We know gas stoves have health risks. What about other gas appliances?

This story originally appeared in Yale Climate Connections and is part of Covering Climate Now, a global journalism collaboration strengthening coverage of the climate story.

Poor air quality is a long-standing problem in Los Angeles, where the first major outbreak of smog during World War II was so intense that some residents thought the city had been attacked by chemical weapons. Cars were eventually discovered to be a leading cause of smog, but they weren’t the only ones. In 1978, the regional air-quality authority created regulations aimed at reducing pollution from a surprising source: gas-powered water heaters found in homes throughout the city.

Gas stoves have become an unlikely front line in the culture wars thanks to growing awareness of their contribution to health problems like childhood asthma, not to mention their links to climate change. But the other gas-fueled appliances found in many American homes — water heaters, furnaces and clothes dryers, to name a few — have received far less attention, although they also pose risks to public health and the environment.

“I’m not here to scare folks,” said Brittany Meyer, the national director of healthy indoor air policy at the American Lung Association. ​“But that said, we do know that there are impacts to both indoor and outdoor air quality that come from the burning of fuel inside the home that is vented outside, which is the appliances you’re talking about.”

Gas appliances can give off toxic carbon monoxide and other air pollutants

Approximately half of American households rely on gas appliances for heat and hot water. According to the U.S. Census Bureau, piped natural gas powered around 61 million water heaters, 58 million furnaces and 20 million clothes dryers in 2021. Other common gas-powered appliances include fireplaces (approximately 7 million), air conditioners (around 2 million), and space heaters.

Health risks associated with gas appliances center on the chemical composition of the fuel they burn. Natural gas consists primarily of methane, a powerful heat-trapping gas that’s partly responsible for climate change, which the World Health Organization has called ​“the single biggest health threat facing humanity.” But it also contains other substances that, when released into the air through leaks or incomplete combustion, can more directly harm human health. The best known of these, carbon monoxide, causes at least 420 accidental poisoning deaths each year in the United States.

Drew Michanowicz, a scientist at research institute PSE Healthy Energy, has carried out several studies to understand exactly what’s in the natural gas that enters American homes, taking samples from more than 200 residences in California and Massachusetts. ​“They pretty much all contain a small suite of hazardous air pollutants that clearly we would not want to be exposed to,” he said. Among them: nitrogen dioxide, benzene, formaldehyde, particulate matter, and additional substances linked to asthma, cancer, heart disease and other health problems.

Venting reduces indoor pollution from gas appliances — but it’s not foolproof

The growing concern over gas stoves stems largely from the fact that cooking appliances release these chemicals directly into areas of the home where residents spend much of their time. Stoves often aren’t vented to the outdoors when in use, either because the kitchen isn’t equipped with an exterior vent or because occupants don’t turn it on.

Outside of the kitchen, however, the most common gas-powered appliances — water heaters, furnaces and clothes dryers — are required by building codes to be automatically vented outdoors.

This divergence in venting regulations reflects the fact that non-cooking appliances burn much more gas than stoves do, and therefore emit much more pollution.

“An average home in the Northeast uses like 60 MMBtu [metric million British thermal units, a standard unit for measuring the heat content of energy sources] per year for space heating and another 20 for water heating,” said Matt Rusteika, the director of market transformation at the Building Decarbonization Coalition, a nonprofit focused on building electrification. ​“By comparison, you only use about 2 MMBtus per year of gas if you have a gas stove.”

A robust set of government regulations and industry standards has been developed to counter the threat of indoor air contamination from these appliances, said Iain Walker, a building scientist at Lawrence Berkeley National Laboratory who specializes in residential ventilation and home decarbonization. ​“Effectively, no matter where you go in the country, the same requirements are going to be met about the size of the vent and so on and how it’s all connected,” he said.

As a result, as long as these appliances are installed properly, the risk that they will release harmful pollution into homes is low, Walker said. ​“The vast majority of times, [for] a heating device, including water heaters, with a correctly installed flue, you are not going to have any problems.” Groups like the Lawrence Berkeley National Laboratory and weatherization programs throughout the country have amassed a substantial body of evidence confirming that this approach is effective, he said.

But though the system generally works, sometimes it doesn’t. Walker cautioned against undue alarmism, but he said that indoor air pollution from gas appliances is very difficult to eliminate completely. ​“Unfortunately, we know enough to know that it’s still an intractable problem.”

Joe Roy of energy-efficiency company CMC Energy Services said that over the course of thousands of home-energy assessments conducted in Connecticut, the company’s technicians have found malfunctioning gas appliances or gas lines in approximately 15% of all residences built after 1980. In homes dating from before the Reagan era, this figure climbs to 25%.

Leaking gas lines are the most common source of unwanted chemicals that enter the living space, Roy said, particularly in older homes.

But a variety of issues can lead to pollution from vented appliances themselves. A furnace component known as a heat exchanger may crack over time, which can lead to dangerous levels of carbon monoxide being distributed through a home’s duct system. Water heaters can experience a phenomenon known as backdrafting that causes combustion byproducts to enter the living space. Residents sometimes move clothes dryers without realizing that an easily damaged gas line lies behind them, with predictable results. And a practice known as sidewall venting, in which high-efficiency gas equipment is vented through a hole in the wall rather than through the chimney, as was standard practice in the past, occasionally leads to problems with debris, bird nests, snow or other substances blocking the outlet air.

