Summary

The COVID-19 pandemic taught many hard lessons. Among the most prominent of these was the tight connection between the indoor environment and health. People learned that schools, workplaces, businesses, and even homes were places where someone could be subjected to a hazard simply by breathing.

Among the indoor exposures that presents a concern. is 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 (PM_2.5)¹, 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 PM_2.5 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. Although it has been relatively poorly studied to date, indoor exposure to PM_2.5 is gaining increased attention, particularly given that Americans spend the vast majority of their lives indoors and that indoor PM_2.5 levels can exceed outdoor² levels.

Against this backdrop, the U.S. Environmental Protection Agency (EPA) asked the National Academies of Sciences, Engineering, and Medicine to convene a committee of scientific experts to consider 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. EPA requested that the committee focus on residential settings but also consider schools and other non-industrial indoor environments where appropriate.

FRAMEWORK AND ORGANIZATION

Chapters 1 and 2 of the report provide introductory and background information, including the committee’s full statement of task and synopses of previous National Academies studies on the indoor environment. Chapter 3 discusses the sources and composition of indoor particulate matter, while Chapter 4 covers particle dynamics and building characteristics that influence indoor PM. Chapters 5, 6, and 7 describe, respectively, exposure to indoor PM, the

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¹ “Fine particulate matter”, “fine PM”, and “PM_2.5” are used interchangeably throughout this document.

² “Outdoor” and “ambient” are used interchangeably throughout this document.

health effects of that exposure, and practical mitigation solutions to such exposure. Social, economic, and cultural differences affect every aspect of these issues—what and how much fine PM people are exposed to, the circumstances in which that exposure occurs, the effects that exposure has on their health, and the opportunities for employing mitigation measures—and it thus addressed throughout the report. Major findings, conclusions, and recommendations are collated in Chapter 8.

REPORT SYNOPSIS

Human exposure to fine PM and the subsequent health effects are complex and require a systems approach for understanding and developing appropriate practical mitigation strategies. The cascade of events leading to health impacts starts with PM_2.5 sources (intrusion of outdoor sources into the indoor environment or PM generated from indoor sources), so understanding the nature of these sources as well as their size distributions and compositions is important. Once particles are emitted, they are transported away from the source, mixed into the indoor space, distributed to different parts of a building, and may be exhausted to outdoor air, deposited onto indoor surfaces, or captured by filters in a mechanical system, standalone air cleaners, or even personal protection equipment (PPE) in the breathing zone of an individual. Particles may also be transformed as they migrate through an indoor atmosphere or following deposition onto surfaces. Such processes affect the concentration, size distribution, and composition of PM_2.5 that building occupants are exposed to. These factors, along with the frequency and duration of exposure, influence the health effects caused by the inhalation of indoor fine PM, including ultrafine particles.

Sources of Indoor Fine Particulate Matter

Fine PM sources are the drivers for exposure, health effects, and the need for mitigation. As noted, they include outdoor particles that penetrate indoors as well as those emitted directly from indoor sources. Data and modeling suggest that particles of both outdoor and indoor origin contribute almost equally to indoor fine PM concentrations when measured by mass. In contrast, the number concentrations for indoor UFPs are dominated by indoor sources. During acute events such as wildfires, particles of outdoor origin may dominate indoor PM_2.5 concentrations. Inhabitants of homes with underlying housing quality issues often have larger exposure both to outdoor-origin fine PM—because of such factors as proximity to sources of industrial emissions and highways and increased natural ventilation and leakage—and to indoor-origin PM, owing to greater occupant and indoor source densities.

Indoor sources are dynamic and vary by frequency of occurrence, frequency of use, emission source strength, composition, the size distribution of emitted particles, their locations within buildings, the existence or effectiveness of local exhaust or capture, proximity to occupants, and socioeconomic factors.

Indoor combustion sources emit significant amounts of PM_2.5. Natural gas combustion is a substantial source of UFPs, particularly if the particles are not properly exhausted above a stove or vented from appliances such as water heaters, dryers, or heating systems. Wood combustion in fireplaces and wood-heating stoves can also be a prominent source via direct emissions to indoor spaces and the accumulation of outdoor PM_2.5 that penetrates back into buildings. Smoking of tobacco or other products is another major combustion source.

