1
Introduction
This chapter provides information about the motivation for the report and the committee’s conduct of the study. It begins with an overview of why indoor chemistry is a subject that deserves examination. This report builds on prior efforts in the area, which are summarized in this chapter to help frame the issue. It provides the statement of task for the National Academies of Sciences, Engineering, and Medicine (the National Academies) committee responsible for this report and the committee’s approach to completing its task. The chapter concludes with a guide to the organization of the report.
WHY INDOOR CHEMISTRY MATTERS
Indoor chemistry describes the complex collection of reactions involving chemicals indoors (Weschler and Carslaw, 2018). This includes chemical reactions occurring in the gas phase, on particles, or on surfaces (Weschler et al., 2006). Understanding this complex chemistry matters for three prime reasons: (1) indoor chemistry is complex and may lead to the creation of new chemicals of concern, (2) indoor chemicals can adversely impact indoor air quality and the indoor environment, and (3) indoor chemicals are an important source of human exposure that may result in adverse health effects.
Indoor Chemistry Is Complex
Indoors, chemicals can undergo oxidative and other types of reactions with other chemicals in the air or on surfaces, creating potentially more or less toxic products. These chemicals can then move within the indoor environment. For example, chemicals emitted into the gas phase can partition to particles suspended in the air or collect on surfaces. Under different environmental conditions (e.g., temperature and relative humidity), this process may be reversed, leading to re-emission of chemicals into the gas phase. Both environmental conditions and indoor chemistry can vary between buildings depending in part on the building use, design, construction materials, operation, maintenance, occupant density, chemical use, contents, and air handling and purification systems.
For example, ventilation can vary between different types of buildings. Mechanical ventilation is commonly used in office buildings in the United States, while many homes are naturally ventilated through windows, doors, and infiltration (IOM, 2011). Buildings also differ in the types of chemicals that may be emitted owing to the types of materials present and the various activities that occur in different types of spaces. For example, chemical emissions from cooking and candles are common in restaurants and residences but are uncommon in other public buildings. The use of disinfectants and cleaning products occurs more often in health care facilities and schools than in many other types of buildings. Each type of indoor space could therefore have very different indoor chemistry.
Indoor Chemistry Influences the Indoor Environment
Chemicals found indoors are a significant risk factor that can modify or degrade the indoor environment. Direct emission of chemicals into the indoor environment can occur from various sources, including building materials, paints, stoves, cleaning products, pesticides, furnishings, electronics, and personal care products (see Chapter 2 and Figure 2-1). Biologic sources including microorganisms, plants, pets, and other animals can also contribute to indoor chemistry. Some chemicals present indoors can be from outdoor sources, often because of ventilation. Chemicals that contaminate occupants’ skin, clothing, or belongings can be brought indoors. People are also an important source of indoor aerosolized particles, carbon dioxide, ammonia, methane, and other organic chemicals (Ampollini et al., 2019; Li et al., 2020; Wang et al., 2021; Yang et al., 2021). Collectively these chemicals can affect indoor air quality and degrade the indoor environment in other ways.
Indoor Chemistry Can Impact Human Health
The indoor environment influences the behavior, comfort, productivity, and health of its occupants. The opportunity for human exposure to indoor chemicals is significant: exposure is the product of chemical concentration and time, and people spend most of their time indoors. Data from the National Human Activity Pattern Survey showed that Americans spent an average of nearly 69 percent of a 24-hour day in a residence and more than 18 percent in other indoor locations (Klepeis et al., 2001). A Canadian counterpart, the Canadian Human Activity Pattern Survey, revealed similar activity and location patterns except for seasonal differences. Canadians spend less time outdoors in winter and less time indoors in summer than their U.S. counterparts (Leech et al., 2002). On daily average, German children spend 65 percent of a 24-hour day in a residence and nearly 20 percent in other indoor locations (Conrad et al., 2013). Whether exposure to an indoor chemical results in an adverse effect is dependent on exposure duration and additional factors, including the inherent toxicity of the chemical (or mixture), its concentration in the environment, the route of exposure, and the susceptibility of the person. Box 1-1 provides several examples in which indoor chemistry is associated with adverse human health effects. Exposure to indoor-sourced chemicals can generally be reduced by increasing ventilation, using appropriate air-cleaning processes, selecting low-emission materials for building surfaces and furnishings, and avoiding certain cleaning and personal care products. Exposure can also be impacted by human factors, including the timing of when a building is occupied, occupant density, and occupant behaviors such as cooking and cleaning that may influence both chemical sources and exposure.
