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Introduction1
Overwhelming evidence exists that exposure to outdoor fine particulate matter (PM2.5)2 is associated with a range of short-term and chronic health impacts, including asthma exacerbation, acute and chronic bronchitis, heart attacks, increased susceptibility to respiratory infections, and premature death (Chen et al., 2007), with the burden of these health effects falling more heavily on underserved and marginalized communities (Mikati et al., 2018; Parker et al., 2018). Although less studied to date, indoor exposure to PM2.5 is also gaining attention as a potential source of adverse health effects, particularly given that Americans spend 90 percent of their lives indoors (Klepeis et al., 2001) and indoor PM2.5 levels can exceed outdoor levels (Logue et al., 2011). PM2.5 found indoors can be particles of outdoor origin that migrate indoors or originate from indoor sources such as cooking, candle burning, and other occupant activities, including cleaning.
To better understand the sources of indoor PM2.5, the possible health effects of exposure to indoor PM2.5, and engineering approaches and interventions to reduce those exposure risks, the National Academies of Sciences, Engineering, and Medicine (National Academies) convened a virtual workshop with the following statement of task:
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1 This proceedings has been prepared by the workshop rapporteurs as a factual summary of what occurred at the workshop. Statements, recommendations, and opinions expressed are those of individual presenters and participants and are not necessarily endorsed or verified by the National Academies of Sciences, Engineering, and Medicine, and they should not be construed as reflecting any group consensus.
2 “Fine particulate matter” and “PM2.5” are used interchangeably throughout this document.
The National Academies shall convene a planning committee of scientific experts to conduct a workshop on the state of the science on exposure to fine particulate matter (PM2.5) indoors, its health impacts, and engineering approaches and interventions to reduce exposure risks, including practical mitigation solutions in residential settings. The workshop will feature invited presentations and panel discussions on these topics. It shall include consideration of
- the key implications of scientific research and engineering practice for public health, including potential near-term opportunities for incorporating what is known into practice; and
- where additional research will be most critical to understanding indoor exposure to PM2.5 and the effectiveness of interventions.
Opportunities for advancing research by addressing methodological and technological barriers and enhancing coordination and collaboration between the science, medical, and engineering communities will also be given attention. The indoor environments considered in this workshop will be limited to nonindustrial exposure within buildings.
The resulting workshop, “Indoor Exposure to Fine Particulate Matter and Practical Mitigation Approaches,” was held on April 14, 21, and 28, 2021 and was sponsored by the US Environmental Protection Agency’s (EPA) Office of Radiation and Indoor Air. It was intended in part to further the discussion about indoor PM2.5 that began at the 2016 National Academies workshop, Health Risks of Indoor Exposure to Particulate Matter: Workshop Summary (NASEM, 2016).3 As planning committee chair Richard Corsi (University of California, Davis) noted, the committee decided to focus the scope of these workshops on exposures that occur in residential and school buildings and on existing and practical mitigation technologies and approaches rather than novel, emerging, or unproven technologies. In keeping with its statement of task, the planning committee also strove to center attention on PM2.5, although the workshop did address some sources of both PM2.5 and larger particles.4
In his introduction to the workshop, Jonathan Edwards (director, EPA Office of Radiation and Indoor Air) explained that EPA’s indoor air program was launched and authorized by statute in the 1980s in response to the growing concern in the scientific and public health communities about the risks associated with poor indoor air quality. Since then, EPA has developed a robust, nonregulatory program that works to reduce exposure to high-priority, indoor contaminants, including radon, indoor asthma
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3 A summary of the 2016 workshop and additional resources are available at https://www.nap.edu/catalog/23531/health-risks-of-indoor-exposure-to-particulate-matter-workshop-summary.
4 PM2.5 refers to particles equal to or smaller than 2.5 microns in diameter. Ultrafine particles, which the speakers also discussed, are a subset of PM2.5, just as PM2.5 is a subset of PM10, or particles 10 microns in diameter and smaller.
triggers, volatile organic compounds, environmental tobacco smoke, biological contaminants, and particulate matter, in homes, schools, and other commercial buildings.
