flow rate
  • Outdoor air fraction
  • Efficiency of filtration on ventilation air
  • Proximity to sources
  • Air mixing in the building
  • Ambient fine PM (and other pollutant) concentration
  • Building/system capabilities
  • Operational timing
  • User operation (e.g., range hood fans, open windows)
  • Maintenance of fans and other equipment
Filtration and Air Cleaning
  • Filter efficiency
  • Flow rate
  • Magnitude of other loss processes
  • Runtime (central)
  • User operation and placement (portable)
  • Filter replacement and maintenance
Personal Protective Equipment (PPE)
  • Efficiency
  • Fit
(not applicable)
  • Use/compliance
  • Replacement
  • Time spent in proximity to source

LITERATURE REVIEW METHODOLOGY

The committee was charged to focus on practical intervention approaches for PM2.5 indoors. As this topic has not been addressed in detail in previous National Academies reports on the indoor environment and health, the methodology underlying the literature search is presented here.

The search was conducted in Fall 2022 in the Science Citation Index using the set of terms listed in Table 7-2. These terms were established using an iterative process that was designed to be broad and inclusive of all relevant articles for which the committee was aware. In addition, a separate search was conducted in PubMed to capture additional articles, and committee members consulted their personal libraries to ensure as thorough an examination as possible. No strict date limitation was placed on the search, but the committee focused on the most recent work (generally speaking, papers published from 2010 onward) addressing the range of mitigation measures under consideration.

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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 7-2 Search Terms Used in the Review of Practical Mitigation Measures

(personal protective equipment OR PPE OR mask OR respirator) OR (source control OR source removal OR emissions reduction) OR (rangehood OR kitchen OR Ventilation OR ventilated OR air exchange rate OR air change rate OR airflow OR exhaust OR window OR HVAC OR mechanical ventilation) OR (filter OR filtration OR air cleaning)
(intervention OR trial)
(cardiovascular OR respiratory OR cognitive OR asthma)
(fine PM OR PM2.5 OR particulate OR ultrafine)
(indoor OR home OR school)

This process identified 471 articles. From this master list, the committee conducted a relevance assessment and classified papers into five categories:

  1. Papers that addressed PM mitigation and a health outcome and that were relevant to one or more of the four mitigation measure categories (source control, ventilation, PPE, or filtration/air cleaning).11 Owing to its charge and the guidance provided by the sponsor, the committee excluded papers that addressed cookstove combustion source control and direct assessments of environmental tobacco smoke (ETS) source control (e.g., smoking cessation), although a small number of papers that investigated reducing ETS through ventilation or filtration were assessed. Investigations from global environments with much higher ambient PM than is typically found in U.S. locations were included with the location of the investigation noted.
  2. Papers that otherwise fit Category 1 but that did not address health effects (e.g., they focused on characterizing concentration or on exposure reduction).
  3. Papers that otherwise fit Category 1 but that did not fit into one of the mitigation measure categories (e.g., they examine cookstove emissions but not emission mitigation).
  4. Papers that addressed indoor PM but did not examine a health outcome or a mitigation strategy.
  5. Papers that did not fit into an above category, such as, for example, a paper that was intended for a specific audience (e.g., clinicians) that addressed mitigation of fine PM but did not report research results.

For the first 100 papers, two committee members completed the categorization, and any differences were resolved and the approach standardized. The remaining 371 papers were categorized by at least one committee member. The primary focus was placed on Category 1 papers, but papers in categories 2–5 were included where they offer insight on topics not addressed in the Category 1 literature.

Following the categorization process, all Category 1 papers were identified by their primary (and secondary, if relevant) mitigation measure, and a committee member was assigned to each category to collect data on Category 1 papers including sample size, study population, study duration, building type, location, health outcomes considered, and findings. Following this

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11 As noted in Table 7-1, behavioral interventions are a part of all of these mitigation strategies and thus are not called out separately.

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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.
×

categorization, a committee member with health expertise separated the papers into different health endpoints to correspond with the discussion in Chapter 6.

An obvious initial finding is that there is a wide variation in the number of articles that address the different framework measures, with fewer than 10 articles on PPE, source control, and ventilation and (by far) the largest number of articles on filtration and air cleaning and specifically on portable filtration. Similarly, some populations and health effects have received disproportionate attention relative to others. As would be anticipated, there is a wide variety in quality metrics and study approaches, with almost all investigations being very short term. A consistent theme through most of the investigations is that the broader context is often not considered with much depth, which complicates assessments of measurement effectiveness and the generalizability of findings.

The literature search also identified a number of review papers (R. W. Allen and Barn, 2020; Cheek et al., 2021; Fisk, 2013; Kelly and Fussell, 2019; Morishita et al., 2015; H. Park et al., 2021; Rajagopalan et al., 2020; Sandel et al., 2010; Sublett, 2011; Walzer et al., 2020; Warner, 2017; Xia et al., 2021) that met the criteria for inclusion in Category 1. These papers were evaluated and included in the discussion where warranted. They were useful for identifying issues that arose in previous reviews as well as informing the recommendations from this chapter.

LITERATURE REVIEW RESULTS

This section discusses the literature examining mitigation of indoor PM exposure and health outcomes. It focuses on the most recently published papers and reports but reviews older research where relevant. Findings are grouped by practical mitigation measure category. The purpose of this examination is to highlight what is known about the effectiveness of various mitigation approaches along with the broad themes and gaps found in the course of the literature review. Mitigation efforts focused on limiting exposures to toxic substances are not addressed here; such interventions are examined in the National Academies’ Why Indoor Chemistry Matters report (NASEM, 2022).

Table 7-3 summarizes the results of a selection of the studies considered by the committee, some of which are not otherwise addressed in the text. This table is grouped by health outcome. It is intended to illustrate the state of the literature and to summarize the findings of representative research efforts.

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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 7-3 Summary of Selected Studies Addressing Mitigation of Indoor PM Exposure, Grouped by Health Outcomes

