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US wildfire smoke may have contributed to thousands of extra COVID-19 cases and deaths

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, according to a Harvard study.

This 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 as the nation was contending with the COVID-19 pandemic, huge wildfires swept across the western US, 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 John A. Paulson School of Engineering and Applied Sciences at Harvard University, and the Environmental Systems Research Institute in Redlands, California, 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 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 metre 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 September 14 to September 17, 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, 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 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, California. and Calaveras, California, 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.

Study details

Excess of COVID-19 cases and deaths due to fine particulate matter exposure during the 2020 wildfires in the United States
Xiaodan Zhou, Kevin Josey, Leila Kamareddine, Miah C. Caine, Tianjia Liu, Loretta J. Mickley, Matthew Cooper, Francesca Dominici

Published in Science Advances 13 August 2021

Abstract

Introduction
According to the National Interagency Fire Center, approximately 7 million acres of land burn every year in the United States (1). As of December 2020, more than 10 million acres were burnt in the western United States alone. In 2020, California and Washington both recorded their largest wildfires in state history.

The warming climate is expected to increase wildfire risk and, consequently, exposure to smoke. In the past four years, the United States has experienced record-breaking wildfires, leading to an increase of more than 470,000 daily exposures per year and 1.85 billion more person-days of exposure to high wildfire risk compared to 2001–2004.

Wildfire smoke contains high levels of fine particulate matter (PM2.5) the pollutant in smoke that poses the greatest risk to health. Short-term exposure to PM2.5 is associated with adverse health outcomes. According to recent research by Burke et al., wildfires contribute to up to 25% of the PM2.5 concentration in the atmosphere in the United States and up to half of PM2.5 in some regions of the western United States.

Exposure to PM2.5, specifically from wildfires, has been associated with negative health outcomes, including all-cause mortality and respiratory morbidity, as well as asthma, chronic obstructive pulmonary diseases, and others. In particular, studies have shown that short-term wildfire-specific PM2.5 exposure is linked to increases in asthma symptoms, emergency department visits for respiratory symptoms, and respiratory hospital admissions, as well as increases in risk and severity of respiratory viral infections.

Certain populations are at higher risk from exposure to PM2.5 from wildfires, including people with heart or lung disease, the elderly, children, and foetuses.

Method

We assembled a multisite time series study of 133 counties in three states (California, Washington, and Oregon) for the period from 15 March to 16 December 2020 (a total of 277 days).

We excluded from the analysis 41 counties that had missing PM2.5 daily values during wildfire days in the study period, as it would not be appropriate to impute these missing values using historical data. The remaining 92 counties cover 95.1% of population of the three states (48.8 million), with a total of 25,484 daily records at the county level.

The year 2020 brought unimaginable challenges in public health, with the confluence of the COVID-19 pandemic and wildfires across the western United States. Wildfires produce high levels of fine particulate matter (PM2.5). Recent studies reported that short-term exposure to PM2.5 is associated with increased risk of COVID-19 cases and deaths. We acquired and linked publicly available daily data on PM2.5, the number of COVID-19 cases and deaths, and other confounders for 92 western U.S. counties that were affected by the 2020 wildfires.

Conclusion

We estimated the association between short-term exposure to PM2.5 during the wildfires and the epidemiological dynamics of COVID-19 cases and deaths. We adjusted for several time-varying confounding factors (e.g., weather, seasonality, long-term trends, mobility, and population size). We found strong evidence that wildfires amplified the effect of short-term exposure to PM2.5 on COVID-19 cases and deaths, although with substantial heterogeneity across counties.

 

Harvard University TH Chan article – Wildfire smoke may have contributed to thousands of extra COVID-19 cases and deaths in western U.S. in 2020 (Open access)

 

ScienceAdvances article – Excess of COVID-19 cases and deaths due to fine particulate matter exposure during the 2020 wildfires in the United States (Open access)

 

See more from MedicalBrief archives:

 

Australian wildfires spark worrying public health effects

 

Heavy wildfire smoke may increase risk of out-of-hospital cardiac arrest

 

Cooking with wood may cause extensive lung damage — Radiological Society

 

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