Edwards noted that the COVID-19 pandemic has increased the public’s awareness of the importance of indoor air quality. He added that since the 2016 workshop—which highlighted the fact that indoor exposure to PM2.5 is a serious public health issue—EPA, the research community, and indoor environment-related industries have made substantial progress in developing tools and resources that can help reduce the risks posed by exposure. Examples of these tools and resources include revised EPA guidance on residential air cleaners and filters (US EPA, 2021), improved filter and air cleaning technologies, additional analyses of the health effects of indoor-generated PM2.5, and the increasing use of sensor-based technologies to detect PM2.5.
Concluding his remarks, Edwards said EPA will use the results of the workshop to continue to build a scientific foundation on the impacts of indoor exposure to particulate matter. The workshop will also help EPA communicate the importance of the science, to develop and share new and revised guidance on engineering and behavioral approaches, to mitigate exposure to indoor particulate matter, and to help drive advances in indoor particulate matter mitigation technology.
CONDUCT OF THE WORKSHOP
The three-webinar workshop (see Appendix A for the agenda) was organized by an independent planning committee in accordance with National Academies procedures.5 The planning committee members were Seema Bhangar, Wanyu R. Chan, Richard L. Corsi (chair), Elizabeth C. Matsui, Linda A. McCauley, Kimberly A. Prather, David Y. Pui, Jeffrey A. Siegel, and Marina E. Vance (see Appendix B for biographic sketches of the committee members and workshop speakers). The workshop was broadcast live online. Webcast analytics reported more than 600 unique views for each of the three broadcasts, with observers in nearly all the US states and over 30 countries on 6 continents. The workshop presentations were subsequently posted to the Web along with links to the videos of the talks.6
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5 The role of the planning committee was limited to planning the workshop.
6 Available for April 14: https://www.nationalacademies.org/event/04-14-2021/indoor-exposure-to-fine-particulate-matter-and-practical-mitigation-approaches-workshop-on-sources-of-indoor-fine-particulate-matter, April 21: https://www.nationalacademies.org/event/04-21-2021/indoor-exposure-to-fine-particulate-matter-and-practical-mitigation-approaches-workshop-on-indoor-pm-exposure-health-metrics-and-assessment, and April 28: https://www.nationalacademies.org/event/04-28-2021/indoor-exposure-to-fine-particulate-matter-and-practical-mitigation-approaches-workshop-on-mitigation-of-indoor-exposure-to-fine-particulate-matter.
ORGANIZATION OF THE PROCEEDINGS
This publication summaries the presentations and discussions that took place during the workshop; it is divided into nine chapters plus supporting appendices. Chapters 2 and 3 describe the major outdoor and indoor sources of PM2.5, respectively, and Chapter 4 recaps the first day’s presentations and discussion. Chapter 5 discusses the health effects of exposure to indoor PM2.5, Chapter 6 explores the measurement of PM2.5 and the challenges of evaluating an individual’s actual exposure to PM2.5, and Chapter 7 summarizes the second day’s presentations and discussions. Chapter 8 examines engineering approaches to control and mitigate indoor PM2.5, and Chapter 9 reviews how individuals respond to information about their possible exposure to indoor PM2.5 and public health approaches to reduce community exposure to indoor PM2.5. Chapter 10 summarizes the workshop’s key points as noted by the workshop planning committee chair.
Several speakers offered personal observations regarding actions that might be taken by individuals or governmental entities. However, in accordance with National Academies policies, the planning committee did not attempt to establish any conclusions or recommendations about needs and future research directions, focusing instead on issues identified by individual speakers and workshop participants. This proceedings was drafted by rapporteur Joe Alper in collaboration with National Academies staff member David A. Butler as a factual summary of what occurred at the workshop, and the National Academies does not endorse or verify the statements.
The National Academies is, at EPA’s behest, conducting a study7 that will address the issues discussed in the workshop in greater detail and will offer findings, conclusions, and recommendations regarding them. This study will be released in early 2023.
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7 Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions: https://www.nationalacademies.org/our-work/health-risks-of-indoor-exposures-to-fine-particulate-matter-and-practical-mitigation-solutions.
