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In 2020, at the same time the nation was contending with the COVID-19 pandemic, huge wildfires swept across the western U.S., including some of the largest ever in California and Washington. Wildfires produce high levels of fine particulate matter (PM2.5), which has been linked with a host of negative health outcomes, including premature death, asthma, chronic obstructive pulmonary diseases (COPD), and other respiratory illnesses. In addition, recent studies have found a link between short- and long-term exposure to PM2.5 and COVID-19 cases and deaths.The researchers from Harvard Chan School, the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), the Department of Earth and Planetary Sciences at Harvard, and the Environmental Systems Research Institute in Redlands, Calif.built and validated a statistical model to quantify the extent to which wildfire smoke may have contributed to excess COVID-19 cases and deaths in California, Oregon, and Washington, three states that bore the brunt of the 2020 wildfires. They looked at the connection between county- and daily-level data on PM2.5 air concentrations from monitoring data, wildfire days from satellite data, and the number of COVID-19 cases and deaths in 92 counties, which represented 95% of the population across the three states. The researchers accounted for factors such as weather, population size, and societal patterns of social distancing and mass gatherings.

The researchers relied on satellite data of smoke plumes to identify the locations and days affected by wildfires.

By combining satellite data with ground measurements of total PM2.5, we could more confidently distinguish smoke from other types of particles, said co-author Tianjia (Tina) Liu, a fifth-year graduate student in the Department of Earth and Planetary Sciences, who led the validation of the satellite data.The study found that from August 15 to October 15, 2020, when fire activity was greatest, daily levels of PM2.5 during wildfire days were significantly higher than on non-wildfire days, with a median of 31.2 micrograms per cubic meter of air (g/m3) versus 6.4 (g/m3). In some counties, the levels of PM2.5 on wildfire days reached extremely high levels. For instance, from September 14 to September 17, 2020, Mono County, Calif., experienced four days in a row with PM2.5 levels higher than 500 g/m3 as a result of the Creek Fire. Such levels are deemed hazardous by the U.S. Environmental Protection Agency.Wildfires amplified the effect of exposure to PM2.5 on COVID-19 cases and deaths, up to four weeks after the exposure, the study found. In some counties, the percentage of the total number of COVID-19 cases and deaths attributable to high PM2.5 levels was substantial.On average across all counties, the study found that a daily increase of 10 g/m3 in PM2.5 each day for 28 subsequent days was associated with an 11.7% increase in COVID-19 cases, and an 8.4% increase in COVID-19 deaths. The biggest effects for the COVID-19 cases were in the counties of Sonoma, Calif., and Whitman, Wash., with a 65.3% and 71.6% increase, respectively. The biggest effects for the COVID-19 deaths were in Calaveras, Calif., and San Bernardino, Calif., with a 52.8% and 65.9% increase, respectively.

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Wildfire smoke may have contributed to thousands of extra COVID-19 cases and deaths in western US in 2020 - Harvard School of Engineering and Applied...