Indoor cooking activities are an important source and can lead to short-term but intense increases in both PM_2.5 and UFPs. Emissions depend on numerous factors, with higher temperature cooking and higher fat content foods leading to higher emissions. Countertop appliances such as toasters can also be important sources of PM_2.5 and are generally unvented.

Other sources of indoor PM_2.5 and UFPs include candles; incense; office equipment such as photocopy machines, laser printers, and 3-D printers; scenting products that are heated with oils; spray products used for cleaning and personal care; building occupants (via shedding and respiratory aerosols); and aerosolized water sources that leave behind mineral particles or microbial agents like bacteria or fungal spores. Fine PM resuspension from indoor surfaces can cause short-term spikes in particle concentrations, often near the building occupants responsible for the resuspension. Finally, indoor secondary organic aerosols can be formed from the reactions of organic compounds (in cleaning products, for example) with oxidants such as ozone that enters from outdoors or are emitted from electronic equipment including ionic and other air cleaners.

Less research has been conducted on sources of PM_2.5 in schools. However, classrooms with closer proximity to cafeterias and windows facing bus loading zones have been associated with higher classroom PM_2.5, and lack of furnace cleaning has been associated with higher black carbon concentrations. Student movement in and around classrooms also leads to the resuspension of particles from flooring and other surfaces.

Particle Dynamics and Building Characteristics that Influence Indoor PM

Once it has entered a building from outdoors or been emitted directly indoors, fine PM mixes into room air and may be transported to other spaces within the building. The rate and extent of such transport depends on the building layout; heating, ventilation, and air conditioning (HVAC) design and operation; and open doors and other connections between zones. Activities such as walking affect the direction of particle movement and dispersion, suggesting that measurements in occupied versus unoccupied spaces will result in different transport outcomes. Recent advances in consumer-grade sensors have allowed for the deployment of more monitors to investigate PM transport at higher spatial and temporal resolutions. The deployment of such monitors in homes has been used to show that emissions from cooking in kitchens can be detected in bedrooms within minutes following emission, depending on location. Inter-zonal transport can also be a source of indoor fine PM from neighboring units in multi-family buildings—secondhand smoke transfer between adjacent units, for example.

During transport, particles may undergo physical, chemical, and biological processes such as aerosol aging, oxidation, evaporation, condensation, and partitioning, which collectively interact to influence important indoor PM properties, including chemical composition, size, phase state, surface charge, and, for some biological particles, viability. Particles are also removed from indoor air by deposition onto surfaces, ventilation/exfiltration to the outdoors, and capture by filtration systems. A challenge in understanding individual sources, sinks, and transformations is that measurements of indoor fine particulate matter concentrations, size distributions, or composition alone generally do not yield insights into the presence or magnitude of specific mechanisms. Rather, indoor concentration measurements produce a measure of the net result of any number of competing or interacting processes. It is possible, though, to use a combination of mathematical models and measurements to estimate the magnitude of particle sources, sinks, and transformations.

Building Occupant Exposures to Fine PM

Despite advances in assessment methodology and instruments, there remain significant challenges to accurately quantifying exposure to PM_2.5 indoors and linking such exposures to specific sources. This hinders the ability to make informed decisions on mitigation strategies and identify disparities associated with such exposure.

Due to improvements in outdoor air quality and advances in building design and construction, indoor exposure to PM_2.5 of outdoor origin has generally been decreasing in the United States over the past decades. However, this reduction is not occurring uniformly, and communities affected by wildfire smoke and those near localized outdoor sources such as congested roadways or industrial facilities can still be significantly burdened. Furthermore, limiting the penetration and infiltration of outdoor fine particulate matter by reducing air exchange between outdoors and indoors can lead to an increase in the concentration of PM_2.5 from indoor sources.

There are several methods for quantifying inhalation exposure. Personal monitors are regarded as the gold standard for measuring individual exposure at the point of contact. However, the equipment can be cumbersome and expensive for large-scale studies. Exposure models rely on data on time–activity patterns combined with measured or modeled particle concentrations in various locations to predict exposure when personal monitoring is not feasible. Exposure reconstruction uses internal body measurements, or biomarkers, to directly measure the absorbed dose and to infer exposure from multiple pathways and sources. The literature on the utility of using biomarkers of exposure to indoor PM_2.5 is limited; however, extant studies indicate a relationship between outdoor PM_2.5 and DNA alteration.

