Are gas stoves worse than fireplaces? Here’s how some other appliances can also pollute your home.

A contractor worked on a natural gas furnace in a home in Spanish Fork, Utah.George Frey/Bloomberg

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For weeks, pundits, public officials, and people all over the Internet have been discussing the dangers of gas stoves and ovens. And with good reason: Research shows they spew emissions that warm the planet and threaten human health.

But cooking appliances aren’t the only household items that run on methane gas. The fossil fuel also commonly powers furnaces, water heaters, and fireplaces like the ones you may have relied on throughout a recent weekend’s brutal cold. And like gas stoves, these appliances release pollution.

“We should be very concerned,” said Leah Louis-Prescott, a manager at nonprofit RMI who focuses on sustainability and building electrification. “These fossil fuel appliances are really harming our air quality and our health and of course the climate, and it has largely gone under the radar how big of an issue these appliances are.”

Are gas stoves more harmful than other appliances?

There’s evidence that gas furnaces and hot water heaters have a smaller effect on indoor air quality than gas stoves, though they do create other problems.

Less research has been done comparing the relative dangers of gas stoves to gas fireplaces. According to the Environmental Protection Agency, gas fireplaces produce far less indoor air pollution than wood-burning ones. But Louis-Prescott said no appliances that run on gas should be considered truly safe.

“At the end of the day, burning gas is burning gas. We’re emitting the same pollutants,” she said. “That pollution is going somewhere.”

Venting makes a difference

Like stoves and ovens, gas-powered heating appliances emit carbon monoxide, a toxic byproduct of combustion that can cause brain damage and heart problems that can sometimes be fatal. They also emit a family of gases such as nitrogen oxide and nitrogen dioxide, which can damage the lungs and respiratory system, and may increase cancer risk.

But if they’re properly vented, allowing those toxins to disperse into the air outside, the impact of gas furnaces, water heaters, and fireplaces on indoor air quality should be minimal, said Jonathan Buonocore, a professor of environmental health at the Boston University School of Public Health.

“Venting is the most common way to get the air pollution out of the home, and works fairly well,” he said. Still, more research should be done into how well that venting is really happening, Buonocore said. Without knowledge of how well ventilated each individual appliance is in your home, it’s hard to know which poses the most risk.

In Massachusetts, gas furnaces and water heaters must always be vented, according to the state’s Chapter 68 code on chimneys and vents.

“You’re going to have carbon monoxide, otherwise,” said Adi Rosa, of the HVAC installation company Presidential Electric and HVAC Services of Boston. “If we don’t vent it, somebody’s going to die.”

But unvented gas fireplaces and other space heaters — which are more likely than their vented counterparts to pour carbon monoxide and nitrogen dioxide into the home — are permitted if they meet specific safety conditions.

“Unvented propane or natural gas-fired space heaters were introduced into the fire code in 2004 and are acceptable only when certain safety conditions are met, including permits, inspections, [and carbon monoxide] alarms in the same room,” said Department of Fire Services spokesman Jake Wark. “Unvented natural gas fireplaces must be listed by a nationally recognized testing lab or the flue gases must be properly vented according to the gas and building codes.”

Those regulations may help mitigate risk. But Shelly Miller, author of a 2011 study on the appliances’ emissions, said unvented fireplaces shouldn’t be allowed anywhere under any circumstances.

“Unvented natural gas fireplaces should be banned due to the elevated risk for adverse health effects,” she said.

Scott MacFarlane, who owns Dedham heating and cooling company MacFarlane Energy, said he sometimes sees improper venting of gas appliances in Massachusetts buildings.

“Over the years, I’ve see a lot of stupid things go on,” he said.

Either way, emissions still flow outdoors

Even vented gas fireplaces, furnaces, and water heaters can harm indoor air quality if they are installed improperly, or if vents and chimneys get blocked.

“The performance of chimneys and ventilation systems is really important,” Buonocore said.

And while venting can improve indoor air quality, it still degrades the air we breathe outdoors.

“When we talk about indoor and outdoor air, there’s not a hard line that’s so easy to delineate, because you can open a window and the outdoor air comes in,” said Louis-Prescott. “They’re constantly interchanging, and you’re breathing in both.”

In fact, in Massachusetts, gas appliances produce more than nine times as much nitrogen oxides pollution as gas power plants, she said, citing federal data.

“We as a country have taken meaningful steps forward to help curb power plant pollution,” said Louis-Prescott. “But we’ve yet to take meaningful steps to curb appliance pollution.”

Different levels of pollution

Different appliances emit various pollutants at different levels. For example, a 2020 study found that gas water heaters emit more carbon monoxide than any other gas appliances, while gas furnaces emit the most nitrogen oxides.

These appliances may be exposing us to other dangerous emissions as well, Buonocore said. He contributed to a study last year that found gas used for stoves across Boston contains benzene, a carcinogen deemed unsafe at any level of exposure.

“We specifically looked at stoves,” he said, “but there’s no reason that I can think of for why the gas going to your stove would be different from the gas going into your furnace.”

Unequally harmful

Not everyone is equally affected by degraded air quality from gas appliances. In Massachusetts, people of color are exposed to 55 percent more pollution from residential appliances compared to white communities, a 2021 paper found.

Another group facing greater risk: renters, who often have less control to choose their own appliances. A Lawrence Berkeley National Laboratory report found that residents living in small apartments with gas furnaces are more likely to face unsafe levels of carbon monoxide and backdrafting, which is when exhaust gas moves back into the home instead of being vented outside.

The impact on climate change

In addition to their harsh effects on air quality, gas appliances — which run on methane, a planet-warming greenhouse gas — contribute to climate change.

A 2022 report concluded that gas stoves emit methane even when they’re switched off, which might suggest that leaks in stove fittings and connections with gas service lines are ubiquitous. Researchers are working to determine the planet-warming effect of other appliances’ leaks.

Gas appliances also have a broader climate footprint. Before it is piped into homes, methane must be extracted from oil and gas wells, sent through pipelines, processed, and then sent back through distribution pipelines to reach homes. Each step in the process can lead to leaks. Reports show the planet-warming impact of methane leaks has been routinely undercounted.

“Mitigating climate change involves, you know, not burning fossil fuels,” Buonocore said. While ventilation can improve your indoor air quality, the best possible solution, he said, is to switch to electric appliances.