Summary
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. Biologic sources including microorganisms, plants, pets, and other animals can also contribute to indoor chemistry.
Why does indoor chemistry matter? It matters because people spend most of their time at home or in other indoor locations. Complex mixtures of chemicals in indoor environments may adversely impact indoor air quality and human health. Whether exposures to indoor chemicals result in an adverse effect is dependent on exposure duration and additional factors, including the inherent toxicity of the chemical mixture, chemical concentrations in the environment, the route of exposure, and the susceptibility of the person.
This report explores indoor chemistry from different perspectives including sources and reservoirs of indoor chemicals and the ability of these chemicals to undergo transformations and partitioning in the indoor environment. In some cases, indoor chemistry can result in the creation of potentially more or less toxic products, reactive intermediates, or products with different physiochemical properties.
This study provides a status report for indoor chemistry research. The overarching goal of this project was 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, air quality, and human health. The report also identifies future research needs.
PRIMARY SOURCES AND RESERVOIRS OF CHEMICALS INDOORS
Thousands of chemical compounds have been detected in the indoor environment, including in air, particles, and settled dust, or on surfaces. These chemicals are intermittently or chronically emitted into the indoor environment from primary sources that originate either indoors or outdoors. Materials used to construct buildings, the furnishings that are brought into those buildings, and
building occupants and their activities are important indoor sources of chemicals. Cooking is one of the most notable sources of indoor chemicals, with emissions varying depending on cooking style and ingredients. Cooking generates gases and particles, both from the heat source and the food. The use of personal care and consumer products contributes chemicals to the indoor environment, as does the use of disinfectants and cleaning products in health care facilities and schools.
Over the past few decades, new materials and chemicals have been introduced in the indoor environment—for example, new types of building insulation and greater use of electronics and smart devices in indoor environments, particularly in homes. These devices and materials can be sources of chemicals to the indoor environment, such as plastics, plasticizers, antioxidants, ultraviolet stabilizers, and flame retardants. Increased use of cleaning and disinfection agents, particularly during the COVID-19 pandemic, has led to increased levels of some chemicals, such as quaternary ammonium compounds, in the indoor environment. Over time, some chemicals that were emitted by primary sources in the indoor environment have been phased out of use owing to concerns about their elevated exposure and/or toxicity. However, recycling of certain materials may lead to ongoing exposure to phased-out chemicals. For example, polyurethane foam from discarded furniture is sometimes recycled into bonded carpet padding. Unfortunately, phased-out chemicals are sometimes replaced with chemicals that have less data available on their emissions, exposure, and potential hazards. Some legacy contaminants that were phased out, such as polychlorinated biphenyls, chlordane, and chlorpyrifos, are persistent in indoor environments and contribute to prolonged and chronic exposure in older buildings and apartments, which in some cases can lead to social justice issues.
A wide range of analytical techniques are currently being used to identify new chemicals, in both bottom-up and top-down approaches, that may be released into the indoor environment, but these approaches are costly and time-consuming. Challenges in identifying chemical sources in the indoor environment, including a lack of transparency in chemical use in consumer products, continues to be a major obstacle to chemical inventory and risk evaluation. Another risk management challenge relates to the lack of information on the health effects or distribution of many of the chemicals found in the indoor environment.
To address these challenges, the committee identified a need to prioritize research linking sources with exposures and to characterize the impacts of mixtures on health. To support discovery, improved analytical methods and non-targeted approaches are needed, as well as harmonized databases for chemical information. Although the study of indoor environments has recently increased, data are lacking for nonresidential settings and underrepresented countries and contexts.
PARTITIONING OF CHEMICALS IN INDOOR ENVIRONMENTS
Partitioning of chemicals plays an important role in indoor chemistry and indoor air quality. Partitioning refers to both the thermodynamic state of chemicals distributed among phases in a system and the processes that transfer chemicals among phases, generally with a net tendency to approach equilibrium. At the thermodynamic state of equilibrium, partitioning determines the concentration of a chemical in air, on surfaces, or elsewhere.