PRIOR EFFORTS ON WHICH THIS REPORT BUILDS
Research into indoor chemistry extends back decades and has often leveraged advances in atmospheric chemistry. Many initial investigations focused on the measurement of individual chemicals in indoor air and the release of chemicals from building materials and contents into the
indoor environment. Studies performed in the mid-20th century examined lead, radon, asbestos, and cigarette smoke levels in homes (Samet and Spengler, 2003). One of the first studies showed that sulfur dioxide concentrations found in Dutch homes with gas-fired combustion devices differed from those found in outdoor air (Biersteker, 1965). Since that time, interest has grown not only in the presence or absence of individual chemicals but also in the various chemical processes that occur indoors. Researchers recognized that hydrolysis chemistry taking place in building materials increased emissions of formaldehyde as far back as 1962 (Wittmann, 1962). Studies examining the reaction of ozone with surfaces in the indoor environment began in the early 1970s (Mueller et al., 1973; Sabersky et al., 1973; Shair and Heitner, 1974). Comprehensive chemistry models of indoor air (Nazaroff and Cass, 1986), evidence of gas-phase chemistry (Weschler et al., 1994; Weschler and Shields, 1996, 1999), and surface reaction products (Pitts et al., 1985; Weschler et al., 1992), as well as indoor surface reservoirs (Tichenor et al., 1991), followed. The U.S. Environmental Protection Agency (EPA) launched the Total Exposure Assessment Methodology (TEAM) Project in 1979. This project focused on volatile organic compounds (VOCs), carbon monoxide, and pesticides (Wallace, 1989; Wallace et al., 1987). This study used a probabilistic sampling approach and personal air monitors and breath analysis to measure daily personal exposures in air and drinking water. This study showed that most exposures occurred indoors, and pollutant levels in the indoor environment were higher than levels measured outdoors. In 1990, EPA partnered with the California Air Resources Board to conduct a follow-up study measuring personal exposure to particles (PM10). This Particle Total Exposure Assessment Methodology (PTEAM) study found that vacuuming, dusting, cooking, and living with a smoker were associated with higher PM10 exposures (Clayton et al., 1993). The Relationship of Indoor, Outdoor, and Personal Air (RIOPA) study measured
VOCs, semivolatile organic compounds, and fine particulate matter (PM2.5) outdoors and in homes (Weisel et al., 2005). Additionally, it was shown that indoor concentrations of formaldehyde and acetaldehyde were higher indoors than outdoors, while acrolein and crotonaldehyde found in homes without tobacco use came primarily from outdoor air (Liu et al., 2006). The goal of the RIOPA study was to assess exposure, not to understand chemistry; it relied on collecting time-integrated samples that provided detailed snapshots of chemical composition of personal and indoor air but did not capture real-time changes in indoor chemistry.
This report builds on the results of a number of prior studies supported by foundations, agencies, and societies, including EPA, the National Science Foundation, the National Institute of Standards and Technology, the National Institute for Occupational Safety and Health, the National Institutes of Health, and ASHRAE, among others in the United States and internationally (Andersen and Gyntelberg, 2011; WHO, 2010). The Alfred P. Sloan Foundation (Sloan Foundation) launched the Microbiology of the Built Environment and Chemistry of Indoor Environments programs in 2004 and 2016, respectively. Unlike federal funders of research, who typically fund research within a single discipline or related group of disciplines, the Sloan Foundation had the flexibility to fund highly interdisciplinary research, aiming to break down siloes that commonly limit research progress in many fields. The foundation’s ability to support multi-year programs led to the creation of a community of investigators with an interest in indoor chemistry who have formed consortia and partnered to conduct large field campaigns. The SURFace Consortium for Chemistry of Indoor Environments provides a forum for discussing research on the mechanisms and kinetics of chemical reactions that occur on indoor surfaces (Ault et al., 2020). The House Observations of Microbial and Environmental Chemistry (HOMEChem) experiment explored how human activities affected indoor sources of hydroxyl radicals, ozone, and other chemical oxidants and the formation of organic compounds (Farmer et al., 2019). The Modelling Consortium for Chemistry of Indoor Environments (MOCCIE) develops quantitative physical-chemical models that describe gas-phase and surface chemistry in the indoor environment and consider how occupants, indoor activities, and building conditions influence various molecular processes occurring indoors (Shiraiwa et al., 2019). In March 2022, the White House announced the Clean Air in Buildings Challenge, an EPA-led initiative to implement strategies for improving indoor air quality.
