7
A Path Forward for Indoor Chemistry
This report has focused on different aspects of indoor chemistry, including new findings related to underreported chemical species, chemical reactions, and sources of chemicals and their distribution in indoor spaces. An understanding of how indoor chemistry fits into the context of what is known about the links among chemical exposure, air quality, and human health continues to evolve. Each chapter of the report highlights key research needs related to Primary Sources and Reservoirs of Chemicals Indoors (Chapter 2), Partitioning of Chemicals in Indoor Environments (Chapter 3), Chemical Transformations (Chapter 4), Management of Chemicals in Indoor Environments (Chapter 5), and Indoor Chemistry and Exposure (Chapter 6). These needs are not reiterated here; instead, this final chapter focuses on four emerging, crosscutting issues that span the topics discussed in earlier chapters. The committee provides its recommendations for critical needs to advance research, enhance coordination and collaboration, and overcome barriers for implementation of new research findings into practice in indoor environments.
This chapter also provides the committee’s vision for the future of indoor chemistry research. A critical cornerstone of this vision is increased awareness within the scientific community of the challenges and opportunities for innovation in indoor chemistry research as well as the need to fund research in indoor chemistry. It is also critical to translate the emerging knowledge on indoor chemistry into practice that benefits public health and the environment.
CHEMICAL COMPLEXITY IN THE INDOOR ENVIRONMENT
Complex Chemical Mixtures and Processes
An emerging theme in indoor chemistry is the high degree of chemical complexity in indoor environments where people spend, on average, more than 80 percent of their time. People are often in close proximity to sources and processes that emit chemicals. Recent studies have demonstrated the importance of indoor exposure to, for example, polychlorinated biphenyls (Meyer et al., 2013), tris(1,3-dichloro-2-propyl) phosphate (Meeker et al., 2013), Firemaster 550 (Hoffman et al., 2014), and di(n-butyl) phthalate (Lorber et al., 2017). Additional data come from the National Health and
Nutrition Examination Survey, an effort undertaken by the Centers for Disease Control and Prevention to collect survey and biomonitoring data that many researchers use to characterize exposures to important pollutants. Indoor exposure is discussed in Chapter 6 of this report.
Despite the importance of indoor exposure, very little is known about how humans get exposed to multiple chemicals across phases and pathways, how these joint exposures interact across timescales, and the cumulative and long-term impacts of the indoor chemical environment on human health. Humans are rarely exposed to single chemicals and instead are usually in contact with mixtures of chemicals that may have additive, synergistic, or antagonistic modes of action and effects on health. Many of these mixtures are not chemically characterized or quantified. Studies of exposure to mixtures in the indoor environment and their health effects are lacking, in part owing to the complexity and dynamics of indoor chemistry.
Early indoor chemistry studies typically focused on a small number of chemical contaminants, such as the mass of fine particulate matter (PM2.5) and ozone, small aldehydes, lead, and polycyclic aromatic hydrocarbon concentrations. Yet recent research has demonstrated that a much higher diversity of chemical species is present than previously recognized, including many highly functionalized organic compounds. As described in Chapter 2, these contaminants arise from different primary sources, such as influx of outdoor air and emissions from building materials and consumer products. The study of indoor chemistry is further complicated by the role human occupants themselves play and how their behaviors and time-activity patterns influence or modify their exposures. For example, human activities such as cooking and cleaning lead to significant and varied primary emissions. Most studies examining human influences on indoor environments have focused on chemical exposures in developed countries and with communities of higher socioeconomic status, or in underdeveloped countries relying on solid or fossil fuels for indoor heating and cooking (termed “household air pollution” by the World Health Organization). The full range of indoor settings is understudied with respect to indoor chemistry and indoor exposures.
Recommendation 1: Researchers should further investigate the chemical composition of complex mixtures present indoors in a wide range of residential and nonresidential settings and how these mixtures impact chemical exposure and health.
Chemical Reactivity
A second major emerging theme is the considerable degree to which many indoor contaminants are chemically reactive, largely via oxidative processes but also via photochemistry, hydrolysis, and other reaction mechanisms (see Chapter 4). These reactions can occur via interactions of gas-phase species with indoor surfaces, or homogeneously within surface reservoirs. As is now known for gas-surface ozonolysis reactions, the transformations of one precursor molecule lead to numerous reaction products, increasing the chemical complexity of the indoor environment multifold beyond that generated by the primary sources alone. Yet our current understanding of the relative magnitude and duration of exposure to these reaction byproducts and their toxicity relative to parent chemicals in the indoor environment is very limited. This is especially important because reaction products often have higher redox activity or oxidative potential and may elicit larger effects on health on their own, enhance or exacerbate the toxicity of the overall mixture, or even explain adverse health effects attributed to precursors or other chemicals indoors.
