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

Chapter: 8 Key Findings, Conclusions, and Recommendations

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Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." 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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8
Key Findings, Conclusions, and Recommendations

This final chapter of the report builds on the preceding text, recapitulating the elements of the committee’s work, identifying the major themes, and highlighting the key findings, conclusions, and recommendations.

OVERVIEW OF THE COMMITTEE’S WORK

The committee’s statement of task charged it to consider the state of the science on the health risks of exposure to fine particulate matter (PM2.5) indoors and engineering solutions and interventions to reduce the risks of exposure to this particulate matter indoors, including practical mitigation solutions to reduce exposure in residential settings. The U.S. Environmental Protection Agency (EPA), the report’s sponsor, identified two areas of emphasis in this work:

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

The committee was further directed to develop findings and recommendations regarding the key implications of the scientific research for public health, including potential near-term opportunities for incorporating what is known into public health practice, and to identify where additional research would be most critical to understanding indoor exposure to PM2.5 and the effectiveness of interventions. Opportunities for advancing such research by addressing methodological or technological barriers or enhancing coordination or collaboration among governmental bodies and organizations were also to be noted.

The committee approached the task by conducting a wide-ranging review of the available science, focused on the literature it deemed to have been influential in shaping understanding at the time it completed its task in summer 2023. It divided its review into five major categories: analyses of the published research on the sources of indoor fine PM; particle dynamics and building characteristics that influence indoor PM; building occupant exposures and the means of characterizing them; the health effects associated with that exposure; and practical approaches to mitigating it. So vast is the topic that, with even with the limitations it imposed on the scope of the review, the committee cites over 800 papers and reports.

Key conclusions and overarching recommendations resulting from this effort and the findings that underlie them are summarized below. Box 8-1 contains a synopsis of what is and isn’t known about the health risks of indoor exposure to fine particulate matter and practical mitigation solutions. Citations to the literature supporting this information may be found in Chapters 37.

Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." 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.
×

KEY CONCLUSIONS

Five overarching conclusions stem from the committee’s literature review.

There is ample evidence that exposure to indoor fine particulate matter causes adverse health effects.

The epidemiologic literature strongly supports the conclusion that exposure to indoor PM2.5 has adverse effects on the respiratory and cardiovascular systems and likely other organ

Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." 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.
×

systems. Evidence for a role of indoor PM2.5 in neurologic, metabolic, and reproductive outcomes is less well developed but emerging. There is, however, only a limited understanding of how inequities in indoor PM exposure (in terms of concentration as well as other particle characteristics) contribute to health disparities; and of the health effects of PM2.5 exposure at school, where children—a cohort that is more vulnerable to adverse effects—spend a significant amount of time.

This understanding stems in part from the knowledge that PM2.5 of outdoor origin generally makes up a large fraction of indoor PM2.5 and that a greater amount of PM2.5 of outdoor origin is inhaled indoors than outdoors as well as from the wealth of literature that associates outdoor PM2.5 with adverse health outcomes. Compared with the evidence supporting the adverse health consequences of PM measured outdoors, there are fewer studies of the health effects of PM measured indoors, and these have substantially smaller sample sizes.

People report spending nearly 90 percent of their time indoors, on average, including nearly 70 percent in residences and approximately 10 percent in other indoor environments, including schools. Estimates vary widely, but, generally speaking, indoor sources account for approximately half of total indoor PM2.5 concentrations in homes, with the remainder originating from outdoors. Because outdoor PM2.5 infiltrates and persists indoors, the bulk of human exposure to PM of outdoor origin is likely to take place indoors.

Disparities exist in population exposure to indoor fine particulate matter of both outdoor and indoor origin.

Exposure to PM2.5 and related health impacts may be greater for people living in economically disadvantaged circumstances and marginalized communities near heavy industry or busy highways, along with populations such as seniors, children, those with underlying chronic diseases, those living in older and smaller homes, and those lacking resources to purchase lower-emitting appliances or to maintain air cleaning technologies.

