Study Links Excess COVID-19 Cases and Deaths to Wildfire Smoke

Firefighter Raymond Vasquez battles the Silverado Fire in Irvine, Calif., on Oct. 26, 2020. Jae C. Hong/AP Photo

New research indicates that exposure to pollutants in wildfire smoke may have led to thousands more COVID-19 cases and deaths.

The new study, published in the journal Science Advances, links fine particle pollution known as PM2.5, which measures 2.5 micrometers across and is produced by wildfires, among other sources, to a significant increase in COVID-19 cases and deaths in California, Oregon, and Washington state.

“The wildfires exacerbated the pandemic substantially,” Francesca Dominici, a Harvard biostatistician and author of the study, said in remarks to National Geographic.

The study sought to gauge the effect of last year’s wildfires in the three states on excess COVID-19 cases and deaths by evaluating their correlation with data on short-term PM2.5 exposure, while seeking to account for a number of confounding factors, including weather, seasonality, long-term trends, mobility, and population size.

Evaluating data from 92 counties affected by fires, the study’s authors “found strong evidence that wildfires amplified the effect of short-term exposure to PM2.5 on COVID-19 cases and deaths, although with substantial heterogeneity across counties.”

Heterogeneity refers to clinical, statistical, or methodological variability among studies in a systematic review.

Members of the Cal Fire San Mateo-Santa Cruz Unit (CZU) march along Old Lawley Toll Road during the Glass Fire in Calistoga, Calif., on Oct. 2, 2020. Stephen Lam/Reuters

For COVID-19 cases, the study found that 52 of 92 counties had “strong evidence” of a positive association between PM2.5 and a heightened risk of contracting COVID-19, which is caused by the CCP (Chinese Communist Party) virus.

While there were sharp differences across counties, when pooled across multiple counties, the results indicated that a daily increase of 10 micrograms per cubic meter in PM2.5 for 28 subsequent days was associated with an 11.7 percent increase in COVID-19 cases. In two counties with the highest impact—Sonoma, California, and Whitman, Washington—the researchers concluded that the same level of PM2.5 over the same time horizon was associated with a 65.3 percent and 71.6 percent increase in infections, respectively.

There was also significant variability among counties in terms of COVID-19 deaths, with 17 of 92 counties reflecting “strong evidence of a positive association” between PM2.5 and fatalities. Under the same PM2.5 exposure parameters as above, the researchers found an 8.4 percent overall increase in deaths, with two California counties—Calaveras and San Bernardino—coming in at 52.8 percent and 65.9 percent, respectively.

The study concluded that the PM2.5 in wildfire smoke likely accounted for 19,742 more COVID-19 cases and 748 more deaths than would have occurred absent the fine pollutant.

“We found strong evidence of a positive association between daily increases in PM2.5 and increased risks of COVID-19 cases and deaths, cumulatively up to 4 weeks,” the researchers wrote.

While scientists continue to learn more about how wildfires affect human health, an expert told National Geographic that PM2.5 could make it easier for the CCP virus to enter the body by compromising certain cells that help expel various pathogens.