The dependence of multiphase reaction kinetics on oxidant concentrations, condensed-phase water abundance, light levels, and substrate chemical composition is poorly understood. Moreover, as reaction products form within different surface reservoirs, they may react with each other, forming even more complex products with less well-known exposure and health consequences. In contrast,
due to factors such as lower concentrations of oxidants indoors, the persistence of some chemicals may be higher indoors.
Recommendation 2: Researchers should focus on understanding chemical transformations that occur indoors, using advanced analytical techniques to decipher the underlying fundamental reaction kinetics and mechanisms both in the laboratory and in indoor environments.
Distribution of Indoor Chemicals
Another challenge is to accurately describe the phase distribution of chemical contaminants (see Chapter 3). Whereas the field of indoor chemistry has largely focused on the measurement of gas-phase and aerosol particle abundance, it is now clear that most molecules are largely present in a variety of surface reservoirs. Our incomplete quantitative understanding of the partitioning of semivolatile molecules limits the ability of models to accurately describe the removal rate and exposure levels, especially for near-field exposures. Additional measurements are needed of the spatially and temporally dependent abundance of contaminants in all surface reservoirs in a range of indoor environments, and of the rates at which such gas-surface partitioning processes occur. This information then needs to be integrated into partitioning models to more accurately predict the phase distributions of a wide variety of contaminants. Paired with knowledge of human behavior and time-activity patterns indoors, this can greatly enhance our understanding of exposure across phases and pathways. In the case of a single chemical of interest or concern, this would advance our understanding of aggregate exposure and complete health effects across exposure pathways. In the case of multiple chemicals or mixtures of concern, these advances would bring us closer to a cumulative risk assessment paradigm to better understand the combined and overall toxicity of multiphase, multipathway contributions of the indoor chemical environment on health.
Recommendation 3: Researchers should prioritize understanding the phase distribution of indoor chemicals between all indoor reservoirs and incorporate these findings into exposure models.
Overall, integration of our knowledge of partitioning processes, transformation chemistry, environmental conditions, human influences, and building and heating, ventilation, and air-conditioning (HVAC) parameters is essential to more accurately represent these complex processes and enable more accurate chemical exposure and health risk assessments. This field is ripe for multidisciplinary collaboration to significantly advance knowledge of the indoor chemical environment and its importance for human exposures, health, and well-being. It requires expertise from a variety of disciplines, including chemistry, engineering, building science, toxicology, exposure science, epidemiology, social sciences, urban planning, environmental regulation, and risk assessment. The complexity of the indoor environment and how it interacts with the outdoor environment and with humans is too diverse and broad for siloed approaches to make a significant contribution to advancing the field.
Recommendation 4: All stakeholders should proactively engage across disciplines to further the development of knowledge on the fundamental aspects of complex indoor chemistry and its impact on indoor environmental quality, exposure assessment, and human health.
INDOOR CHEMISTRY IN A CHANGING WORLD
There are unprecedented changes occurring to the outdoor environment due to climate change, wildfires, and urbanization, standing in contrast to improvements derived from environmental
regulations and advancements in technology. This section examines the impacts of those changes on indoor environments.
