(Bloomberg) -- The science is overwhelmingly clear: Gas stoves are a source of air pollution located inside your home. They can emit climate pollution, as well as pollution linked to asthma and other health problems. They can release pollution when turned on, and even when they’re off. In some cases, the exposure to pollution is low; in others, the exposure rivals what can occur outside, such as from traffic on busy intersections and highways.

While the scientific studies on this subject span continents and decades, the conventional wisdom is that these appliances, found in about a third of US households, are perfectly safe. “Gas stoves have gotten pretty much a free pass as a source of air pollution in homes,” says Drew Michanowicz, senior scientist at the nonprofit research institute PSE Healthy Energy. 

It’s a disconnect driven in part by successful marketing from the gas industry, which pitches this fossil fuel as a “natural” product and sells Americans on gas stoves as the best for cooking — through television ads, product placement, social media influencers and even by crashing Nextdoor neighborhood chats.

That PR campaign is up against very little public awareness about the risks and sources of indoor air pollution. “I think a lot of people, when they think about pollution they immediately think about big industrial facilities or major highways or power plants,” says Jonathan Levy, an environmental health professor at Boston University. “Fewer people think about what are the sources in our homes or our office buildings or wherever we spend most of our time during the day.”

But here’s the simple truth, says Michanowicz: “If you have an open flame, you’re going to have pollution.”

As state and local officials, and perhaps even federal agencies, discuss banning new natural gas infrastructure in homes and other buildings, here’s a roundup of the science helping inform those decisions.

The climate impact

One form of pollution from gas stoves is methane, the main component of natural gas. Although methane does not pose a direct threat to human health, it is a greenhouse gas 80 times more potent than carbon dioxide during its first 20 years in the atmosphere. 

When researchers measured emissions in 53 stoves across California, they found that all but one of them leaked methane when the burners and oven were turned off, according to a 2022 study published in the journal Environmental Science and Technology. In fact, most of the methane emissions were observed when the stove was turned off, according to study author Eric Lebel. From there, the researchers estimated the climate impact of gas stoves emitting methane was equivalent to the annual carbon dioxide emissions of roughly half a million gas-powered cars on the road.

Gas stoves and asthma 

Lebel and his colleagues also measured stove-top emission rates of nitrogen dioxide (NO2), a byproduct of burning gas. “What we found was that the rates of NO2 emissions were proportional to the amount of gas being burned, meaning that if you use a bigger burner, more than one burner at once, or turn a burner higher, you will get higher rates of emissions,” says Lebel, who worked on this research as a PhD candidate at Stanford and is now a senior scientist at PSE Healthy Energy. In certain cases, especially if the kitchen is small and there’s poor ventilation, the resulting nitrogen dioxide concentrations could exceed the Environmental Protection Agency’s standard for safe one-hour outdoor NO2 exposure, Lebel explains. There is no health standard for indoor nitrogen dioxide emissions.

Another 2005 study by Levy and colleagues looking at public housing developments in the Boston area found that the highest concentrations of nitrogen dioxide inside apartments were often measured inside the kitchen, and that those concentrations were consistently higher than what was measured outside. Gas stoves were located in every participating apartment unit. 

Public health studies (unrelated to stoves) have found that nitrogen dioxide exposure can cause asthma and other breathing issues, as well as exacerbate asthma in people with existing diagnoses. A 2013 analysis of what was then the latest research worldwide suggested that cooking with gas in homes was linked to a 42% increased risk of having current asthma and a 24% higher risk of having asthma for life. This correlation was statistically significant, but the study did not find any meaningful increase in kids with asthma in homes with higher nitrogen dioxide levels. Its authors warned that the finding “should be interpreted with caution” because of limited data.

Multiple studies published more recently show a link between outdoor nitrogen dioxide exposure and increased asthma risk in children, says Bert Brunekreef, emeritus professor of environmental epidemiology at Utrecht University in the Netherlands, and a co-author of the 2013 study. “There’s reason to be suspicious of what’s happening indoors,” he says. “Nitrogen dioxide is nitrogen dioxide.”

A study published just last month in the International Journal of Environmental Research and Public Health analyzed past science on gas stoves, including the 2013 study, and used statistical modeling to estimate the proportion of childhood asthma related to gas stove use in the US. That team of researchers’ best estimate is that 12.7% of current childhood asthma nationwide is tied to gas stove use, with some uncertainty ranging from 6.3% to 19.3%. The link is estimated to be even higher in states with higher gas stove use, such as California, Illinois, Massachusetts and New York. 

The study does not say that gas stoves are causing this level of asthma, author Brady Seals stresses, but that the connection is very strong. The important takeaway is “to know it’s a risk and risk that can be mitigated and prevented — the intention is not to be alarmist,” says Seals, manager of the Carbon-Free Buildings program at green-energy nonprofit RMI.In a statement, gas trade group the American Gas Association described the most recent study as “advancing bad science” and “recklessly creating fear for homeowners without cause.” The group also pointed to a study that found no difference in the level of particulate pollution between electric and gas stoves. While a separate 2001 study, funded by the California Environmental Protection Agency’s Air Resources Board, found that both electric and gas stoves can produce particulate pollution, the researchers noted that gas cooking could generate nitrogen dioxide levels that exceeded their indoor air quality guideline.

There’s more

While nitrogen dioxide is the main gas connected to the health risk of gas stoves, it’s not the only one. Other studies have shown stoves can release carbon monoxide, which can be life-threatening at high levels, and a suite of known carcinogens and other hazardous chemicals called volatile organic compounds, such as benzene, hexane and toluene. A 2022 study in Boston tested for non-methane gases coming directly out of gas stoves and found more than 200 other compounds. Some of the most toxic ones, such as benzene, were found in more than 90% of samples. The same study also found that there can be low levels of gas leakage, too small to be detected by the nose.“We can’t really say what the actual exposures are” to these other chemicals and estimate their health impacts, says Michanowicz, an author of the 2022 study. But the research advances understanding of what’s in the gas piped directly into homes. “The natural gas stream is more than just methane,” he says. “There are other air pollutants in there that we really don’t want to be breathing in.”Despite the many possible risks tied to gas stoves, “not everyone’s exposure is the same and we can take conscious steps to reduce our exposure,” Levy says. Smaller homes, or homes with small or enclosed kitchens, tend to have higher exposure. Opening windows or using working range hoods can help, especially if those hoods vent outside, he says. And those with the ability and funds can cut their risk to zero by switching to an electric alternative.

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