• Posted August 16, 2021

Double Trouble: Wildfires Can Raise COVID Risks

The wildfire smoke now smothering wide portions of the United States isn't just stinging eyes and tightening chests -- it also might be contributing to the current surge of severe COVID-19 cases.

Data from three Western states subject to frequent wildfires shows that COVID-19 cases and deaths increase with the amount of smoke pollution in the air, according to a new study.

As wildfires raged last year, a daily increase in fine particle pollution of just 10 micrograms per cubic meter (µg/m3) of air was associated with an average 12% increase in COVID-19 cases and an 8% increase in COVID-related deaths over the course of a month, said lead researcher Xiaodan Zhou. She is a statistician with Environmental Systems Research Institute (ESRI) in Redlands, Calif.

In other words, thousands of COVID-19 cases and deaths in California, Oregon and Washington between March and December 2020 could be attributable to air pollution caused by wildfire smoke, the researchers concluded, though the study did not prove cause and effect.

The link between wildfire smoke and a respiratory disease like COVID-19 makes sense to Dr. Amesh Adalja, a senior scholar with the Johns Hopkins Center for Health Security, in Baltimore.

"It's biologically plausible that particulate matter [PM] in wildfires, which are lung irritants that causes inflammation, could synergize with COVID making symptoms more likely and more severe," Adalja said. "Not surprisingly, this study demonstrates an association."

The findings suggest yet another reason for people to get vaccinated, Zhou advised.

"We show that wildfire smoke can exacerbate COVID-19 conditions, that is once people contract COVID-19 in the first place," Zhou said. "We are fortunate to have available safe and effective vaccines that prevent COVID-19 in the first place. The most direct action we can take in ending this pandemic is by getting as many people vaccinated as possible."

Wildfire smoke already has been linked to premature death, asthma, COPD and other respiratory illnesses, the researchers noted.

For this study, published Aug. 13 in the journal Science Advances, Zhou and colleagues created a statistical model that evaluated county-level pollution data and COVID-19 counts from 92 counties in the three states. The counties represented 95% of the population of the states.

The investigators found that from Aug. 15 to Oct. 15, 2020, the greatest fire activity of the burn season had filled the skies with significantly higher levels of fine particle pollution for days and weeks on end.

For instance, Mono County, Calif., experienced four days in a row in September 2020 with PM 2.5 levels higher than 500 µg/m3 -- levels deemed "hazardous" by the U.S. Environmental Protection Agency.

Those same counties also experienced an increase in COVID-19 cases and deaths. After controlling for other factors like weather and population size, the researchers found an association between long-term increases in air pollution from wildfire smoke and the number and severity of COVID-19 cases.

In some cases, wildfires had an effect on COVID-19 cases and deaths that lasted up to four weeks, the study authors added.

A daily increase in particle pollution of 10 µg/m3 of air was associated with a 53% increase in COVID-19 deaths in the counties of Calaveras, Calif., and a 66% increase in San Bernardino, Calif. -- the largest effect researchers observed.

The same level of increased air pollution was linked to the greatest rise in COVID-19 infections in the smoke-choked counties of Sonoma, Calif., and Whitman, Wash., with increases of 65% and 72%, respectively.

On days when wildfires were raging, the smoke they produced caused the highest percentage of excess deaths in the counties of Butte, Calif., and Calaveras, Calif., according to the report

Among the total number of COVID-19 deaths that occurred in these counties, 41% and 137%, respectively, were attributable to high levels of particle pollution on wildfire days.

The counties of Butte, Calif., and Whitman, Wash., had the highest percentages of total COVID-19 cases attributable to high levels of fine particle pollution on wildfire days, the findings showed.

Among the total number of COVID-19 cases that occurred in these counties, 17% and 18%, respectively, were attributable to high levels of fine particle pollution during wildfires.

More information

The U.S. Centers for Disease Control and Prevention has more about COVID-19.

SOURCES: Xiaodan Zhou, statistician, ESRI, Redlands, Calif.; Amesh Adalja, MD, senior scholar, Johns Hopkins Center for Health Security, Baltimore; Science Advances, Aug. 13, 2021, online