Stoves and ovens in Colorado running on natural gas produce toxic benzene at levels worse than secondhand tobacco smoke and comparable to notorious oil and gas production leaks, according to a new Stanford University study researchers call the first of its kind.
Benzene produced while gas appliances are in use can linger for hours if not properly vented to the outdoors, and drifts to bedrooms and other parts of the house, the researchers said, after they tested dozens of homes in California and Colorado. The scientists chose Colorado in part because of high oil and gas production that could contribute to higher “background” benzene that researchers need to know for comparison purposes, Stanford officials said in a media briefing.
But they also chose Colorado for more prosaic reasons: Their equipment is heavy and it’s an easier road trip from California than, say, Rhode Island.
The researchers said they are not encouraging all families with gas stoves to sound alarms and worry over decades of past exposure to the benzene emissions apparently common in many homes. But they did include medical commentary from a retired oncologist who underlined just how toxic benzene is in the human body. Benzene can cause cancer, and a safe level has never been set.
The researchers also urged cooks to check their venting. Even venting kitchen fumes to the outdoors may not completely eliminate benzene dangers, but short of switching immediately to electric induction cooktops, it helps, they said.
“The idea is obviously not to cause panic,” said Dr. Jan Kirsch, a retired hematologist and oncologist in occupational and environmental medicine, who led a childhood leukemia study for the University of California Berkeley. “The idea is there are risks, and we want to reduce them.”
One piece of good news about benzene, a component of fossil fuels, is that it leaves the body and stops doing damage when exposure is cut off, Kirsch said.
Families suddenly worried they’ve exposed children to decades of gas stove cooking should be relieved to know that other outdoors sources of benzene have dropped over the years, said Yannai Kashtan, a lead researcher on the study and a graduate student in Stanford’s Earth system science division at the Doerr School of Sustainability.
“And that is a happy story,” Kashtan said.
Catalytic converters in cars, and the increasing pace of electrification of transportation, have made a difference. “Other pollution control measures have actually done a really good job of reducing those outdoor benzene levels,” said Kashtan, who added he was nevertheless happy to realize the stove in his rented apartment is electric.
Colorado’s environmental groups are likely to use Stanford’s results to accelerate efforts to electrify home appliances and trade out stoves, furnaces and water heaters that burn natural gas. Vent hoods on gas stoves and ovens are not enough, said Danny Katz of the consumer nonprofit CoPIRG.
“Gas stoves should never be sold without a hood, but many people don’t know that and don’t have one,” Katz said. “While hoods vent some of the toxic benzene outside, it doesn’t entirely remove this harmful gas from your kitchen. The best way to protect yourself and your family is to use an emissions-free electric or induction stove.”
Federal, state and utility rebates are available to many consumers who swap gas appliances for new models running on electricity increasingly generated by renewable sources.
The Stanford report notes that studies detailing the dangers of indoor gas combustion are now piling up.
“A 2013 meta-analysis concluded that children who live in homes with gas stoves had a 42% greater risk of asthma than children living in homes without gas stoves, and a 2022 analysis calculated that 12.7% of childhood asthma in the U.S. is attributable to gas stoves,” the study said.
For the new benzene study, researchers hooked up monitoring equipment in 87 homes across multiple counties in Colorado and California. They sought a mix of large and small homes, as well as apartments.
They wanted to quantify the emissions directly related to burning natural gas or propane — heated food can itself release polycylic aromatics, especially fatty substances like cooking oils. The research first found that those foods did not release measurable benzene when cooked in a pan. They then neutralized any confusion from food results by measuring emissions from boiling water, in the same pot each time, and by measuring emissions in empty ovens, the study said.
In nine of 33 cases in one subset of the study, or 29%, “a single gas burner on high or an oven set to 350°F raised kitchen benzene concentrations above the upper range of indoor benzene concentrations attributable to secondhand tobacco smoke.” The indoor emissions even approached outdoor benzene levels recorded from nearby oil and gas operations in separate California and Colorado incidents in 2020, which prompted investigations about dangers to schoolchildren, the study notes.
While outdoor benzene may have dropped in many Western nations, indoor measurements may be even more important when studies show many Americans spend 87% of their lifetime indoors, the Stanford researchers said. Venting of appliances is not an automatic fix, either.
Many vents underneath microwaves mounted over ranges simply recirculate room air through an ineffective filter, said study senior author Rob Jackson, a Stanford professor of Earth system science. Those that do vent to the outdoors may not be completely effective, he added.
“These results highlight the importance of combustion by gas stoves for indoor air quality and human exposure in future policies designed to protect people from air pollution, particularly people in lower-income neighborhoods with smaller home sizes,” the study concluded.
The Stanford results also indicate the EPA is hugely underestimating total U.S. benzene emissions from indoor appliances. Applying the Stanford research on a national scale reaches an estimate of 7,200 kilograms of benzene from annual gas stove emissions in the U.S., the study said, while the EPA’s estimates from all gas appliances including water heaters and furnaces is only 4,300 kilograms.
The EPA has only measured benzene from large utility boilers, and the results are likely skewed, the Stanford authors said.