Study Location Study Population and Sample Size Mitigation Approach Health Outcome[s] Resulting from Mitigation Approach
Children—General respiratory focus
Singleton et al. (2018) US: AK; homes 214 children ages 0–12 Source control and ventilation Home remediation and education reduced respiratory symptoms, low respiratory tract infection visits, and school absenteeism in children with lung conditions.
Walker et al. (2022) US & Navajo Nation: AK, AZ, MT, NM; homes 461 children ages 0–5 Filtration and air cleaning No significant difference with education or HEPA air cleaner intervention on lower respiratory tract infections.
Children and young adults with asthma
Noonan et al. (2017) US: AK, ID, MT; homes 114 rural children ages 6–18 Filtration and air cleaning Filter use resulted in PM2.5 reduction: no treatment effect on Pediatric Asthma Quality of Life Questionnaire or other outcomes.
Phipatanakul et al. (2021) US: Northeast; schools 236 elementary school age children Filtration and air cleaning No statistically significant improvement in asthma symptom days from HEPA filters.
Lee et al. (2020) South Korea: Incheon and Gyeonggi-do; homes 30 elementary school age children Filtration and air cleaning Air cleaners associated with less frequent use of asthma medications. Asthma severity assessed by symptoms and medication use, lung function, airway inflammatory, and urine microbiome.
Kim et al. (2020) South Korea: homes 26 children ages 6–11 Filtration and air cleaning HEPA air filters reduced PM2.5; lower PM2.5 associated with reduction in peak expiratory flow rate, but no significant difference between filter and control groups.
H.-K. Park et al. (2017) US: Fresno, CA; homes 16 children ages 6–18 Filtration and air cleaning Improvement in childhood asthma control test scores, peak flow rates, nasal symptoms, but only statistically significant for self-reported nasal symptoms.
Cui et al. (2020) China: Shanghai; homes 43 children ages 5–13 Filtration and air cleaning Improvements in total airway resistance, small airway resistance, resonant frequency, small airway reactance, fractional exhaled nitric oxide, peak expiratory flow.
Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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.
×
Antonicelli et al. (1991) Italy; homes 9 children and adults ages 10–28 Filtration and air cleaning No improvements in symptom score or bronchial hyperresponsiveness with intervention. Dust mites may have been a factor as all participants were allergic to house dust mites.
James et al. (2020) US: Cincinnati, OH; homes 43 children ages 5–13 Filtration and air cleaning Asthma control and quality-of-life scores significantly improved following HEPA filter intervention.
Eggleston et al. (2005) US: “inner-city” homes 100 children ages 6–12 Source control and filtration and air cleaning Asthma daytime symptoms significantly increased in the control group and decreased in the treatment group. Other measures of morbidity, such as spirometry, nighttime symptoms, and emergency department use, were not significantly changed.
Moreno-Rangel et al. (2020) US: McAllen, TX; homes 13 children ages 7–12 Filtration and air cleaning Four survey tools used, including Home Environmental Personal Well-Being Survey, Pediatric Quality of Life Inventory Asthma Module (PedsQL), the Asthma Control Test, and Healthy Homes and Asthma Test. All surveys showed better health outcomes after intervention, but only the PedsQL showed a statistically significant improvement.
Sulser et al. (2008) UK: homes 36 children ages 6–17 Filtration and air cleaning No significant change in forced expiratory volume in 1 second (FEV1) after cold air challenge or in the use of medication and serum eosinophil cationic protein levels. Trend observed in the active group towards an improvement of bronchial hyperresponsiveness (BHR), whereas the sham filter group showed a deterioration of BHR.
Jhun et al. (2017) US: school 25 children ages 6–10 Filtration and air cleaning Classroom PM2.5 levels reduced compared to intervention control group; modest improvement in peak flow, but no significant changes in FEV1 and asthma symptoms.
Thiam et al. (1999) Singapore: homes 24 children ages 6–14 with asthma/dust mite sensitivity Source control and filtration and air cleaning Mattress covers improved FEV1 and reduced diurnal peak expiratory flow rate; both mattress covers and HEPA filters improved mean symptom scores.
Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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.
×
Xu et al. (2010) China: Beijing; homes 30 children ages 5–16 Filtration and air cleaning As a measure of pulmonary inflammation, the exhaled breath condensate (EBC) nitrate concentrations decreased significantly, and the EBC pH and PEF values increased significantly with operation of the filtration and air cleaning unit.
Butz et al. (2011) US: MD; homes 126 children ages 6–12 Filtration and air cleaning Air cleaner intervention group experienced fewer days of asthma symptoms in homes that were in an inner-city and included a household smoker.
Grant et al. (2023) US: MD; homes 155 children ages 5–17 Filtration and air cleaning Multifaceted intervention, including air purifiers, did not change asthma medication use. Indoor PM concentrations were not significantly reduced with the intervention.
Lajoie et al. (2015) Canada; homes 83 children Ventilation Intervention group showed significant decrease in average levels of formaldehyde, airborne mold spores, toluene, styrene, limonene, and α-pinene concentrations. There was no significant change in number of symptoms days/2-week period. However, there was a significant decrease in number of children experiencing wheezing episodes over 12-month period.
Kile et al. (2014) US; homes 12,570 children ages 2–16 Ventilation Found that children whose parents reported using ventilation when operating their stove had higher lung function and lower odds of asthma, wheeze, and bronchitis; it is not clear if the health benefits were due to fine PM reduction or other indoor air pollutants, such as NO2.
Adults—General respiratory focus
Yoda et al. (2020) Japan: homes 32 healthy adults Filtration and air cleaning No statistically significant findings on respiratory or pulmonary function or fractional exhaled nitric oxide (FeNO), but indoor PM was not reduced with filtration.
Hansel et al. (2022) US: homes 94 adults, former smokers Filtration and air cleaning Air cleaners improved respiratory symptoms: lower rate of moderate exacerbations and lower rescue medication use. Adherence to intervention was associated with greater magnitude of improvement. Air cleaner intervention did not significantly improve primary outcome of respiratory status, measured by St. George's Respiratory Questionnaire.
Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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.
×
