Smoke from last year’s wildfires in California, Oregon, and Washington, contributed to a significant increase in COVID-19 cases and deaths in those states, according to a new study.

“The wildfires exacerbated the pandemic substantially,” says Francesca Dominici, a Harvard biostatistician and author of the study published Friday in Science Advances. Without the smoke from the fires, there might have been 19,742 fewer COVID-19 cases and 748 fewer deaths from COVID-19, the study found.

Wildfire smoke contains thousands of different compounds, but one of the most prevalent is particulate matter measuring 2.5 micrometers across—PM2.5. A well-established health threat, PM2.5 is closely monitored by the Environmental Protection Agency (EPA), providing enough data to examine its population impact.

While PM2.5 emissions from traffic and