After declining for a decade, pollution in the US is now seeing an increase in the years following the 2016 election, a “worrisome” trend that researchers say can have dire consequences for human health.

According to a working paper released by the National Bureau of Economic Research, annual average fine particulate matter (PM2.5) in the US increased in 2017 and again in 2018 by 5.5 percent after declining 24.2 percent between 2009 and 2016.

“The health implications of this increase in PM2.5 between 2016 and 2018 are significant. The increase was associated with 9,700 additional premature deaths in 2018,” the authors write. “At conventional valuations, these deaths represent damages of $89 billion.”

Drawing over 1.8 million daily air quality readings from the Air Quality System database provided by the US Environmental Protection Agency, the researchers analyzed three major types of pollution – ammonium nitrate, sulfate, and elemental carbon – allowing to trace the sources of pollution. For example, nitrogen is associated with industrial, household, and mobile sources while sulfur dioxide can be traced to coal-fired power plants.

Over the course of the study period, PM2.5 increased at monitoring stations in the West and Midwestern parts of the country and flatlined in the Northeast and South. Three factors may have played a role in the increase: changes in economic activities, increased wildfire activity, and a lack of regulatory enforcement.

Firstly, increases in economic activities can influence air quality. Over the timeframe studied, the US saw decreases in coal consumption but increases in natural gas and fuel used by vehicles. Coal consumption fell by nearly one-third but natural gas consumption increased across the country in households, power plants, and other industry-related sources by that same amount.

Increase in wildfires “frequently occur in the West and affect air quality in the Midwest,” notes the study. Last year, California saw the deadliest wildfires in the state’s history that resulted in air quality alerts across the continent from toxic smoke and more than 150,000 evacuations. Experts warn that such levels of destruction may soon become the norm.

Lastly, the researchers note that decreases in Clean Air Act enforcement actions from 2013 have similarly contributed to an increase of PM2.5 across the nation.

“Enforcement may influence firms’ compliance with the Clean Air Act,” wrote the authors in the study. To explore enforcement, they used a database from the EPA’s ECHO database focused on actions resulting in a penalty for violations of section 113d of the Act with over 3,000 incidents in the database since 2009.

“These declines could have been driven by many factors including increased compliance levels or changes in enforcement practices. The decline in enforcement actions, however, is concerning in light of the increases in air pollution… after 2016,” the authors note.

Eighty percent of the burden of air pollution is felt by the elderly, but its impacts can be felt across all ages. A study published earlier this year found that air pollution now kills more people than smoking and has been linked to a variety of health effects, including obesity, miscarriage, and autism.