Cui et al. (2018) China: Shanghai; homes 70 non-smoking healthy adults Filtration and air cleaning Filtration significantly lowered airway impedance, airway resistance, and small airway resistance, reflecting improved airway mechanics especially for the small airways. However, no significant improvements for spirometry indicators (FEV1, FVC) were observed. Filtration also significantly lowered von Willebrand factor (VWF) 24 h after the end of filtration, indicating reduced risk for thrombosis.
Francis et al. (2003) UK: homes 30 adults with asthma + pets Filtration and air cleaning Beneficial clinical response observed in reduction in asthma treatment in 10/15 subjects in the active group compared with 3/15 in the control group after 12 months of intervention. No significant differences between the active and control groups were detected for changes in measures of lung function, reservoir pet allergen, or airborne pet allergen during the study.
Warburton et al. (1994) UK: homes 12 adults with asthma Filtration and air cleaning No difference in subjective symptom scoring, spirometry, or bronchial reactivity demonstrated.
Skulberg et al. (2005) Norway: offices 80 adults with airway symptoms Filtration and air cleaning Irritation and general symptom indices (acoustic rhinometry and peak expiratory flow) decreased in both groups, but there was no improvement in the intervention group compared with the control group.
van der Heide et al. (1997) The Netherlands: homes 45 adults with asthma/allergies Source control and filtration and air cleaning Statistically significant improvement of provocative concentration of histamine only in group with both mattress cover and filter. Dust and dust-mite allergen collected in filter was significantly correlated with improvement in peak flow variation.
Allergic diseases
K. H. Park et al. (2020) South Korea: homes 44 adults with house dust mite–induced allergic rhinitis Filtration and air cleaning For allergic rhinitis, medication scores improved significantly, while subjective measures (symptoms, visual analog scale, and quality-of-life scores) did not differ.
Stillerman et al. (2010) US: homes 35 adults with perennial allergic rhinoconjunctivitis Source control and filtration and air cleaning Significant improvements in nocturnal nasal and ocular allergy symptoms and quality of life for the active vs. placebo device.
Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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.
×
(dog, cat, or dust mite sensitivity)
Jia-Ying et al. (2018) China: Guangzhou; homes 32 children and adults with allergic rhinitis Filtration and air cleaning HEPA filtration was associated with improvements in activity limitation, non-nasal-eye symptoms, practical problems, and nasal symptoms via the Rhinitis Quality of Life Questionnaire.
Wood et al. (1998) US: homes 35 adults (cat-allergic) Filtration and air cleaning No differences in morning, afternoon, or nighttime nasal-symptom scores, chest-symptom scores, sleep disturbance, morning or afternoon peak-flow rates, or rescue medication use.
Reisman et al. (1990) US: homes 32 adults with perennial rhinitis and/or asthma + dust-mite-positive skin test Filtration and air cleaning No difference in the total symptom/medication scores or individual symptom scores during the placebo and active-filter periods over the full study period. Analysis of the last 2 weeks of each filter period in which respiratory infection was absent demonstrated definite differences in total and individual symptoms, suggesting active-filter benefit. Patients' subjective responses also suggested benefit from the filter.
Adults—Cardiovascular diseases
Day et al. (2018) China: Hunan Province; offices and dormitories 89 healthy adults Filtration and air cleaning No statistically significant findings from electrostatic precipitator (ESP) alone. ESP+ HEPA: change in plasma-soluble P-selectin and a −3.0% change in systolic blood pressure, suggesting reduced cardiovascular risks.
Padró-Martínez et al. (2015) US: MA; homes 20 adults living near highway Filtration and air cleaning Blood pressure, high sensitivity C-reactive protein (hsCRP), interleukin-6 (IL-6), tumor necrosis factor alpha-receptor II (TNF-RII) and fibrinogen levels assessed. Il-6 concentration higher; no statistically significant change in all other tested parameters.
L.-Y. Lin et al. (2011) Taiwan: homes 60 young healthy university students Filtration and air cleaning Blood pressure and heart rate elevated in subjects with no filter.
Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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.
×
Karottki et al. (2013) Denmark: Copenhagen; homes 48 non-smoking senior adults, some taking vasoactive drugs Filtration and air cleaning No statistically significant effects of filtration as category were observed on microvascular and lung function or the biomarkers of systemic inflammation.
S. Liu et al. (2018) China: Beijing; homes 35 non-smoking senior adults Filtration and air cleaning Benefits of cardiovascular of short-term air filtration intervention included reduction in SDNN12 during filtration, further effects of black carbon, 12-hour daytime ambulatory heart rate variability and blood pressure.
Chuang et al. (2017) Taiwan: Taipei; homes 200 adult “homemakers” Filtration and air cleaning Improvements in systemic inflammation, oxidative stress and elevated blood pressure.
Bräuner et al. (2008) Denmark: Copenhagen; homes 42 adults living in proximity to major roads Filtration and air cleaning Indoor air filtration significantly improved microvascular function (MVF–biomarker of inflammation). MVF was significantly associated with personal exposure to iron, potassium, copper, zinc, arsenic, and lead in PM2.5.
Eom et al. (2022) South Korea: homes 38 adults with coronary artery disease Filtration and air cleaning Improved baroreflex sensitivity, and decrease in the indicator of oxidative stress represented as 8-hydroxy-2′-deoxyguanosine. Blood pressure, heart rate variability, baroreflex sensitivity, autonomic function test results, and endothelial function tested.
Morishita et al. (2018) US: Detroit; homes 40 non-smoking elderly adults in low-income housing Filtration and air cleaning Decreased brachial systolic and diastolic blood pressure; improvements in aortic hemodynamics, pulse-wave velocity, and heart rate variability measures.
Langrish et al. (2012) China: Beijing; walking outdoors 98 adults with coronary artery disease Personal protective equipment (PPE) 12-lead electrocardiography for 24-hour period showed reduced symptoms, reduced maximal ST depression, reduced blood pressure and improvement in heart rate variability.
Vieira et al. (2016) Brazil: São Paulo; 26 adults with heart failure and Filtration and air cleaning In patients with heart failure, diesel exhaust (DE) adversely affected reactive hyperemia index; 6-minute walking test; O2 pulse; and arterial stiffness. Compared