While there is a knowledge base addressing socioeconomic and cultural disparities in ambient PM2.5 sources, concentrations, and compositions, less is known about how such differences manifest in differences in indoor PM2.5 Moreover, while it is expected that there is high variability in the types and magnitudes of indoor PM2.5 sources that is likely attributable to socioeconomic and cultural differences, robust characterizations of the presence, types, and frequency of indoor emission sources—as well as technologies to mitigate exposures—for specific populations do not readily exist.

Technological advances have great potential for quantifying and reducing exposures to fine particulate matter.

There has been great progress in recent years in the development of small, easy-to-use, and relatively inexpensive devices for measuring airborne PM levels and in the capacity to share such information over the web. Consumer-grade sensors that can be used by non-technical people to measure PM2.5 and track location, and also be used 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 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 also make it possible to provide real-time alerts to inform exposure-avoiding behavior. There is a specific need for such monitoring approaches to identify and quantify important parameters that potentially affect the effectiveness of practical mitigation measures.

Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." 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.
×

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. Generally speaking, PM exposure mitigation may be implemented with a combination of source reduction, ventilation, central or in-room filtration, and personal protective equipment (PPE). It is reasonable to assume that reductions in indoor PM2.5 concentration will have health benefits, even if based solely on reducing exposures to PM2.5 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.

However, it is not possible 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 PM2.5 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. The hierarchy of controls identified by the committee provides a guideline for determining the order in which alternatives should be pursued.

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 PM2.5, including indoor and ambient sources, air handling and cleaning technologies, building-related features, and occupant behaviors. Currently, though, there is no single entity with the authority to apply an integrated system approach toward lowering population exposure to PM2.5.

While EPA exercises considerable responsibility for conducting and sponsoring research on the indoor environment and communicating the results of that work to the public, it has no regulatory authority regarding indoor particulate matter. Other federal agencies also have interests. The Department of Energy promotes energy efficiency and sustainability, which has material impacts on indoor environmental quality through guidance in such areas as energy-efficient building design and, heating, ventilation, and air conditioning (HVAC) systems. The Centers for Disease Control and Prevention focuses on public health issues, including those related to indoor environments. That agency provides provide guidance on preventing and addressing issues like mold, respiratory diseases, and exposure to environmental hazards in indoor spaces. The Department of Housing and Urban Development and the Department of Defense, among others, manage huge portfolios of building stock and are generally responsible for the health of the people who live in them. The Consumer Product Safety Commission investigates safety issues regarding and develops standards for products that include those that generate indoor PM and develops standards for such products. And the Occupational Safety and Health Administration and the National Institute for Occupational Safety and Health deal with indoor exposures and health in occupational environments. Most of these federal agencies have their state and sometimes tribal, territorial, and local counterparts. They are joined in responsibility by those public and private entities that develop and, in some cases, enforce building codes, standards, and guidelines.

Consequently, the 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.

Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." 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.
×

OVERARCHING RECOMMENDATIONS

Four primary recommendations are offered to advance reductions in population exposure to PM2.5 to lessen health impacts on susceptible populations including the elderly, young children, and those with pre-existing conditions; and to address important knowledge gaps.

Prioritize the mitigation of PM exposures among susceptible populations and do so with urgency.

Disparities exist in population exposure to indoor fine particulate matter of both outdoor and indoor origin. These occur not only because of higher indoor exposure concentrations due to more activities happening in smaller, densely occupied, and interconnected (multi-family) homes, or outdated appliances that have higher emissions or ventilation equipment that are less effective at removing PM, but also because of the greater susceptibility of the exposed populations leading to excess health burdens. Settings where indoor PM exposures and their associated health impacts are enhanced and mitigation opportunities are limited include schools and early childhood education facilities, and institutional housing such as homeless shelters, transitional homes, skilled nursing facilities, and prisons.

Public health professionals and federal, state, local, tribal, and territorial agencies should thus prioritize immediate, multilevel, easily implementable, cost-accessible, and effective interventions relying on currently available evidence and tools to address this situation. In doing so, collaboration with community-based organizations and communication professionals to address the non-technical aspects of fine and ultrafine particle mitigation, including messaging, education, and public engagement, will be important, as will a consideration of the factors that drive 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 exposure of susceptible populations to PM2.5, it is important to provide informative and understandable outreach materials through trusted sources as a means of modifying possible behavior and decision making in order to reduce exposures, particularly in residences where individuals or families have some control over their exposure.