Consumer-grade sensors are improving the ability to measure PM_2.5 exposure, but important limitations remain. Beyond improving instrument accuracy, cost, ease of use, and other performance aspects, it will be is critically important to advance understanding of how measured values are useful for determining the health impacts from exposure to fine particles and benefits of mitigation.

There are numerous complexities associated with exposure assessments, starting with the selection of the appropriate metric. PM may be characterized by such parameters as mass, number count, composition, and surface area, and it is not necessarily clear which parameter may be most useful for informing a specific exposure-related question. This issue is further complicated for UFPs, which make up a small amount of the mass of PM_2.5 but are dominant in number.

An additional complexity entails temporal variations in exposure and the resolution of short-term (acute) versus long-term (chronic) exposures. The knowledge base related to acute exposure to indoor PM_2.5 and UFPs is particularly limited, but such knowledge is important to understanding the risks related to exposures from cooking and emerging sources.

Our understanding of the potential health impacts from various indoor sources in different built environments is partly restricted by limitations in the instrumentation available to characterize exposure. Complexities associated with the varying composition of PM_2.5 (which may have health implications), and the nature of exposed populations and their vulnerability to PM_2.5 or its chemical or biological components, also complicate analyses.

Exposure studies point to the potential for large disparities among populations. Disparities occur not only because of higher indoor PM_2.5 concentrations that are associated with activities happening in smaller, densely occupied and interconnected (multi-family) homes, outdated appliances that have higher emissions, and ventilation equipment that is not present or

is less effective at removing PM_2.5, but also because of elevated outdoor PM_2.5 concentrations. These large disparities in exposure can lead to excessive health burdens on some populations.

Health Impacts of Exposure to Fine PM in Buildings

Knowledge of the health effects of PM_2.5 is dominated by studies of exposure to PM of outdoor origin. Such studies are useful in understanding health effects from indoor exposures because of the encroachment that outdoor pollution makes into indoor spaces. There is also a growing base of literature on the effects of indoor PM_2.5 of combined outdoor and indoor origin on human health. Generally speaking, the literature on the respiratory and cardiovascular effects associated with indoor air pollution is more established than that of the effects on neurological and reproductive systems as well as cancer.

Respiratory effects.

Indoor PM_2.5 exposure has been implicated in a range of adverse acute clinical, biologic, and physiologic manifestations of asthma and chronic obstructive pulmonary disease (COPD), including symptoms, exacerbations, inflammation, and degraded lung function and quality of life. Indoor PM_2.5 has also been linked to symptoms in populations without lung disease. The biologic components of indoor particulate matter also cause acute respiratory effects among people with and without asthma. Although there are few studies examining the health effects of PM_2.5 exposures in schools, investigations have reported associations between exposures to some airborne biologic agents—a fraction of which are fine PM—in schools and adverse health effects among children with asthma.

Cardiovascular effects.

There is strong evidence that elevated outdoor PM_2.5 concentrations are associated with adverse cardiovascular health outcomes. Short-term increases in outdoor PM_2.5 are associated with increased risks of mortality and heart failure events requiring emergency department visits or hospitalizations. Studies of outdoor PM have consistently demonstrated that both short and long-term increases in PM_2.5 are associated with increases in blood pressure, a physiologic measurement that is one of the major risk factors for cardiovascular disease, including coronary artery disease, heart failure, stroke, chronic kidney disease, and diabetes, as well as for other chronic conditions. However, studies investigating indoor PM_2.5 and blood pressure, heart rate, heart rate variability, and electrocardiogram changes have demonstrated mixed results.

Cancer.

There is substantial evidence to support a causal link between particulate matter of outdoor origin and lung cancer incidence and mortality. Most of what is known about the association between pollution of indoor origin and cancer has been generated from studies of secondhand smoke and biomass burning in low- and middle-income countries and has been primarily focused on lung cancer. Using an indoor wood-burning stove or fireplace has been associated with a higher risk of breast cancer in women with a family history of breast cancer.

Neurological effects.

Elevated exposure to ambient PM_2.5 and also indoor work-related exposure has been associated with neurodegeneration risks such as dementia and cognitive decline and with an increased risk of depression for long-term exposure to outdoor PM_2.5. Living in areas with higher levels of particle components was associated with an increased risk of psychiatric hospitalization. In school environments, increased classroom PM_2.5 exposure has been associated with decreased performance on neurobehavioral tests for children. Children attending schools with higher levels of UFPs both indoors and outdoors experienced diminished growth in cognitive measurements. Multiple studies have demonstrated that UFPs penetrate the brain via the nose and olfactory nerve, and animal models have shown that UFPs alter neurotransmitter levels, triggering oxidative stress, inflammation, and other biochemical changes. Links between

such biochemical changes and health outcomes such as cognitive decline, autism, and depression have been hypothesized.

