When we fire up a gas stove, we’re releasing a powerful climate pollutant into kitchens and beyond. But a new study found that this isn’t just happening when the stove is on. Even when turned off, a typical gas stove will send methane up to the atmosphere.

The new peer-reviewed study, published in the journal Environmental Science & Technology, helps answer a particular question that’s been nagging scientists for years. The puzzle has been accounting for all the sources of methane as concentrations in the atmosphere have risen to record levels. They know the natural gas industry, and specifically leaks from its pipelines, is the biggest contributor (natural gas is mostly methane). Other well-documented sources are livestock and landfills.

But there was a mystery when it came to urban environments: In one study in Boston, researchers noted that pipeline leaks couldn’t explain the high levels of methane emissions they detected. There had to be other leaks, most likely from gas-burning appliances inside homes.

So Stanford scientist Robert Jackson, one of the study’s coauthors, set out to track down this missing methane inside homes and buildings. And he was surprised at what his team found.

Basically all stoves “leak a bit when they’re burning,” Jackson said. “And they all leak a bit when you turn them on and off, because there’s a period of time before the flame kicks in. The most surprising was almost three-quarters of the methane that we found emitting from the stoves came from when they weren’t running.”

In other words, the gas stove, a feature of 40 million American homes, is likely always releasing a greenhouse gas. Gas stoves are still a relatively small source of methane compared to pipelines and refineries, and they aren’t even the biggest gas-guzzling appliance in buildings — gas furnaces and water heaters use much more of the fuel through the day and night. But the methane emissions from stoves are roughly equivalent to the carbon dioxide released by half a million gas-powered cars in a year, the researchers found.

Methane also contributes to ground-level ozone that harms human health. Inside the home, the level of methane is low enough that the researchers don’t consider leaks to be a health threat. The bigger health problem is when the gas is lit, because that produces nitrogen dioxide as a byproduct. (The Stanford study didn’t measure other pollutants associated with gas stoves, like formaldehyde, particulate matter, and carbon monoxide.)

The Stanford study joins a small but growing body of work showing how gas appliances in buildings are releasing more climate and air pollution than previously understood.

“This is a really important study,” said Maryann Sargent, a Harvard research scientist unaffiliated with the study who has published research on methane readings in cities. “[It] is one piece that says these stoves are actually a pretty significant emitter. It’s filling in this gap of unknown emissions.”

The new data doesn’t just help scientists piece together a better understanding of gas stoves’ impact on climate change. It bolsters climate advocates’ arguments that the natural gas system is too leaky to continue. It could even hasten the transition away from gas reliance in cities.

The gas stove is responsible for methane, carbon dioxide, and nitrogen oxides in the kitchen

To figure out what was going on in the typical American kitchen, Stanford’s scientists knew they would have to go to people’s homes to test a variety of stove models to get as close to real-world conditions as they could. They ended up collecting measurements inside 53 California homes, rentals, and Airbnbs (because of Covid-19 precautions), using instruments that measure wavelengths of light to determine gas concentrations.

After installing big plastic partitions between the kitchens and other rooms (to block out other potential leak sources), they set out to measure three major pollutants coming from gas stoves and sometimes ovens when the appliances were off, turned on, and switched back off.

The main pollutant the scientists were looking for was methane, because that’s primarily what’s in natural gas. Methane has the power to warm the climate more than 80 times in a few decades compared to carbon dioxide, and it’s the second-biggest contributor to climate change. The stove emits the most methane with the puff of gas when it first turns on, and again when it turns off. Overall, they found that up to 1.3 percent of the gas that stoves use is leaked.

In addition to methane, the scientists also looked at how the gas stoves, when turned on, released some carbon dioxide, the single-biggest contributor to warming, too.

The third pollutant they looked at does not contribute to climate change, but it can harm human health. Nitrogen oxides are produced as a byproduct of burning methane, and increase risks of cardiovascular problems and respiratory disease when inhaled.

The study found that the bigger the burner, the worse the nitrogen oxide emissions. But in a small kitchen it only took a few minutes of usage for stoves (without range hoods) to generate levels above national health standards. (A separate study found that children in homes with gas stoves are 42 percent more likely to have asthma than children whose families use electric stoves because of relatively high levels of nitrogen dioxide.)

Even when the burner is off, it emits a small level of methane that the scientists called “steady-state emissions.” Jackson told me he suspected these steady-state emissions were coming from ill-fitting connections and leaky pipes feeding into the stove, though the study didn’t look for the exact source. The scientists didn’t find a correlation between methane leakage and how old the stove was or the model. And since most people only use their stove an hour or less a day, this slow trickle of pollution when the stove is off ends up adding up to most of the methane found leaking from the kitchen.

But finding these leaks is not so easy. Multiplied by 40 million kitchens that run on gas in the US, study coauthor Jackson said, “that’s a lot of methane. That’s a lot of natural gas reaching our air. And a lot of nitrogen oxides entering our homes for people who have asthma and health conditions.”

The Stanford researchers just looked at 53 homes. To figure out what the impact was nationwide, they extrapolated by using national survey data on how Americans typically cook.

The measurements from stoves alone suggest the Environmental Protection Agency is severely undercounting climate pollution coming from all residences, not to mention likely leaks coming from other gas appliances like furnaces, fireplaces, and water heaters.

Cities are starting to move to phase out the gas stove. And the gas industry is fighting back.

We don’t have to fix every leaky stove to eliminate this source of emissions because there’s already a better solution: Replace these stoves with ones that run on electricity. The best contender to replace the gas stove is the induction stove, which uses a magnetic field to heat pans.

But this transition to electric-powered cooking won’t happen without city, state, and even federal policy.

City councils have started looking at phasing out gas in new buildings across the country. In December 2021, New York became the largest city yet to begin to transition off of gas, banning hookups to gas in construction by the end of 2023. Now some states, including New York and California, are considering similar measures. These policies do not yet address how to transition away from the nation’s existing gas infrastructure, a much thornier topic that deals with retrofitting homes and remodels.

Though the gas industry is facing greater scrutiny of all of its operations, the stove is an important part of its strategy to beat back city climate action. The industry is counting on a steady stream of new customers for decades to come, and Americans’ devotion to the stove is its best hope to lock in new gas infrastructure. In the past, I’ve reported on the industry’s battle to stall city efforts banning gas hookups. In one case, the industry hired social media influencers to tout the benefits of cooking with gas. In other cases, gas lobby groups have worked to pass state laws that prevent cities from changing their building codes to discourage gas.

Not everyone agrees that the newly identified methane leaks from stoves are a significant factor in climate change. University of Illinois engineer Zachary Merrin, who was not affiliated with the Stanford study, noted these emissions overall are relatively small. Merrin did say the study yielded similar results as his own research, which looked at a stove while it was running, not when it was turned off. “This is very reassuring regarding the accuracy of the measurements,” he said over email. But, he added, “in my opinion, ultimately these emissions are small, and addressing them would be difficult.”

Merrin pointed out that some relatively larger leaks are coming from a small number of the stoves they measure: The Stanford study found that less than 10 percent of sites accounted for nearly half of the leaks, and those could be easily dealt with, he said.

“Stoves resonate with people,” Stanford’s Jackson said. “It is the only fossil fuel that we use where you’re standing right over the flame breathing everything that the stoves are emitting from their flame and from pipelines.” The advantage to electric or induction cooking is two-fold: “It not only cuts greenhouse gases, it makes indoor air safer to breathe.”