Reduce exposure to fine PM in schools.

School is a unique indoor environment where children and young adults spend considerable time. Reducing exposures to fine PM, including infectious aerosols, in schools has 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, for example, spur improvements in indoor air quality in schools with opt-in by school districts and assistance from federal and state governments 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.

As part of this effort, the committee recommends that EPA, in collaboration with other governmental entities and private funders, should prioritize the support of studies designed to characterize differences in indoor PM2.5 exposure—including differences in PM2.5 characteristics—in home and school settings across communities and also characterize their

Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." 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.
×

contribution to health disparities. As already noted, significant disparities exist in PM2.5 exposures and exposure impacts. It will not be possible to identify and to formulate practical mitigation strategies for disproportionately affected populations or to assess the efficacy of their implementation until there is a clear understanding of who is affected by the disparate exposures and how these individuals’ circumstances shape the effectiveness of interventions.

Continue to support research necessary to fill important knowledge gaps.

While the existing knowledge base is sufficient to draw conclusions about some health outcomes related to indoor PM exposure and to recommend practical mitigation strategies for lowering exposure to PM2.5, significant gaps in knowledge remain and should be prioritized for future research. Several important knowledge gaps and research needs were noted by the committee. Some of these are highlighted below; additional observations are offered in chapters 37.

There are a few general efforts that would greatly advance knowledge and provide the groundwork for advances in the understanding of adverse health impacts from indoor PM exposures and interventions that would ameliorate them.

Toward this end, the committee recommends that EPA, in collaboration with other governmental entities, private funders, and standards and professional organizations, foster additional research on methods for measuring PM in the indoor environment. Studies of indoor sources of PM may take place in controlled laboratory chambers or actual indoor spaces. Both environments present research challenges and limitations. The deployment of large, research-grade instrumentation into occupied indoor spaces offers some of the greatest exposure assessment challenges because of such factors as noise, space requirements, and safety limitations. Recent advances in lower-cost, consumer-grade sensors have made it possible to deploy sensors effectively in a wide variety of indoor environments. However, to capture the true diversity of indoor sources and indoor environments, advances must be made in miniaturized research-grade instrumentation that can characterize PM in terms of size, concentration, chemical composition, and the like at the large scales needed to advance our understanding of health effects of indoor PM2.5. In concert with this, the indoor air research community should continue to build and maintain capacity for identifying, quantifying, and measuring new mechanisms for sources, sinks, and transformations of indoor PM as they arise and to subsequently understand the potential impacts of such mechanisms on the toxicity of indoor PM.

A national effort is needed to measure and report indoor exposure to PM using validated methods and sufficient characterization of the built environment, occupancy, and activity patterns to identify key determinants of indoor exposure to fine particles (and other indoor air pollutants) so that source-specific exposure can be assessed, which can in turn help guide mitigation efforts for subpopulations overburdened with exposure to fine particles in homes, schools, and other building types. The data would greatly improve our understanding of the exposure and potential health impacts of indoor PM on the U.S. population in key indoor environments: homes, schools, and other vulnerable settings.

The committee also offers recommendations aimed at creating baseline standards for information gathering in some critical areas. The first of these is the fostering of additional research on establishing uniform criteria for the information needed on indoor sources to inform the assessment of exposure, health effects, and mitigation. It is impractical to address all indoor sources of PM2.5 because they continually evolve and change along with the consumer market. If uniform criteria existed for characterizing indoor sources, it could provide a pathway for harmonizing future studies in indoor particle physics and chemistry as well as helping with the

Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." 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.
×

development of mitigation strategies and associated communications to the public. As an initial step in this process, compiling a comprehensive indoor emissions inventory (including outdoor sources) across a wide range of particle sizes, mass and number concentrations, and compositions would help researchers and policy makers to better evaluate data regarding different source categories and their exposures. This recommendation would be best carried out by EPA in collaboration with other governmental entities, private funders, and standards and professional organizations.