In new ruling by key engineering standards body, unvented gas appliances are deemed unsafe

A separate class of gas appliances that aren’t required to be vented to the exterior, including fireplaces and heaters, poses a different set of challenges. These machines can be cheaper and more flexible than their vented cousins due to the lack of need for a flue, but concerns have been raised about their safety.

The regulatory environment for these appliances is changing because of a recent update to the engineering standards that underlie most local building-code ventilation requirements in the United States.

The Lawrence Berkeley National Laboratory’s Walker, who recently served as chair of the committee tasked with this update, said that one of the core debates during the process centered on how long users leave these appliances on during a given session — and therefore how much pollution might accumulate in the living space.

“Historically, these things have been allowed because the concept was that they would be low-capacity and not operate very much,” he said. ​“A lot of the wrangling [in the recent committee] has not been over, ​‘Does combustion produce contaminants?’ [Instead] the discussions were about, ​‘Well, what if it turns on and operates for several hours? Then what?’”

The committee’s final recommendation was to prohibit the use of unvented gas space heaters or fireplaces in homes seeking to meet the engineering standard, Walker said.

Gas furnaces and other appliances can cause outdoor air pollution and harm health

Although venting gas appliances is the best way to reduce risk within the home, emissions don’t simply disappear when they make it outdoors. Instead, they circulate within communities.

“There are millions of boilers and furnaces and water heaters, and they’re all pumping combustion fumes up into the air, and it’s bound to affect people’s health,” said the Building Decarbonization Coalition’s Rusteika.

One key area of concern is nitrogen oxides, or NO_x, a family of chemicals formed by fuel combustion in air that has been linked to respiratory problems like asthma, in addition to cardiovascular and other health issues. NO_x also reacts with other chemicals in the air to form smog, which is itself associated with a variety of health problems ranging from reproductive harm to early death.

Several recent reports have called attention to gas appliances’ role in outdoor air pollution in California. One 2022 publication jointly produced by the San Francisco Bay Area Planning and Urban Research Association, or SPUR, and environmental nonprofits Sierra Club and RMI explored the links between residential and commercial appliances and smog. It found that these machines generate approximately four times as much NO_x as the state’s electric utilities and around two-thirds as much as its light-duty passenger vehicles.

A 2020 report from the UCLA Fielding School of Public Health found that if all residential gas appliances were replaced by clean electric models, the reductions of NO_x and particulate matter in the outdoor air would lead to 354 fewer deaths and 596 fewer cases of acute bronchitis each year in the state.

Detailed information from other parts of the nation is harder to come by. The American Lung Association’s Brittany Meyer said that there’s a need for more research on how gas appliance emissions affect outdoor air quality. One of the surprises of the association’s 2022 literature review on the health impacts of residential combustion was ​“the lack of studies on some of the health impacts of vented combustion-based appliances on people not directly using the appliance,” she said.

But Seth Hartley, an atmospheric scientist at consultancy ICF and a co-author of the 2022 report, said that existing data is clear on the broad outlines of the issue, if not the details. ​“We know that [these appliances] emit NO_x. We know that NO_x causes smog. We know that NO_x is a lung irritant and causes other things like that. But to quantify that relationship — still, there’s a lot missing.”

A growing focus on the health risks of gas appliances

To date, building-code-mandated ventilation has been the primary mechanism through which U.S. governments at any level have sought to limit health risks from gas-powered appliances.

“The U.S. EPA doesn’t regulate indoor air quality, so there hasn’t been that much focus, really, on controlling the indoor air quality aspects of these [appliances]. And generally, I don’t think there has been that much focus on the outdoor impacts, either,” Hartley said.

There are exceptions, however. Other regions of California have joined L.A. in setting limits on NO_x emissions from appliances, and the state’s recent ban on sales of new gas furnaces and water heaters, which goes into effect in 2030, was motivated in part by air pollution concerns. Utah has NO_x emission limits for water heaters, as does Texas for small water heaters and boilers. And in New York state, a recently passed law seems likely to lead to new emissions limits on appliances, according to the Building Decarbonization Coalition’s Rusteika.

In general, policy responses to health impacts from gas appliances are ​“starting to bubble up,” Rusteika said. ​“The overlap with climate is maybe mainly what’s driving that movement in some places. In other places, the focus is on NO_x, which we’ve been regulating in other sectors for a long time.”

How to make sure gas appliances aren’t making you sick in your home

People who are concerned about the impacts of these appliances in their homes can take steps to reduce the risk of indoor air pollution. The Centers for Disease Control and Prevention recommends installing a battery-powered carbon monoxide detector on each level of the home and having all gas-powered appliances professionally serviced each year.

According to Joe Roy of CMC Energy Services, it’s critical to ensure that this regular maintenance includes an assessment of the home’s gas lines. So make sure that the HVAC contractor’s scope of work includes checking gas lines for leaks, Roy said.

Consumer-grade NO_x monitors can also help provide peace of mind within the home, he said.

Another option for mitigating the risks posed by gas appliances is to take advantage of Inflation Reduction Act incentives for buying and installing their electric counterparts.

Rusteika said that rising sales of heat pumps show that ordinary Americans are increasingly convinced of the benefits of moving away from gas.

“People are used to burning fuel in their house, but people used to be used to sitting next to people smoking in restaurants as well,” he said. ​“We know that electrification is essential for the climate. We also know that electrification improves indoor air quality. We know that it’s more comfortable. We know that it can save you a lot of money in certain contexts. And so now is the time, right?”