Is the air inside your home more dangerous than the air outside it?

For millions of Americans, staying indoors offers little protection from dangerous air pollution, according to a new Stanford University-led study. The paper, published Dec. 2 in PNAS Nexus, reveals that gas and propane stoves expose people to substantial amounts of nitrogen dioxide, a pollutant linked to health problems that include asthma, obstructive pulmonary disease, preterm birth, diabetes, and lung cancer. Replacing gas stoves with electric reduces nitrogen dioxide exposure by over a quarter on average across the U.S. and by half for the heaviest stove users, according to the findings. Previous studies have measured nitrogen dioxide pollution from gas stoves, but this is the first study to measure exposure to nitrogen dioxide outdoors and indoors nationally.

“We know that outdoor air pollution harms our health, but we assume our indoor air is safe.” said study senior author Rob Jackson, the Michelle and Kevin Douglas Provostial Professor in Earth System Science at the Stanford Doerr School of Sustainability. “Our research shows that if you use a gas stove, you’re often breathing as much nitrogen dioxide pollution indoors from your stove as you are from all outdoor sources combined.”

Every year, outdoor air pollution kills hundreds of thousands of Americans and causes millions of new cases of childhood asthma globally. While the U.S. Clean Air Act and other regulatory efforts have helped curb the threat, few regulations apply to indoor pollution, which can be just as dangerous. The study is the first comprehensive national assessment of exposure to nitrogen dioxide, a key pollutant, from both indoor and outdoor sources, such as fossil fuel-burning vehicles, electricity generation, and gas stoves.

A 2024 study of air pollution in U.S. homes by the same researchers found that gas stoves emit unsafe levels of nitrogen dioxide that linger in the air for hours after burners and ovens are turned off.

Additional research by some of the same authors has shown that gas stoves also emit dangerous levels of benzene, a carcinogen linked to leukemia and other blood-related cancers.

“It’s time to redirect our focus to what’s happening inside our homes, especially as families spend more time indoors,” said study lead author Yannai Kashtan, an air quality scientist at PSE Healthy Energy who was a graduate student in Jackson’s lab while doing research for the study.

Prioritizing indoor air quality

The researchers combined measurements of indoor air quality with data for outdoor air quality, building characteristics for 133 million residential dwellings, and statistical samplings of occupant behavior. They were able to paint a clear picture of where pollutants come from and the effects on human health. They also created U.S.-wide maps that quantify long- and short-term exposure to nitrogen dioxide indoors and outdoors by zip code.

For most Americans, most exposure to nitrogen dioxide still comes from outdoor sources like cars and trucks burning fossil fuels. However, the maps revealed that for 22 million Americans – especially those living in smaller homes and in rural areas––cooking with gas leads to nitrogen dioxide levels that surpass recommended long-term safety thresholds when outdoor exposure alone would not. While stoves are proportionally responsible for the most nitrogen dioxide exposure in rural areas, total exposures were highest in large cities, where outdoor levels of nitrogen dioxide tend to be high and living spaces tend to be small, thus concentrating nitrogen dioxide produced by stoves.

The researchers also found that the highest short-term exposures in the home are all attributable to gas stove usage rather than to outdoor sources because gas stove pollution comes in concentrated bursts. 

Communities stand to benefit from interventions, such as rebates and tax incentives, that encourage cleaner cooking technologies and reduce exposure to harmful indoor pollutants. Benefits may be strongest for people living smaller homes, in rented units where landlords may not otherwise have an incentive to install electric stoves, and in communities where few families can afford the upfront cost of a new electric stove. A previous Stanford-led study showed that long-term NO₂ exposure is 60% higher among American Indian and Alaska Native households, and 20% higher among Black and Hispanic or Latino households compared to the national average. Many of these communities already face higher levels of nitrogen dioxide pollution from outdoor sources, such as vehicle exhaust and fossil industries.

"As we strive for cleaner air and healthier living, we should prioritize indoor air quality,” Jackson said. “Switching to electric stoves is a positive step towards cleaner cooking and better health."

Jackson is also a senior fellow at the Stanford Woods Institute for the Environment and the Precourt Institute for Energy. 

Other coauthors of the study include Chenghao Wang of the University of Oklahoma; and Kari Nadeau of the Harvard T.H. Chan School of Public Health.

The study was funded by the Stanford Doerr School of Sustainability and its Department of Earth System Science, and Stanford’s Knight-Hennessy Scholars Program.