Wildfire smoke may have led to thousands of additional COVID cases and deaths

BOSTON, Mass. — Wildfires are becoming more and more frequent across the west coast of the United States. These devastating infernos destroy everything in their path and leave lingering clouds of polluted air to boot. Now, researchers from the Harvard T.H. Chan School of Public Health report that west coast wildfires in 2020 may have also contributed greatly to the COVID-19 pandemic. Their study concludes that thousands of COVID-19 cases and deaths in Oregon, California, and Washington between March and December 2020 may be attributable to increases in smog (PM 2.5) caused by wildfire smoke.

“The year 2020 brought unimaginable challenges in public health, with the convergence of the COVID-19 pandemic and wildfires across the western United States. In this study we are providing evidence that climate change—which increases the frequency and the intensity of wildfires—and the pandemic are a disastrous combination,” says senior study author Francesca Dominici, Clarence James Gamble Professor of Biostatistics, Population and Data Science at Harvard Chan School, in a university release.

The year 2020 started like any other, but within a matter of months the U.S. found itself in a heated battle with COVID-19. At the same time, huge wildfires were also burning across the Pacific coast. Some of the largest wildfires ever recorded in California and Washington occurred in Spring 2020. All of the smoke from wildfires produce high levels of fine particulate matter (PM 2.5), or smog, which contribute to a number of health problems including asthma, chronic obstructive pulmonary diseases (COPD), and other respiratory illnesses.

Smog’s impact on the pandemic

Importantly, recent research has also uncovered a connection between PM 2.5 exposure and COVID-19 cases and deaths.

To better understand this relationship, study authors created their own statistical model capable of quantifying just how much wildfire smoke may have contributed to excess COVID cases and deaths in those three states. Specifically, the team used satellite data to examine the connection between daily readings on PM2.5 air concentrations and the number of COVID-19 cases and deaths across 92 local counties. Those areas account for 95 percent of the region’s population. Researchers also accounted for population size, weather patterns, and local social distancing and mass gathering trends.

While the wildfires were at their peak (between Aug. 15 and Oct. 15, 2020), smog levels were much higher on wildfire days than on non-wildfire days. On average, wildfire days showed 31.2 micrograms of smog per cubic meter of air (µg/m3) versus 6.4 (µg/m3). Sometimes smog recordings would reach extremely high levels. In mid-September, Mono County, California saw four straight days of PM 2.5 levels higher than 500 µg/m3. Scientists with the United States Environmental Protection Agency consider such levels to be hazardous to human health.

The study also concludes the wildfires amplified the effect of exposure to PM2.5 on COVID-19 cases and deaths up to four weeks after the fact. Across these counties, researchers linked a daily increase of 10 µg/m3 in PM 2.5 every day for 28 days to an 11.7 percent uptick in COVID-19 cases and an 8.4 percentage increase in COVID-19 related deaths. Sonoma county in California and Whitman county in Washington experienced the biggest smog-related increases in COVID cases, with a 65.3 percent and 71.6 percent increase, respectively. Meanwhile, Calaveras county in California saw a 52.8 percent increase in deaths and San Bernardino county recorded a 65.9 percent uptick.

Where is air quality doing the most damage?

Regarding individual counties and days, researchers report that Butte, California and Whitman, Washington had the highest percentages of COVID-19 cases with a link to smog. For deaths, 41 percent of COVID-19 deaths in Butte, California can be attributed to high smog levels, as well as 137.4 percent of COVID-19 deaths in Calaveras, California.

All together, study authors conclude that 19,700 COVID-19 cases and 770 coronavirus-related deaths across those three states can be attributed to daily increases in PM 2.5 from wildfires.

“Climate change will likely bring warmer and drier conditions to the West, providing more fuel for fires to consume and further enhancing fire activity. This study provides policymakers with key information regarding how the effects of one global crisis—climate change—can have cascading effects on concurrent global crises—in this case, the COVID-19 pandemic,” Prof. Dominici concludes.

The study is published in Science Advances.