The study focuses on fine-particle air pollution, known as PM2.5, which is of particular concern to regulators and public health experts because its microscopic size means it can be inhaled and absorbed into the bloodstream. Its ill effects are only now starting to be fully understood — the Environmental Protection Agency didn’t even have a regulatory standard for it until 1997.

Overall, concentrations of the pollutant have risen about 5.5 percent since 2016, and the Carnegie Mellon researchers identified several reasons for this, including rising natural gas use and people doing more driving. The corresponding rise in emissions from those sources more than offsets the falling levels being realized by the decline in coal being burned by electricity-generating plants in the United States.

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An increase in wildfires is another factor because they release large amounts of smoke and fine particles into the atmosphere. Big fires, particularly in California in 2018, played a role in driving up total national air pollution. Removing those fires from the analysis lessens the increase in pollution in 2017 and 2018 but doesn’t eliminate it, the researchers write.

A final potential driver of rising pollution is the rollback of regulatory enforcement by the Environmental Protection Agency. Clean Air Act enforcement actions fell in the first two years of the Trump administration, although the researchers note that the trend toward lax enforcement started well before 2017.

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One thing that’s clear at the moment is the effect that rising pollution is having on mortality and life expectancy. Using commonly accepted formulas for translating air pollution exposure to death rates, the Carnegie Mellon researchers estimate that in 2018, nearly 10,000 lives would have been saved had pollution levels remained at their 2016 numbers. Nearly 43 percent of those additional deaths would have happened in California, largely because of the wildfires there. Back-of-the-envelope calculations suggest the Camp Fire alone caused more than 1,400 deaths due to air pollution exposure.