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Harvard study finds small increase in long-term exposure to PM2.5 leads to a large increase in COVID-19 death rate

A new study by researchers at Harvard T.H. Chan School of Public Health concludes that a small increase in long-term exposure to PM2.5 leads to a large increase in COVID-19 death rate, with the magnitude of increase 20 times that observed for PM2.5 and all-cause mortality.

US government scientists estimate that COVID-19 may kill between 100,000 and 240,000 Americans. The majority of the pre-existing conditions that increase the risk of death for COVID-19 are the same diseases that are affected by long-term exposure to air pollution.

The Harvard team investigated whether long-term average exposure to fine particulate matter (PM2.5) increases the risk of COVID-19 deaths in the United States.

They collected data for approximately 3,000 counties in the United States (98% of the population) up to 4 April 2020. They fit zero-inflated negative binomial mixed models using county-level COVID-19 deaths as the outcome and county level long-term average of PM2.5 as the exposure.

They adjusted by population size, hospital beds, number of individuals tested, weather, and socioeconomic and behavioral variables including, but not limited to obesity and smoking. They also included a random intercept by state to account for potential correlation in counties within the same state.

They found that an increase of only 1 μg/m3 in PM2.5 is associated with a 15% increase in the COVID-19 death rate, 95% confidence interval (CI) (5%, 25%). Results are statistically significant and robust to secondary and sensitivity analyses.


  • Xiao Wu, Rachel C. Nethery, Benjamin M. Sabath, Danielle Braun, Francesca Dominici (2020) “Exposure to air pollution and COVID-19 mortality in the United States.” medRxiv 2020.04.05.20054502; doi: 10.1101/2020.04.05.20054502


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