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Exposure to air pollution and COVID-19 mortality in the United States

Our take —

This ecological study of 3,087 counties in the United States (covering ~98% of the population), published as a preprint and thus not yet peer reviewed, found a statistically significant, positive association between average levels of fine particulate matter (PM-2.5) for the period 2000-2016 and COVID-19 death rates of counties up to April 22, 2020. Findings remained robust even after several sensitivity analyses. However, given the ecological study design and the possibility that confounding factors may be unaccounted for, particularly as COVID-19 is still a new and evolving disease, caution should be taken in interpreting the study findings. These ecological results should be interpreted with caution and followed up with individual-level designs and exploration of potential mechanisms for the observed associations.

Study design


Study population and setting

The aim of this study was to examine the association between long-term air pollution exposure (2000-2016) and COVID-19 mortality during the first wave of the COVID-19 pandemic in the United States. The pollutant of interest was particulate matter <2.5 micrometers in diameter (PM-2.5). Particulate matter, which refers to the mix of solid particles and liquid droplets found in the air, come in various shapes and sizes, and can contain hundreds of different chemicals. PM-2.5 are of particular interest because their small size allows them to penetrate deep into the lungs, and even into the bloodstream. In order to estimate average PM-2.5 levels per county, the authors first estimated monthly PM-2.5 exposure levels at 0.01 degree X 0.01 degree grid resolution across the US—combining satellite, modeled and monitored PM-2.5 data in a geographically weighted regression. Values for all grid points within a zip code were then averaged. Then, the PM-2.5 levels were averaged across all zip codes in each county. The authors obtained COVID-19 death counts for 3,087 counties in the United States (covering ~98% of the population) up to April 22, 2020. Death counts were obtained from the Johns Hopkins Center for Systems Science and Engineering Coronavirus Resource Center which summarizes data reported by CDC and state health departments. Mortality rate per county was calculated as the total COVID-19 deaths in the county divided by the county’s population size. A negative binomial mixed model incorporating a random intercept by state to account for correlation in counties within the same state, and several potential confounding factors (e.g., time since stay at home orders, population density, median household income, percent black, percent Hispanic, percent current smokers) was fit to assess the relationship between the PM-2.5 level and COVID-19 deaths outcome.

Summary of Main Findings

Higher COVID-19 death rates were observed in the Mid-Atlantic, upper Midwest, and Gulf Coast regions of the US, similar to patterns for high population density and high PM-2.5 levels. After adjustment for covariates, the mortality rate ratio was 1.08 (95% CI: 1.02, 1.15), indicating that for each 1 ug/m^3 increase in average PM-2.5 levels, there was an associated statistically significant 8% increase in the COVID-19 death rate.

Study Strengths

Use of an extensively cross-validated PM-2.5 prediction model to estimate each county’s long-term average PM-2.5 level, and several sensitivity analyses to examine robustness of the findings.


This was an ecological study, with data available at the county level only. Thus, these findings cannot be used to make individual-level inferences. In addition, limited testing capacity in the country at the time of the analyses made it difficult to accurately quantify number of COVID-19 cases, a limitation somewhat diminished by utilization of death versus diagnosis. It is unclear whether the period examined for PM-2.5 (2000-2016) may be most relevant for the COVID-19 death outcome (assessed in 2020).

Value added

This was the first nationwide study to examine associations between long-term PM-2.5 exposure and COVID-19 death rates in the United States. This study provides a base for further investigations into how air pollution might impact infection and mortality from COVID-19.

This review was posted on: 22 September 2020