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New research points to another potential factor that might play into a person’s risk of death due to Covid-19: prolonged exposure to high levels of air pollution.

In a study published Wednesday in Science Advances, researchers estimated long-term air pollution levels for more than 3,000 U.S. counties, which also had Covid-19 mortality data available through June 2020. While the study wasn’t designed to show whether pollution exposure directly affected a person’s risk of death due to Covid-19, it did demonstrate an association between increased pollution levels and higher Covid-19 death tolls.

“The results of our study suggest that in counties with high levels of pollution is where we need to implement social distancing measures now more than ever, knowing that people here will be more susceptible to die from Covid-19,” said Francesca Dominici, a professor of biostatistics at Harvard University’s T.H. Chan School of Public Health and co-author of the study.

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Previous research has linked elevated concentrations of fine particulate matter in the air, otherwise known as soot, to lung and heart diseases, neurological disorders, and premature deaths after long-term exposure. These particles, which are 2.5 micrometers in diameter or smaller, are easily inhaled and can get deep into the lungs and also enter the bloodstream.

Given that Covid-19 is also a respiratory disease, Dominici and her colleagues hypothesized that exposure to air laden with such particulate matter could exacerbate the severity of Covid-19 symptoms and its prognosis.

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“It could be a double hit to the lung function,” said Xiao Wu, a graduate student at Harvard University’s T.H. Chan School of Public Health and co-lead author of the study.

To assess the potential link between air pollution and Covid-19 death tolls across U.S. counties, the researchers first needed to estimate the fine particulate matter levels for each county.

Although there are 4,000 air monitoring stations spread across the country, these are usually in urban areas or in cities and don’t cover all counties. So, the team adopted a machine learning approach, pulling in data from satellites that capture urbanization, vegetation, agriculture, and other variables that could contribute to local and regional air pollution between 2000 and 2016. They also accounted for factors like the point in the pandemic at which health officials issued stay-at-home orders, population density, household income, education, and race.

The researchers found that even a slight spike in fine particulate matter could be associated with an 11% increase in a county’s Covid-19 death toll. But without knowledge about individual residents — and characteristics such as their age, comorbidities, and smoking status — it’s difficult to tease out the exact role air pollution might play in risk.

“What we can say is that if you’ve been living in a county with a high pollution level for a long time, on average, you might have a higher mortality risk from Covid-19,” Dominici said. “We cannot establish causality at the individual level.” The study comes on the heels of another recent paper, also co-authored by Dominici, which estimated the association between air pollution and Covid-19 mortality worldwide.

Wang-Jian Zhang, an environmental health researcher at the University at Albany who was not involved in the research, said that the role of air pollution and the increase in mortality documented by Dominici and her colleagues in the new paper “seems to be a big effect.” He said there is a question of whether the findings would change if precautionary practices like social distancing, wearing masks, or avoiding gatherings were taken into account.

With researchers across the globe regularly updating and enhancing their models to build a more nuanced understanding of air pollution’s impact on Covid-19 patients, Dominici’s study, previously published as a preprint in April 2020, was among the first to examine this connection.

“Our study speaks once again to the critical importance of environmental regulations and the adverse health effects of really fine particulate matter,” Dominici said. “Now, more than ever, we should impose more stringent regulation on air pollution.”

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