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12 Standard deviation of NN intervals

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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.
×
exposure chamber 15 control adults with DE exposure, filtration reduced particulate concentration, and was associated with an increase in VO2 and O2, an improvement in reactive hyperemia index, and a decrease in B-type natriuretic peptide. In both groups, DE decreased the 6-min walking distance and arterial stiffness, although filter did not change these responses. DE had no effect on heart rate variability or exercise testing.
Han et al. (2021) China: Tianjin; walking outdoors 39 healthy university students PPE Short-term exposure to traffic acutely affected blood pressure, heart rate, and heart rate variability, but N95 mask and powered air-purifying respirator interventions generally showed little efficacy in reducing these effects.
Shi et al. (2017) China: Shanghai; respirator use 24 healthy young adults PPE 48-hour respirator use resulted in lower blood pressure and increases in heart rate variability parameters.
Liu et al. (2021) China: Beijing; college dormitories 56 healthy college students Filtration and air cleaning Increases in PM2.5 and negative ion exposure independently associated with increased urinary concentration of malondialdehyde, a biomarker of systemic oxidative stress, resulting in a null net effect of negative ion air purifier (NIAP) on malondialdehyde. No significant net effects of NIAPs observed for other outcomes indicative of lung function, vascular tone, arterial stiffness, and inflammation.
Lin et al. (2013) Taiwan: Taipei; homes 300 healthy adults Ventilation Increases in cardiovascular endpoints and decreases in heart rate variability were associated with increased indoor particle concentration and window opening, while no significant changes in cardiovascular endpoints were observed when windows were closed and AC was on.
Biomarker studies
Brugge et al. (2017) US: Boston & Chelsea, MA; homes 23 low-income Puerto Rican adults; skewed female and elderly Filtration and air cleaning No intervention benefit in terms of reduced inflammation; associations between inflammation biomarkers hsCRP, IL-6, or TNFRII in blood samples, and associations with indoor particle number concentrations (PNC) were inverse and not statistically significant.
Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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.
×
Kajbafzadeh et al. (2015) Canada: British Columbia; homes 83 healthy adults in traffic- or woodsmoke-affected areas Filtration and air cleaning Endothelial function and biomarkers of systematic inflammation showed an increase in C reactive protein per interquartile range increase in indoor PM2.5, only in traffic-affected locations, not woodsmoke-affected locations.
R. W. Allen et al. (2011) Canada: British Columbia; homes 45 healthy adults Filtration and air cleaning Air filtration associated with improved endothelial function and decreased concentrations of inflammatory biomarkers but not markers of oxidative stress.
Li et al. (2017) China: Shanghai; dormitories 55 healthy college students Filtration and air cleaning Significant increases in cortisol, cortisone, epinephrine, and norepinephrine. Between-treatment differences were also observed for glucose, amino acids, fatty acids, and lipids.
Chen et al. (2018) China: Shanghai; dormitories 55 healthy college students Filtration and air cleaning Randomized crossover trial of a sham versus true air filtration intervention and estimated associations with time-weighted PM2.5 indoor and outdoor exposures. Higher PM2.5 exposure positively associated with the expression interleukin-1 (IL1), IL6, tumor necrosis factor (TNF), toll-like receptor 2 (TLR2), coagulation factor 3, and endothelin 1 (EDN1), and negatively associated with miRNAs (miR-21-5p, miR-187-3p, miR-146a-5p, miR-1-3p, and miR-199a-5p) predicted to target mRNAs of IL1, TNF, TLR2, and EDN1.
Yang et al. (2022) China: Beijing; schools 125 children ages 9–12 Filtration and air cleaning 27 biomarkers tested. Air cleaner intervention associated with decreases in fractional exhaled nitric oxide, exhaled breath condensate IL-1β, and IL-6.
Wen et al. (2022) China: Beijing; dormitories 54 healthy college students Filtration and air cleaning 38 inflammatory cytokines tested. No significant alteration in cytokines observed under air filtration intervention.
Cognitive function
Gignac et al. (2021) Spain: Barcelona; schools 2,123 high school students ages 13–16 Filtration and air cleaning No differences found in the median of cognitive attention test (Flanker task) hit reaction time standard error (HRT-SE) between classrooms with cleaned air and normal air.
Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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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B. Du et al. (2022) Canada: Toronto; simulated office 59 healthy university students Source control Exposure to essential oil emissions caused shortened reaction time at the cost of significantly worse response inhibition control and memory sensitivity, indicating potentially more impulsive decision making. Effect of scented lemon-oil and non-scented grapeseed oil were similar.
More than one category of health outcome
Guo et al. (2021) China: Chongqing; nursing home 24 healthy older adults Filtration and air cleaning Air filtration associated with significantly decreased concentrations of inflammatory and coagulation biomarkers, but not of biomarkers of oxidative stress and lung function.
Zhao et al. (2020) China: Beijing; homes 29 healthy young adults Filtration and air cleaning Blood pressure, pulmonary function, fractional exhaled nitric oxide, circulating biomarkers tested. Statistically significant improvements in most measured effects.
Shao et al. (2017) China: Beijing; homes 35 non-smoking senior adults Filtration and air cleaning Filtration lowered PM and systemic inflammation (IL-8); no other demonstrable changes in the cardio-respiratory outcomes of study interest.
Wang et al. (2021) China: Beijing; dormitories 57 healthy students Filtration and air cleaning Increased blood pressure and airway inflammation, and decreased lung function associated with specific phthalic acid esters. Compared with sham air purification, average diastolic blood pressure, fractional exhaled nitric oxide, and 8-isoprostane (8-isoPGF2α) levels decreased significantly in the real purification. The effects of indoor air purification on lung function indicators including forced expiratory volume in one second (FEV1), peak expiratory flow (PEF), and forced expiratory flow were also significant.
Chen et al. (2015) China: Shanghai; dormitories 35 healthy college students Filtration and air cleaning Air purification significantly associated with decreases in geometric means of several circulating inflammatory and thrombogenic biomarkers. Furthermore, systolic blood pressure (BP), diastolic BP, and fractional exhaled nitrous oxide were significantly decreased. The impacts on lung function and vasoconstriction biomarkers were beneficial but not statistically significant.
Weichenthal et al. (2013) Canada: southern Manitoba; homes 37 First Nations adults Filtration and air cleaning Lung function, blood pressure, and endothelial functions measured. Air filter use was associated with an increase in forced expiratory volume in 1 s, a decrease in systolic blood pressure, and a decrease in diastolic blood pressure.
Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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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Dong et al. (2019) China: Beijing; schools 44 children ages 11–14 Filtration and air cleaning Cardiorespiratory metrics showed an increase in forced exhaled volume in 1 s and a decrease in fractional exhaled nitrogen oxide. However, heart rate variability was altered negatively.
Johnston et al. (2013) Australia: Launceston & Hobart; ambient 67,000 Launceston residents, intervention, & 148,000 Hobart residents, controls Source control Before and after intervention, PM10 levels dropped in intervention city, with small, non-significant reductions in annual mortality. In males, mortality reduction significant for all-cause, cardiovascular, and respiratory.
Karottki et al. (2013) Denmark: Copenhagen; homes 48 non-smoking senior adults, some taking vasoactive drugs Filtration and air cleaning No statistically significant effects of filtration were observed on microvascular and lung function or the biomarkers of systemic inflammation among all subjects or in the subgroups taking or not taking vasoactive drugs. However, the filtration efficacy was variable and microvascular function was significantly increased within 2 days with the actual PM2.5 decrease in the bedroom, especially among subjects not taking any drugs.
Jiang et al. (2021) China: Beijing; walking outdoors 52 healthy college students PPE N95 use while walking outdoors improved lung function, reduced nitrate in exhaled breath condensate (reduced respiratory oxidative stress), and reduced serum inflammation biomarkers; effect was stronger during higher pollution conditions.
Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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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Source Control

Source control via elimination or substitution is, when feasible, the best line of defense against exposure, following recommendations on the hierarchy of controls. Elimination may be accomplished by removing a source or relocating the exposed people. As demonstrated in Figure 5-1, mitigating at the source prevents the downstream transport, transformation, and contact processes that result in exposures and health effects. The co-benefits typically extend beyond fine particle reduction, by including the removal of other gaseous and particle components emitted by the source.

Examples of source-control measures at the policy and behavior level are siting a school away from busy roadways, providing residents with access to clean air shelters during wildfires, electrifying heating and cooking in homes, electrifying school buses, banning the use of candles in schools, and enacting regulatory bans or incentives to reduce the use of high-emitting materials or devices (details about these sources are in Chapter 3), along with the banning of smoking in indoor environments (e.g., Repace, 2004). Examples of programs that incentivize innovation and behavior change are the Biden-Harris administration Net-Zero Game Changers initiative (OSTP, 2022), Green Building certifications that address fine PM or programs to educate consumers to drive the use of cleaner options and discourage the use of elective or recreational sources such as candles and incense.

Another class of solutions involves engineering a product or appliance to have a lower emission rate or less toxic emissions (as described in Chapter 3). Either primary or secondary emissions may be targeted. For instance, secondary sources are targeted by designing cleaning products without terpenes or vacuum cleaners that reduce the reservoir of dust and allergens available for resuspension or by using hard flooring instead of carpet. Disinfectants or materials that suppress the growth of dust mites or other allergens can reduce the bioactivity of a source, rather than targeting emissions on a bulk quantitative basis. However, safety regulations mandating limits on or disclosures for particle or precursor emissions from (or amounts of toxic chemicals contained in) consumer equipment are limited. When they exist, the onus is on the consumer to demonstrate harm.

Policy-level measures have the advantage of influencing large populations in a cost-effective manner. They can be targeted to address equity considerations or specifically to address marginalized and susceptible populations, and they can have benefits that persist over time. However, while individuals and communities can advocate for them, they require a political and budgetary coordination that typically takes time, and they are vulnerable to being influenced by special interest groups and systemic bias. Measures relying on individual behavior change have the advantage of being immediately available. Their accessibility depends on cost, and the persistence of benefits relies on continued use. A practical barrier is access to reliable data and recommendations to guide behavior and purchasing decisions, especially in cases where a decision involves a comfort or aesthetic trade-off.

