The COVID-19-wildfire smoke paradox: Reduced risk of all-cause mortality due to wildfire smoke in Colorado during the first year of the COVID-19 pandemic

Abstract

Background: In 2020, the American West faced two competing challenges: the COVID-19 pandemic and the worst wildfire season on record. Several studies have investigated the impact of wildfire smoke (WFS) on COVID-19 morbidity and mortality, but little is known about how these two public health challenges impact mortality risk for other causes.

Objectives: Using a time-series design, we evaluated how daily risk of mortality due to WFS exposure differed for periods before and during the COVID-19 pandemic.

Methods: Our study included daily data for 11 counties in the Front Range region of Colorado (2010-2020). We assessed WFS exposure using data from the National Oceanic and Atmospheric Administration and used mortality counts from the Colorado Department of Public Health and Environment. We estimated the interaction between WFS and the pandemic (an indicator variable) on mortality risk using generalized additive models adjusted for year, day of week, fine particulate matter, ozone, temperature, and a smoothed term for day of year.

Results: WFS impacted the study area on 10% of county-days. We observed a positive association between the presence of WFS and all-cause mortality risk (incidence rate ratio (IRR) = 1.03, 95%CI: 1.01-1.04 for same-day exposures) during the period before the pandemic; however, WFS exposure during the pandemic resulted in decreased risk of all-cause mortality (IRR = 0.90, 95%CI: 0.87-0.93 for same-day exposures).

Discussion: We hypothesize that mitigation efforts during the first year of the pandemic, e.g., mask mandates, along with high ambient WFS levels encouraged health behaviors that reduced exposure to WFS and reduced risk of all-cause mortality. Our results suggest a need to examine how associations between WFS and mortality are impacted by pandemic-related factors and that there may be lessons from the pandemic that could be translated into health-protective policies during future wildfire events.

Keywords: COVID-19; Mortality; Wildfire smoke.

Fig. 1

Trends in weekly crude all-cause mortality rate, 2010–2020.

Fig. 2

Annual crude all-cause mortality rates (deaths per 100,000) by (partial) county for three selected years of the study period.

Fig. 3

Time series of daily population-weighted exposure variables level, 2010–2020.

Fig. 4

Incidence rate ratios for A) all-cause mortality, B) CVD mortality, C) respiratory disease mortality, and D) deaths of despair associated with the COVID-10 pandemic, the presence of wildfire smoke (lag 0) before the pandemic, and the presence of wildfire smoke during the pandemic (lag 0) for each county in the study area and the pooled estimate. Results for models for WFS alone (red dashed line) and adjusted for PM_2.5, ozone, and temperature (lag 0) (blue solid line) are shown. The pooled effect includes the estimated effect from the mixed effect model and the 95% prediction interval. Note: results for Elbert County are not shown; effect estimates for Elbert County are available in Table S2 of the supplemental materials.

Fig. 5

Incidence rate ratios for A) all-cause mortality, B) CVD mortality, C) respiratory disease mortality, and D) deaths of despair associated with the COVID-19 pandemic, an SD increase in wildfire smoke density (lag 0) before the pandemic, and an SD increase in wildfire smoke density (lag 0) during the pandemic for each county in the study area and the pooled estimate. Results for models for WFS alone (red dashed line) and adjusted for PM_2.5, ozone, and temperature (lag 0) (blue solid line) are shown. The pooled effect includes the estimated effect from the mixed effect model and the 95% prediction interval. Note: results for Elbert County are not shown; estimates for Elbert County are available in Table S3 of the supplemental materials.