Chemicals in indoor environments are often not at equilibrium, and net transfer can occur between phases. Partitioning distributes chemicals from their initial sources throughout indoor spaces to air, building materials, furnishings, dust, and so forth. For example, phthalates emitted from plastics can partition to surfaces, porous materials, settled dust, and other compartments. These compartments buffer the air concentrations of chemicals, reducing the short-term effectiveness of controls by ventilation or filtration. Partitioning also influences occupant exposure to chemicals. For example, partitioning of indoor chemicals to aerosols increases inhalation exposure, while partitioning to dust and surfaces increases ingestion exposure, especially by toddlers.
Chemicals can have a greater affinity for one compartment than another, and this affinity is characterized by a “partition coefficient.” Because of the very high surface-area-to-volume ratio of indoor environments, partitioning influences the manner, extent, and duration over which occupants are exposed to contaminants. For example, nicotine has a high affinity for indoor material surfaces; therefore, more nicotine is found on surfaces than in the air. Many molecules that are entirely volatile in the outdoor environment behave like semivolatile organic compounds indoors. Compartment size is also important. By combining partition coefficients with the compartment size, one can determine where most of the mass of a chemical contaminant will be at equilibrium. Equilibrium conditions may change with time as environmental parameters such as temperature and relative humidity change. Partitioning does not occur instantaneously, and it can take time, sometimes years, for compartments to approach equilibrium due to slow rates of molecular transport. Thus, partitioning of semivolatile and low-volatility molecules to indoor surfaces can increase indoor residence times. Partitioning can also result in chemicals moving to locations and conditions favorable for chemical transformations.
Challenges remain in understanding partitioning from the molecular to whole building scales and how partitioning influences exposure and chemistry. These challenges include limited information on the detailed composition of building materials across the building stock; complexity of spatial and temporal variations in environmental conditions; and poor understanding of surfaces and indoor materials at molecular and nanometer length scales, as well as of the molecular interactions that determine the partitioning of a chemical between phases.
Despite a rapidly growing base of knowledge about indoor partitioning, important data gaps remain. The materials that are present in buildings, or comprise buildings, are not physically or chemically well characterized. Partition coefficients have been measured for very few chemical contaminants and materials. Models to predict thermodynamic parameters exist, but their application to real indoor materials has not been widely demonstrated. Furthermore, models have not been successfully applied to some chemical classes important in indoor environments, such as surfactants. The extent to which environmental and other building factors, occupant activities, and control systems influence partitioning and exposure remains to be explored. There is also a need to improve our understanding of partitioning at the molecular level. Addressing these needs should lead to improved predictive models of partitioning and, by extension, exposure.
CHEMICAL TRANSFORMATIONS
Chemical transformations are chemical processes that lead to the loss or removal of certain substances (e.g., reactants) and the generation or formation of new substances (e.g., products). The products that arise from these reactions frequently are very different from the reactants in terms of their partitioning, toxicity, and other properties. For example, chlorinated chemicals found in cleaning products can react with unsaturated organic compounds to produce higher molecular weight products that can contribute to film growth or secondary organic aerosol (SOA) formation.
Different types of chemical reactions are relevant indoors, including photolysis, hydrolysis, acid-base reactions, and redox reactions. Some of these processes are irreversible, leading to a permanent loss of species, while others are reversible, resulting in the temporary loss and eventual regeneration of reactants. These chemical processes are complex and extensive, with numerous species involved as precursors, intermediates, or products.
Chemical transformations occur at different locations indoors, including the gas phase, airborne particles, and indoor surfaces, as well as hidden places such as ducts and the heating, ventilation, and air-conditioning (HVAC) system. Surface-adsorbed molecules may diffuse into the bulk of indoor surfaces and materials, where they may undergo chemical transformations. The relative rates of ventilation, gas-phase loss, and loss to surfaces are important to compare when evaluating the
fate of an indoor air molecule. Reactions on surfaces can be very important, even if relatively slow, if the species preferentially partitions to the surface.
Major findings from the past several years illustrate the complexity of chemical reactions that occur in indoor environments. In particular, gas-phase oxidation reactions, some occurring via auto-oxidation mechanisms, lead to the formation of a suite of highly oxygenated gas-phase species which may form SOA. In addition, much of reactive indoor chemistry occurs on surfaces, via multiphase chemistry. Although long acknowledged to be important, ozonolysis reactions of unsaturated organics have recently been demonstrated to form highly oxygenated species, such as secondary ozonides and volatile oxygenates, on surfaces. This chemistry is known to occur on humans, their clothing, and other surfaces contaminated by cooking or smoking emissions.