The design and selection of a source-control strategy is dependent primarily on the type of source it is directed toward. The range of sources and their characteristics were reviewed in Chapter 3. The attributes considered were: Are the emissions continuous or intermittent? Is the source primary or secondary? Is it bioactive or biologically inert? How toxic are the emitted particles? Are marginalized and susceptible populations disproportionately exposed?

The practicality of a source-control measure depends on how readily it can be deployed, whether it is feasible for an individual or requires collective action or product reengineering, and

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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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also on the relative effort that would be needed to employ downstream measures such as ventilation and filtration to mitigate the resulting exposures and health effects.

The evidence base for source-control measures that demonstrably reduce adverse health impacts is slim for the range of sources that are within the scope of this review. The vast majority of papers in the literature review that presented a source-control strategy addressed either biomass cookstoves used in rural settings in lower- or middle-income countries or environmental tobacco smoke. The single study (Noonan et al., 2017) that met the review criteria had inconclusive findings. The authors evaluated asthma-related outcomes among children in homes using biomass combustion for residential home heating in rural and periurban areas in the United States. The effects of replacing a wood stove with an Environmental Protection Agency (EPA)-certified, improved-technology wood-burning appliance and of implementing an air filter were evaluated. The results indicated that the air filters, but not the improved stoves, resulted in lowered PM2.5 and an improvement in one of the secondary health outcomes tested. Neither intervention resulted in improved quality-of-life measures. Johnston et al. (2013) also evaluated the effect of a wood heater replacement program on multiple health outcomes, but the relevance of their findings is limited because of the lack of indoor air quality data.

A related evidence-base that was not systematically reviewed but that indirectly points at the benefits of source control includes studies on source-specific health effects. Chapters 3 and 6 present studies where the existence of a source was strongly associated with specific health effects, so that it is reasonable to conclude that removing those sources would mitigate the adverse outcomes. As an example, in a controlled exposure study with 34 female and 25 male adults in a simulated office, B. Du et al. (2022) presented evidence of the cognitive impacts of essential oil emissions. Outcomes studied included reasoning, response inhibition, memory, risk taking, and decision making. Essential oil diffuser particle emissions caused shortened reaction time at the cost of significantly worse response inhibition and memory sensitivity, indicating potentially more impulsive decision making. The effects of scented lemon-oil and unscented grapeseed oil were similar. Studies of this nature can be used to infer the health benefits anticipated from eliminating or replacing specific sources.

A challenge associated with this review is that it excluded studies describing exposures that were not clearly attributed to the fine particle fraction as well as nonspecific interventions that included source control among a range of other measures. As a result, hygiene measures such as integrated pest management to reduce exposures to pests, pest control agents, and microbial products and efforts to reduce the use and generation of toxic chemicals such as phthalates and metals for which inhalation of airborne fine particles is one of the exposure routes are not reviewed in depth, in spite of their importance. Though excluded from the literature review on these grounds, studies showing the improvement of symptoms from the reduction of allergens do translate to broad-scale recommendations for household behaviors.

Ventilation

Improving building ventilation has been a component of healthy home and other building interventions. Lajoie et al. (2015) found that installing of mechanical ventilation in homes of children with asthma significantly reduced their symptoms. A 2017 literature review by Fisk and Chan found that increased ventilation rates were associated with reduced respiratory health effects and student absences. Review studies (Jacobs et al., 2010; Sandel et al., 2010) generally found evidence to indicate that some measures of interventions have health benefits through reducing occupant exposures to indoor air pollutants. However, the measures that are most likely

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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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to affect indoor fine PM exposures, such as building ventilation and envelope sealing, require more field evaluation and research to show their effectiveness. Similarly, a review by Fisk et al. (2020) on the impacts of residential energy efficiency retrofits with IEQ, comfort, and health found that there are too few studies (only three were identified that met the inclusion criteria) that measured changes in indoor particle concentrations pre and post retrofits.

Some studies have reported on the impacts of building ventilation on indoor concentrations of fine PM (Kang et al., 2022; Singer et al., 2017; H. Zhao et al., 2021). However, the committee’s review identified only one U.S. study that evaluated the health outcomes. Singleton et al. (2018) studied the impact of home remediation and household education on indoor air quality, respiratory visits, and symptoms in Alaska Native children. The home remediation included improving ventilation (passive vents, bathroom fans, or range hoods in 98 percent of homes) and other measures: replace old leaky woodstoves with more efficient, EPA-certified models (47 percent), fixing or replacing oil-fired furnaces (23 percent), and addressing moisture issues (10 percent). Environmental health professionals provided home-based education about indoor air quality (IAQ) using discussions and informational pamphlets about the proper use of home ventilation systems, burning dry wood, gasoline storage, using best household cleaning practices, and smoking outside the home. Three months after the interventions, a respiratory therapist or case manager made an in-home visit to provide education on respiratory triggers, asthma medication use, and medication compliance. Overall, parents reported decreases in symptoms after remediation. Children had an age-adjusted decrease in lower respiratory tract infection (LRTI) visits. Singleton et al. (2018) concluded that home remediation and education reduced respiratory symptoms, LRTI visits, and school absenteeism in children with lung conditions. However, short-term IAQ monitoring (1 to 4 consecutive days) that was repeated three times (2 weeks before, 2 weeks after, and 12 months after the remediation) found decreases in total volatile organic compounds (VOCs) and benzene, toluene, ethylbenzene, and xylenes, but no changes in PM2.5.

One study in Taipei focused solely on ventilation as the mitigation strategy (L.-Y. Lin et al., 2013). It recruited 300 healthy subjects, and their exposures to PM and total VOCs were monitored under three conditions: windows open, windows closed, and windows closed with air conditioning on. During each of the 24-hour monitoring periods, participants stayed home and refrained from combustion activities. Under these prescribed experimental conditions, the study found that in-home exposure was associated with inflammation, oxidative stress, blood coagulation, and autonomic dysfunction. By closing windows and turning on air conditioning (and under the case where there was no combustion indoors), this study concluded that improvements in cardiovascular health could be expected. However, findings from this study may not apply in conditions where the mixture of indoor and outdoor sources of PM is different and, in particular, in locations with low ambient PM.

Because ventilation will not only alter indoor exposure to fine PM but also change the concentrations of other indoor air pollutants (such as VOCs), intervention studies cannot easily pinpoint the observed health outcomes with fine PM alone. For example, Kile et al. (2014) evaluated the association between ventilation of gas stoves and chronic respiratory illness in U.S. children enrolled in NHANES III. Even though the study found that children whose parents reported using ventilation when operating their stove had higher lung function and lower odds of asthma, wheeze, and bronchitis, it was not clear if the health benefits were due to reductions in fine PM or in other indoor air pollutants, such as NO2.