Outdoor-Indoor Chemistry Interactions
Outdoor air pollution is associated with a wide range of adverse human health impacts. Epidemiological evidence on the relationship between outdoor air pollution and adverse health outcomes at the population level has driven regulatory policies setting outdoor air quality standards. Yet most exposure to outdoor air pollution actually occurs indoors. Large population-based epidemiological studies aiming to understand the health effects of outdoor air pollution generally rely on outdoor air quality measurements. Outdoor concentrations are treated as surrogates of personal exposure to air pollution of outdoor origin, yet this treatment introduces exposure misclassification and is sometimes not clearly described. Indoor chemistry transforms some outdoor air contaminants that enter indoor spaces into new chemical hazards, altering what people are exposed to. For example, ozone concentrations are lower indoors than outdoors, but many oxidation products of ozone are substantially higher indoors. The size distribution of outdoor particulate matter (PM) changes as the ultrafine and coarse particles are preferentially removed by infiltration and deposition, leaving more of the fine particles that most efficiently penetrate deeply into the lungs. Outdoor particles release some volatile components into the gas phase and take up indoor airborne chemicals such as phthalates, plasticizers, flame retardants, and cooking emissions. This chemical complexity contributes to PM’s variable toxicity and impact on health and implies that, for the same PM2.5 mass concentration, health risk could vary based on the origin and chemical composition of PM2.5. It stands to reason that indoor chemistry, both transformations and partitioning, plays a significant—and currently overlooked—role in modifying the outdoor pollution that primarily concerns recent epidemiological studies.
Recommendation 5: Researchers who study toxicology and epidemiology and their funders should prioritize resources toward understanding indoor exposures to contaminants, including those of outdoor origin that undergo subsequent transformations indoors.
Similarly, indoor chemistry influences outdoor air pollution. Indoor emissions from building materials, cooking, and consumer products markedly impact outdoor air pollution. Volatile chemical products (VCPs) made from petrochemical feedstocks are important contributors to ambient photochemical air pollution, comprising half of all petrochemical pollution in industrialized cities, yet many of those products are used and emitted indoors (McDonald et al., 2018). As cities and their resident populations continue to grow, and outdoor pollution regulations continue to reduce other sources, indoor emissions are poised to make up increasingly higher proportions of primary chemicals found outdoors. Buildings may alter the local outdoor air in subtle ways that may be important as outdoor air pollution continues to improve. Beyond emissions from VCPs, other consumer products, building materials, and cooking, other processes may meaningfully alter outdoor air pollution, such as chemical transformations of gases and particles (of outdoor origin) associated with partitioning, hot surfaces, combustion, and use of air and surface cleaners or other activities. Our understanding of the specific impact of indoor chemistry on outdoor air pollution is limited, but it has never been more important to characterize the sources, chemistries, and eventual exposures described in this report.
Recommendation 6: Researchers and their funders should devote resources to creating emissions inventories specific to building types and to identifying indoor transformations that impact outdoor air quality.
A Resilient Built Environment
The way that buildings are designed and operated has to respond to the changing climate and trends in energy efficiency and energy sources. Energy efficiency has often been considered to be in conflict with the desire to improve indoor air quality through increased ventilation. This is especially true in home weatherization programs and in newer construction in the United States with tighter building envelopes. It is possible, however, to reject the energy-ventilation tradeoff paradigm. Tight building envelopes, purposeful ventilation, and reduced energy consumption through heat recovery ventilation are possible routes to improving energy efficiency without impacting ventilation rates. Ventilation with clean outdoor air is an important modifier of human exposure to all the chemical sources present indoors. Epidemiological evidence has consistently demonstrated improved health outcomes in building occupants under increasing outdoor air ventilation rates (Sundell et al., 2011), and increased ventilation has been observed to reduce symptoms associated with sick building syndrome (Fisk et al., 2009). Future building design will need to account for indoor chemistry and contend with environments that are continuously changing due to trends in building regulations and energy choices, including regulated decreases in the installation of natural gas appliances in new construction.
The availability of healthy outdoor air for ventilation cannot be taken for granted, especially with the increasing impacts of climate change. Recent events have shown that exchanging indoor air with outdoor air can be problematic when the concentrations of chemical species and PM in outdoor air pose a health risk. This conundrum traditionally occurs when ventilation is used to reduce indoor contaminant exposures in urban areas with elevated outdoor levels of pollution. A recent and more acute scenario required residents of the western United States to weigh the known benefits of outdoor air ventilation on reducing the risk of airborne transmission of COVID-19 with the known dangers of exposure to elevated PM2.5 concentrations from wildfires. This also extends to the potential for poor COVID-19 outcomes in patients with increased exposures to air polluted by wildfires. Finding solutions will require a continued commitment to improving outdoor air quality. In parallel, it may also include improved capacity for building envelopes to remove contaminants from outdoor air and continued reductions in indoor chemical sources. In the absence of wildfire impacts, as outdoor air quality continues to improve due to more stringent regulations and adoption of electric vehicles and renewable energy sources, its impact on the indoor environment will be less important. In this scenario, the relative importance of indoor sources on human health will become proportionally larger.
