Natural gas leaking out of home appliances like kitchen gas stoves, furnaces, and water heaters may be mostly methane, but recent research discovered that traces of naturally occurring air toxins are also in the mix.
Work by scientists in the Boston area identified 21 toxins in unburned natural gas, including benzene, a carcinogen that harms white and red blood cells in the human body. Other toxins included hexane, toluene, and heptane. Although this is the first time these toxins have been documented directly from stove-top gas, the gas likely poses less risk to most people than other environmental hazards like tobacco smoke.
The researchers collected gas from 69 homes in 15 communities across Greater Boston over 16 months. At each home, scientists siphoned natural gas through Teflon tubes attached to kitchen stoves. During the onset of COVID-19, researchers also measured gas from outdoor grill hookups and fireplaces.
They then sent the samples to an EPA-certified lab to test the gas’s volatile organic compounds (VOCs). Natural gas is typically about 95% methane, but according to the authors, little was known about trace chemicals in the remaining 5% until this new study, published in Environmental Science and Technology. They found that 95% of samples contained low levels of benzene, a chemical used in plastics, lubricants, detergents, and pesticides.
According to the American Cancer Society, about half of benzene exposure in the United States comes from cigarette smoking and secondhand smoke. Massachusetts recommends that benzene exposure be less than 0.2 part per billion by volume over 24 hours. Hexane, a cleaning agent and chemical used to extract oils from seeds and vegetables, appeared in all but 2% of the samples collected. The EPA has stated that chronic hexane exposure is associated with nerve damage.
Gas in the winter had much higher concentrations of toxins than in summer, and the scientists could not explain the cause. Benzene was nearly 8 times more concentrated in homes’ natural gas in the winter than in summer.
“Now that we know there are small quantities of volatile organic compounds (VOCs) present in the gas supply in the Greater Boston area, it is reasonable to conclude that our gas supply is not as clean as we thought it was once,” said Zeyneb Magavi, a co–executive director of the Boston-based Home Energy Efficiency Team who participated in the study.
The state’s three major utilities—Eversource, Columbia Gas (owned by Eversource), and National Grid—supply gas to the homes tested in the study, according to the researchers. But gas utilities are not responsible for leaks that happen in homes—homeowners are—as they are considered “behind the meter.”
National Grid did not respond to a request for comment, and Eversource directed questions to the Northeast Gas Association, a trade association representing natural gas distribution companies in the region.
“All the natural gas samples measured in the study met federal regulations,” said Alana Daly, vice president of communications and public policy at the Northeast Gas Association. “When pipeline natural gas is utilized as intended, in properly vented and maintained end-use equipment, the presence of these trace constituents [does] not pose adverse risks to human health or indoor air quality.”
Questions of Exposure Linger
Although the study provides a much more detailed account of the chemical makeup of consumers’ gas than ever before, it doesn’t determine health risks. The limitation stems from the data themselves: Researchers tested only pure natural gas. They did not test the ambient kitchen air surrounding a leaking stove.
Still, the scientists warn that people could be exposed to small leaks without knowing it. With 43 million homes in the United States cooking with gas, gas stoves are a common staple in American kitchens. And commercial kitchens, pipeline repair, and utility worksites might expose people more frequently, the authors cautioned.
The abundance of VOCs in gas varied across the study area, and although the results may be representative of gas sources in the Northeast, the scientists can’t say whether gas from other regions has similar chemical makeups. The gas source, the variability of the reservoir, processing, transit, and the addition of hydrocarbon from upstream underground gas storage or liquid natural gas facilities could determine its VOC content.
Gas stoves emit much higher levels of methane than initially thought, even while they’re switched off, according to separate work published by Stanford University researchers earlier this year.
For Earth science professor Rob Jackson, who participated in the Stanford study, even though the latest research suggested that natural gas contains only a small amount of benzene relative to other sources, such as smoking, car exhaust, or gasoline, “any unnecessary benzene in a home is too much.”
“Now that we know benzene is found in natural gas, utilities should disclose pollutant data regularly for their service areas,” Jackson said. “I hope this change will come from the new study.”
Ventilation—turning on a hood that exhausts air outside or opening windows and doors—when cooking with gas is important, wrote the authors. In addition, licensed HVAC (heating, ventilating, and air-conditioning) contractors or plumbers can test for leaks too.
“There’s no need to panic,” Magavi said. She recommended that homeowners consider transitioning to electric appliances in the long term. Make a plan for when gas-powered appliances break or require major maintenance, so homeowners can be ready to switch to electric appliances at opportune moments, she said. Induction cooktops run on electricity and have high energy efficiency. The cheapest models are just a few hundred dollars more than midtiered gas and electric ranges.
If gas stove owners don’t want to wait or pay for electric appliances, induction cook plates, which use electricity, cost under $100 and can be used separately from the stove for basic cooking needs, she added.
In the future, the Consumer Product Safety Commission could set more stringent performance standards for gas stoves and ventilation hoods, and gas companies could measure and report publicly what’s in the gas.
Gas suppliers could also make the leaks more noticeable. Methane doesn’t have a smell, so companies add an odorant to help people detect large amounts of the gas, which is flammable. The latest study found that about 1 in 20 homes had a leak that homeowners weren’t aware of that required follow-up with an expert. If gas providers upped the odorant in methane, small leaks could be easier to catch.
Limiting gas leaks could provide more benefits than just a reduction in hazardous air pollutants, said study author and Harvard T.H. Chan School of Public Health’s visiting scientist Drew Michanowicz. Methane warms the atmosphere 80 times more than carbon dioxide over a 20-year period. Reducing methane emissions could shave off a third of a degree of warming by the 2040s, according to the United Nations Environment Programme.
“Cooking over a natural gas flame is probably the most intimate connection with climate change that you never think about,” said Michanowicz.
—Jenessa Duncombe (@jrdscience), Staff Writer