Indoor Air Quality Research

Americans spend about 90 percent of their time indoors, according to the EPA's National Human Activity Pattern Survey. Indoor air pollution concentrations are often two to five times higher than outdoor levels, and during certain activities (cooking, cleaning, smoke events with windows closed) can spike much higher. Indoor air quality (IAQ) research is older than outdoor air pollution epidemiology in some ways, but the modern field has been reshaped by tighter buildings, gas appliances, wildfire smoke infiltration, and the COVID-era rediscovery of ventilation.

Background

EPA established its Indoor Air Quality program after the Radon Gas and Indoor Air Quality Research Act of 1986. Lawrence Berkeley National Lab's Indoor Environment Group, led for decades by Bill Fisk and now by Brett Singer, has produced much of the foundational chamber and field-study data. The WHO's 2010 Guidelines for Indoor Air Quality cover specific pollutants (benzene, CO, formaldehyde, NO2, naphthalene, PAHs, radon, trichloroethylene, tetrachloroethylene) and dampness/mold.

What the studies found

Gas stoves emit NO2 and benzene at meaningful levels. A 2022 study by Eric Lebel and colleagues at Stanford and PSE Healthy Energy, published in Environmental Science and Technology, measured 53 California homes and found gas stoves leak methane even when off and emit NO2 that pushes kitchen levels above the WHO 1-hour guideline of 200 micrograms per cubic meter during normal cooking. A 2023 follow-up found gas and propane burners emit benzene at 10 to 25 times the rate of electric coil or radiant elements, raising indoor benzene in some homes above health benchmarks. A 2024 Stanford paper extended NO2 measurements nationwide and estimated that long-term NO2 exposure from gas stoves contributes to roughly 200,000 cases of childhood asthma in the U.S.

Wildfire smoke gets indoors. Research by Brett Singer (LBNL), Wayne Cascio (EPA), and others using portable HEPA monitors during the 2018 California Camp Fire and the 2020 Oregon fires showed indoor PM2.5 in typical homes runs 30 to 80 percent of outdoor levels during smoke events, depending on tightness and HVAC use. Tight, mechanically ventilated buildings with MERV-13 filters or portable HEPA units saw indoor levels drop to 10 to 20 percent of outdoor.

Radon is the second-leading cause of lung cancer in the U.S. EPA estimates radon exposure contributes to roughly 21,000 lung cancer deaths per year, second only to smoking. The risk is geographic: parts of the upper Midwest, Appalachia, and the Mountain West sit on uranium-bearing geology that pushes radon into basements.

Volatile organic compounds (VOCs) from cleaning, paints, and personal care products. Studies led by Jessica Gilman and Brian McDonald at NOAA (Science 2018) found that volatile chemical products now rival vehicle emissions as a source of urban VOCs. Indoor concentrations are typically much higher than outdoor, especially during and after cleaning.

Ventilation drives almost everything. ASHRAE Standard 62.2 sets minimum residential ventilation rates (currently 0.35 air changes per hour plus a kitchen and bathroom exhaust requirement), but enforcement is weak and many U.S. homes fall below it. The COVID pandemic prompted a rewrite of ASHRAE 241 (Control of Infectious Aerosols, 2023), which specifies equivalent clean air delivery for occupied spaces.

Why it matters

Outdoor air quality is regulated; indoor air mostly is not. Outside of workplace OSHA limits and a handful of state programs (California's CARB indoor air cleaner certification, Washington and California school ventilation rules), indoor exposure is the homeowner's or building operator's problem. Yet indoor exposure dominates total inhaled dose for most pollutants.

Building electrification (heat pumps, induction stoves) is now framed as both a climate and an IAQ play. Massachusetts, New York, and several California cities have moved toward all-electric new construction citing IAQ benefits.

Open questions

The combined effect of low-level chronic exposure to multiple indoor pollutants (formaldehyde from engineered wood, NO2 from cooking, VOCs from products, PM2.5 from infiltration) is poorly quantified compared to single-pollutant outdoor work. Research on filtration in schools (the 2020 Beall school AC unit study, Park et al. 2023) suggests classroom HEPA filtration measurably improves test scores and reduces absenteeism, but the dose-response is still being built. Long-term studies on chronic mold and dampness exposure remain methodologically hard.

Sources

• The Inside Story: A Guide to Indoor Air Quality (EPA)
• Methane and NOx Emissions from Natural Gas Stoves, Cooktops, and Ovens in Residential Homes (Lebel et al., ES&T 2022)
• Gas and Propane Combustion from Stoves Emits Benzene and Increases Indoor Air Pollution (Kashtan et al., ES&T 2023)
• Gas stoves emit unsafe levels of nitrogen dioxide (Stanford 2024)
• Volatile chemical products emerging as largest petrochemical source of urban organic emissions (McDonald et al., Science 2018)
• WHO Guidelines for Indoor Air Quality: Selected Pollutants (2010)
• EPA Radon and lung cancer risk
• ASHRAE Standard 241, Control of Infectious Aerosols (2023)