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Wildfire Smoke Led to Excess COVID-19 Cases and Deaths

A study by researchers from Harvard University reports that thousands of COVID-19 cases and deaths in Oregon, California and Washington, from March 2020 to December 2020, may have been caused by the higher amount of fine particulate air pollutants (PM2.5) from wildfire smoke.

Wildfire Smoke Led to Excess COVID-19 Cases and Deaths.
The Apple Fire burns north of Beaumont. Image Credit: Harvard John A. Paulson School of Engineering and Applied Sciences.

The research is the first to determine the degree to which increases in PM2.5 pollution during the wildfires added to increased COVID-19 cases and deaths in the United States. The study was published in the journal Science Advances.

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.

Francesca Dominici, Study Senior Author and Clarence James Gamble Professor of Biostatistics, Population and Data Science, Harvard T.H. Chan School of Public Health

When the United States was fighting the COVID-19 pandemic in 2020, large wildfires engulfed the western part of the country, including the ones which were regarded as the largest ever in Washington and California.

Wildfires generate high levels of fine particulate matter (PM2.5), which causes several health issues, such as asthma, premature death, chronic obstructive pulmonary diseases (COPD) and other respiratory illness. The research has also traced the link between short-term and long-term exposure of PM2.5 and COVID-19 cases and deaths.

Researchers from the 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, California, had developed and tested a statistical model to determine the level to which wildfire smoke may have added to the additional COVID-19 cases and deaths in Washington, Oregon and California.

These three states were the most affected by the 2020 wildfires.

The researchers observed the relation between daily-level and county-level data on PM2.5 air concentrations from tracing data, wildfire days from satellite data and the number of COVID-19 cases and deaths in 92 counties. This represented 95% of the population throughout the three states. The researchers considered the factors like population size, weather and societal patterns of social distancing and mass gathering.

The team was dependent on satellite data of smoke plumes to find the regions 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.

Tianjia Liu, Study Co-Author, and Fifth-Year Graduate Student, Department of Earth and Planetary Sciences, Harvard University

The research revealed that between August 15th, 2020, to October 15th, 2020, the fire activity was at its highest and the PM levels during the wildfire days were remarkably higher than on non-wildfire days. In numbers, the level had a media of 31.2 μg/m3 of air against 6.4 μg/m3.

In few counties, the PM2.5 levels on wildfire days climbed to extremely high levels. For example, between September 14th to 17th, 2020, the Mono County in California, experienced PM2.5 levels higher than 500 μg/m3 as a result of the Creek Fire that spread continuously for four days. These levels are regarded as “hazardous” by the U.S. Environmental Protection Agency.

According to the study, wildfires elevated the effect of exposure to PM2.5 on COVID-19 cases and deaths, until four weeks after the exposure. In some counties, the percentage of the total count of COVID-19 cases and deaths can be correlated to high PM2.5 levels.

Considering all the counties, the study found that, on average, a daily rise of 10 μg/m3 in PM2.5 each day for 28 continuous days led to an 11.7% increase in COVID-19 cases and an 8.4% rise in COVID deaths. Counties such as Sonoma, California, Whitman and Washington, experienced the highest COVID-19 effects, with a 65.3% and 71.6% increase, respectively.

The highest effects related to COVID-19 deaths were witnessed in Calaveras, California, San Bernardino and California, with a 52.8% and 65.8% increase, respectively. The gross number of COVID-19 cases and deaths subjected to the daily increase in PM2.5 from wildfires in the three states was 19,700 and 750, respectively.

Given projections of rising temperatures under climate change, the current trend of increasing fires in the West is likely to continue, with consequences for respiratory diseases such as COVID-19. Warmer temperatures dry out vegetation, which then becomes fuel for fire. Just this week, the Intergovernmental Panel on Climate Change named the western United States as one of several regions likely to experience enhanced fire activity in future decades.

Loretta J. Mickley, Study Co-Author and Senior Research Fellow, Harvard John A. Paulson School of Engineering and Applied Sciences

The study was co-authored by Xiaodan Zhou of the Environmental Systems Research Institute and Kevin Josey from the Department of Biostatistics at Harvard Chan School. Leila Kamareddine of the Department of Biostatistics at Harvard Chan School also contributed, as did Miah C. Caine from SEAS.

The research was financially supported by the Environmental Protection Agency, the National Institutes of Health and the Alfred P. Sloan Foundation, and the Vice Provost for Research-Harvard University.

Journal Reference:

Zhou, X., et al. (2021) Excess of COVID-19 cases and deaths due to fine particulate matter exposure during the 2020 wildfires in the United States. Science Advances.


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