The complexity of such reactions presently precludes a quantitative understanding of these processes under actual indoor conditions, where substrate composition and environmental parameters (e.g., relative humidity) have been shown to affect the mechanisms and kinetics. The identities and amounts of many indoor chemicals, especially in surface reservoirs, remain incompletely understood. This data gap can lead to incomplete toxicological and epidemiological evaluation of chemical dose and health outcomes in indoor environments. Furthermore, such uncertainties in reactive chemistry, when coupled with uncertainties in partitioning, make it challenging to determine the relative importance of the major exposure pathways for many indoor chemicals.
New chemistry has been identified recently when chemical cleaning agents, such as chlorine bleach, are used on indoor surfaces. The suite of chemical products that arises from such use is only just starting to be studied. The reactive chemistry that occurs with some other common cleaning agents, such as hydrogen peroxide, has yet to be investigated under indoor conditions.
Photochemistry largely occurs in genuine indoor settings where air or surfaces are directly illuminated with sunlight. While infrequent in many indoor settings, high levels of oxidants can be generated, and other reactive photochemistry can occur in such situations. It is possible that important, yet slow, photochemistry occurs elsewhere on indoor surfaces that are not exposed to direct sunlight, but this has yet to be confirmed.
Condensed-phase water is an important medium for facilitating indoor chemical transformations. These can include acid-base reactions, slow hydrolysis of organic compounds such as esters, reactions with Criegee intermediates that form during ozonolysis of unsaturated organics, and the nitrogen dioxide disproportionation reaction that forms nitric and nitrous acids.
Important progress has been made in the past few years to develop models that integrate the growing knowledge of chemical transformations, partitioning between different indoor reservoirs, mass transfer, and indoor-outdoor air exchange. However, these models remain limited in their predictive capabilities owing to uncertainties in the underlying fundamental chemistry, especially on surfaces. Elucidation of the role of occupants on indoor chemistry remains a research need. Addressing these data gaps may require the application of advanced instrumentation and analytical techniques to study chemistry taking place in buildings and on surfaces.
MANAGEMENT OF CHEMICALS IN INDOOR ENVIRONMENTS
Effective management of chemicals in the indoor environment is critical to human health. The management of chemical contaminants in indoor environments includes removal (through ventilation, filtration, sorption, physical cleaning, and passive surface removal) and chemical transformations (including photolysis, ionizers, chemical additions, photocatalysis). No single management approach can remove all contaminants that are present indoors; therefore, source elimination is always the preferred method of control. However, combinations of management approaches can also be effective at reducing exposure, as can situation-specific choices, such as increasing ventilation to reduce air contaminant exposure. Every management approach has different chemical
consequences; for example, approaches that include oxidation are particularly prone to generating products of concern.
Several knowledge gaps remain in the scientific community’s understanding of underlying physical and chemical principles of air cleaning, including the fundamental chemistry of many air-cleaning technologies. Except for ventilation, particle filtration, and sorption, few air-cleaning approaches have been tested in real-world environments, which contain a far more complicated mixture of compounds than most laboratories. Chemical reactions in indoor environments can follow complex mechanisms and result in numerous products. This makes predicting chemical reactions and the efficacy of air-cleaning devices challenging and highlights the need for better testing standards for air-cleaning efficacy and chemistry that account for this complexity. There is insufficient research to truly understand and prioritize chemical byproducts in terms of toxicology and health effects, and to identify safe and effective levels of chemical additives for air-cleaning technologies. The committee also identified real-world testing of management approaches that have potential to induce chemistry as a future research need.
Given the recent increased public interest in indoor air quality, driven in part by COVID-19, device manufacturers, researchers, and public health professionals need to communicate clearly to consumers about the efficacy and chemical consequences of different air-cleaning approaches. The lack of testing and regulation has led to rampant unsubstantiated claims about efficacy and health benefits of devices. The potential health risks and benefits resulting from their use warrant further investigation and potential certification or regulatory oversight. Based on the current state of knowledge, the committee cautions against approaches that induce secondary chemistry in occupied settings, unless the benefits demonstrably outweigh the risks of exposure to chemical reactants and byproducts.