Another central impact of climate change in some areas is increased presence of dampness in buildings due to extreme precipitation events, rising sea levels, and frequent flooding. Moisture in building materials and the building envelope may initiate or accelerate chemical emissions. Increased moisture in buildings also results in mold and pests ranging from dust mites to cockroaches. In addition to the mycotoxins and volatile metabolites produced by mold and pests, occupants respond to their presence by using more disinfectants and pesticides with implications for indoor chemistry and air quality.
Recommendation 7: Researchers and engineers should integrate indoor chemistry considerations into their building system design and mitigation approaches. This can be accomplished in different ways, including by consulting with indoor air scientists.
FUTURE INVESTMENTS IN RESEARCH
Looking towards the future of indoor chemistry research, it is important to acknowledge the need for strategic investments and coordination among funding agencies (Box 7-1). This section highlights several areas where such a coordinated effort could have a major impact on advancing the science.
Investing in Top-Down and Bottom-Up Approaches
Intersecting and integrating top-down and bottom-up approaches is essential for addressing complex chemical processes that cover a wide range of temporal and spatial scales in indoor environments. Top-down approaches may include indoor field observations that use a suite of analytical tools to characterize gas-phase compounds, aerosol particles, and indoor surfaces. Such observations and experimental analysis can provide quantitative information on indoor species and may lead to a discovery of unknown phenomena or shed light on poorly understood processes. However, top-down approaches by themselves would not fully elucidate fundamental and molecular-level understanding of specific processes. Bottom-up approaches are necessary to achieve full process-level understanding. Efforts to integrate laboratory experiments, indoor field observations, and modeling need to be a high priority and have strong potential for impact when coupled with exposure and health studies. This “three-legged stool” approach has been successfully applied in environmental science and needs to continue to be implemented for indoor environments. Applications of indoor models are a critical component of understanding indoor chemistry to develop an in-depth understanding of complex processes. Models can guide measurements through the identification of key chemical species and predictions of expected concentrations as well as to assist in the interpretation of laboratory experiments and field observations. Controlled laboratory measurements are needed to determine kinetics for emerging reactions and to elucidate chemical mechanisms of heterogeneous processes. Quantum chemical calculations and molecular dynamics simulations provide a full molecular picture of complex surface interactions and estimates for physicochemical parameters that may be compared with measurements and used in models. Chemical kinetic and thermodynamic models that resolve gas- and multiphase chemistry can be applied to gain mechanistic and quantitative interpretations of indoor observations by testing current knowledge and different hypotheses. Computational fluid dynamics coupled with mechanistic chemistry models can help resolve the spatial and temporal evolution of species in indoor environments. Application of these indoor models can help extrapolate field observations and experimental results to other indoor conditions and provide reasonable estimates of concentrations of different chemical species that may be inaccessible by measurements.
Recommendation 8: Given the challenges, complexity, knowledge gaps, and importance of indoor chemistry, federal agencies and others that fund research should make the study of indoor chemistry and its impact on indoor air quality and public health a national priority.
Analytical Tools
Characterizing the chemically complex indoor environment presents a wide array of both challenges and opportunities. The recent development of advanced analytical techniques has permitted the identification of a much greater number of indoor contaminants. For example, non-targeted high-resolution mass spectrometry approaches are revealing the myriad chemicals present in indoor samples, such as settled dust. A current limitation to these approaches is that the data are primarily qualitative and focus on accurately identifying chemical features present in the samples, although these data can be used in a semi-quantitative basis in some cases. Accurate quantification can be problematic when authentic standards do not exist (which is the case for many chemical transformation products) or instruments are not calibrated for various types of chemical classes. This imposes a potential limitation on our ability to quantitatively assess chemical exposures when instrument response cannot be “translated” into a concentration. Additionally, integrating on-line methods to quantify indoor versus outdoor sources and how they contribute to indoor air chemistry is important. This can be done by measuring gas-phase and aerosol composition indoors and outdoors at the same time. Investments in novel methods and chemoinformatic resources that increase our ability to identify and quantify the abundances of wide classes of indoor chemicals are needed in order to understand the impact of these exposures on human health. Future research efforts that combine non-targeted analyses with in vitro and in vivo toxicity assessments will provide more insight into the potential health impacts of these complex chemical mixtures.