Relatedly, the indoor air research community should come to a consensus on what minimal information on indoor PM dynamics is needed to meaningfully improve understanding of the health effects of indoor PM exposure, for example, by modeling exposures across the building stock for use in epidemiology studies. This community should also explore what minimal information is needed to meaningfully improve understanding of practical mitigation measures for indoor PM by, for example, adopting a more “building-aware” epidemiology approach whereby research characterizing the effects of a practical mitigation measure would also provide the context of mechanisms that affect the fate, transport, and transformations of indoor PM. In order to enable this contextualization, there is a specific need for clear, practical, and relatively low-cost monitoring approaches to identify and quantify important parameters that potentially affect the effectiveness of practical mitigation strategies.

The committee additionally recommends that indoor air research community should take better advantage of observational field studies to conduct studies that can directly evaluate the effects of reducing PM2.5 exposure on health. Studies conducted under controlled circumstances offer great advantages to researchers in terms of time, effort, and the ability to manage the myriad potential influences on outcomes, but they yield an incomplete answer to what is perhaps the most salient issue for policy makers: Does this provide information about what happens in the real world? Advances in technology now permit investigators to gather information at a scale and with a degree of accuracy that was unthinkable only a few years ago. These advances need to be exploited.

The committee identified five specific areas where additional research would materially advance knowledge: studies related to mitigation and health improvements, studies of indoor aerosol characteristics, studies on the effects of particle origin on health effects, new technologies for real-time indoor particle monitoring, and social and behavioral influences. These are elaborated on below.

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 are needed to establish an evidence base for the health impacts of indoor fine particulate matter exposure and of mitigation measures, including different exposure scenarios, a range of interventions, and multiple health endpoints. Such studies should include acute exposures such as wildfire smoke. They should evaluate co-benefits such as reductions in airborne infectious agent exposures, which may require different target air exchange rates than those focused on reducing PM of other sources. 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.

As part of the effort to address knowledge gaps, the committee recommends that federal and regional agencies fund large-scale, population-level clinical trials to build the evidence-base for the health impacts of indoor PM mitigation measures. A standard of evidence for the

Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." 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.
×

effectiveness of PM control technologies and strategies should be created, based on the health evidence base. 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 to appropriately contextualize their findings and add to our knowledge base on strategies to mitigate the adverse effects. The building factors that should be characterized include ventilation rate, air infiltration, particle loss rates, portable filter clean air delivery rate and location, and parameters such as runtime, flow rate, and in-situ efficiency for central systems.

The committee also recommends that EPA, in collaboration with other governmental entities and private funders, support the conduct of studies to evaluate the impacts of policies on PM2.5 exposure and health, including cost–benefit analyses that incorporate an estimate of the economic and public health costs of not implementing the policy. Governments must balance competing priorities when making policy determinations. Understanding the costs associated with action—and inaction—will allow for better informed decisions on the need for interventions regarding indoor PM2.5 exposure and mitigation.

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 compositions in different indoor residential and school environments. Such research could be a component of intervention studies to better understand the role of aerosol characteristics on health endpoints. Environmental health researchers need to 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 PM.

Ultrafine particles deserve special attention because they are the predominant component of many indoor sources of PM2.5. While they usually contribute a very small portion of the total PM2.5 mass, they but represent a large portion in terms of particle number concentrations. Information on indoor ultrafine particles, especially their composition and health effects, is currently limited.

The committee therefore recommends that EPA, in collaboration with other governmental entities, private funders, and standards and professional organizations, foster additional research on the composition of ultrafine particles from indoor sources. With this knowledge, researchers and the public could prioritize actions where there is greater potential for impact. Mitigation strategies could be developed along with education initiatives to minimize people’s exposure to those indoor sources that lead to worse health outcomes. There is an opportunity to educate the general public about the indoor sources of fine particulate matter to assist decision making when choosing indoor products and activities to minimize exposure.