Reproductive effects.

Prenatal exposure to increased concentrations of ambient PM_2.5 has several adverse associations, including maternal depression after childbirth. Exposure to ambient PM_2.5 from wildfire smoke has been associated with pre-term birth and decreased birth rate. There have been multiple reports associating prenatal and early childhood exposures to PM_2.5 with neurological outcomes in children, primarily behavioral effects and degraded school performance.

Disparities related to health impacts of fine PM are rooted in numerous factors that culminate in the potential for greater health effects for those in economically disadvantaged households. Furthermore, there are disparities associated with the ability to purchase mitigation devices (indoor air filters, for example) or retrofit residences to improve mitigation measures as well as to obtain necessary health care for impacts of exposure to air pollution.

Practical Mitigation

Given the importance of PM_2.5 to human health and the exposures that occur indoors, there is a compelling need to address mitigation approaches to reduce exposure. Research indicates that several practical measures can be taken to effectively reduce indoor PM_2.5 concentrations. It is reasonable to assume that such reductions will result in health benefits, if for no other reason than the large knowledge base concerning the health benefits of lowering ambient PM_2.5 and the fact that a significant fraction of PM_2.5 indoors is of outdoor origin. However, studies of interventions to lower PM_2.5 exposure in residences and schools face methodological and logistical challenges that make it difficult to evaluate the efficacy of these interventions. Part of the failure to associate measured outcomes with PM_2.5 reductions can be attributed to a lack of accounting for competing removal mechanisms and other confounding factors. Furthermore, some studies only report the effect of introducing an exposure mitigation device rather than measuring whether exposures are reduced, and these are two different endpoints.

It must be kept in mind that many means for reducing exposure to PM_2.5 have associated costs and maintenance requirements, and may entail appropriate training on their effective use. Additionally, building factors such as the availability and use of effective kitchen exhaust fans and the availability of well-maintained HVAC systems, interact with mitigation measures in important ways. And mitigation approaches—particularly those related to source control—can have important cultural implications for some groups who may, for example, use candles or incense in religious practices. All of these factors must be considered when formulating strategies for limiting exposures.

Four general forms of mitigation of PM_2.5 were addressed in this report.

Source control.

Source control may include the elimination or substitution of a source or a reduction in its emissions. Some studies have associated specific sources with adverse health effects, making health benefits of source control by elimination clear. However, the evidence base for source-control measures that demonstrably reduce adverse health impacts is modest for the range of sources that are within the scope of this review.

Ventilation.

The amount of outdoor air brought into a residence or school affects the concentration of PM_2.5 and the impacts of changes in ventilation can be complex. For example, ventilation used for PM_2.5 control impacts other pollutants. Increased ventilation reduces indoor exposure to PM_2.5 of indoor origin and also reduces the concentrations of other pollutants of

indoor origin such as volatile organic compounds (VOCs), but it may also increase concentrations of outdoor pollutants, such as ozone and PM of outdoor origin. As such, intervention studies cannot easily pinpoint the observed health outcomes related to PM_2.5 alone.

Filtration and air cleaning.

Central systems and room air cleaners based on media filtration have the potential to significantly reduce indoor PM_2.5, but there is inconsistent evidence that this is an effective mitigation measure for reducing health effects. Much of the inconsistency arises from the consideration of different health outcomes, variations in study designs and study populations, and unassessed contextual factors. Room air cleaner performance varies significantly between environments due to differences in room volume, ventilation, the placement of the device relative to sources, room mixing characteristics, and user behavior such as turning air cleaners off or lowering settings to reduce noise, and intervention studies seldom characterize such effects. Over time, an air cleaner will also decline in efficiency and flow rate as particles accumulate in it.

Personal protective equipment.

Interest in the efficacy of PPE to filter particles and protect human health dramatically increased during the COVID-19 pandemic. The pandemic also influenced cultural norms related to masking, which created opportunities to expand PPE use for reducing inhalation of PM_2.5 during acute events such as wildfires.