Associations among indoor chemistry, exposures, and adverse health effects have also been considered for several decades (Spengler and Sexton, 1983). A National Research Council (NRC) report entitled Indoor Pollutants (NRC, 1981) largely focused on indoor chemicals with a suspected association with negative health effects. This NRC report evaluated asbestos and other fibers, radon, formaldehyde, tobacco smoke, combustion products, and microorganisms, with a focus on allergens. This report also noted that headaches, mucous-membrane irritation, and difficulty concentrating and other signs of “sick building syndrome” were associated with occupancy of certain buildings (NRC, 1981). A later report by the Institute of Medicine (IOM) concluded that indoor allergens in the United States primarily come from house dust mites and cockroaches, microorganisms, and companion animals; exposure to these indoor allergens could contribute to the incidence of asthma and allergies (IOM, 1993). A 2016 National Academies workshop focused on particulate matter (PM) and a growing body of research that shows that human exposure to PM indoors can exceed outdoor levels and may represent an important risk factor (NASEM, 2016). The burning of solid fuel for cooking and heating in the United States (Rogalsky et al., 2014) and in developing countries is an important source of PM and other indoor chemicals and may account for 4 percent of the global burden of disease (Bennitt et al., 2021). The use of portable high-efficiency particulate air (HEPA) filters in homes located in a community with widespread residential wood combustion reduced indoor PM levels and improved microvascular endothelial function (Allen et al., 2011). The indoor environment has been associated with lung cancer, allergies, sick building syndrome and other hypersensitivity reactions, and respiratory infections (Sundell, 2004; Tran et al., 2020).
Finally, the worldwide pandemic of coronavirus disease 2019 (COVID-19) highlighted the importance of ventilation and filtration in reducing airborne disease transmission indoors. While much of the research to date about the relationship between chemical exposures and human health has focused on understanding acute health effects, associations with chronic and delayed illnesses are also important.
Against that backdrop, EPA, the Sloan Foundation, the National Institute of Environmental Health Sciences, and the Centers for Disease Control and Prevention approached the National Academies requesting an assessment of the state of the science regarding chemicals in indoor air in the nonindustrial indoor environment. The Committee on Emerging Science on Indoor Chemistry was formed to respond to that request. The overarching goal of this project is to identify new findings about previously under-reported chemical species, chemical reactions, and sources of chemicals, as well as the distribution of chemicals; and improve our understanding of how indoor chemistry is linked with chemical exposure, indoor air quality, and human health.
STATEMENT OF TASK
Faced with the challenges described above, the National Academies convened an ad hoc committee to summarize the state of the science regarding chemicals in indoor environments. Biographical information on the committee members is presented in Appendix B. The statement of task is provided in Box 1-2.
COMMITTEE’S APPROACH
The statement of task had several components that could be interpreted in a variety of ways. This section provides the committee’s interpretation of the task, including how it approached and addressed each of the components. The committee interpreted “chemicals in indoor air” to include gas- and particle-phase species, elemental species, and PM. The committee discussed the extent to which it should also include toxins of biological origin. Because this topic was the subject of a recent National Academies report (NASEM, 2017), the committee chose to focus its primary attention on other chemical sources. The committee debated whether nonindustrial exposures
applied to all buildings associated with occupational exposures. The committee decided to include certain indoor environments, including schools, office buildings, and medical facilities, where occupational exposures occur. The committee also considered what “new findings about previously under-reported chemical species” would constitute. Although no strict time frame was enforced, the committee generally considered work published in the past decade to be “new.” Likewise, the committee also considered the relevancy of certain well-studied chemicals (e.g., ozone, radon). The committee included well-studied chemicals when there was emerging evidence of their role in indoor chemistry.
The committee stayed informed about several ongoing National Academies activities, including the work of the Committee on Indoor Exposure to Fine Particulate Matter and Practical Mitigation Approaches. This National Academies effort resulted in several workshops given in 2021 that discussed the state-of-the-science on exposure to PM2.5 indoors, its health impacts, and engineering approaches and interventions to reduce exposure risks, including practical mitigation solutions in residential settings (NASEM, 2021).