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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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The role of ventilation in mitigating the risk of airborne infectious transmission has recently been reviewed (Allen et al., 2021; Fox et al., 2021). Several studies point to the potential effectiveness of ventilation (and filtration) in reducing infection risk in classrooms (Buonanno et al., 2022; Gettings, 2021; Muecke et al., 2006). Despite the general consensus that more ventilation is beneficial for infectious disease outcomes, there is a need for more deliberate considerations on ventilation metrics and acceptable risk for different building types and their occupants. Studies that gather sufficiently detailed data about ventilation and other building factors when testing the effectiveness of mitigation could also help inform this discussion. The 2023 recommendations from the Centers for Disease Control and Prevention (CDC, 2023) to aim for at least effective five air changes per hour and in ASHRAE Standard 241(ASHRAE, 2023) are based on the principle that more ventilation will lower infection risk from reducing exposure to potentially infectious respiratory aerosols, though in practice the effectiveness of increasing ventilation will depend greatly on the existing conditions of a given building. Further, increasing ventilation in the absence of filtration or available thermal conditioning will increase the indoor exposure to fine PM of outdoor origin as well as increase the risk of overheating during extreme heat events. The intersection between ventilation and energy use is particularly important as conditioning of ventilation air often drives building energy use.

Filtration and Air Cleaning

Air cleaning as a mitigation measure for fine PM has a relatively short history (less than 70 years) for use with fine PM (Burroughs, 2020), and the formal study of the health effects of air cleaning measures has an even shorter history. There are a variety of scales for air cleaning of fine PM (e.g., central, room, personal) and a variety of technologies (e.g., media filtration, ionization, electrostatic) which are summarized elsewhere (e.g., Siegel, 2016); however, almost all of the published literature that met the criteria for inclusion in this review focused on room filters that use media filtration (e.g., HEPA filters). Background and contextual information on other scales of air cleaning and other technologies are included here for completeness and to guide their potential inclusion in fine PM mitigation efforts in the future.

Room air cleaners for fine PM are devices that have two general mechanisms: something that moves air (usually a fan) and something that removes fine PM from the air. By far the most common instantiation is a portable unit that contains a high-efficiency particulate air (HEPA) filter and a fan. Room air cleaner performance is best described by a clean air delivery rate (CADR, often standardized according to the American National Standards Institute/Association of Home Appliance Manufacturers AC-1 standard), which is the product of the filter efficiency for fine PM and the air flow rate through the filter. Both of these parameters are independently important. An air cleaner that has a small flow rate cannot be an effective air cleaner, even if it has a high single-pass removal efficiency, simply because it does not treat enough air to compete with other loss mechanisms (namely deposition and ventilation, see Chapter 4). Similarly, an air cleaner that has a low efficiency for fine PM will not provide a competitive removal sink. A room air cleaner will be less effective for removal of fine PM in a larger room and will also be less effective in a room with more ventilation because of competition with other loss processes for fine PM. There are a variety of sizing practices for room air cleaners, including an effective floor area defined in the Association of Home Appliance Manufacturers standard (AHAM, 2020) and a total removal from all loss mechanisms, including room filtration, for reducing risk of transmission of infectious disease (e.g., the recommendation of five effective air changes per

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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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hour to reduce transmission of COVID-19 from the Centers for Disease Control and Prevention (CDC, 2023)).

How well a room air cleaner performs at removing fine PM is highly variable between environments (because of building contextual factors described above) and over time for the same air cleaner (e.g., Barkjohn et al., 2021; Pei et al., 2020). Some of the major factors behind this variability are declines in efficiency or flow rate owing to particle loading (e.g., Pei et al., 2020), placement especially relative to source (e.g., Dai and Zhao, 2022; He et al., 2021; Novoselac and Siegel, 2009), short circuiting and room mixing issues (locally cleaning the air near the air cleaner more than the broader room), user behavior (turning air cleaner to a lower speed, turning off, or moving to edge of room, especially in response to noise) (e.g., L. Du et al., 2011), and the dynamic nature of indoor sources and indoor penetration of outdoor particles (Chapter 3). Importantly, the CADR of room air cleaners is generally highest on the highest fan speed setting, which is also generally when the air cleaner produces the most noise (Peck et al., 2016). A critical consideration of almost all of the articles about portable air cleaners in this review is that this context is not assessed and therefore the results often reflect the unknown context of the research setting and time of experiments.

Fundamentally, the fine PM removal performance of air cleaning installed in a central system is the product of flow rate and removal efficiency and is subject to the same building contextual factors as room air cleaning (namely, competition with background loss rates; see, for example, Alavy and Siegel, 2020). Also, similar to room air cleaners, many charged media central air cleaning filters change in performance over time because of loading (Lehtimaki et al., 1994; Li and Siegel, 2020a) or because of declines in flow (Alavy and Siegal, 2019). There are also some fundamental differences from room air cleaners, factors such as bypass related to gaps around the filter (Li and Siegel, 2020a,b), interactions between air face velocity and efficiency (Hanley et al., 1994), and air flow control for conditioning and ventilation in variable air volume and multiple-speed systems.

Both room and central air cleaners can use other cleaning strategies besides media filtration. Generally, the most common class includes electrically connected air cleaners, which include a variety of technologies (often inconsistently named between manufacturers) including bipolar and unipolar ionization, plasma, and polarized media, among other technologies. Ionization is the most common, with some devices intentionally releasing ions into the air for a purported perception benefit or to increase deposition onto room surfaces, others charging particles and removing them to oppositely charged plates in the device (often called an electrostatic precipitator or electronic air cleaner), and others using ions to enhance removal to a conventional media filter. Many of these approaches have a variety of performance issues ranging from low CADRs (e.g., Waring et al., 2008), degradation in performance over time due to loading (e.g., Zuraimi et al., 2017), as well as the potential negative health effects associated with ions (W. Liu et al., 2021). Some electrically connected air cleaners also emit ozone, a respiratory hazard and chemical oxidant which can lead to a variety of compounds with potentially negative health outcomes, increased concentrations of odorous or irritating compounds, and increased concentrations of fine PM through secondary organic aerosol (SOA) formation. Although in general there are very few studies in the literature review that evaluated these technologies in the context of specific health effects, the precautionary principle should be followed. In particular, the production of primary (e.g., ions or ozone) or secondary (e.g., SOA or gas-phase byproducts) emissions should be avoided until deemed safe.

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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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No matter the scale or type of air cleaning, there are common factors that affect that practicality of air cleaning measures. Common issues include cost (both initial cost as well as replacement filter costs), electricity use/cost (and specifically, the perception of high energy use), noise, negative outcomes from loading or the lack of maintenance (e.g., increased emissions from used/dirty air cleaners, diminished performance because of flow or efficiency degradations), and aesthetics, among other factors. Many of these issues translate to phenomena and behaviors that diminish the effectiveness of air cleaners over time, and this points to a potential bias that results from short study periods in many of the investigations in the literature review because air cleaners likely have a high initial effectiveness and also points to the importance of the messaging for study participants concerning the benefits of air cleaning and effective air cleaner use. Such messaging is rarely described and is an important area for future efforts (see recommendations). Given the importance of these contextual factors, it is unclear whether the generalizability of the findings of any individual investigation on the health effects of air cleaning apply to any other application, particularly if the results are not contextualized with metadata on the air cleaner, building, and behavioral aspects of the air cleaner’s use. However, the existing literature is worth exploring for the identification of general benefits of air cleaning and benefits for specific populations as well as specific opportunities for application and needs for future research.

