It’s not just COVID: There’s a growing awareness that poor air quality is behind all sorts of maladies.

Emily Krieger bought her air filter about five years ago in order to fend off her allergies. The unit, roughly the size of a surround-sound speaker, ran every night. Soon she found she was sleeping better. Then, when wildfire smoke became an even more regular occurrence in the outdoor skies above her Seattle home, she ran the machine to clear the air inside of any smoke that managed to pass through gaps near windows and doors.

But for Krieger, the machine she purchased at first to filter out allergens only became an essential gadget during the pandemic, when the health and safety of her two toddlers was that much more important. These days, she says, it’s often humming on the lowest setting.

“I thought: Let’s keep that bad boy running. It can make a significant impact with regard to keeping the air flowing,” she says.

Air quality is certainly top of mind for many people in a way it was not only two years ago. As the public’s awareness of how viral particles can easily circulate throughout indoor environments, getting some insight into the cleanliness of the air inside has become ever more important.

Yet even before the pandemic coronavirus, greater attention was beginning to be paid to the quality of air inside our homes and workplaces—for good reason. The Environmental Protection Agency recently published a booklet on “indoor air pollution,” a reference to the thousands of chemicals and particles floating around or digging into our countertops, kitchens, bedrooms, carpets, and floors every hour of every day. And a new study from Texas A&M University School of Public Health, published this February, showed that prolonged exposure to poorly-ventilated indoor air is associated with a variety of poor health outcomes, including mundane ones like headaches and dry eyes as well as more severe ones, like lung cancer.

“A silver lining of the pandemic is that people realize that indoor air is an issue,” says environmental engineer Dustin Poppendieck. “I think it’s one of those things we take for granted: We assume it’s healthy and safe. But indoor air is not always healthy.”

We all take about 20,000 breaths per day, and we spend upwards of 90% of our time inside. Pet dander, human hair and skin, pollen, dust mites, particulates from cooking oils, fragrances, phthalates from plastic: All of these and more are among the roughly 30,000 chemicals that make up specks of dust. Many of them are linked to various skin and respiratory problems, chronic disease, and even behavioral changes in children. (Lead paint, still a contaminant in many American homes, has been shown to cause ADHD, irritability, and developmental delays in kids under age 5.)

“Indoor air quality can actually be two to five times worse than outdoor air,” says Kenneth Mendez, CEO and president of the Asthma and Allergy Foundation of America.

Take the everyday gas-burning stove, for instance. According to one study published earlier this year, gas stoves are emitting bits of methane constantly into the air. Climate considerations aside, other studies have found that gas-burning stoves are correlated with a 32% higher risk of asthma in children and adults.

Ozone, too, can have an adverse effect on our health. Up high in the atmosphere, it plays an important role in protecting us from ultraviolet radiation. But at ground level, especially indoors, it can react with various chemicals. The result, sometimes, can be temporary paralysis of the cilia that help expel phlegm from your throat.

It’s no wonder, then, that trying to get a better understanding of the air we breathe is catching on more and more. In 2018, an article in Bloomberg CityLab described how residents in Utah and Colorado, concerned with wildfire smoke and increasing levels of truck pollution, turned to tiny monitors to measure the cleanliness of the air in and around their homes. Last fall, The New York Times reported how concerned parents were even hiding air-quality monitors in backpacks as a way to gauge how well ventilated were the school buildings where they sent their kids every day.

Beyond these individual measures, there are other moves happening to glean more insight into the air we breathe when we’re not outside.

Poppendieck is a member of the Indoor Air Quality and Ventilation Group within the federal government’s National Institute of Standards and Technology. Since February, he and about a dozen other researchers have been involved in one of the largest studies ever conducted on the quality of the air inside, an effort that wraps up this month. A key part of the study, which is happening at a residential test facility in Maryland, is trying to determine the effects of pollutants such as wildfire smoke, urban smog, and common commercial products used in cooking and cleaning. How do these various elements affect us as they permeate the atmosphere in our apartments, houses, and places of business?

“That’s the million-dollar question,” Poppendieck says. “Indoor air matters, but we don’t understand it well.”

To be clear, there is some correlation between indoor air and human health. It’s not difficult for the person with allergies to asthma to know when, or when not, the air inside where they live or work is messing with their health, Mendez says. And who wants mold, commonly linked to allergic reactions like sneezing, runny noses, and itchy, red eyes, inside their home? The question is over, precisely, what connections can be drawn between crummy indoor air and personal health.

On that point, Poppendieck cautions people against relying too heavily on air-quality monitors. “Consumer-grade sensors are really good at measuring changes, but they’re not going to tell you if [the air is] healthy or safe,” he says. “You could have a well-ventilated space, but if you have a gas station next door? No consumer-grade sensors would detect that.”

More important, according to Mendez, is doing your best to ventilate indoor spaces. Turn on the range hood when you’re cooking. Open windows to circulate indoor air. And running an indoor air filter can also help—as it did for Krieger.

“People that I talk to, if you have the money to get an air filter, you get one, you keep it on hand, and you run it,” she says. “That’s just our normal part of life now.”