INDOOR CHEMISTRY AND EXPOSURE
To date, the foremost goal of exposure science has been to identify and characterize the inhalation, ingestion, and dermal uptake by people of harmful chemicals that can cause acute or chronic health effects. The application of exposure science to the study of indoor environments and exposures that occur therein is relatively nascent but rapidly evolving. Cost-effective policies and guidance suitable for diverse indoor environments and indoor-dwelling populations demand a thorough understanding of indoor exposure profiles. Understanding large differences in indoor exposures requires deeper insight on the societal and systemic context in which exposures occur in residential and nonresidential environments. Environmental health disparities that are persistently observed in the United States and around the world too often remain understudied.
The evidence base and toolkits for developing a robust and comprehensive understanding of indoor exposure profiles are growing rapidly. This evidence base has grown through multiple research channels, including field-based, laboratory-based, and modeling studies. Among field-based studies, emergent tools are addressing long-standing challenges of assessing spatial and temporal resolution on concentrations of airborne hazards, as well as diversity of chemical species in indoor air. Consumer-grade measurement tools and research-grade, high-resolution instrumentation are achieving wider use in indoor environments.
Researchers are working to understand exposure to chemical mixtures. These efforts complement strategic priorities of federal agencies, such as the National Institutes of Health. For example, the National Institute of Environmental Health Sciences has identified strengthening understanding of combined exposures as a strategic priority. Measurement science advances applied to indoor environments and personal sampling are helping to better understand discrepancies—for example, between personal exposures and stationary monitors or indoor and outdoor area concentrations. Yet, inconsistency in chemical identifiers remains a challenge. Exposure data are collected across diverse
sampling platforms, ranging from transient, short-duration exposures to chronic and longitudinal exposures, among populations that vary greatly in size and participant composition. This leads to diverse data that are not standardized and therefore not readily available to support modeling efforts.
One of the most important and fundamental needs for improving the utility of exposure models is to form linkages between physical process and exposure and uptake models. Integrating frameworks can lead to an improved understanding of the relationship among indoor air chemistry, exposure, and internal dose. Exposure model improvements will rest upon an enhanced understanding of indoor exposures to chemicals and a deeper understanding of human behavior as it relates to indoor chemistry.
A PATH FORWARD FOR INDOOR CHEMISTRY
This report focuses on different aspects of indoor chemistry, including new findings related to under-reported 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. 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. A critical cornerstone of the committee’s vision for the future of indoor chemistry research is increased awareness on the part of 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 critical to translate the emerging science on indoor chemistry into practice that benefits public health and the environment.
Chemical Complexity in the Indoor Environment
An emerging theme in indoor chemistry is the high degree of chemical complexity in indoor environments. People are often in close proximity to sources and processes that, respectively, emit and transform chemicals. Environmental conditions and indoor chemistry 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, chemical emissions from cooking are common in restaurants and residences but are uncommon in other public buildings.
Despite the importance of indoor exposure, researchers know very little about how humans are exposed to multiple indoor 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 in contact with mixtures of chemicals with potentially synergistic or antagonistic modes of action and effects on health. Many of these mixtures are not chemically characterized or quantified, nor have their chemical transformations or partitioning between different indoor reservoirs been studied. Studies of exposure to mixtures in the indoor environment and their health effects are lacking, in part due to the complexity and dynamics of indoor chemistry.
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.
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
Unprecedented changes are occurring to the outdoor environment owing to climate change, wildfires, and urbanization, standing in contrast to improvements derived from environmental regulations and advancements in technology. These changes have impacts on indoor environments, many of which have yet to be fully characterized.
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.
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.
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
The emerging picture of indoor environments indicates chemical complexity in gas, particle, and surface phases. Although new analytical tools have been instrumental in improving understanding of indoor chemistry, several key challenges remain that will require strategic investments.
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.
Recommendation 11: Federal agencies should design and regularly implement an updated National Human Activity Pattern 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
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. The monumental effort during the COVID-19 pandemic to bring scientific tools to bear on means of mitigating its effects exemplifies what can happen when this connection is made. Making the same connection between science and application is essential in indoor chemistry.
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.
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.
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.
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.
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.
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 communities has focused on 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.
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 and contents of products and materials, and addressing 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.
