Smoke from the West Coast’s wildfires last year didn’t only tinge skies an apocalyptic shade of orange; it increased the number of COVID-19 cases and deaths.

According to research unveiled on Friday, the heavy wildfire smoke that blanketed Washington, Oregon, and California in 2020 significantly exacerbated the pandemic, causing 19,742 additional cases of COVID-19 and 748 additional deaths across the three states.

Although previous research has documented the connection between wildfire smoke and COVID-19, this is the first time that scientists have calculated the specific toll from the pandemic that is attributable to last year’s wildfires.

“We weren’t terribly surprised by the results as scientists,” said Kevin Josey, a postdoctoral fellow at the Harvard T.H. Chan School of Public Health who co-authored the new research. “But as humans, we are dismayed about the impacts.”

The study, which was published in the journal Science Advances, chalked those impacts up to fine particulate matter — called PM 2.5, because the particles are 2.5 micrometers or less in diameter — in wildfire smoke. Those tiny pieces of ash can comprise everything that a fire has burned in its wake, including heavy metals like iron, zinc, and nickel. When inhaled, these particles can lodge deep into the lungs, making people more susceptible to respiratory disease. PM 2.5 can even make their way into the bloodstream, increasing the risk of negative cardiovascular and neurological outcomes.

Previous research has suggested that this system-wide stress caused by inhaling small particulate pollution can make it easier for a mild case of COVID-19 to become a severe one, potentially causing more adverse symptoms in cases that would otherwise have been asymptomatic. Other studies have found evidence that PM 2.5 may weaken our antiviral immune response systems, allowing infections like the novel coronavirus to take greater hold in certain individuals.

Read the full article about wildfire smoke by Joseph Winters at Grist.