Smoke from massive wildfires burning in the U.S. Pacific Northwest significantly increased coronavirus infections and death rates, a Harvard study shows.

Without the fires, there would have been almost 20,000 fewer cases and almost 750 fewer deaths, according to study author Francesca Dominici, a Harvard biostatistician. The study examined COVID-19 cases following wildfires in California, Oregon and Washington in 2020.

“The wildfires exacerbated the pandemic substantially,” Dominici told National Geographic, which reviewed the study. One culprit is a dangerous type of particulate matter called PM 2.5, which is present in vehicle emissions and smoke from forest fires.

When it enters your lungs, it interferes with the mucus that naturally filters pollutants and makes it easier for the coronavirus to enter the body, according to Sarah Henderson with the British Columbia Centre for Disease Control. 

Henderson told National Geographic that PM 2.5 also travels deep into the lungs and generates an immune response, “distracting” immune cells that would otherwise fight the coronavirus.

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