Exposure to Wildfire Smoke Increased Number of Bay Area COVID Deaths

As COVID-19 swept through the United States in 2020, epidemiological evidence shows that smoke from record-setting wildfires exacerbated the severity of the virus in several Northern California counties.

A study by University of California San Diego, San Diego State University, and California Environmental Protection Agency (CalEPA) researchers shows that for every 1,000 COVID cases from March to early November 2020 in Alameda County just outside San Francisco, an additional 58 people died from the virus because of additional respiratory damage caused by exposure to wildfire smoke. The effect was most pronounced in Alameda and San Francisco counties as a string of wildfires burned throughout Northern California.

“Investigating how environmental drivers such as wildfire smoke drive severity of COVID-19 contributes to a better understanding of the causes of severe disease and informs health systems and increases preparedness during this pandemic,” said study lead author Lara Schwarz, M.P.H., a student in the joint doctoral program in public health at the Herbert Wertheim School of Public Health and Human Longevity Science at UC San Diego and San Diego State University. “The results highlight the compounded impacts of respiratory infectious diseases and extreme weather events, two major public health issues in the context of climate change and the ongoing COVID-19 emergency.”

The August Complex fire from 2020 burned more than 1,600 square miles in Northern California in August and September, setting the record for the largest wildfire in state history only two years after the record had been set by the Mendocino Complex fire, which followed the record set by the Thomas Fire less than a year prior. Smoke plumes can reduce air quality across large geographic areas far beyond the original wildfire perimeters producing adverse health effects for populations living in these areas. Heavy wildfire smoke covered at least 70 percent of the area of the six counties studied in Northern California at various times between Aug. 19 and Aug. 22, 2020 and persisted into the following month.

To investigate the impacts of wildfire smoke on COVID-19 severity, the study design capitalized on the random timing of the wildfire occurrence and compared the case fatality ratio trends in impacted counties to the trends in control counties using an approach called synthetic control method. This method aims at building a control group, using information from a large set of unexposed counties in the rest of the U.S., for each county in the San Francisco Bay Area that would mimic the trends in case fatality ratios before the wildfire smoke and then compare such ratios to the synthetic control group after the smoke occurred. This allowed the authors to estimate a trend that represented the case fatality ratios that would have been observed in the six counties of interest if wildfire smoke had not occurred in these counties.

For example, between four and 58 additional deaths per 1,000 COVID cases were observed in Alameda County that were attributable to exposure to wildfire smoke for 35 days after the fire started. With this methodology, as long as an adequate synthetic control is found and assuming that no other event occurred in Bay Area counties with the same timing as the wildfire smoke, the change in COVID case fatality ratios can be attributed to the wildfire smoke.

The researchers said the coincidental overlap between the fires and the pandemic presented an opportunity to conduct a natural experiment of their combined effects on a population. According to the authors, the study’s results highlight “the need to further study these colliding crises to increase preparedness for future pandemic threats in a changing climate.”

The CalEPA-funded study appears in the journal Environmental Research Letters. Co-authors include Anna Dimitrova, Rosana Aguliera, and Alexander Gershunov from Scripps Institution of Oceanography at UC San Diego, Rupa Basu of CalEPA, and Tarik Benmarhnia, an epidemiologist who has joint appointments at Scripps Oceanography and the Herbert Wertheim School of Public Health.