Fine particulate matter has been on the rise in the U.S. since 2016, a disturbing reversal in recent U.S. air pollution trends for environmentalists.

A new study released this October by the National Bureau of Economic Research (N.B.E.R.) found that fine particle air pollution in the U.S. has been on the rise since 2016. The upward trend in air pollution comes after a period of consistent improvement in air quality from 2009 to 2016.

Notably, the study shows air pollution improved during the Obama administration but has shown a reversal under President Trump. The Trump administration has been heavily criticized by environmentalists for rolling back environmental protections, opening the Arctic National Wildlife Refuge to drilling and withdrawing the U.S. from the Paris Climate Accord, among other grievances.

Air Pollution On the Rise

According to the N.B.E.R. report, “Annual average fine particulate matter (called PM2.5) in the United States in counties with monitors increased by 5.5% between 2016 and 2018.” The 5.5% increase comes after a decline in PM2.5 by 24.2% during the 2009 to 2016 period.

Per the Clean Air Act, the Environmental Protection Agency (E.P.A.) monitors PM2.5 levels with air quality monitors at locations throughout the country. The N.B.E.R. study was conducted using the E.P.A.’s data by two Carnegie Mellon economists, Karen Clay and Nicholas Z. Muller.

The E.P.A. defines PM2.5 as “fine inhalable particles, with diameters that are generally 2.5 micrometers and smaller,” meaning particles that can be breathed in by humans and travel through the respiratory tract into the lungs.

PM2.5 particles can cause short term health problems like eye, nose and throat irritation, coughing, sneezing, runny nose and shortness of breath. However, PM2.5 can also have long term health implications and affect lung function, asthma and heart disease.

The N.B.E.R. report described the health implications of the uptick in PM2.5 particles as “significant,” explaining the increase was “associated with 9,700 additional premature deaths in 2018.”

“At conventional valuations, these deaths represent damages of $89 billion,” added the report.

What’s Causing the Rise in Air Pollution?

Clay and Muller pointed to three possible reasons for the increase in air pollution: stronger economic growth in recent years, an increase in wildfires and weakened federal clean air protections. But the authors acknowledged they had not found any direct links between these possible factors and the increase in air pollution.

Economic growth may be leading to more trucks and vehicles on the road emitting more polluting particles as well as an increase 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, the economists wrote. “We conclude that the effect is due to diesel vehicles as well as some industrial boilers.”

The study also noted California had a disproportionately large share of the casualties that resulted from the increase in air pollution due, the study’s authors believed, in part to California’s recent wildfires and vehicle pollution.

“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. But the authors noted that while wildfires may have contributed to the increase in air particulates, they don’t account for the overall pattern and downward trend.

Cal Streets Blog notes that the rise in pollution may be due to a switch from coal to natural gas:

“Between 2009 and 2018, coal consumption by the electricity sector fell by about thirty percent, which cut the amount of sulfur in particulates. In many cases, coal was replaced by natural gas. Over the same time period, the electricity sector’s consumption of gas increased by over fifty percent. This is reflected in a decrease in sulfate – which comes from burning coal – and an increase in nitrates, from burning natural gas.”

As for the impact of the Trump administration’s relaxing of environmental regulations, the study’s authors found that between 2013 and 2018 the number of EPA enforcement actions had “decreased dramatically over time.” But the decrease in enforcement actions could have numerous explanations from increased compliance with EPA laws to decreased monitoring and enforcement.