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The U.S. goes through a lot of natural gas, consuming more than 30 trillion cubic feet of it last year, according to the U.S. Energy Information Administration—about 15% of which is burned by home appliances. On top of climate concerns—natural gas may be less carbon polluting than coal, but the methane released during production is a powerful greenhouse gas in its own right—there is a growing awareness of the risk this gas poses to our health.

According to a new study published today in the journal Environmental Science and Technology, not only the atmosphere, but your kitchen may be a victim of contamination, with low levels of gas leaking from stoves even when they are turned off. More than a third of homes in the U.S. have gas stoves, and these leaks are exposing people to a range of toxic chemicals, including toluene, hexene, xylenes, and especially benzene—a pollutant that has been linked to anemia, reproductive disorders, and various forms of cancer.

The study adds to a mounting body of research examining the environmental and health impact of gas stoves. Another study released in January found that the methane leaking from natural gas stoves in the U.S. is equal to the emissions released by half-a-million gasoline-powered cars every year. Today’s study drills deeper into the health impacts.

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“We found that the emissions from a gas stove while it’s off can produce, in some cases, concentrations of benzene in your house that are equivalent to living with a smoker,” said Eric Lebel, an environmental scientist at the policy institute PSE Healthy Energy and a coauthor of the new paper, at a press conference announcing the results. “For cancer, it’s pretty clearly stated by the World Health Organization (WHO) that there’s no safe level of benzene exposure.”

How big a health risk are gas-stove benzene leaks?

In the new study, the researchers did not venture a guess as to just how serious a health risk the benzene leaks pose. But the U.S. Centers for Disease Control and Prevention (CDC), like the WHO, warns that benzene does all manner of damage to the body. In acute cases, it can affect the central nervous system, leading to headache, dizziness, impaired gait, and nausea. Systemically, it can interfere with blood cell production, potentially leading to both anemia and leukemia. It can also cause eye inflammation, visual blurring, heart arrhythmia, respiratory inflammation, and damage to the immune system.

When the results of yet another study, in June, of gas leaks from stoves in Boston homes were published, the American Gas Association issued a statement arguing that, “while combustion emissions from gas ranges, ovens, and cooktops can contribute to some degree to emissions of recognized pollutants, there are no documented risks to respiratory health from natural gas stoves from the regulatory and advisory agencies and organizations responsible for protecting residential consumer health and safety.” The organization added that the amounts of benzene that leak from stoves are “below conservative health-based screening levels” and would be “only a tiny fraction of the typical background levels of benzene in outdoor and indoor air.”

While the group is a trade organization and thus has skin in the game, the warnings the CDC issues do point out that the greatest danger comes from higher levels of exposure, with concentrations great enough that a telltale sweet odor could be detected—which is not the case in the low-level, odorless emissions reported in the new paper.

What we know about benzene leaks in California

Lebel’s study involved sampling gas leakage from stoves in 159 homes across seven geographical regions in California, including the San Francisco Bay Area, San Diego, and Greater Los Angeles. The sample group cut across different socioeconomic neighborhoods and surveyed different brands and models of stoves and found that in every case, there were detectable levels of leaking benzene.

The researchers expected some leakage of unburned gas when the stoves are in use, especially the pulses of gas produced in the few seconds between the time a burner is turned on and the gas ignites. “Sometimes they go click, click, click while they’re sparking and there’ll be some gas that escapes then,” said Lebel. Still, an estimated 75% of the benzene released into the home is produced when the stove is not in use. And that 75% adds up. Extrapolated across the state as a whole, the benzene leakage from gas appliances is the equivalent of adding 60,000 passenger cars to the road—venting their exhaust directly into your kitchen.

Just which regions of the state are hit hardest by benzene emissions depends in part on which energy company pipes supplies to any one neighborhood. The three main companies that serve California are Pacific Gas and Electric, Southern California Gas, and San Diego Gas and Electric. Los Angeles and the North San Fernando Valley area—both served by Southern California Gas—had the highest level of benzene concentrations in the study. Over 22% of the samples from this supplier were above the median levels in homes supplied by Pacific Gas and Electric and San Diego Gas and Electric.

“We’re not sure exactly why this is,” says Lebel, though the findings had nothing to do with the type of stoves used in the higher-benzene homes. “We think it has something to do with where the gas is being sourced from. California has two major pipelines for importing gas: one coming from the Rockies, and one coming in from Canada.” Southern California Gas draws from both sources, and determining which of the two is dirtier when it comes to benzene emissions would require a finer analysis than the study conducted, looking at which towns or neighborhoods were getting Canadian gas and which were getting Rocky Mountain gas.

Read more: The Best Stove for Your Health and the Environment

California is not the only state in which a study of this kind has been conducted—but it is one of just two. Earlier this year, Drew Michanowicz, a senior scientist at PSE Healthy Energy and a co-author of the new paper, published the similar study of gas stoves in the Boston area and found dangerous leakage of benzene there too—but nowhere near on the scale of California’s.

“Even some of the lowest concentrations we detected in California are 10 times higher than the average we saw in Boston,” Michanowicz says. “Some of the highest concentrations that we found in California are about 66 times higher than the highest levels that we saw in Boston.” Again, the explanation may be the source of the natural gas, but that’s even harder to pin down in Boston than it is in California, because the companies serving the metropolitan area import their natural gas from a mishmash of sources, including Canada and Texas, as well as liquefied natural gas from Algeria and Trinidad.

What you can do to limit benzene in your kitchen

If stoves are leaking benzene in Boston and California, they are surely doing so everywhere else as well, and just what consumers can do to stay safe is limited. Opening windows and turning on range hoods while cooking certainly helps vent pulses of gas, but that does nothing for the vast majority of the time that stoves are not in use.

Sniffing for leaks does not do much either. Sulfur-based compounds known as mercaptans are added to natural gas to give it its signature rotten-egg odor, specifically to alert consumers to leaks, but the quantity of gas leaked when stoves are not in use typically falls below what most people can detect by nose. Any leak that does cross the smell threshold requires immediate action, since it can pose an explosion risk.

“​​You should immediately leave your house and call the gas company,” says Lebel. “It’s not good to be smelling gas in your house.”

Long-term solutions involve doing away with gas appliances altogether and switching to cleaner, safer electric models. In 2022, California environmental regulators approved a plan to phase out the sale of natural-gas appliances by 2030, in a bid to curb fossil fuel use overall. But that is just one state in a nation of 50—all of which would have to follow California’s lead, and most of which are not likely to. In the meantime, the benzene—invisibly, odorlessly—continues to flow.

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