Several US states are again experiencing an influx of wildfire smoke as Canada’s summer fire season gets underway. Due to the scale of the wildfires and natural weather patterns, enormous amounts of smoke are drifting southward — much like last year.

North Dakota, South Dakota, Montana, and Minnesota are among the earliest states to receive air quality warnings this week. Those alerts are a signal that it’s unhealthy for people, particularly vulnerable populations like children and the elderly, to go outside due to the pollution in the air.

More US states and cities could see similar alerts over the coming months as scientists anticipate another intense Canadian wildfire season.

The greatest hazards of wildfire smoke come from fine particulate matter that it carries, like soot, also known as PM 2.5 for its size. Because they’re so small, these pollutants can travel into people’s lungs and bloodstreams, making breathing more difficult and exacerbating other health conditions from asthma to chronic pulmonary problems. Hazardous gasses and chemicals in the smoke like carbon monoxide and benzene can also endanger people’s health.

Health authorities encourage people to stay indoors if possible when they receive severe air quality warnings and to take serious precautions if they need to go outside. Below are some steps to keep in mind — and ways to protect yourself — while navigating this upcoming wildfire season.

Check the air quality index

The first step is checking the Air Quality Index, or AQI, which is run by the Environmental Protection Agency (EPA). People can visit AirNow.gov, and enter their location in order to see what the air quality is like in their neighborhood.

The AQI rates air quality on a scale of 0 to 500, with 0 being the least hazardous and 500 being the most. It also breaks the rating down into six color categories, with green representing the healthiest level of air quality and maroon representing the most dangerous. These calculations are based on the pollutants the EPA detects in the air, including the concentrations of ozone, particulate matter (PM 2.5), carbon monoxide, sulfur dioxide, and nitrogen dioxide.

Any rating of 50 or below indicates that the air is good quality, while any rating higher than 300 signals that the air poses a risk to everyone, according to the agency. The various categories in between those extremes detail who is more at risk as the air quality worsens: For example, sensitive groups could experience negative health effects once the AQI is higher than 100.

If an area experiences a heavy concentration of wildfire smoke, the state government typically issues an air quality warning to make people aware of the risks. In Minnesota this week, the state issued an air quality alert to signal that the air would reach the red AQI category — which is a rating between 151 to 200 — and be hazardous for all people.

Purchase an indoor air filter and recirculate air

When the local Air Quality Index indicates the outside air could be hazardous, people should stay indoors as much as they can.

“The outdoor levels are reduced by anywhere from 20 percent to 50 percent just coming indoors,” Steven Chillrud, a research professor at Lamont-Doherty Earth Observatory of Columbia University, told NY1 in 2023.

Experts urge people to close doors and windows to keep the smoke out. They should also put their air conditioning unit on the “recirculation” function if possible, so that it doesn’t bring in air from outside. People can even put wet towels under their windows and doors to provide another barrier to smoke coming in.

In addition, scientists say people can purchase portable HEPA air filters for their homes, which will further clean the air of particulate matter that might get inside a house or apartment.

Limit time and activity outside

For people who have to go outside, including for work and other urgent reasons, experts say they should try to limit their time outdoors. (Employers can help by curbing the amount of time employees need to spend outside or reschedule tasks.)

The American Lung Association encourages people to keep outdoor exposures to under 30 minutes if the AQI is high. “The chances of being affected by unhealthy levels of air pollution increase the longer a person is active outdoors and the more strenuous the activity,” the Association notes.

Per a CBS News report, people are safe to go outside if the AQI is 100 or below, but ratings higher than that could require more precautions.

Wear a mask

When the AQI is high, people can protect themselves by wearing a mask like a well-fitted KN95 or N95. Those can filter out some particulate matter, while a P100 mask is seen as the most effective option because it can keep out the finest particles. Cloth and surgical masks, however, won’t be as effective.

Individuals with heart and lung conditions should also be especially careful because they could experience the worst health effects from wildfire smoke.

“People with asthma and people who already have lung disease or underlying lung problems — it can exacerbate that, it can irritate that. And if the air quality is bad enough, it can even cause some symptoms of feeling unwell and respiratory symptoms in people who are healthy,” Stephanie Widmer, a member of ABC News’ Medical Unit, previously said.