Studies investigating PPE and protection from PM_2.5 exposure are challenging to design. There are few studies of PPE use in residential or other non-occupational indoor spaces. Studies in non-occupational settings in China have indicated that respirator use can result in lower blood pressure and improved heart rate variability parameters and lung function improvements in those who wore N95 masks relative to those who did not. The beneficial effects of N95 masking were more pronounced on high pollution days.

KEY CONCLUSIONS AND OVERARCHING RECOMMENDATIONS

Five key conclusions stem from this review.

1. There is ample evidence that exposure to indoor fine particulate matter causes adverse health effects. Deleterious effects on respiratory health are clear, evidence of adverse cardiovascular disease effects is growing, and evidence for adverse effects on other organ systems and health conditions is emerging. Furthermore, PM_2.5 of outdoor origin generally makes up a significant fraction of indoor PM_2.5, a greater amount of PM_2.5 of outdoor origin is inhaled indoors than outdoors, and there is a wealth of literature that associates outdoor PM_2.5 with adverse health outcomes.
2. Disparities exist in population exposure to indoor fine particulate matter of both outdoor and indoor origin. Examples of people who are more likely to be affected adversely by such exposures include people living in economically disadvantaged circumstances and in marginalized communities near heavy industry or busy highways. Exposure to PM_2.5 and related health impacts may also be greater for populations living in older and smaller homes, and those lacking the resources to purchase lower-emitting appliances or maintain air cleaning technologies.
3. Technological advancements have great potential for quantifying and reducing exposures to fine particulate matter. Consumer-grade sensors that can be used by nontechnical people to measure PM_2.5 and track location as well as advances in environmental data management, analysis, and modeling, enable new approaches to exposure assessment and control. These technologies—which will continue to evolve in accuracy, capabilities, and

3. lower cost—permit community-based participatory research that can build awareness and address critical data gaps, especially in communities that are disproportionately exposed and under-examined and can also help to provide real-time alerts to inform exposure-avoiding behavior.
4. Effective and practical mitigation of exposure to fine particulate matter in homes and schools is currently possible. Truly practical mitigation strategies must be affordable, available, feasible to implement, perform consistently over product life, and be devoid of adverse secondary consequences. As the report details, there are several actions that can be taken immediately, using some combination of source reduction, ventilation, central or in-room filtration, and PPE. It is reasonable to assume that reductions in indoor PM_2.5 concentration will yield health benefits, even if based solely on reduction in exposure to PM_2.5 of outdoor origin, although the literature related to the specific health benefits of such mitigation is sparse and mixed owing to the numerous confounding and limiting factors. It is not possible, though, to offer generic observations regarding which specific mitigation measures will be most practical to implement because there are myriad variables characterizing the sources of indoor PM_2.5 and ultrafine particles (UFPs): their fate, transport, and transformations indoors; the circumstances and level of exposure to them; and the health effects associated with that exposure. Different circumstances will necessarily dictate different choices.
5. The lack of centralized responsibility for indoor fine PM policy is hindering reductions in population exposure at scale. There are many factors that influence population exposure to indoor PM_2.5, ranging from the types and magnitudes of indoor and ambient sources, air handling and cleaning technologies, building-related features, and occupant behaviors. Currently, there is no single entity with the authority to apply an integrated systems approach toward lowering population exposure to PM_2.5. Consequently, opportunities to implement mitigation strategies where most needed and to support related research are fragmented. There has thus been limited progress to reduce exposure to indoor fine PM, even though effective and practical mitigation approaches exist.

The following overarching recommendations are offered to reduce population exposure to PM_2.5, to reduce health impacts on susceptible populations including the elderly, young children, and those with pre-existing conditions, and to address important knowledge gaps.

1. Prioritize the mitigation of PM exposures amongst susceptible populations and do so with urgency. Public health professionals and federal, state, local, tribal, and territorial agencies should prioritize immediate, multilevel, easily implementable, affordable, and effective interventions to mitigate exposure of economically disadvantaged and marginalized communities to fine PM. In doing so, it will be important to collaborate with community-based organizations and communication scientists to address the non-technical aspects of fine and ultrafine particle mitigation, including messaging, education, and public engagement. Consideration of behavioral factors will be critical moving forward, accounting for user behaviors related to air cleaners, HVAC systems, range hood fans, window use, source usage and frequency, choice of appliances, and more.