Recommendation 9: Researchers and their funders should invest in developing novel methods and chemoinformatic resources that increase our ability to identify and quantify the abundances of wide classes of indoor chemicals, both primary emissions and secondary chemical reaction products.
Our emerging picture of indoor environments indicates chemical complexity in gas, particle, and surface phases. Although new analytical tools have been instrumental in improving our understanding of indoor chemistry, several key challenges remain. For example, indoor surface measurements of real-world samples have been limited to off-line measurements, typically involving sample collection and subsequent analysis. Development of real-time surface measurements is an emerging direction. Comprehensive chemical characterization with real-time instrumentation can provide insight into indoor systems, but the technical expertise, logistical constraints, and instrument costs restrict these measurements to short-term measurements in a limited number of settings. Because of these constraints, researchers commonly conduct field studies in manipulated test houses or in convenient and accessible locations, such as university classrooms or academic employee residences. Future studies will need to include an array of buildings that are more representative of the U.S. population’s experience. Two important directions include the following: (1) expanding comprehensive indoor chemical studies to different buildings and environments across building use, age, and location to increase the diversity of stakeholders and occupants; and (2) developing and applying lower-cost chemical sensors for more widespread research applications. While measurement tools for atmospheric chemistry have developed rapidly over the past decade, the complexity of the indoor environment warrants both targeted and non-targeted analyses, with particular attention paid to potential instrument interferences and calibrations to the novel compounds present in indoor settings.
Recommendation 10: Researchers measuring indoor environments should apply and develop new analytical tools that can probe the chemical complexity of gases, aerosols, and surfaces.
Human Activity Data and Indoor Chemistry
Indoor chemistry is not only heavily influenced by chemicals used in indoor spaces and building characteristics but also influenced by the activities of daily living. Human activities in indoor environments are incredibly diverse, and many of these activities contribute to indoor chemistry. Therefore, it is important to gather human activity data, in combination with other metadata regarding buildings, to capture contributions of human activity to indoor chemistry in a wide-ranging manner (Li et al., 2019; Nguyen et al., 2014). Nationally representative surveys (e.g., the American Census Survey, American Housing Survey, and Residential Energy Consumption Survey) already exist to collect probability-based samples of the U.S. population and include some questions that provide insight on indoor activities of daily living that are associated with environmental exposures. These surveys are deployed at varying frequencies and scales, and none of them expressly include questions designed to characterize activities associated with indoor chemistry (e.g., sources, sinks, and patterns of occupancy). The National Human Activity Patterns Survey (NHAPS) was purposefully designed to document the time, location, and characteristics of activities relevant to estimating contaminant exposures, but this survey was only implemented from 1992 to 1994 as a probability-based national telephone interview survey of approximately 10,000 people. Detailed surveys of occupancy patterns, activities, and materials are essential to link activities with chemical implications, so efforts such as the NHAPS warrant more sustained implementation moving forward. In addition to national-level surveys, there are numerous other ways to gather information on people’s activities in indoor environments as they relate to indoor chemistry and indoor chemical exposures, including in-home sensing of environmental and air quality parameters, energy and water use monitoring in homes and buildings, and data on consumer choices and expenditures. One challenge with these types of data is that they vary with respect to their availability and the extent to which privacy and confidentiality issues arise.
Recommendation 11: Federal agencies should design and regularly implement an updated National Human Activity Patterns Survey. Federal and state agencies should add survey questions in existing surveys that capture people’s activities in indoor environments as they relate to indoor chemistry and indoor chemical exposures.
COMMUNICATING SCIENCE AND RISKS: INDOOR CHEMISTRY AND ENVIRONMENTAL QUALITY
In this section, the committee seeks to convey the importance of communicating emerging information about indoor chemistry to stakeholders. To many stakeholders, science concerns itself mainly with discovery (“what?” and “how?”) and leaves questions of relevance (“why does it matter?”) and application (“how can it be used?”) to others. The process of creating scientific knowledge and transferring it into the spheres of practice and policy can be inefficient and slow. There is also a need for science to address research questions that are derived from practical concerns. The monumental effort during the COVID-19 pandemic to apply scientific tools to mitigate its effects exemplifies what can happen when this connection is made. Making the same connection between science and application is essential in indoor chemistry.