To address the statement of task, the committee consulted several sources of information on buildings, indoor chemistry, and occupant health. A comprehensive discussion of indoor chemicals and human health was considered beyond the scope of this report. Instead, the committee focused on providing emerging data showing associations between under-reported chemical species and human health. The primary source used for health outcomes was human epidemiologic studies that examined exposures to chemicals in homes and other nonindustrial settings. A systematic review of the research literature was deemed an undertaking beyond the scope of this report. Committee members independently compiled lists of potential citations based on their scientific expertise. The committee considered the most influential scientific work available at the time it completed its task in early 2022, encompassing observations in real indoor environments, laboratory experiments, and theoretical modeling studies. This is often referred to as the “three-legged stool” model and has been a very successful approach in outdoor atmospheric chemistry (Box 1-3). The committee also referred to the research and conclusions of prior National Academies committees that addressed indoor environments and health issues. The 2007 NRC report Green Schools: Attributes for Health and Learning (NRC, 2007); the 2011 IOM report Climate Change, the Indoor Environment, and Health (IOM, 2011); and the 2017 National Academies report Microbiomes of the Built Environment: A Research Agenda for Indoor Microbiology, Human Health, and Buildings (NASEM, 2017) were influential. Finally, the committee organized a public virtual information-gathering workshop that was held on April 5, 2021. The agenda for this workshop is available in Appendix C.
ORGANIZATION OF THIS CONSENSUS REPORT
The subsequent chapters of this report explore aspects of the dynamic, interacting systems connecting humans, nonindustrial built environments, and indoor chemistry. Chapter 2 provides an overview of major primary sources and reservoirs of chemicals in the indoor environment, their emission rates, and emerging science on detecting and quantifying them. Chemical emissions are linked to both chronic and episodic sources and are modified by behavioral, environmental, and physical factors. Building materials, consumer products, infiltration of outdoor air, and human behavior (particularly cooking) can strongly influence the chemicals present in the indoor environment. Dust and indoor surfaces also serve as reservoirs of chemicals that are slowly released from primary sources, and they play a role in understanding human exposure in the indoor environment.
Chapter 3 focuses on chemical partitioning between surfaces and the air. Partitioning refers to the transfer of molecules from one phase to another, such as from air to an indoor surface. This process is often reversible, and a material can act as a reservoir that can absorb or release chemicals depending on conditions. In addition to building materials, furnishings, and personal
items, chemicals can partition into dust, condensed water, and the organic films that coat indoor surfaces. Partitioning strongly influences the timing and intensity of occupant exposure to chemicals and can limit efforts to reduce exposure by increasing building air exchange or the use of active air cleaning.
Chapter 4 addresses recent advances in our understanding of the nature of chemical reactions that occur in indoor environments, both in the air and on surfaces. These chemical transformations lead not only to the loss of reactants but also to the formation of secondary products that are potentially more reactive and/or harmful than their precursors. Although early studies in the field had highlighted specific classes of reactions as being important in indoor spaces, recent work has delved into determining their nature in sufficient detail and new assessments of the impacts on human exposure can now be made.
Chapter 5 addresses the active management of chemicals in indoor environments using interventions to clean air and surfaces. The chapter first provides an overview of the hierarchy of controls as a framework for considering risk-reduction strategies. With this framework in mind, the chapter next considers management approaches that result in minimal changes in indoor chemistry followed by management approaches that induce chemical transformations. This chapter also describes environmental factors, human behavior, and other considerations to keep in mind when selecting and using management approaches.
Chapter 6 places the emerging science on the chemistry of indoor environments in context with exposure and the environmental health paradigm. It begins with an overview of fundamental exposure concepts and further describes features of indoor settings that influence exposure levels, timing, and duration. The chapter summarizes a broad range of exposure determinants, with a focus on factors that may drive or contribute to environmental exposure and health disparities. This chapter
further considers the intersection of indoor chemistry with exposure modeling, advances in exposure assessment through measurement science, and emerging science on exposure to mixtures as it relates to indoor environments.
Chapters 2 through 6 each conclude with a short list of research recommendations that identify potential opportunities to advance the science. Finally, Chapter 7 organizes the committee’s vision for the future of indoor chemistry as a field of study under four major themes: (1) chemical complexity in the indoor environment, (2) indoor chemistry in a changing world, (3) future investments in research, and (4) communicating science and risks. This final chapter of the report considers not only what research advances are highest priority but also how new findings can be translated into practice to reduce adverse indoor exposures and improve public health.
Appendix A contains a glossary of terms used in the report. Biographies of the committee members responsible for this study are provided in Appendix B. The agenda for the public workshop held by the committee can be found in Appendix C. A table summarizing a set of exposure models commonly used for predicting near-field exposures is presented in Appendix D.
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