As the U.S. economy has grown, the gains have coincided with a reverse in improvements in the nation's air quality.

A new study released Tuesday found that the amount of pollutants in U.S. air rose between 2016 and 2018 after seven straight years of improvement. Two economists who conducted a study of Environmental Protection Agency data found that particulate matter air pollution dropped 24 percent in the U.S. from 2009 to 2016 but increased 5.5 percent the next two years, which correlated with thousands of premature deaths.

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“That increase was associated with 9,700 premature deaths in 2018,” Karen Clay and Nicholas Muller, economists with Carnegie Mellon, wrote in a paper published by the National Bureau of Economic Research.

The researchers concluded that the deaths represent $89 billion worth in damages.

Clay and Muller attributed the rise in air pollution to a recent stronger economy, a rise in wildfires and weakened federal “clean air” rules, according to an expanded version of the paper reviewed by CBS News.

The researchers found that a rise in economic activity has led to more trucks and other pollution-emitting vehicles on the roads along with a rise in manufacturers' carbon emissions.

“The chemical composition of particulates point to increased use of natural gas and to vehicle miles traveled as likely contributors to the increase” in pollution, they wrote. “We conclude that the effect is due to diesel vehicles as well as some industrial boilers.”

Clay and Muller also said an increase in wildfires in the West since 2016 could be linked to a rise in airborne carbon particles.

“Because of these large increases and the large exposed population in California, we find that nearly 43% of the increase in deaths nationally from 2016 to 2018 occurred in California,” they said.

According to government data, wildfires release high amounts of carbon dioxide and other damaging chemicals into the atmosphere as well as pollutants that are harmful to humans.

Lastly, Clay and Muller cited a decline in federal enforcement of the Clean Air Act as one cause of decreased air quality. The Union of Concerned Scientists, an advocacy group, credits the 1970 law with preventing hundreds of thousands of premature deaths and hundreds of millions of cases of respiratory and heart disease.