When air pollution meets comorbidities: evidence on the causal effect of PM2.5 on COVID-19 hospitalizations and deaths.

BACKGROUND: Despite growing evidence on the health effects of air pollution, few studies have examined its impact on COVID-19 hospitalizations and deaths, and none have explicitly accounted for preexisting comorbidities in a causal framework. This study addresses this gap by investigating the relationship between PM2.5 exposure and COVID-19 severity. We place particular emphasis on the role of comorbidities, which can increase vulnerability to COVID-19 and may interact with air pollution exposure. METHODS: We use an exhaustive dataset covering the entire French territory over 2020-1, totaling more than 3.5 million weekly observations. The data include information on COVID-19 mortality and hospitalizations, disaggregated by age, sex, and municipality, along with detailed records on preexisting conditions. To address endogeneity, we employ a two-stage least squares approach, using wind direction as an instrument for local PM2.5 concentrations. RESULTS: We find no significant effect of PM2.5 exposure on COVID-19 mortality or hospitalizations among individuals without preexisting conditions. However, when accounting for comorbidities, a 10 μg/m3 increase in PM 2.5 was associated with higher COVID-19 mortality rates (1.5 per 100 000, 95% CI: 0.8-2.2), particularly among patients with respiratory (0.9 per 100 000, 95% CI: 0.4-1.4), and cardiovascular diseases (0.6 per 100 000, 95% CI: 0.1-1.1). CONCLUSIONS: These findings underscore the importance of incorporating comorbidities when assessing the health effects of air pollution during a pandemic. Our results also show that the effects of PM2.5 are driven by the oldest age groups, who are both more vulnerable to COVID-19 and more likely to have comorbidities.