Actively Engaging Stakeholders
Stakeholders with an interest in emerging issues in indoor chemistry include building occupants; researchers in ambient and indoor air quality, indoor chemistry, toxicology, epidemiology, and environmental and public health; practitioners in health care, healthy housing, air quality, HVAC, weatherization, and construction; government agencies that address public health; communities impacted by indoor
chemistry-related exposures; and policy analysts and legislators. Some stakeholders are involved in creating scientific knowledge, while others focus on applying this knowledge in practice and to regulation. This section focuses on how those in the former group can most effectively dialog with the latter.
The stakeholder who most affects, and who is most affected by, indoor chemistry is the general public. Effectively engaging this audience requires scientific and technical professionals to communicate two overarching messages: first, how indoor air quality and indoor chemistry contribute to exposure and their personal health outcomes; and second, how their own actions and behaviors could mitigate or exacerbate exposure. For example, the increased use of consumer-grade indoor air quality monitors may help improve indoor air quality and present opportunities for citizen science but not unless users are also equipped with enough knowledge to interpret the information they provide. Empowering the public with this knowledge is essential, but community engagement is challenging and requires resources (see Chapter 8 in NASEM, 2016). Conveying these often technical messages in a way that brings about changes in behavior will require risk communication from front-line professionals who work directly with the public. Health care providers can increase air quality awareness among patients through consultation; yet, the health care industry has not widely adopted the practice of consulting patients about health risks from ambient or indoor air quality, suggesting that the scientific community could do more to engage with these important intermediaries. Communication with stakeholders in accessible terms that emphasize the personal relevance of indoor chemistry is central to translation of new research findings into practice.
Recommendation 12: Researchers should proactively engage in links that connect research to application throughout the indoor chemistry research process—for example, at the dissemination stage, by engaging with technical and standard-writing committees, presenting at conferences attended by practitioners, and disseminating the significance of research findings in social and mass media.
Environmental Justice and Indoor Chemistry
Indoor chemistry is a complex and emergent field of research. Moving forward, careful consideration is warranted for the potentially unique indoor environments documented in low-income, rural, and cold-climate areas as well as in communities of color. Foremost, researchers and practitioners can directly engage environmental justice (EJ) communities in formulating future research priorities and forge collaborations with social scientists who work with EJ communities. In addition, the development of a conceptual framework that holistically captures the scope and scale of variables that are unique to substandard housing may be useful. Such a framework would contextualize and ground the broader field of indoor chemistry within an equity lens. Factors and variables for consideration in the framework include
- unique sources and distribution of chemical contaminants in substandard housing and the influence of different types and variable quality of heating, cooling, ventilation, and filtration systems;
- health effects of indoor exposures in substandard versus standard housing due to building materials and maintenance practices;
- source, proximity, and scale of outdoor contaminants to which EJ communities may have greater indoor exposure;
- unique behaviors, chemical emissions, and chemical interactions that may occur due to a variety of reasons including locus of controls, differences in activity patterns, occupant density, and type of housing, such as multifamily versus single family; and
- how chemical interactions in a changing world may affect EJ communities in unique ways.
Recommendation 13: Researchers and practitioners should include environmental justice communities in the wide range of indoor environments they study and engage these communities in formulating research priorities and recommendations for future indoor air quality standards.
Consumer Products, Sensors, and Services
A wide variety of products and services are marketed to consumers to improve indoor air quality. This report has highlighted several types of products that are known or suspected to influence indoor chemistry. For example, there are oxidant-emitting air cleaners and ultraviolet light-based air disinfection systems. There are also services that remediate buildings with smoke damage or mold that use strong oxidants in the process. Low-cost carbon dioxide or particle sensors inform consumers of conditions that may prompt action (e.g., opening a window). The impacts of these kinds of products, services, and sensor-prompted behaviors on indoor chemistry are poorly understood.
Recommendation 14: Funding agencies should support interdisciplinary research to investigate the impact of products and services on indoor chemistry, especially under realistic conditions. There is also a need to determine how occupant access to air quality data leads to behavior that influences indoor chemistry.
It is important to note that manufacturers continue to market novel air-cleaning products and remediation services that are of uncertain value and that may adversely impact indoor air quality and occupant health. This has been especially the case during the COVID-19 pandemic. These products may range from useful to useless and from beneficial to harmful. Some claims about these products misrepresent benefits or worse. Indoor chemistry is complex, and consumers and facility managers currently do not have useful tools for evaluating these products, services, or related manufacturer claims. Even the experts on this committee do not yet have sufficient information about most of these products and services to make informed decisions about their utility or potential for harm.
