Wildfire smoke as a driver of covid-19 fatality rates? A synthetic control analysis of San Francisco Bay Area Counties

Abstract

BACKGROUND AND AIM: The 2020 wildfire season broke records for the most active fire year on the West Coast, resulting in the worst air quality observed in decades. Concurrently, the public health threat of COVID-19 has caused approximately 3.6 million cases and 59,000 deaths in California to date. Due to the association between air pollution and respiratory disease outcomes, wildfire-specific particulate matter exposure is a hypothesized driver of COVID-19 severity. While several studies have considered this association, methodological challenges exist to study this question due to the ecological fallacy and threat of unmeasured confounders in this proposed etiological question. Additionally, no study has considered wildfire smoke as a natural experiment. This study proposes synthetic control methods (SCM) to study the impact of wildfire smoke on COVID-19 mortality rates, considering smoke exposure as a natural experiment. The San Francisco Bay Area (SFBA)—a densely populated region with record-high smoke exposure—is ideal to study this question. METHODS: The CDC COVID data tracker was used to compute weekly case-fatality ratios for each county in the United States from May 11th to November 10th, 2020. The NOAA hazard mapping Smoke Product was used to identify county-level wildfire smoke exposure; counties with area covered by heavy smoke surpassing 70% on average in a given week were considered exposed. Unexposed counties were considered as potential controls. Counties were considered eligible if more than 100 COVID-19 deaths were reported during the study period. RESULTS:Exposure to heavy smoke started between the weeks starting August 26th to September 7th in six eligible counties in which 1,307 COVID-19 deaths occurred within the study period. Results indicate some counties observed an increase in case fatality ratios, while other counties didn’t exhibit change. CONCLUSIONS:Results can be used to understand environmental drivers of COVID-19 mortality, and protect vulnerable populations from concurrent and potentially concomitant public health threats. KEYWORDS: Covid-19, wildfire smoke, synthetic control method