As the historically deadly wildfires continue to rage in parts of Northern and Southern California, the resulting smoke poses a significant risk to people in the surrounding areas.

Between the fires in the San Fernando Valley outside of San Francisco and those in Ventura County near Los Angeles, millions of people have been exposed to air pollution from the smoke. The Woolsey fires are consuming the towns of Thousand Oaks and Malibu, among others, putting the residents of Los Angeles at an even greater health risk due to wind patterns.

“In Ventura, a lot of the cities in the nearby areas are upwind of the fires. So there were times over the weekend that they were getting pretty bad smoke exposure, but the smoke exposure was actually worse in the city of Los Angeles,” Dr. Suzanne Paulson, professor of atmospheric and oceanic sciences and director of the Center for Clean Air at UCLA, tells PEOPLE.

Paulson explains how different wind types may affect air pollution: “We have two typical wind regimes in Los Angeles. One is the Santa Ana winds that blow very strongly from the land to the ocean, and unfortunately, they’re also associated with very dry weather, so they make the fire much worse,” she says. “What happened this weekend was a big smoke cloud over the ocean from the Santa Ana winds, and then the winds reversed to the other normal regime, the sea breeze, which brings air from the ocean over the city. So the sea breeze brought the smoke flume over the city in a huge way.”

And when smoke comes into the area, it’s “absolutely dangerous” for people to be outside, she says.

That’s why she advises people to stay indoors when smoke levels are high. If that’s not an option, she suggests using a protective device — beyond just a thin mouth covering.

“Most of the things people wear are a waste,” she says. “A bandana does nothing, and a surgical mask does nothing.”

Instead, Paulson recommends N95 respirators, which are available at hardware stores for around $10.

“If they fit tightly around the face so the air goes through them and not around them, they are quite useful at reducing exposure,” she says. “They’re not a great inconvenience, and people should absolutely be wearing them if the air quality is bad and they’re outside and moving around.”

Otherwise, she says, exposure to the smoke may become problematic. “It causes coughing, and it may feel like your eyes are burning. It can feel tight in the chest and like you have trouble breathing,” Paulson says. “It’s absolutely dangerous to go outside when the smoke is at its worst. It’s not going to make you drop dead on the spot, but exposure to elevated air pollution degrades your health slightly, and the more air pollution exposure, the more your overall health is degraded over your life.”

While most people will not experience lasting effects from the smoke, Paulson says, the particles will remain in their lungs for “many years,” which, combined with other health issues, could create bigger problems. And the outlook is worse for people with pre-existing conditions.

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“People who have pre-existing conditions could have more significant health effects, like people who are asthmatic, or have cardiovascular conditions,” she says. “If they go outside and move around a lot, they could potentially have an episode.”