Of the four areas considered in this review, air cleaning as a mitigation measure has by far the most research. This review identified 55 articles that met the criteria for inclusion in the literature review (Table 7-3) and addressed air cleaning. The very big picture of this literature is that there is clear evidence that air cleaning is an effective mitigation measure for fine PM. However, there is also considerable variation in findings between studies and within studies for multiple health outcomes. Much of the inconsistency arises from the consideration of different health outcomes, variations in study designs, variations in study populations, and, critically, the often unassessed contextual factors described above. Each of these larger categories is discussed below, with a broader view towards practical guidance on air cleaning mitigation for fine PM that can be made based on available evidence as well as towards identifying areas for future research.

Table 7-3 divides health outcomes into broad groupings. One of the largest health outcomes investigated in the literature in response to an air cleaning intervention is childhood asthma. All investigations consider portable air cleaning generally with HEPA filters. Although there is encouraging evidence that filters improve at least one of the studied parameters, in all but two (Antonicelli et al., 1991; Phipatanakul et al., 2021) of the 16 included investigations have decidedly mixed evidence, with over half showing no improvement for at least one of the considered outcomes. Many of the investigations are a few weeks in total length, which may limit the ability to observe an impact. Although many of the included investigations measured fine PM, only some make specific measurements of likely asthma triggers. Notably, Antonicelli et al. (1991)—an investigation that found no significant benefit for the use of portable filters on any of the considered symptoms—also found no change in dust mite concentrations between operating and sham air cleaners. This may point to a particle size interaction (e.g., dust mites may be associated with larger particles which are already effectively removed by other loss processes) or to an unmeasured contextual factor. Fine particle concentrations are actually measured in only some of the studies, and none of the included studies measured exposure. Thus, it is unclear if the measure was appropriate for the space (e.g., undersized or poorly placed air cleaner) or whether the air cleaner was actually used.

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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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These same patterns exist for the other health outcomes in Table 7-3. Although there is considerable evidence that filters improve health outcomes, the majority of investigations found significant improvement in at least one measured health outcome. Similarly, most investigations also found no significant improvement in at least one measured health outcome. Taken together, these findings suggest that the use of filters is likely beneficial for some health outcomes but a consistent finding beyond this is challenged by the wide variety of study methodologies, health outcome assessments, and (often unmeasured) contextual factors that change the performance of filters. Of note, many articles in Table 7-3 associate beneficial health outcomes related to reductions in PM2.5, but it is unclear how much of the particle reduction was due to the filters used versus changes in contextual factors.

Personal Protective Equipment

Global interest in the efficacy of personal protective equipment (PPE) to filter particles and protect human health dramatically increased during the COVID-19 pandemic. This also influenced cultural norms related to masking, which could create opportunities to expand PPE use. There is a robust literature on PPE and protection from infection, particularly with a focus on COVID-19. Particles of different sizes are relevant for unique infectious diseases. The review did not explicitly include this literature but rather focused on the role of PPE in protecting from fine particulate matter exposure. Although PPE has been studied in occupational settings, that is also beyond the scope of this document.

Studies investigating PPE and protection from the risk of PM exposure are challenging from a design standpoint. It is difficult to measure the exposure reduction aspect as this occurs at the level of the individual, as opposed to source reduction, ventilation, or air cleaning, which can be assessed by measuring airborne particles in the relevant space. Efficacy measurements are also confounded by the amount of time used and by the fit of the mask, which can be influenced by PPE characteristics, individual characteristics, and behavior.

There are few studies of PPE use in residential or other indoor spaces that do not pose an occupational hazard related to exposure. Studies that have investigated PPE as an intervention have included chamber studies as well as experimental designs in which PPE is used while walking outdoors or during a prespecified amount of time. These have often been conducted in international settings in countries that have higher ambient concentrations of PM than the United States. Study populations range from younger healthy populations to elderly populations with specific chronic medical conditions.

Shi et al. (2017) designed a study to measure the effect of wearing masks in which they randomized 24 healthy young adult participants to wearing particulate respirators for 48 hours versus no respirator, with a 3-week washout period in between. The investigators noted the need for practical approaches to protect individuals from particulate exposure in developing countries and conducted this study in Shanghai. Participants were instructed to use the respirators when spending time outdoors, including a 1-hour walk outdoors, and as much as possible indoors. Respirator use resulted in lower blood pressure and improved heart rate variability parameters (high-frequency power, RSSD, pNN50).

Other studies in international settings used an experimental design of exercising outdoors to evaluate the health benefits of PPE. Jiang et al. (2021) used a randomized crossover trial to assess the effect of wearing N95 face masks versus sham masks among 52 college students in Beijing, China. The participants had lung function and cardiopulmonary blood biomarker assessment at baseline and after a 2-hour walk. The analyses compared the differences on high-

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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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pollution versus lower-pollution days (PM2.5 > 75 μg/m3 versus PM2.5 < 75 μg/m3). The N95 mask was associated with lower cytokine concentrations post-exposure for IL-6, IL-10, IL13, IL-17A, IFN-γ, and TNF-α. Lung function improved in the N95 group and did not change in the sham group. Beneficial effects of N95 were more pronounced on high-pollution days.

Han et al. (2021) also studied university students in China using a study design of walking outdoors near a busy road for 2 hours. Participants were assigned PPE interventions in a crossover design with powered air-purifying respiratory (PAPR) placebo, PAPR with PM filter, PAPR with PM and VOC filter, and an N95 respirator. The researchers demonstrated that short-term exposure to traffic acutely affects heart rate variability, blood pressure, and heart rate, but that N95 mask and PAPR interventions generally show little efficacy in reducing these effects. Langrish et al. (2010) applied a similar study design to investigate 98 individuals with coronary heart disease. Participants were randomised in an open crossover trial to a highly efficient mask versus no mask. The use of the mask was associated with a reduction in symptoms, reduction in blood pressure, and improvement in heart rate variability. These studies provide examples of attempts to study the health benefits of PPE in typical settings but have limited or no representation of the indoor environment.

Exposure chamber studies have evaluated the impact of air filtration through a polypropylene filter face mask. The FILTER-HR trial (a double-blind, randomized to order controlled, crossover trial) examined the effects of clean air, unfiltered diesel exhaust, and filtered diesel exhaust exposure in 26 individuals with heart failure and 15 control volunteers (Vieira et al., 2016). The filtration was implemented at the level of the chamber rather than the individual in the experimental design and demonstrated that filtration attenuated the adverse effect of diesel exhaust with respect to reactive hyperemia index and B-type natriuretic peptide. Filtration did not improve the effect of diesel exhaust (DE) on reduced 6-minute walk distance or arterial stiffness. DE has no effect on heart rate variability. The filtration of particles was associated with an increase in maximal oxygen consumption (VO2 max, maximal oxygen consumption) and O2 pulse during exercise testing compared with DE exposure. While this study examined the potential benefit of an air filter, the filter was not applied to the face, so extrapolation of findings is limited.

