According to a new study published in the journal Science Advances on Friday, Aug. 13, exposure to wildfire smoke during last summer’s wildfire season was linked to thousands of more coronavirus infections and hundreds of deaths, posing a larger challenge to public health officials.

The authors discovered that over 20% of COVID-19 cases were linked to an increase in wildfire smoke in certain counties, all of which were affected by wildfires. Smoke was connected to an even higher percentage of deaths in several counties, Insider reported.

From March to December 2020, Harvard University researchers examined data from 92 counties in California, Oregon, and Washington. They discovered “strong evidence” that short-term exposure to fine particulate matter from wildfires increased COVID-19 cases and deaths. Researchers also estimated that exposure to wildfire smoke caused almost 20,000 more coronavirus infections and 750 COVID-19 deaths, per the study, and was published in the journal Science Advances.

“These results provide strong evidence that, in many counties, the high levels of PM 2.5 that occurred during the 2020 wildfires substantially exacerbated the health burden of COVID-19,” the authors wrote.

To reach their findings, the researchers examined data from the National Oceanic and Atmospheric Administration’s satellite data on smoke plumes to identify the areas and days that wildfires occurred. These values were combined with PM 2.5 data from ground-level air quality monitors in each of the counties, as well as COVID-19 cases and death rates from Centers for Disease Control and Prevention data, according to The Seattle Times.

Their statistical model took into consideration other characteristics, including weather and how much time people spent at home and a four-week lag to account for the virus’s incubation period and the time it takes for infected people’s health to deteriorate.

John Balmes, a professor of medicine at the University of California, San Francisco, and an expert on the health effects of pollution who was not involved in the study, said the new study includes reported infections, not just deaths, which makes it particularly interesting. “It’s one thing for air pollution to be increasing the severity of the coronavirus infection, it’s another for it to be increasing reported cases,” he said.

Air pollution not only worsens COVID-19 symptoms but also increases transmission.

“When there are more particles in the air, these microbes actually have a greater chance of getting into your lungs,” Irva Hertz-Picciotto, an environmental epidemiologist at the University of California at Davis, told The Washington Post. “There’s a lot of plausibility that the wildfires, by massively increasing the amount of PM2.5 that people are breathing, could promote transmission of the virus.”

According to the study’s authors, climate change has increased the danger of wildfires and thus exposure to smoke.

“This illustrates the systemic and contingent nature of crises and how the effects of one global crisis (climate change) can have cascading effects on concurrent global crises (the COVID-19 pandemic) that play out in location-specific ways (increased COVID-19 cases and deaths due to wildfire),” the authors wrote.