Cooking at home can be healthier than fast food. However, gas and propane stoves may pose a danger by emitting high levels of nitrogen dioxide (NO2) according to a new study published in Science Advances.

The research was conducted by environmental scientist Rob Jackson and graduate student Yannai Kashtan at Stanford University. Jackson has previously shown that methane leaking from U.S. residential stoves is equal to the emissions from half a million cars per year.

Gas and propane stoves create NO2 when heat causes two oxygen atoms to combine with one nitrogen atom. Electric stoves do not get as hot and do not cause this reaction. NO2 irritates the airways, reduces lung function, and exacerbates coughing and wheezing according to the American Lung Association. Exposure can occur from stoves as well as and .

To study the severity of NO2 from stoves, Jackson and Kashtan placed sensors in over 100 homes. Variables considered included home size from 800 to over 3,000 square feet, use of ventilation hoods, number of burners or oven used, cooking time, window opening, and city air quality.

Findings showed that while kitchens saw first contamination, most other rooms were also affected within an hour. Ventilation hoods reduced NO2 only 10-70% depending on fan speed and suction of all burners. Hoods recycling indoor air did nothing to reduce molecular pollutants. Smaller homes and apartments saw up to four times more exposure than larger homes due to dose and time. Gases remained above health thresholds for hours after stove use.

On average, gas and propane stoves increased indoor NO2 by 4 ppb, using three-quarters of the World Health Organization’s 5.3 ppb limit before outdoor exposure. Minority and low-income groups facing dirtier outdoor air experienced twice as much chronic and three times as much acute NO2 exposure versus wealthier households.

Options to reduce exposure include electric burners, hood use, and opening windows during cooking. Risk is long-term and cumulative rather than negligible.