A new study discovered that gas stoves contribute more to global warming than previously thought due to constant tiny methane leaks while they are turned off.

Because of the levels of nitrogen oxides measured in the same study that tested emissions around stoves in homes, new concerns about indoor air quality and health were raised.

Even when not in use, gas stoves in the United States emit 2.6 million tons (2.4 million metric tons) of methane — in carbon dioxide equivalent units — into the atmosphere each year, according to a study published Thursday in the journal Environmental Science & Technology. That is the annual amount of greenhouse gases produced by 500,000 cars, or what the United States emits into the atmosphere every three and a half hours.

“They’re constantly bleeding a little bit of methane into the atmosphere all the time,” said co-author and Stanford University climate scientist Rob Jackson.

According to the study, this methane is in addition to the 6.8 million tons (6.2 million metric tons) of carbon dioxide that gas stoves emit into the air when they are in use and the gas is burned. Methane is a greenhouse gas that is dozens of times more potent than carbon dioxide but does not last nearly as long in the atmosphere and is not as abundant in the atmosphere.

The researchers examined 53 home kitchens in California, many of which were rented from bed and breakfasts. They covered most of the rooms in plastic tarps and then measured emissions when the stoves were turned on and off. And what was surprising was that three-quarters of the methane released occurred while the stoves were turned off, according to Jackson. He claims that the government does not account for these emissions releases.

“That’s significant because we’re attempting to reduce our carbon footprint and we claim that gas is cleaner than coal, which it is,” said study lead author Eric Lebel, a scientist at PSE Healthy Energy, an Oakland nonprofit. However, he claims that when leaks are factored in, much of the benefit is lost. Many communities, including New York City and the Bay Area cities of San Francisco, Oakland, San Jose, and Berkeley, have bans on the use of gas stoves in new construction that will go into effect in the coming years, according to Jackson.

“If people want, they can already choose electric appliances,” said Frank Maisano, a Washington policy and public relations expert who represents gas and appliance interests. “People prefer gas appliances because they are more efficient, especially in colder climates.”

“In general, natural gas appliances are more energy- and cost-effective than electric counterparts,” Maisano said.

When all natural gas use and extraction are considered, Jackson estimates that approximately 100 million tons (91 million metric tons) of gas leaks into the atmosphere. And the couple million tons from gas stoves are “significant.” That’s a significant part, and it’s one we haven’t accurately included in the past.”

According to Zachary Merrin, a research engineer with the Illinois Applied Research Institute’s Indoor Climate Research & Training group, the leakage finding is “a very important takeaway” and fits with other work that found there are often large leaks that account for a large portion of the emissions.

Merrin, who was not involved in the study, stated that the emission of unburned methane is “clearly bad.” Cooking directly with gas reduces emissions more than a fossil fuel-powered electric stove but more than a solar-powered electric stove.”

According to Jackson, the methane leak is not hazardous to human health or a potential explosive. However, when the tests were carried out, the researchers discovered high levels of nitrogen oxides, greater than 100 parts per billion. The EPA does not have indoor air quality standards for that gas, according to Jackson, but the measurements they took exceed its outdoor air quality standards. While methane does not contain nitrogen, nitrogen oxides are byproducts of natural gas oven combustion, he explained.

According to Maisano, people should always use hood ranges and ensure proper ventilation. Jackson, who owns a gas stove that he intends to replace, stated that he had never used ventilation prior to this study, but that he now does so every time.