People with COVID-19 who live in U.S. regions with high levels of air pollution are more likely to die from the disease than people who live in less polluted areas, according to a new nationwide study from Harvard T.H. Chan School of Public Health.

The study, which has not yet been peer reviewed, was published on the preprint server MedRxiv. It is the first study to look at the link between long-term exposure to fine particulate air pollution (PM2.5)—generated largely from fuel combustion from cars, refineries, and power plants—and the risk of death from COVID-19 in the U.S.

The study looked at more than 3,000 counties across the country, comparing levels of fine particulate air pollution with coronavirus death counts for each area. Adjusting for population size, hospital beds, number of people tested for COVID-19, weather, and socioeconomic and behavioral variables such as obesity and smoking, the researchers found that a small increase in long-term exposure to PM2.5 leads to a large increase in the COVID-19 death rate.

The study found, for example, that someone who lives for decades in a county with high levels of fine particulate pollution is 8%* more likely to die from COVID-19 than someone who lives in a region that has just one unit (one microgram per cubic meter) less of such pollution.

The study suggests that counties with higher pollution levels “will be the ones that have higher numbers of hospitalizations, higher numbers of deaths and where many of the resources should be concentrated,” said senior study author Francesca Dominici, Clarence James Gamble Professor of Biostatistics, Population, and Data Science at Harvard Chan School, in an April 7, 2020 New York Times article.

The new findings align with known connections between PM2.5 exposure and higher risk of death from many other cardiovascular and respiratory ailments. The researchers wrote, “The study results underscore the importance of continuing to enforce existing air pollution regulations to protect human health both during and after the COVID-19 crisis.”

Lead authors of the study were Xiao Wu, doctoral student, and Rachel Nethery, assistant professor of biostatistics. Other Harvard Chan School authors included research assistant Benjamin Sabath and research scientist Danielle Braun.

Read the New York Times article: New Research Links Air Pollution to Higher Coronavirus Death Rates

*This article was updated on May 5, 2020 based on updated analyses from the researchers, using data through April 22, that adjust for epidemic characteristics, including time since outbreak began and time since states’ stay-at-home orders were issued.