June 28, 2022: This report has been updated to reflect additional comment from the American Gas Association.

Samples of natural gas taken before it was burned for cooking in homes in the Boston area contained 296 chemical compounds, including 21 that are toxic to humans, researchers led by the Harvard T.H. Chan School of Public Health report. 

The toxic compounds included hexane, toluene, heptane, cyclohexane and benzene, a known carcinogen. The researchers also found the so-called odorant level added to natural gas as a safety precaution varied and could be undetectable by human noses, suggesting that federal guidelines need to be adjusted to address this inconsistency, they wrote. 

The results were published Tuesday in the journal Environmental Science and Technology and added to a growing body of research into the potential health and climate risks of natural gas, even as reliance on this fuel has increased.

The study comes as some cities and states move to ban gas-fired appliances in new construction in favor of electric appliances. In January, researchers at Stanford reported that  methane leaks from residential gas stoves were far greater than previously thought and had a 20-year climate impact comparable to carbon dioxide emissions of half a million cars. They also wrote that combustion pollutants released inside homes from burning gas could trigger respiratory diseases. 

About half of all United States’ homes use natural gas for cooking and heating every day. With every click of the stove and turn of the thermostat, consumers are harnessing the energy of natural gas piped hundreds, even thousands, of miles from its original source.

Natural gas is mostly methane, a greenhouse gas with more than 80 times the warming power of carbon dioxide over a 20-year period. But, when burned, methane produces less carbon dioxide than burning coal, leading to its promotion as a “bridge fuel” in the energy transition. 

Of the 234 samples collected from 69 homes, 95 percent had low levels of benzene, which is linked to anemia, decreased immune function and cancer. The presence of benzene and other toxic compounds raises the question of the health impact of natural gas in homes but is not a cause for panic, said Drew Michanowicz, a visiting scientist with the Center for Climate, Health and Global Environment and lead author on the new study. 

The study only looked at the presence of hazards like benzene in the natural gas itself before it was burned, not whether home users are exposed to these compounds in the ambient air.

Michanowicz said the findings should prompt scientists and energy experts to consider whether natural gas is harming the public in ways health researchers now understand. “What are the costs of this system?” he asked.  

Rob Jackson, professor of earth system science at Stanford University and co-author of the January study that found higher than anticipated methane leaks from gas stoves, said the new Harvard research would help give scientists and policymakers better data on overall indoor air pollution coming from natural gas sources. 

“This [new] study is a fascinating compliment to other studies on indoor pollutants generated in gas combustion,” Jackson said.

Ongoing studies are looking at how much of each toxic compound in natural gas used at the Boston-area homes their owners are actually exposed to and whether those amounts pose a significant health risk. The researchers said this is the first step in determining what the health risks are for gas consumers, both during stove use and when there are natural gas leaks inside a home. One in 20 homes visited in the study had gas leaks that prompted researchers to recommend that consumers follow-up with experts. 

“There may be low level leaks in a lot of homes,” said Curtis Nordgaard, senior scientist at the research and policy institute PSE Healthy Energy, and a co-author on the new study. When the sum of them is added up across a metro area like Boston, he said, “it may be significant.” 

The American Gas Association’s senior director of public relations, Jake Rubin, representing local energy companies that deliver natural gas, wrote in an email on Monday 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.”

A more extensive statement from the association Rubin released on Tuesday said any trace of benzene that might leak into a room would be “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.”

The statement said that natural gas industry performs “standard testing” of natural gas which has “always shown that natural gas also contains trace amounts of volatile organic compounds (VOCs).”

“Given the very low levels of VOCs found in natural gas,” the statement said, “the temporal and spatial variabilities reported by the authors are primarily of academic interest.”

Occupational hazards may be more consequential than home use, Nordgaard said. People working in commercial kitchens, for example, might have greater risk of exposure to hazardous compounds in the gas. The time of year may also increase potential exposure. In Boston, the concentrations of hazardous compounds peaked in the winter and were eight-times higher than the concentrations in the summer. 

This winter peak could be for a variety of reasons, said Michanowicz. There is increased demand for natural gas to heat homes in the winter, coupled with people spending more time inside and with less ventilation. Natural gas sources may also change through the season, and different wells may naturally have higher concentrations of benzene.

Beyond the potential public health impact, characterizing the gas coming into homes can influence decisions around climate change. “Cooking over natural gas flame is the most intimate connection with climate change that we never think about,” said Michanowicz.

The pipelines that start at fracked wells across the country “literally end where the kitchen begins,” he said. As a greenhouse gas, methane is the second largest contributor to global warming after carbon dioxide and reached record levels in the atmosphere last year. Stopping methane leaks into the atmosphere from stoves, water heaters and—most prevalently—pipelines and oil and gas wells, is critical to countering climate change. 

In Boston, natural gas is piped from gas wells located in the southern United States or western Canada. For the study, researchers focused on three major natural gas suppliers: the Algonquin Gas Transmission System operated by Spectra Energy Partners, Kinder Morgan’s Tennessee Gas Pipeline Company and the Maritime & Northeast Pipeline owned by Spectra Energy Partners, Emera Inc. and ExxonMobil.

While methane is about 80 times more potent than carbon dioxide, it is relatively short-lived, remaining in the atmosphere for only 20 years (carbon dioxide persists in the atmosphere for centuries). So cutting methane emissions now would more quickly help nations meet their goals for curbing global warming by 2050. 

What Can be Done? 

While transitioning to electric and induction stoves is the most complete way to protect against any potential harms of gas use, there are simple, more affordable actions consumers can take right away to reduce their exposure, said Zeyneb Magavi, co-executive director of Home Energy Efficiency Team and co-author on the Harvard study. 

Magavi recommended using a range hood with ventilation to the outside when cooking with a gas stove. In the absence of vents, opening windows also increases ventilation. 

Rather than replacing their gas stove right away, consumers can plan to transition to an electric stove over time and save up for a new appliance, she said. In homes with poor ventilation, buying an inexpensive induction heat plate is another option, Magavi said. 

Increasing ventilation and air filtration in new and existing buildings can also be good preventative measures, Magavi said. 

A better understanding of indoor air quality and gas consumption must be driven by more data and transparency, said Magavi. This includes information from natural gas companies on what is actually in consumers’ gas.

Jackson, the Stanford researcher, hopes that utilities make more gas composition data publicly available as a public service. Companies currently publish information of the large components in their gas, like hydrogen, oxygen and sulfur. Other contents are categorized as general impurities, but the chemical concentrations are not described as in the Harvard study. 

Finding and fixing indoor gas leaks can limit potential health impacts and increase safety, said Magavi. Because the new study found that the sulfur odor of natural gas is not always smelled, residents and first responders may need more sensitive gas detectors to alert them of any leaks.

All samples in the study met federal requirements for odorant levels, which is added to the gas to make dangerous leaks easier to detect. Even so, the authors used previous natural gas leakage data to estimate the average methane concentration. They estimated that current standard odor levels may not be high enough for humans to smell the average methane concentration leaked from stoves. This may suggest that regulators need to increase odorant levels in order to find leaks that currently go undetected, the study concluded.