   While education of stakeholders is insufficient in and of itself to significantly reduce the exposure of susceptible populations to PM_2.5, it will be important to provide informative and understandable educational materials through trusted sources as a means of assisting with possible behavior modification and decision making aimed at reducing exposures,

1. particularly in residences where individuals or families have some control over their exposure.

2. Reduce exposure to fine PM in schools. Reductions in exposures to fine particulate matter, including infectious aerosols, in schools have the potential to improve acute and chronic health impacts, reduce absences, and improve student performance. An immediate and highly visible program, perhaps analogous to “Green School” designations, could spur improvements in indoor air quality in schools, with opt-in by school districts and assistance from governmental entities for impoverished school districts. Clear goals should be established and effectively communicated with guidance on source reduction, ventilation, central filtration, effective and right-sized air cleaning, fine PM monitoring, and frequency of monitoring. District or school-specific improvements in measured fine PM and health outcomes, including reductions in absences, should be monitored for schools that implement the guidance and compared against national averages in order to assess the effectiveness of particular interventions.
3. Continue to support research necessary to fill important knowledge gaps. While the existing knowledge base is sufficient to recommend practical mitigation strategies for lowering exposure to PM_2.5 and related health effects, significant gaps in knowledge remain and should be prioritized for future research. Several important knowledge gaps and research needs are noted below. Additional research needs are included in each chapter.
   1. Mitigation and health improvements. Research is needed to quantify the efficacy of mitigation efforts to reduce exposure and the health benefits of practical mitigation strategies. Large-scale intervention studies should be conducted to establish an evidence base for the health impacts of indoor fine particulate matter exposure and mitigation measures, including different exposure scenarios, a range of interventions, and multiple health endpoints. Such studies should include acute exposures such as wildfire smoke and should evaluate co-benefits such as reductions in airborne infectious agent exposures. The inclusion of economically disadvantaged and marginalized communities in these studies is critical, as is the appropriate characterization of building factors such as indoor space geometry, ventilation, recirculated air flows, use of local exhaust, nature of filtration, indoor sources, proximity to outdoor sources, and the like.
   2. Indoor aerosol characteristics. Additional research needs to be conducted to identify and understand the variations in aerosol characteristics, including size (particularly, UFPs), concentrations, sources, and composition in different indoor residential and school environments. Such research could serve as a complementary effort to intervention studies to better understand the role of aerosol characteristics on health endpoints.
   3. Effects of particle origin on health effects. Understanding the relative health effects of indoor fine particulate matter of both outdoor origin and indoor origin is important for defining appropriate mitigation strategies. Advancing understanding of the source[s] associated with specific health effects is also important for informing source control measures.
   4. New technologies for real-time indoor particle monitoring. New technologies—particularly lower-cost and real-time sensors that capture key aerosol characteristics—would benefit future exposure and health studies as well as serve as sentinels for mitigation feedback systems or actions by building occupants to reduce exposure. Research and development are needed to expand features and improve quality control and

4. 4. consistency at the single-sensor level and to aid in the installation, maintenance, and data interpretation from networks of sensors.
   5. Affordable, quiet, and effective air cleaning technologies. While there are standalone air cleaners based on media filtration that lower indoor fine PM concentrations, research is still needed to develop cleaners that are priced in a range that allows for their widespread use; that are effective at lowering exposure to, and the health effects of, indoor aerosols; and that have features such as quiet operation that make them more likely to be used.
   6. Social and behavioral influences. Social science and behavioral health perspectives should be included in future studies of indoor fine PM to understand how social, cultural, and behavioral factors influence exposure, health effects, and the implementation of mitigation strategies, particularly in susceptible populations. Such research should be a part of the intervention initiatives proposed above.
5. Magnify and unify efforts to reduce population exposure to indoor fine particulate matter. The broad recommendations listed above cannot be effectively enacted without coordinated support and action. However, as already noted, the lack of centralized responsibility has to date hindered a significant reduction in population exposure to indoor fine PM at scale. Such a reduction will require unification and integration of efforts across federal, state, local, tribal, and territorial entities. A concerted effort will be needed that spans environmental, building code, public health, and social service agencies, in collaboration with community, school-based, and other organizations that can aid with implementation. The form and details of this effort will need to be worked out among the involved parties and, while it might not be simple to bring about, the rewards in terms of improved population health will be great.