Wildfire season is fully upon us—and with it, the smoke. The most spectacular threat of a wildfire is of course the possibility it might directly threaten lives and homes, like the wildfires last year that devastated Los Angeles.

But it is the smoke and the stinging haze—and especially the invisible particulate matter borne aloft for hundreds of miles—that is most likely to pose grave health risks to millions across the country, as wildfires rage in Canada and across the American West during summer months. Cardiac deaths spike after wildfires. So do visits to lung doctors. Smoke is bad for you.

The more we know about the health effects of wildfire particulates in the air, the worse the effects seem to be. Many of the tiny particulates in wildfire smoke are both highly inflammatory and small enough to enter the bloodstream after entering through the lungs. They might then swim in your blood for weeks. High concentrations of particles smaller than 2.5 microns (PM 2.5) can harm developing lungs, lower fertility, damage mental health and ability to concentrate, and increase risks of health conditions ranging from stroke to heart disease to lung cancer.

Health risks from wildfire smoke are especially troubling for immunocompromised people and people with asthma, says Heidi Huber-Stearns, director of the Center for Wildfire Smoke Research and Practice at the University of Oregon. But even healthy children are vulnerable.

“This is an issue for developing lungs, whether or not they seem to be visibly suffering or wheezy,” Huber-Stearns told WIRED. “It doesn’t have to be a kid who has other health issues. It’s all kids.”

The best policy during hazardous air quality warnings is to stay at home near a good air purifier. But this isn’t possible for all people. Here’s how to keep your lungs and your family safe as the air fills with health-damaging soot. If you’re more worried about the fire than the smoke, check out WIRED’s guide to emergency preparedness for tips on how to safeguard your home and plan a bug-out bag.

Know the Air Quality Where You Live

The first line of defense is always intelligence. Wildfire haze and taint have become an unavoidable fact of summer life in the western United States—and increasingly in the Northeast, as Canadian wildfires send a stream of eye-squinting haze into America’s most populous corridor.

Airnow.gov is a partnership among multiple federal agencies, aggregating data from thousands of air quality monitoring stations. It’s also likely the most up-to-date source of information on air quality in any given neighborhood, including forecasts for the coming days and weeks. Punch in your zip code to see the air quality index in your area and the most prominent pollutants.

In general, if the air quality index is higher than 150, sensitive individuals should avoid being outdoors for prolonged periods and consider masking with N95 respirators. This group can include children, older people, people with lung disorders, and immunocompromised people.

At an AQI above 200, denoted by the color red on Airnow’s map, this advice applies to everyone, regardless of health status. Wear masks. Stay in clean air spaces when possible. Outdoor physical exertion should especially be avoided.

Use an Air Purifier to Create a Clean Air Space

If you can afford it, the best tool for keeping your home particulate-free is a high-quality air purifier to remove dangerous particulate matter from your air.