Furthermore, EPA, in collaboration with other governmental entities, private funders, and standards and professional organizations, should foster additional research on spatiotemporal PM2.5 variability indoors. This variability, which results from the everchanging nature of indoor sources in indoor environments—particularly residences and schools—may significantly affect the exposure of indoor occupants. Specifically, questions remain on how acute exposures (high concentrations, short time periods) cause health effects and can be influenced by practical mitigation choices. This knowledge could help inform the type and location of mitigation strategies contextually. In other words, not all mitigation strategies may work for all indoor PM2.5 sources, but if there is an understanding of which sources play a larger role in the exposure of indoor occupants, decisions can be made to optimize mitigation strategies.

Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." 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.
×

Effects of particle origin on health effects.

While understanding the relative health effects of indoor fine particulate matter of both outdoor origin and indoor origin is important for defining appropriate mitigation strategies, research in this area is still lacking. Advancing understanding of the source[s] associated with specific health effects is also important for informing source control measures.

EPA, in collaboration with other governmental entities, private funders, and standards and professional organizations, should thus foster additional research on ambient air pollution as a source of indoor particles. Although the penetration of outdoor air pollutants into the indoor environments is relatively well understood, knowledge gaps remain in terms of the health effects of ambient particles that infiltrate and persist indoors. Particularly, questions remain on how to contextualize the evidence linking exposure to ambient PM2.5 levels and their health outcomes given that people in many societies spend the majority of their time indoors, so they are likely to be predominantly exposed to ambient particles indoors.

New technologies for real-time indoor particle monitoring.

New technologies—particularly low-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 consistency, both at the single-sensor level and in relation to installation, maintenance, and data interpretation from networks of sensors.

Such research is needed because, while consumer-grade sensors and personal monitoring are advancing abilities to measure exposure, important limitations remain. The accessibility of these lower-cost sensors has greatly expanded monitoring capabilities, but the efforts have been mainly outdoors. Beyond improving instrument accuracy, cost, form factor (ease of use, connectivity), and other performance aspects, it is critically important to advance our understanding of how measured values are useful for determining the health impacts from exposure to fine particles or mitigation effectiveness. While indoor PM is generally expected to contribute to excess morbidity and mortality, the lack of a standardized approach to readily obtain indoor fine PM exposure levels, especially in historically marginalized communities, limits the advancement of our understanding of the connection between exposure and disease.

Furthermore, our understanding of the acute exposure to indoor PM, while improving, is still limited. There are emerging concerns about new sources, such as vaping, as well as about more frequent cleaning and disinfection, and electronic air cleaners. Very high exposure to indoor fine PM is occurring in some microenvironments. Many indoor sources are intermittent and can lead to localized, short-lived, and high concentrations of UFPs and PM2.5. Indoor sources of particles, such as cooking, personal care products, and some office products, can emit copious amounts of UFPs and PM2.5 for the duration of the emitting activity, leading to high, sometimes short-lived, PM concentrations in the vicinity of the activity. This can lead to elevated exposure to the people performing the activity.

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

Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." 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.
×

Our understanding of the potential health impacts of these indoor sources in different built environments is partly restricted by the available instrumentation used to characterize exposure. In particular, the understanding of indoor exposure to some specific types of PM, such as UFPs and specific PM compositional constituents, remains poor. Beyond exposure concentrations, intake from all routes (inhalation, ingestion, and dermal), lung deposition, and dose are also highly variable and difficult to quantify, and they add to the uncertainty in characterizations of health impacts. Studies point to a need for innovation to improve measurement techniques and study methods and thus to enable better characterization of the total exposure and health impacts to fine particles in indoor environments.

Researchers should therefore use emerging consumer-grade sensors and statistical modeling to estimate indoor PM exposure at a larger scale to facilitate the conduct of large-scale population-based epidemiologic studies. Such studies are critical to advancing the understanding of (1) the effects of indoor PM on less common health outcomes and on health disparities; (2) the effects of particle characteristics—beyond mass concentration and including composition, size, shape, and sources—on health; (3) individual and population characteristics that confer susceptibility to indoor PM exposure or certain “types” of indoor PM; and (4) the contribution of particles of outdoor origin to the health effects of indoor PM.

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; are effective at lowering exposure to, and health effects of, indoor aerosols; are easier to maintain; are more intuitive to operate; and have features like quiet operation that make them convenient and comfortable to use. This has become especially important in recent times as exposures to emissions from indoor appliances and from wildfire smoke penetrating the indoor environment have reached the public consciousness.

