Natural gas stoves emit far more methane than previously thought, as well as harmful nitrogen oxides in concentrations that can quickly exceed federal safety standards, researchers at Stanford University report. 

The findings, published Thursday in the journal Environmental Science and Technology, come as a growing number of cities and states look to phase out gas-fueled appliances in homes in favor of more climate-friendly electric alternatives.

The researchers took detailed measurements of methane, the primary component of natural gas, and nitrogen oxides, harmful pollutants that result from burning natural gas, from 53 homes across California. Based on their findings, the study authors extrapolate the collective methane emissions from all residential stoves nationwide would be equal to the annual greenhouse gas emissions of half a million cars when considering the climate impact of methane over a 20-year period.

“We’re systematically underestimating the climate impact of gas appliances,” said Rob Jackson, an earth system science professor at Stanford University who helped lead the research. “And we’re standing over stoves that are emitting pollutants that we breathe.”

Running gas ovens and stove top burners in small kitchens with poor ventilation resulted in emissions that within a few minutes surpassed the Environmental Protection Agency’s safety standards for outdoor air concentrations of nitrogen dioxide, an irritant that can aggravate asthma and may contribute to the development of the disease. The agency does not have a separate safety standard for indoor air concentrations of nitrogen dioxide.

The current research is part of a series of ongoing studies led by Jackson and his colleagues that takes one of the first detailed looks at emissions of methane and other pollutants from gas-fired home appliances. A 2020 study by the group found methane emissions from hot water heaters were far higher than previously thought. A study looking at emissions from furnaces is ongoing.

Carbon dioxide emissions from burning methane or natural gas are the primary driver of climate change from gas stoves. However, methane leaking from the appliances and their piping  increase stoves’ climate impact by about one third, the study noted.

One of the most surprising findings of the current research was that the vast majority of methane emissions—76 percent—came from slow but steady leaks in stove piping and fittings when the stove was not in use.  

Current EPA estimates of emissions from the residential sector only consider methane resulting from the incomplete combustion of natural gas in stoves and other appliances while they are operating.

“While post-meter leak emissions (including leak emissions from stoves) are not currently included in the [Greenhouse Gas] Inventory, EPA plans to incorporate an estimate for these post-meter emissions in the upcoming 2022 GHG Inventory,” EPA spokeswoman Enesta Jones said.  

The agency reports that average nitrogen dioxide levels in homes without gas-fired appliances is about half that of outdoors while, in homes with gas stoves, “indoor levels often exceed outdoor levels.” To reduce exposure to nitrogen dioxide, EPA recommends that individuals “install and use an exhaust fan vented to outdoors [sic] over gas stoves.”

Renters who do not have an exhaust fan and low-income homeowners who cannot afford to install one can also open their windows to reduce nitrogen dioxide concentrations, Pierre Delforge, clean buildings director for the Natural Resources Defense Council, said.  

The American Gas Association, an industry group representing companies that deliver natural gas, challenged the study and its findings.

The study, which took air samples inside kitchens which were blocked off from the rest of the home with plastic “is in-no-way a realistic measure of the circumstances in a typical home…or any home,” said AGA spokesperson Jake Rubin. “As it pertains to NO2, the study did not include emissions from the cooking process. Indoor air quality studies have consistently found that emissions from the cooking process—not solely from the burner or heat source operation—represent the chief source of concern with respect to indoor air quality.”

One limitation noted in the study itself was the lack of stoves sampled in low-income, multi-family homes. The research was conducted during the ongoing Covid-19 pandemic when Stanford University regulations prevented the study’s authors from working in homes that were occupied at the time of the research.

To work around this limitation, the researchers relied on Airbnb rentals which skewed toward higher income, single family homes.

Eric Lebel, the study’s lead author and currently a senior scientist with PSE Healthy Energy, a non-profit research institute, said it’s important that future studies look at low-income households. “Low-income families are more likely to have smaller kitchens with less well-maintained stoves and so are likely exceeding the NOx [nitrogen oxides] thresholds even quicker and with higher concentrations,” he said.

The California Energy Commission is currently offering $2 million in grant money for further research into methane emissions from the state’s residential sector. The research “must include multi-family units and households from under-resourced communities such as low-income and/or disadvantaged communities,” the Commission stated.   

Zachary Merrin, a research engineer with the indoor climate research and training program at the University of Illinois’ Applied Research Institute, said in an email that the current study was “well executed.” However, Merrin, who has conducted prior research on methane emissions from stoves, noted that a small fraction of the sites, or homes, monitored in the current study, just 9 percent, accounted for 49 percent of total methane emissions documented by the research.  

“Finding more of those outliers could significantly influence the results,” Merrin said. “The most valuable thing from a societal point of view would be to develop a way to quickly and easily identify those outlier sites with the large gas leaks and be able to locate them for repair.”

The current study comes amid a push by cities and states across the country to incentivize the use of electric appliances or ban gas-fired appliances in new construction.  

New York City became the largest U.S. city to pass such a ban in December, when it prohibited the use of natural gas appliances in newly constructed buildings by 2027. Oakland, San Francisco and San Jose have passed similar bans and New York State is considering what would be the first statewide gas ban.

The bans have also met with opposition. Twenty states have passed preemptive legislation that would ban cities or counties from restricting the use of gas appliances, a move supported by natural gas industry groups.

Delforge, of NRDC, said such bans by state legislatures don’t make sense as gas stoves are both a climate and health concern, which the current study underscored.

“It just doesn’t make sense for the legislature to prevent a city from doing that, from taking action to protect its own citizens’ health,” he said.