The wildfires that ripped across the Western United States last year are said to have contributed to thousands of COVID-19 cases and deaths, as researchers warn climate change will have a profound impact in human health.
A study by Harvard Chan School, the John A. Paulson School of Engineering and Applied Sciences at Harvard University, and the Environmental Systems Research Institute in Redlands, California has found that thousands of COVID-19 cases and deaths in California, Oregon, and Washington between March and December 2020 may be attributable to increases in fine particulate air pollution (PM2.5) from wildfire smoke.
The study is the first to quantify the degree to which increases in PM2.5 pollution during the wildfires contributed to excess COVID-19 cases and deaths in the US.
“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,” said Francesca Dominici, Clarence James Gamble Professor of Biostatistics, Population and Data Science at Harvard Chan School and senior author of the study.
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 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 study found that from 15 August to 15 October 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 (µg/m3) versus 6.4 (µg/m3). In some counties, the levels of PM2.5 on wildfire days reached extremely high levels. For instance, from 14 September to 17 September 2020, Mono County, California, experienced four days in a row with PM2.5 levels higher than 500 µg/m3 as a result of the Creek Fire. Such levels are deemed “hazardous” by the US 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,” according to the study. “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 µg/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, California and Whitman, Washington, with a 65.3% and 71.6% increase, respectively. The biggest effects for the COVID-19 deaths were in Calaveras, California, and San Bernardino, California with a 52.8% and 65.9% increase, respectively.
When the researchers looked at individual wildfire days and at individual counties, they found that Butte, California and Whitman, Washington had the highest percentages of total COVID-19 cases attributable to high levels of PM2.5 during the wildfires: Among the total number of COVID-19 cases that occurred in these counties, 17.3% and 18.2%, respectively, were attributable to high levels of PM2.5. Butte, and Calaveras, had the highest percentages of total COVID-19 deaths attributable to high levels of PM2.5 during the wildfires: Among the total number of COVID-19 deaths that occurred in these counties, 41% and 137.4%, respectively, were directly attributable to high levels of PM2.5.
Across the three states studied, the cumulative number of COVID-19 cases and deaths attributable to daily increases in PM2.5 from wildfires was, respectively, 19,700 and 750, the study found.
“Climate change will likely bring warmer and drier conditions to the West, providing more fuel for fires to consume and further enhancing fire activity. This study provides policymakers with key information regarding how the effects of one global crisis, climate change, can have cascading effects on concurrent global crises—in this case, the COVID-19 pandemic,” said Dominici.