Accordingly, engineering and technology researchers and industry should endeavor to optimize existing and develop new air cleaning and ventilation technologies that have these health-conscious, consumer-friendly attributes. 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 in addition to assessing primary and secondary byproducts of air cleaning.

Social and behavioral influences.

The indoor air research community should explicitly incorporate social science and behavioral health science perspectives and expertise into studies of the health impacts of indoor PM2.5 to better understand how social, cultural, and behavioral factors may influence PM2.5 exposure and health effects and the implementation of practical mitigation strategies. As this report makes clear, there are systematic differences in exposure to indoor PM and in susceptibility to adverse effects of that exposure that result in disparate health outcome risks for different populations. The research in this area is still relatively sparse, however, and much more needs to be done in order to formulate effective interventions. One straightforward way to address this gap would be to make consideration of social, cultural, and behavioral factors a standard element of studies by including people with such expertise in research teams.

Furthermore, public health professionals and researchers should consider behavioral factors in their development of control strategies in order to ensure effective implementation and maximize impact. Examples of behaviors that can mitigate or exacerbate exposure include

Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." 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.
×

adjusting air cleaner speed and operation to control noise levels or electricity use, HVAC or furnace runtime, the use of range hood fans, window use, the use of primary and secondary sources such as candles or terpenes in cleaning products, and the choice of electric or gas appliances for cooking and heating.

Magnify and unify efforts to reduce population exposure to indoor fine particulate matter.

The literature review presented in this report establishes that reducing PM2.5 exposure would have a significant public health benefit. The three broad recommendations offered above would have a material effect in realizing that benefit, but they cannot be effectively enacted without coordinated support and action. However, as already noted, the lack of centralized responsibility for indoor air quality hinders the ability to take the steps that would result in a significant reduction in population exposure to indoor fine PM at scale. Such a reduction will require a 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 might include such interventions as woodstove replacement, healthy home retrofits, school HVAC upgrades, and portable air cleaner deployments. Another effort could involve changes to building standards and practices, which have the potential to bring about wide-ranging and long-lasting benefits. And effective communication with the public will be required. In the end, the effectiveness of a mitigation measure is often determined by the quality of the implementation guidance that accompanies it. There is a need to make indoor exposure to fine PM more “visible,” in the sense of raising awareness of its importance to health and well-being. This can motivate people to take actions that reduce indoor sources and increase the use of mitigation measures.

Programs such as these will require evaluation of the outcomes achieved in order to identify best practices and motivate their funding and continued support. Guidance on how to measure the potential reduction in indoor fine PM exposure and what metrics to use is needed so that these programs can adjust and improve over time to bring more benefits to communities.

Collaborations to study indoor PM exposure and implement interventions in susceptible, underserved, and disproportionately exposed communities should be particularly encouraged. Indoor environments and the people who live in them are diverse. They have unique characteristics that may lead to high indoor fine PM exposures that require focused attention. More targeted data on such exposures are necessary to improve our understanding of them and, ultimately, to protect susceptible populations. Indoor environment researchers need to collaborate with community- based organizations and community members if they are to conduct the kinds of culturally sensitive studies that will produce information relevant to these populations and develop effective messaging on PM exposure issues to help motivate practical mitigation.

While it might not be simple to bring these measures about, the rewards in terms of improved population health will be great.

Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." 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:"8 Key Findings, Conclusions, and Recommendations." 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:"8 Key Findings, Conclusions, and Recommendations." 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:"8 Key Findings, Conclusions, and Recommendations." 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:"8 Key Findings, Conclusions, and Recommendations." 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:"8 Key Findings, Conclusions, and Recommendations." 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:"8 Key Findings, Conclusions, and Recommendations." 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:"8 Key Findings, Conclusions, and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 223
Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 224
Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 225
Suggested Citation:"8 Key Findings, Conclusions, and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2024. Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions. Washington, DC: The National Academies Press. doi: 10.17226/27341.
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Page 226
Next: Appendix A: Agendas 2021 Workshop Series on Indoor Exposure to Fine Particulate Matter and Practical Mitigation Approaches »
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 Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions
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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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