N95 masks helped protect against the coronavirus. They also work against wildfire smoke.

Today is the day to put that mask back on.

As smoke from wildfires in Canada drifts over large parts of the United States, the best thing you can do on Wednesday to avoid inhaling pollutants is to stay indoors. Of course, this is impossible for many people. So if you have to brave the outdoors, donning a mask is the next best thing.

But beware, not all masks work equally well. A surgical mask, scarf, or bandana won’t do much to prevent you from inhaling pollutants.

N95 masks can filter out tiny particles smaller than 0.3 microns. For comparison, the width of a single human hair is about 60 micrometers, according to the California health department.

Instead, use N95 face masks (you may have some left over from the pandemic), or respirators, which can be found at hardware stores or online. Make sure to cover both your nose and mouth.

“Air quality in New York is at an unhealthy level today because of smoke from wildfires drifting down from Canada,” said Lucky Tran, a scientist, wrote on Twitter. “Reminder that N95 masks also help protect against smoke. It’s a good idea to wear a mask outside today!” He also advised people to turn on indoor air purifiers.

People in affected areas should take the smoke seriously, experts warned. “Wear N95 or better masks and stay indoors if you can, with air purifiers,” Mask Oakland, a project that responds to smoke in the Bay Area-based community organization, advised.