For years, the quality of the air Americans breathe had been steadily improving, but that trend is showing signs of unraveling thanks in part to U.S. economic gains that come with potentially deadly ramifications.

After a seven-year stretch of improvement, between 2016 and 2018 the amount of pollutants in the nation's air rose, according to a new analysis of Environmental Protection Agency data by two economists at Carnegie Mellon. They found that particulate matter air pollution fell 24% in the U.S. from 2009 to 2016, but it increased 5.5% the following two years.

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

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Behind the rise in air pollution

The researchers say three factors could explain the increase in air pollution: stronger economic growth in recent years, an increase in wildfires and weakened federal "clean air" rules.

The pickup in economic activity over the last few years — while a plus for workers and businesses — has resulted in more trucks and other polluting-emitting vehicles on the roads, as well as boosted manufacturers' carbon emissions, Clay and Muller noted.

"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, the economists wrote. "We conclude that the effect is due to diesel vehicles as well as some industrial boilers."

The findings could bolster theories advanced by experts such as Oxford University economist Kate Raworth who argue that unrestrained economic growth is contributing to climate change.

Clay and Muller also linked increased airborne carbon particles partly to an increase in wildfires in the West since 2016.

"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 the National Oceanic and Atmospheric Administration, wildfires release large amounts of carbon dioxide and other ozone precursors into the atmosphere, along with toxic pollutants that can harm people living both nearby and far away.

The researchers also point to a decline in EPA enforcement of the Clean Air Act as a factor that could explain worsening air quality. The law, established in 1970, is responsible for preventing hundreds of thousands of premature deaths and hundreds of millions of cases of respiratory and heart disease, according to the Union of Concerned Scientists, an advocacy group.