CONSIDERATIONS ACROSS ALL MITIGATION MEASURES

Despite differences between the intrinsic, building, and behavioral factors that make up the details of various mitigation measures, there are some common considerations. The first is that the effectiveness of a mitigation measure is often determined by the quality of the implementation guidance that accompanies it. Educational efforts—including those about important sources of fine PM, the effective use of user-controlled ventilation systems (such as rangehood fans), the placement and operation of portable filters, and the use of PPE—are often lacking and likely account for some of the variation observed in study outcomes. Although educational interventions themselves are not always effective in reducing adverse health outcomes (e.g., Walker et al., 2022), there have been few efforts to develop and test educational interventions that are responsive to building and behavioral contexts. Practical considerations associated with the use of mitigation measures, including when to change central or room filters and how to maintain ventilation and filtration systems, are often not well known by building occupants, and there are thus substantial opportunities for initiatives such as community-sourced

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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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and innovative educational materials and approaches to improve outcomes (for example, see Burke, 2020).

A second issue entails the disparities in which communities have been a part of mitigation research investigations. These disparities lead to results that cannot be generalized to different populations and contexts. There are disparities in the ability to implement mitigation measures, including cost, maintenance requirements, and appropriate training on effective use, and in the building factors that interact with mitigation measures, such as the inter-apartment transport of fine PM in multi-unit residential buildings, the availability of effective kitchen exhaust fans, and the presence of HVAC systems that use central air cleaning or mechanical ventilation. There are mitigation approaches (such as the banning of smudging, incense, or cannabis) that have important cultural implications for some groups and the historical use of source control to target specific groups (e.g., messaging about the harms of menthol cigarettes to African American communities). A robust approach to the practical mitigation of fine PM should not increase disparities and instead should be designed to prioritize reducing fine PM exposure for racialized, marginalized, and susceptible communities.

Finally, fine PM mitigation has historically been considered through the lens of cost. Capital and operating costs are often real or perceived barriers to specific mitigation measures. However, mitigation benefits are much larger than mitigation costs (Bekö et al., 2008; Fisk and Chan, 2017; Montgomery et al., 2015; Zuraimi and Tan, 2015), with avoided health care costs making up a large and often unaccounted portion of benefits. The cost of not mitigating fine PM includes these health care costs as well as the likely negative impacts of exposure on productivity, decision making, student learning, and cognitive performance. There is limited research estimating such costs, making this an important area for future efforts.

CONCLUSIONS AND RECOMMENDATIONS

The information reviewed by the committee in the course of their work leads them to conclude that effective and practical mitigation of exposure to fine particulate matter in homes and schools is currently possible. Such mitigation is possible with a proper combination of source reduction, ventilation, central or in-room filtration, and PPE. It is reasonable to assume that reductions in indoor PM2.5 concentration will have health benefits, even if based solely on reduction in exposure to PM2.5 of outdoor origin, although the literature related to the specific health benefits of such mitigation is sparse and sometimes mixed owing to the numerous confounding and limiting factors described in this and preceding chapters.

It is not possible to offer generic observations regarding which specific mitigation measures will be most practical to implement because, as this report has made clear, there are myriad variables characterizing the sources of indoor PM2.5 and UFPs; their fate, transport, and transformations indoors; the circumstances and level of exposure to them; the health effects associated with that exposure; and the context and details of how mitigation measures are used. Different circumstances will necessarily dictate different choices. Generally speaking, though, the hierarchy of controls explored by the committee—again, indoor source control, ventilation, filtration and air cleaning, and personal protective equipment—provides a guideline for determining the order in which alternatives should be pursued, and consideration should be given to layered or combined approaches.

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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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The following recommendations flow from the chapter’s analysis:

Public health professionals should prioritize the implementation of immediate, multilevel interventions to mitigate exposure, relying on currently available evidence and tools, for economically disadvantaged, historically marginalized, underserved, and disproportionately exposed populations. These prioritized and community focused efforts should form the basis for the studies of effectiveness and cost–benefit needed to expand exposure mitigation efforts to the general public.

Federal and regional agencies should fund large-scale, population-level clinical trials to build the evidence base concerning the health impacts of indoor PM mitigation measures. A standard of evidence for the effectiveness of PM control technologies and strategies should be created, based on positive health outcomes. The trials need to consider exposure scenarios related to indoor versus outdoor sources and acute versus chronic effects as well as a range of interventions, including filtration, ventilation, source control, and personal protective equipment.

Researchers should characterize building factors in studies of PM mitigation to appropriately contextualize findings and add to the existing knowledge on strategies to mitigate adverse effects. These factors should include ventilation rate, air infiltration, particle loss rates, portable filter clean air delivery rate and location, and such parameters as runtime, flow rate, and in-situ efficiency for central systems.

Public health professionals and researchers should consider behavioral factors in their development of control strategies to assure effective implementation and to maximize impact. Examples of behaviors that can mitigate or exacerbate exposure include adjusting air cleaner speed and operation to control noise levels or electricity use, adjusting HVAC or furnace runtime, using range hood fans, opening and closing windows, using primary and secondary sources such as candles or terpenes in cleaning products, and selecting electric or gas appliances for cooking and heating.

Environmental health researchers should consider the effects of composition and other particle attributes and use this knowledge to harness mitigation options that may be more practical in some settings than reduction of PM2.5 defined in conventional, mass terms.

Engineering and technology researchers and industry should endeavor to optimize existing air cleaning and ventilation technologies and also develop new one that are more effective, energy-efficient, quieter, easier to maintain, and more intuitive to operate. Special attention should be paid to lower-cost solutions that are more accessible and likely to be used by marginalized and susceptible individuals and communities. Additionally, in-situ air cleaning test approaches should be developed and promulgated that capture contextual factors as well as assess primary and secondary byproducts of air cleaning.

Coalitions of public health, engineering, and social science and public policy researchers should partner with community-based organizations to better characterize and address potential non-technical components of a successful PM mitigation implementation effort, such as messaging, education, and community engagement. Efforts should be made to better understand implementation strategies that can bring the most benefits to vulnerable, underserved, or disproportionately exposed populations.

Suggested Citation:"7 Practical Mitigation Solutions for Indoor PM." 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:"7 Practical Mitigation Solutions for Indoor PM." 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:"7 Practical Mitigation Solutions for Indoor PM." 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:"7 Practical Mitigation Solutions for Indoor PM." 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:"7 Practical Mitigation Solutions for Indoor PM." 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:"7 Practical Mitigation Solutions for Indoor PM." 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:"7 Practical Mitigation Solutions for Indoor PM." 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:"7 Practical Mitigation Solutions for Indoor PM." 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:"7 Practical Mitigation Solutions for Indoor PM." 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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