Conclusion 1: Standardized consensus test methods could enable potential certification programs for air-cleaning products and services. Such test methods could help regulators determine whether action on these products and services is warranted.
Standards for Indoor Environmental Quality
Unlike regulation of outdoor chemistry, the management of indoor chemistry is at a nascent stage. Regulation of outdoor air has followed scientific discoveries for years. Many outdoor air hazards have been greatly reduced based on effective, science-based approaches to regulation and public risk communication. In the United States, the National Ambient Air Quality Standards regulate the concentration of carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, ozone, and PM2.5 in outdoor air, leading to demonstrable reductions in morbidity and mortality.
An effective regulatory framework for indoor air has not been established in the United States. The committee recognizes the inherent challenges in regulating non-occupational indoor air quality, such as privacy, personal liberty, and property rights. Indoor spaces contain thousands of chemical species at a wide range of concentrations where little to no data exist on acute or chronic health impacts, especially for children and other vulnerable populations. There is also a broad diversity of building stock and uses, occupant behavior, and other factors that further contribute to the complexity of indoor chemistry.
Globally, regulators are currently working to determine how best to use indoor chemistry findings to create guidance for a range of stakeholders. For example, in 2021, within its resolution
on the implementation of the Ambient Air Quality Directives, the European Parliament called on the European Commission to regulate indoor air quality. Canada has enacted indoor air quality guidelines for numerous contaminants, as well as Indoor Air Reference Levels. The World Health Organization has published guidelines for nine contaminants, and ASHRAE recently published threshold values for 14 compounds and PM2.5.
While these recent steps indicate a growing desire to reduce contaminants found indoors, it is important to note that, just as the challenges in implementing regulations differ from those for outdoor air quality, threshold levels may also differ. For example, those deciding where to set a maximum indoor standard for ozone may want to consider not only the health effects of ozone itself but also the products formed by ozone-initiated indoor chemistry (Xiang et al., 2019).
Establishing standards for the indoor chemical environment that are protective of public health across multiple settings requires crosscutting, multipronged collaborations and solutions. For example, while the U.S. Environmental Protection Agency does not have the authority to regulate indoor concentrations of chemicals in air or dust as it does for outdoor air, it has the authority to regulate emission factors of new and recycled products introduced indoors. Building codes, standards, and guidelines all have a role to play in eliminating hazards resulting from the indoor chemical environment.
Recommendation 15: Researchers and their funders should prioritize understanding the health impacts from exposure to specific classes and mixtures of chemicals in a wide range of indoor settings. Such understanding is needed to inform any future standards, guidelines, or regulatory efforts.
CLOSING COMMENTS
There is a growing awareness that exposure to environmental contaminants contributes to the burden of human disease. For decades, much of the attention of the scientific and regulatory community has focused on ambient (outdoor) air quality and drinking water. These efforts have contributed to improvements in outdoor air and water quality that have measurably protected human health and the environment. An important contributor to human health, namely the indoor environment, has received far less attention, although its potential consequences have been documented.
The paucity of advanced chemical studies on the indoor environment directly hinders our ability to predict and mitigate both health and environmental effects. A better understanding of sources, sinks, and transformations of chemicals found indoors is needed. It is clear that the chemical complexity of the indoor environment—present in its sources, chemical transformations, and loss mechanisms—currently precludes accurate and complete assessments of the exposure rates for many of the chemicals present indoors. Effective integration of laboratory experiments, indoor measurements, and modeling is necessary to determine the impacts of this chemistry on indoor environmental quality and chemical exposures. As more attention is deservedly focused on indoor chemistry, more chemicals and their reactions will be identified, adding to an already complex problem. Many of these chemicals may have little to no information regarding their toxicity, either as individual agents or in combination with other chemicals present in the environment. Mitigating chemical hazards will depend on many factors and needs to be done in a manner that considers the impacts of any mitigation strategy itself on the indoor environment. This will require efforts in changing building design and operation, altering the use of products and materials, and minimizing the impact of human activity on indoor chemistry.
The immensity of this daunting task need not lead to inaction. Rather, investment at this time in a holistic approach that considers chemistry, biology, and social contributions to health will pay dividends in the future.
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