August 2020

Taking a look at Indoor Air Quality while stuck inside

In 1989, the U.S. Environmental Protection Agency (EPA) submitted a report to Congress on indoor air quality, which found that people spend approximately 93% of their time indoors, 2% outdoors and 5% in transit (e.g., car, train, bus). After spending a significant part of this year under quarantine due to the COVID-19 pandemic, it’s likely that many of us have spent even more time than that indoors.

Between 1992–1994, the EPA sponsored a National Human Activity Pattern Survey (NHAPS) to obtain improved estimates of the time spent in various environments, including homes, restaurants, bars, automobiles and outdoors for the EPA to use as data inputs in human exposure models. The results from the NHAPS showed that humans spend approximately 87% of their time indoors, between 5–6% of their time in a vehicle and the remaining 7–8% of their time outdoors, but that is highly dependent on age and type of activity. The EPA uses this information as a basis for the Exposure Factors Handbook, which combines information on various physiological and behavioral factors commonly used in assessing exposure to environmental chemicals.

In light of the COVID-19 pandemic and the stay-at-home orders that were implemented in the U.S. to help flatten the curve, it’s likely that we’re spending more time indoors than what these two sources would suggest. In light of this, it seems reasonable that we would know more about how our everyday activities impact our indoor air quality. However, there’s much we don’t yet fully understand.

In the February 2020 edition of SPRAY, I wrote an article about The Home Observations of Microbial & Environmental Chemistry (HOMEChem) project. The HOMEChem project aimed to measure and record how everyday activities—such as cooking, cleaning and personal hygiene—influence emissions, chemical reactions and removal of trace gases and particles in indoor air. Scientists equipped a home with sensors to collect and record the indoor air quality as a result of these everyday activities.

There have been several new publications that have reported on the results of the HOMEChem project since our last column on this subject. The first looked at indoor air during and after cleaning with bleach, measuring several chlorinated and nitrogenated compounds as a result of the activity. Solutions of bleach, which were prepared according to the directions on the label, were used to mop the floors of the research home. The measurements found that several reactions occur after the bleach solution is applied, and the authors speculated that the amount of some of the substances that were generated (in the parts per billion) could be a health concern, especially with prolonged exposure. However, the research notes that proper ventilation helps avoid concentrations of these substances that may cause concern and, as such, exposure can be avoided. Beyond good ventilation, it’s critical that consumers follow label directions to properly and safely use a product.

The second publication explored several gaseous substances that occur from cooking, cleaning and personal hygiene cleanliness and how they interact with the various surfaces found in homes, such as walls, flooring and furniture. While ventilation controls the removal process for substances in indoor air, there are gaseous molecules that can interact with materials at faster rates than the air exchange. According to the data, substances that are fully volatile under typical outdoor conditions may show less volatility indoors and, as a result, deposit on the available surface area. This experiment needs to be explored further to better understand how different substances interact with various surfaces.

The third publication analyzed the behavior of semi-volatile organic compounds (SVOCs) in the research home. SVOCs are organic substances that can have meaningful abundances in both the gas and condensed phases. The transition between the gas phase and when SVOCs are deposited on surfaces is dependent on a number of factors, including volatility, surface temperatures and airborne particle concentrations. While the data suggests that the behavior of a specific SVOC can be predicted if the volatility of the substance is known, more research is needed to determine if the volatility can be predicted across multiple types of SVOC substances.

The most recent publication investigated particulate matter (PM) within the home. The data showed that cooking is the largest source of indoor PM. Other activities, such as cleaning, do produce PMs of a larger size, which could be attributed to certain human movements (such as dust resuspension, shedding skin cells or emission of clothing fibers), rather than the cleaning substances themselves.

There are many studies of indoor air quality that have either already been conducted or are ongoing. These studies routinely show that there are a number of factors that can impact indoor air quality, including the intended use of the room, the actions that occur within the room, the inhabitants’ lifestyle and social status, the presence of equipment and store items and the intensity of the air exchange rate. All of these factors need to be considered when examining indoor air quality.

As we spend more time indoors, it becomes even more important to understand the quality of our indoor air. If you’re interested in learning more, I encourage you to get involved with HCPA’s Scientific Affairs Council. If you would like to discuss indoor air quality further, please contact me at [email protected]SPRAY


1)       U.S. Environmental Protection Agency. 1989. Report to Congress on indoor air quality: Volume 2. EPA/400/1-89/001C. Washington, DC. Can be found here.

(2)       Klepeis, N.E. et al. The National Human Activity Pattern Survey (NHAPS): A Resource for Assessing Exposure to Environmental Pollutants. Lawrence Berkeley National Laboratory. 2001. Can be found here.  

(3)       Link: EPA

(4)       Georges, Nicholas. HOMEChem for the Holidays. SPRAY Technology & Marketing, February 2020, p. 11.

(5)       James M. Mattila et al. Multiphase Chemistry Controls Inorganic Chlorinated and Nitrogenated Compounds in Indoor Air During Bleaching Cleaning. Environmental Science & Technology, 54(3), 2020.

(6)       Chen Wang et al. Surface Reservoirs Dominate Dynamic Gas-Surface Partitioning of Many Indoor Air Constituents. Science Advances, 6(8), 2020.

(7)       David Lunderberg et al. Surface Emissions Modulate Indoor SVOC Concentrations Through Volatility Dependent Partitioning. Environmental Science & Technology, 54, 2020.

(8)       C.J. Weschler and W.W. Nazaroff. Semivolatile Organic Compounds in Indoor Environments. Atmospheric Environment, 42, 2008.

(9)       Sameer Patel et al. Indoor Particulate Matter during HOMEChem: Concentrations, Size, Distributions, and Exposures. Environmental Science & Technology, 54, 2020.

(10)     Mariusz Marc et al. Indoor Air Quality of Everyday Use Spaces Dedicated to Specific Purposes – A Review. Environmental Science and Pollution Research, 25, 2018.