Recent Study Suggests Association between Exposure to Wildfire Particulate Matter and Excess COVID-19 Cases and Deaths

Posted on Behalf of  Claire M. Hamaji, Brook Hemphill

Severe wildfires are once again raging in the western United States and Canada, with the resulting wildfire smoke stretching across the continent and covering large swaths of the country (AP, 2021; Popovich & Katz, 2021; Isai, 2021). As noted in our previous blog on Wildfires and Respiratory Illness: Implications for the COVID-19 Pandemic, an increasing number of studies suggest that fine particulate matter 2.5 (PM2.5) from wildfire smoke can exacerbate numerous respiratory illnesses, including COVID-19. A recent study by Zhou et al. (2021) adds to the body of research on this topic.

Zhou et al. (2021) quantified the degree to which COVID-19 cases and deaths were associated with increased PM2.5 concentrations throughout the western United States during the 2020 wildfire season.  The authors hypothesized that short-term exposure to PM2.5 may increase the possibility of:

  1. Asymptomatic COVID-19 infections developing into symptomatic infections and documented cases;
  2. More severe COVID-19 infections that lead to death.

In this study, daily data of PM2.5 concentrations and COVID-19 cases and deaths spanning from March 15, 2020 to December 16, 2020 from counties in Oregon, Washington and California were collected and analyzed.  Specifically, the authors:

  1. Estimated the percent increase of COVID-19 cases and deaths correlating to the daily increase of PM5 up to 28 days after exposure;
  2. Estimated the percentage of all COVID-19 cases and deaths attributed to high PM5 concentrations during the wildfire season within each county;
  3. Conducted several sensitivity analyses using different lag periods, as well as adjustments for confounding environmental effects (e.g., mobility; weather; seasonality).

The authors state that their results suggest (1) “strong evidence of a positive association” between short-term exposure to PM2.5 and likelihood of COVID-19 cases and mortalities four weeks later; (2) high PM2.5 concentrations caused by the 2020 wildfire season “substantially exacerbated the health burden of COVID-19” and finally; (3) the data and results were “fully reproducible” as demonstrated through the numerous sensitivity analyses. The authors concluded that high PM2.5 concentrations from wildfire days (defined as days in which PM2.5 concentrations exceeded 21 ug/m3) in 2020 in the numerous counties among the three states analyzed were a key factor in the observed association between daily increases in PM2.5 and increased risk of COVID-19 cases and deaths.   

Several limitations to the Zhou et al. study should be noted, however, including, but not limited to:

  1. All sources of PM5 (e.g., wildfire; traffic; ports; agricultural; soil; industrial plant emissions) were treated as having the same health impacts, even though emerging studies suggest that wildfire-specific PM2.5 is potentially more toxic to human health than equivalent doses from other PM2.5  sources (e.g., Aguilera et al., 2021);
  2. 41 out of 133 counties, including some Northern California counties where wildfires occurred in 2020, were omitted from the analysis because of missing wildfire PM5 data;
  3. Information on COVID-19 testing rates was not available, thereby introducing uncertainty regarding COVID testing data completeness.
  4. A wide range of heterogeneity was observed in the results, indicating a protective effect in a small number of counties where increased wildfire PM5 concentrations were correlated with decreased COVID-19 cases and deaths.

Additional research is needed to assess the biologic plausibility of these findings. This study presents further evidence of an association between wildfire smoke PM2.5 concentrations and COVID-19 cases and mortality rates. Employers and individuals can take action now, however, to develop and implement risk management strategies in order to reduce the exposure risk to SARS-CoV-2 and wildfire smoke. In their recent paper, for example, Zisook et al. (2020) illustrate key risk management concepts that could be adapted to address the combined risks of COVID and wildfire PM2.5 exposure.

Cardno ChemRisk is experienced at characterizing and addressing potential health risks to workers and community members from wildfire smoke and COVID-19. To learn more about the ways Cardno ChemRisk can support your business, please contact Claire Hamaji, Melanie Nembhard, or Rachel Zisook, or visit our website.