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Introduction¹

The health effects of outdoor exposure to particulate matter (PM) are the subject of both research attention and regulatory action. Although much less studied to date, indoor exposure to PM is gaining attention as a potential source of adverse health effects. Indoor PM can originate from outdoor particles and also from various indoor sources, including heating, cooking, and smoking. Levels of indoor PM have the potential to exceed outdoor PM levels (Chen and Zhao, 2011).

The U.S. Environmental Protection Agency (EPA) defines PM as a mixture of extremely small particles and liquid droplets comprising a number of components, including “acids (such as nitrates and sulfates), organic chemicals, metals, soil or dust particles, and allergens (such as fragments of pollen and mold spores)” (EPA, 2003). When considering PM and health, it is appropriate to consider particulate matter as a class or category rather than as a single species, said William Nazaroff, the Daniel Tellep Distinguished Professor of civil and environmental engineering at the University of California, Berkeley. Sorting PM along one or more of its important attributes, he said, can assist in clarifying how sources and building parameters influence exposures and health consequences. Among the possible attributes useful for thinking about PM, he said, are its size, source, and composition.

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¹ The planning committee’s role was limited to planning the workshop, and the workshop summary 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.

Nazaroff said that a size classification might consider three clusters: ultrafine particles (UFPs) that are less than 0.1 microns—or 100 nanometers—in diameter; fine particles that are less than 2.5 microns in diameter (PM_2.5); and coarse particles between 2.5 microns and 10 microns in diameter (PM₁₀). UFPs are found in greater quantities but have negligible influence on mass concentrations. Outdoors, PM_2.5 tends to be dominated by primary aerosols² (those that are emitted directly into the air, such as diesel soot) and the conversion of gaseous species to particulate matter in the atmosphere. In the latter case, the major air processes in the outdoor environment include sulfur oxides becoming particulate sulfate, nitrogen oxides becoming particulate nitrate, ammonia becoming ammonium in combining with nitrate and sulfate, and organic gases that become oxidized in the atmosphere into species with lower volatility and higher polarity. Coarse particles tend to be mechanically generated from such sources as crustal elements, tire and brake wear, and sea salt near the coasts (Masri et al., 2015).

Some of the evidence regarding the health effects of PM exposure derives from large-scale epidemiological studies, Nazaroff said in his introductory remarks. In the context of this workshop, he said, it is important to consider the effects of indoor PM—particularly PM_2.5—even though the majority of studies have focused on outdoor PM levels and their impacts. Understanding the major features and subtleties of indoor exposures to particles of outdoor origin can improve our understanding of the exposure–response relationship on which ambient air pollutant standards are based.

Other types of health risk studies also contribute to the overall state of knowledge. One important category, Nazaroff said, involves studying sources of PM. Examples include studies of the health risks of exposure to environmental tobacco smoke, cooking aerosols, and bioaerosols. Nazaroff said that most of the health-related exposure to PM is believed to occur through inhalation, and he added that the physiologic response to PM exposure is complex, nuanced, and involves much more than just the respiratory tract.

Nazaroff said that another broad theme to consider is the nature of indoor spaces and how these influence exposures and health risks associated with indoor PM. The spaces themselves are diverse along many dimensions, including the type of ventilation system, the density of occupancy, the types of indoor sources, the presence and quality of particle filtration, and the rates of particle deposition. In addition, individual behavior can play a predominant role in determining the ultimate exposure a person experiences, Nazaroff said. Consequently, indoor conditions that pose a negligible risk to one person can result in an adverse outcome for another.

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² An aerosol is a suspension of tiny particles or droplets in the air.

As a concluding note in his opening remarks, Nazaroff said that the features that might be considered for a rational public policy addressing indoor PM include public education, interventions targeting reduced exposures for vulnerable populations, and standards or guidelines for the design and operation of building factors that influence indoor PM levels and exposures.

EPA INDOOR ENVIRONMENTS DIVISION

EPA does not regulate indoor air. David Rowson, the director of the agency’s Indoor Environments Division, explained that his division, which sponsored this workshop, is responsible for providing non-regulatory guidance, technical assistance, outreach, and education programs to protect the public from harmful exposure to indoor pollutants. This differs from EPA’s outdoor programs, which work primarily, though not solely, in a regulatory capacity. The division’s major priorities currently involve pollutants that present high public health risks and include radon, indoor environmental asthma triggers, secondhand smoke, mold, and moisture. The division is also involved with addressing exposure to formaldehyde, polychlorinated biphenyls, and other chemicals and biological contaminants found indoors. The division’s non-regulatory activities focus on promoting voluntary interventions to reduce exposure to specific indoor pollutants of high concern. Rowson said that the division works to develop solutions that are holistic in nature and that are intended to address indoor air quality (IAQ) in homes, schools, and commercial buildings by focusing on their design, construction, operations, and maintenance. These holistic approaches include an important focus on PM.

“Providing information to the public on significant sources of indoor PM and how to reduce exposures is part of our ongoing work,” Rowson said. However, given the growing body of literature related to indoor PM and because EPA was already aware that a number of indoor sources of PM present public health risks, his office commissioned the National Academies of Sciences, Engineering, and Medicine to hold a workshop examining the issue of indoor exposure to PM more comprehensively and considering both the health risks and possible intervention strategies. In particular, this workshop was held to address the following task:

EPA requested that the discussion of the consequences of indoor exposure to PM give special attention to emerging health concerns and to the populations that it exercises responsibility for.

CONDUCT OF THE WORKSHOP

The workshop (see Appendix A for the agenda) was organized by an independent planning committee in accordance with the procedures of the National Academies of Sciences, Engineering, and Medicine. The planning committee members were Terry Brennan, Richard Corsi, Howard Kipen, William Nazaroff (Chair), and Tiina Reponen. The workshop took place in Washington, DC, on February 10–11, 2016 and was broadcast live over the Web.

About 60 people attended the workshop in person. The webcast analytics reported more than 400 unique viewers from 12 countries: Canada, Finland, France, India, Indonesia, Iran, Ireland, Mexico, Saudi Arabia, Sweden, the United Kingdom, and the United States (including 38 states and the District of Columbia). All workshop presentations were subsequently posted to the Web along with links to videos of the talks.³

ORGANIZATION OF THE SUMMARY

This publication summarizes the discussions that occurred throughout the workshop. It is divided into seven additional chapters plus supporting appendixes. Chapter 2 describes the major sources of indoor PM, while Chapter 3 explores the chemistry and dynamics of PM. Chapter 4 discusses issues related to exposure levels to indoor PM, and Chapter 5 describes some of the strategies for mitigating exposure to indoor PM. Chapter 6 recounts the discussion held among the workshop participants after the workshop’s first day of presentations. Chapter 7 discusses some of the health risks associated with exposure to PM, and Chapter 8 takes up the issue of how to engage the public in matters related to the risks of exposure to indoor PM. The workshop agenda is provided in Appendix A. Appendixes B and C provide biographic information on the speakers and on the planning committee and staff.

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³ See http://www.nationalacademies.org/hmd/Activities/PublicHealth/Health-Risks-IndoorExposure-ParticulateMatter/2016-FEB-10.aspx (accessed July 28, 2016).

In accordance with the policies of the National Academies of Sciences, Engineering, and Medicine, the workshop did not attempt to establish any conclusions or recommendations about needs and future directions, focusing instead on issues identified by individual speakers and workshop participants. In addition, the organizing committee’s role was limited to planning the workshop. The workshop summary was drafted by rapporteur Joe Alper in collaboration with Academies staff members David A. Butler and Guru Madhavan as a factual summary of what occurred at the workshop.

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