This article was reviewed by a Caltech faculty member.
In recent months, debate has swirled around gas stoves and their relationship to health conditions such as childhood asthma. Paul Wennberg is Caltech's R. Stanton Avery Professor of Atmospheric Chemistry and Environmental Science and Engineering, a Resnick Sustainability Institute investigator, and director of the Ronald and Maxine Linde Center for Global Environmental Science. He uses satellites, aircraft, and specialized laboratory instruments to study the global atmosphere and air quality—and how human activity affects both.
Wennberg recently discussed gas stoves and other factors that can contribute to poor indoor air quality.
Often when we think about air quality, we're thinking about outdoor air. What should we be thinking about when it comes to indoor air quality?
It's a really big question. And, of course, one that has been so paramount the last couple of years as people have understood better, for example, how airborne disease is transmitted.
We spend most of our time indoors. In a laboratory or even in a commercial building, typically the air is exchanged about every 15 minutes; every 15 minutes, most of the air in a room has been exchanged with outdoor air that's been brought into the building by fans. At home, generally, the ventilation is much worse. Typically, the air exchange rates in the home might be one per hour or even less, so pollutants tend to build up. The major sources of contaminants to indoor air result from the people living inside. Indirectly, from cooking and cleaning, but also directly—we all give off chemicals when we are breathing or sweating. We may also breathe out viruses such as the coronavirus or flu virus if we happen to be sick.
How do gas stoves affect indoor air quality?
Besides people—and their pets—the major indoor pollutants come from burning stuff. As you know if you've ever left a slice of bread in the toaster too long, you go into your kitchen and you see that pall, that haze of smoke. Those are all small particles in the air. The same thing happens when you're cooking with oils. You often notice if you turn the heat up too much, you'll get smoke coming off. Those are particles as well. Those particles are not good to breathe, and that's why we try to ventilate our stoves.
Part of the problem is that some stoves in apartments and homes don't have ventilators, and for those that do have ventilation above them, many of these hoods don't work particularly well. They can trap oil, but many just blow the air right back into the kitchen. The recommendation is that people should have proper externally ventilating exhausts over their cooking. But of course if you live in a home you don't own it is generally not possible to add such exhausts—and even if you own your home, adding exhausts can be prohibitively expensive.
The challenge is that when you're cooking, you are effectively adding a lot of things to the indoor air. And because, again, homes are poorly ventilated, you end up building up significant amounts of contaminants, including NO2 [nitrogen dioxide]. The NO2 comes from burning at higher temperatures and is typically associated with truck and car emissions in the outdoors. Indoors, a gas stove will produce NO2. Electric stoves will generally not generate NO2. That's one of the major contaminants that comes from using gas stoves.
But other things will produce NO2 as well, such as burning candles.
What does the research tell us about the impact of indoor air quality and the relationship between gas stoves and conditions like asthma?
There are a number of epidemiological studies that point to an association between asthma and breathing NO2 as well as asthma and breathing harmful particles. We also know that any combustion source indoors leads to higher NO2 concentrations. So, there's a link in that we know NO2 and particles are not good to breathe, and we know that you end up producing lots of NO2 and lots of particles when you use gas stoves. But, at a population level, it's hard to disentangle all of the different sources of NO2 and particles people may be exposed to, even indoors.
Nonetheless, the evidence is strong that we need better indoor air quality. The focus of the conversation right now is on gas stoves, but if you clean up your gas stove and you're still enjoying candles every night, you've missed the message.
What else, besides NO2, is emitted when we turn on a gas stove or light a candle inside a home?
Particles—that's the other really big one—particles are very small and are suspended in the air. They are harmful to our lungs and can penetrate deeply into the body after we breathe them. With gas appliances, there's also unburned natural gas, which we can call fossil fuel gas. Fossil fuel gas includes a lot of chemicals in addition to methane. It includes, for example, small amounts of benzene and other compounds that are not healthy to breathe. These tend to burn up when you start the fire.
There's a lot of evidence, however, that the gas pipelines in people's homes can leak. If you look at the gas pipe coming into your home, you'll see that it gets divided to run to a lot of different places. It goes to your hot water heater, your dryer. Each of those connections has a valve, and each valve is a place where small amounts of gas can leak.
What can people do to limit exposure to NO2 and other contaminants in the home?
The upshot to all of this is that we should stop burning stuff indoors. This includes fossil gas, candles, cigarettes, and marijuana. We should also improve the ventilation in our homes—even just opening windows helps.
You can also add filtration for particles in particular. That's something people can do even if they rent their homes because filters can be either purchased or built. There are simple and inexpensive do-it-yourself methods of making these filters.
People suffer from indoor air pollution. A lot of the allergies people have are because indoor air has biological materials, dander, and other things floating around that can be removed with indoor air filters.
How do scientists study air quality? When reading about air quality, we frequently encounter the term "ppm." What does it mean?
Parts per million, or ppm, is a unit of what's called a mixing ratio. It tells you what fraction of the air is made up of the constituent you're looking at. One part per million of NO2, for example, means that in every million molecules of air you breathe in, one would be NO2. Readers may also encounter "ppb" or parts per billion. These may seem like very small units, but only 10 to 100 parts per billion of NO2 is enough to negatively affect our air quality resulting in adverse health outcomes.
And so, to study air quality, we need really sensitive instruments. The concentrations we're studying, while toxic or potentially toxic, can be very, very low. There are a class of instruments that have been developed over the years, some at Caltech, to measure the concentration of pollutants in the air.
Curious about the impact of gas stoves and other methane gas infrastructure on the global environment? Read more about where greenhouse gas emissions come from on the Caltech Science Exchange.