Smoke from hundreds of ongoing wildfires in eastern Canada has floated down into the U.S., prompting poor air quality alerts across the Northeast and Midwest, and even as far south as South Carolina.

One of the main reasons this kind of pollution can cause breathing issues is because the particulate matter—called PM 2.5, since the diameter of each particle is 2.5 micrometers or smaller—is so fine. (For reference, the diameter of a single human hair is nearly 30 times larger than a PM 2.5 particle.)

“We have defenses in our upper airway to trap larger particles and prevent them from getting down into the lungs. These are sort of the right size to get past those defenses,” Dr. David Hill, a Connecticut-based pulmonologist and member of the American Lung Association’s National Board of Directors, told PBS NewsHour.

People with existing respiratory conditions like chronic obstructive pulmonary disease or asthma—as well as people with heart conditions, older people, and children—should be extra careful when wildfires diminish the air quality. But what exactly does “extra careful” mean? Here are four ways to stay safe.

1. Stay inside.

The easiest way to avoid the air pollution outside is simply to avoid being outside. Skip the strenuous outdoor workout in favor of an indoor one, and keep doors and windows closed so the pollution stays outside.

The directive to remain inside goes for pets, too—especially birds and any animals with cardiovascular or respiratory diseases. The American Veterinary Medical Association recommends keeping outdoor bathroom breaks as short as possible. And if your pet has irritated or overly watery eyes, nasal discharge, an increased breathing rate, or any of these other symptoms, it’s best to call your veterinarian.

2. Filter the air.

If you have an air filter, now’s the time to turn it on. Or, you can turn on your air conditioner, which also filters the air (as long as you replace the HVAC filter often enough).

If you do have to head somewhere in the car, take advantage of your vehicle’s air recirculation button. As its name suggests, this button continually recirculates the already-filtered air inside your car, rather than pulling in new, polluted air from outside.

3. Wear a mask.

N95 and KN95 masks don’t just keep out pathogens—they’re also heavy-duty enough to block PM 2.5 and other pollutants from entering your lungs. It’s not a bad idea to don one before venturing outside on a hazy day.

4. Monitor the Air Quality Index.

The Air Quality Index (AQI) is a color-coded and numbered system based on the volume of pollutants in the air. Weather apps and local news sites often have information about what the current AQI is in your area, along with helpful details about what it means and what precautions you should take to stay healthy.

AirNow—an air quality monitoring site run by a number of federal agencies, including NOAA, NASA, the EPA, and the CDC—also has a mobile app that tells you all of that information, too. Find out more and download it here.

This article was originally published on www.mentalfloss.com as 4 Ways to Stay Safe When Wildfire Smoke Causes Poor Air Quality.