Spring 2023

Waiting to Exhale: The Impact of People on Indoor Air Quality

By Amy I. H. Hrdina, Ph.D.

In the wake of the COVID-19 pandemic and its stay-at-home orders, indoor air quality is receiving greater attention with the increased public awareness of what constitutes “clean air.”  Despite this recent attention, indoor air quality has been a research topic since the early 1980s.  In fact, the main motivation behind indoor air research that “many people spend large amounts of each day indoors…in many cases 80-90%” dates back to the 1981 report “Indoor Pollutants” by the National Research Council, Committee on Indoor Pollutants, which is still in use today (National Research Council, Committee on Indoor Pollutants, 1981).  In addition, the Federal Interagency Committee on Indoor Air Quality (CIAQ) was established in 1983, and the journal Indoor Air, created nearly a decade later, recently celebrated the 32^nd anniversary of its first issue.  Yet, The White House only recently held its first summit on Improving Indoor Air Quality in October 2022, as a response to post-pandemic public awareness.  In essence, as people began to understand how COVID-19 spread, the pandemic reinforced the importance of indoor air quality and its direct impact on human health.

Despite more than four decades of research, characterizing the quality of air in an indoor environment remains a highly complex challenge.  Potential sources of indoor air pollution (see Figure) generally fall into one of two categories.  The first category derives from human behavior, which can vary, such as smoking, cooking habits, owning pets and plants, and hygiene routines.  The second category is more difficult to pinpoint and mitigate because it relates to the structure and condition of the indoor environment itself, including emissions from building materials and building “leakiness,” which can lead to the infiltration of outdoor pollutants and the accumulation of pollutants generated indoors.  Further, the list of known indoor air pollution sources has not changed drastically over time; yet, it has become more refined, as the understanding of pollution sources and their underlying chemistry has improved.  For example, studies in the early 1980s discussed the indoor air impacts of radioactivity in general, emphasizing the need for more research on radon and its progeny (National Research Council, Committee on Indoor Pollutants, 1981).  Since then, the sources of radon are better understood along with the health risks associated with radon exposure; thus, radon is explicitly listed as an indoor pollutant instead of the generic term “radioactive sources.”  In addition, research on the health effects of tobacco smoke has matured from recognizing second-hand smoke exposure to the implications of third-hand smoke, in which deposited toxicants from indoor tobacco smoke persist long after smoking has occurred.

Source of Indoor Pollutants

In recent years, the field of indoor air sciences has expanded to include the complex chemistry of the indoor atmosphere.  For example, “[t]he Alfred P. Sloan Foundation has developed a multi-million dollar program…focused on understanding the fundamental chemistry taking place in indoor environments and how that chemistry is shaped by building attributes and human occupancy” (Indoor Chem, 2018).  This program, which began in 2018, has funded a variety of indoor air chemistry campaigns, including the Indoor Chemical Human Emissions and Reactivity project (ICHEAR), a series of intensive chamber experiments studying the impact human emissions (exhaled and emitted from skin) have on the chemical composition of the indoor air (Beko et al., 2020).  ICHEAR used an array of instrumentation developed for atmospheric chemistry, providing real-time, high-resolution measurements.  Among other findings, the study observed that increases in ozone (O₃) concentration indoors in hot and humid conditions (i.e., 37 parts per billion [ppb], which is below the National Ambient Air Quality Standard [NAAQS] of 70 ppb), doubled whole-body emission rates of volatile organic compounds (VOCs) (Wang et al., 2022).  Other recently funded projects include the Chemical Assessment of Surface and Air (CASA) experiment and the House Observations of Microbial and Environmental Chemistry (HOMEChem) study.

[T]he impact human beings have on their own indoor environment is so chemically diverse that it affects how each individual experiences the air quality around them, creating, in a sense, one’s own personal atmosphere.”

Ultimately, the impact human beings have on their own indoor environment is so chemically diverse that it affects how each individual experiences the air quality around them, creating, in a sense, one’s own personal atmosphere.  Along with behavioral choices that influence indoor activities, the use of personal care products, age, and diet can influence the composition of VOCs emitted from each individual (Williams et al., 2016).  Since the mixture of VOCs emitted from humans differs among individuals, so does the chemistry that occurs around each person.  Given this complexity, the research examining how the human body interacts with the indoor environment is still relatively novel, and recent studies have focused on general trends in human emissions.  For example, work published by Zannoni et al. (2022) observed the elaborate production of OH radical (a known oxidant in the atmosphere) when people were exposed to O₃.  Based on the compounds measured in the study, O₃ reacts with a natural oil VOC (squalene, which is used on the skin) to form an oxidized product that goes on to react with another molecule of O₃ to generate OH radical (Zannoni et al., 2022).  The extent of this chemistry all depends on the concentration of O₃ indoors, but it is an important step in understanding how people have a direct impact on the air quality around them.