When Alex Huffman, an aerosol scientist and associate professor at the University of Denver, recently outfitted his house with CO2 monitors—part of a home experiment to see how well different spaces were ventilated—he found out his home was pretty tightly sealed against the outside air, but carbon dioxide exhaled from his family would build up a bit as they lived their lives. There was a bigger surprise, though: When they cooked, the CO2 levels skyrocketed—not only in the kitchen, but throughout the house.
“I was amazed to see how high the CO2 went,” Huffman says, noting that his home has no externally venting fan. “The CO2 built up very quickly and stayed high. As long as we didn’t open the windows, it stayed high for hours.”
Huffman was monitoring CO2 not out of environmental or health concerns, but as a proxy for COVID-19 transmission. When we breathe, we exhale both CO2 and aerosol droplets, so the concentration of CO2 in a space can give you an idea of how many aerosols, which may contain COVID-19, have built up. But CO2 is also a pollutant, and the readings he got when cooking show how polluting the activity is.
Cooking emissions also contain small particles known as PM2.5 (particulate matter smaller than 2.5 micrometers). These particles, which are also prevalent in wildfire smoke, can penetrate into our lungs and even enter our bloodstream. They can make asthma more severe, worsen chronic lung conditions such as COPD and emphysema, and there’s also evidence these particles can increase heart attacks, strokes, and lead to premature deaths. High CO2 levels cause health effects too, such as rapid heart rate, headaches, fatigue, dizziness, and nausea. The burning natural gas in gas stoves also produces amounts of nitrogen dioxide, another potent pollutant.
In London, cooking emissions account for 10% of particulate pollution. In megacities like China, that share can go up to 22%. A recent study out of the University of Birmingham found that these emissions remain in the atmosphere for up to several days—not breaking up and dispersing sooner like previously thought—contributing to poor air quality and impacting human health. “If these particles have longer residence times in the atmosphere, they will travel further and affect air quality in a wider area over a longer period of time,” says Christian Pfrang, a researcher at the University of Birmingham who worked on that study, over email.
Cooking emissions don’t only affect the outside air; they can also hang around inside. “These cooking emissions will have longer residence times wherever they are—so if homes are poorly ventilated, this will significantly impact on indoor air quality in particular in the kitchen/dining area,” Pfrang says. With COVID-19 lockdowns forcing people inside their homes more, and maybe spurring them to cook at home more often, this means people could be exposed to much higher levels of indoor air pollution than usual.
Just how much more, though, is hard to say. There’s still a lot we don’t know about our indoor air, and its effects. Does this added indoor pollution mean we’ve been exposed to more CO2 than when we could be in any other year, when we go outside more often and commute to work? It really depends on what your living space is like, how much you cook, where you live, and how much pollution you’re normally exposed to, says Delphine Farmer, an associate professor of chemistry at Colorado State University who studies the air we breathe indoors. (Farmer is also one of the principal investigators with HOMEChem, a first-of-its-kind experiment into understanding the chemistry of our indoor air and the impacts of activities such as cooking and cleaning.) Still, even with those variables, she thinks there has certainly been “enhanced exposure” to these particular cooking emissions with people staying inside.
Your specific exposure depends on a few factors. Huffman notes he has a gas stove, which is particularly polluting. With that flame, gas stoves put out both gases and pollutant particles. Emissions from electric stoves are “likely to be negligible,” Pfrang says, though the cooking done on them can produce its own emissions. And different types of cooking produce different levels of emissions. Any time you fry or sauté with oil in a pan, you’re going to generate “a large portion of particles,” Farmer says. Any time you toast, “it’s like a miniature smoldering forest fire.” Things like boiling and making rice produce less pollutants. When roasting, though, “when you can smell those really strong roasted scents,” Farmer says, “you’re certainly releasing a large number of gases and particles.”
It also depends on how well ventilated your home is. “Historically in America we have really focused on sealing houses well. We’ve been kind of afraid of the outside air,” Huffman says. “But what we’ve done is trap all of that into the breathing space where we spend most of our time, so it doesn’t go away very quickly, whatever it is that you’ve produced.” Older homes may be leakier—which is bad for your heating bill and energy efficiency, Farmer notes, but good for flushing out indoor air pollutants. Newer homes built for high energy-efficient standards will have worse ventilation. It’s not always good to bring the outside air in to flush out cooking pollutants, either. For those in the West who dealt with forest fires this year, opening windows to clear out cooking emissions would have been worse than keeping them shut to keep out the smoke.
Ultimately, there’s still a lot we don’t know about indoor air and how it affects us—but we’re learning. There’s been a newfound interest in indoor-air research, especially because of how much time we’re spending indoors. Farmer thinks in the next few years we’ll have a better understanding of how short-term exposure to high emissions levels from cooking compares to long-term exposure to outdoor pollution such as smog.
Even if we don’t know if indoor pollutants are as harmful, Farmer says she believes in the precautionary principle: “If there are simple things that you can do to avoid potential exposure, then I believe in doing them,” she says. “And they really are very simple.” She suggests cracking open a window when you’re cooking, getting into the habit of turning on your kitchen hood vent, and utilizing outdoor-air vents in your house, even if it’s the one in your bathroom. Farmer also recommends everyone get a simple, cheap air purifier, one with just a filter and a fan. (Avoid the fancier ones with UV light or ozone ions, which can actually make air worse, she says.)
And don’t only use these tools when you’ve burnt your toast or attempted to flambé. “Everyone has something that they do in their kitchen, either you run and you turn on a bathroom fan or you crack open a window or you turn on an air purifier. . . . Whatever you do when you smell that burnt toast or the overdone stir-fry, that’s what you should be doing all the time when you cook,” Farmer says. “While you might not be smelling the smoke and the air pollutants, they’re certainly there.”