Air quality in the United States improved significantly between 2010 and 2016, saving thousands of lives each year, but pollution levels have started to rise again, according to a new annual review published in the Annals of Thoracic Society.

The researchers found that 12,600 people died prematurely from particulate matter and ozone pollution in 2010. By 2016, deaths more than halved, falling to 6,180. The next year, deaths rose again to 7,140.

Kevin Cromar, co-author of the study, said the Trump administration's rollback of key environmental regulations — including the Clean Power Plan, standards limiting car emissions, and rules cutting methane, a highly potent greenhouse gas — have likely caused air pollution to surge.

As of the end of 2018, the administration had eliminated or weakened at least 78 environmental regulations, according to the New York Times.

“At the federal level we’re heading in the exact opposite direction, and these [efforts] are going to result in more air pollution, worse health impacts,” Cromar told Global Citizen.  

“We need strong federal action,” he added. “But we’ve seen efforts to roll back regulation when we really need to be moving forward.”

The report looks at levels of particulate matter and ozone in the atmosphere, two hazardous forms of pollution, and found that only particulate matter levels have fallen in any significant way.

Read More: Air Pollution Harms Every Part of Your Body, Research Shows

Particulate matter includes extremely small particles of varying sizes released into the atmosphere from power plants, vehicle exhaust, construction, chemical facilities, and other forms of industrial activity. These tiny particles get breathed in, penetrate the lungs, and enter the bloodstream, where they cause damage to every part of the body.

Ozone is released by largely the same industrial activities, but it’s a gaseous molecule that primarily causes harm to the lungs when breathed in. At higher levels in the atmosphere, ozone is critical for shielding the surface of the Earth from ultraviolet rays, but high levels of ozone in the troposphere are dangerous to humans.

Deaths from ozone have hovered in the range of 4,000 per year between 2010 and 2017, while deaths from particulate matter 2.5 microns in size or smaller have fallen by nearly two-thirds, from 8,330 in 2010 to 3,260 in 2017.

Cromar said that various things can be done to lower air pollution and improve health outcomes.

Read More: 3 Ways People in Poverty Suffer the Most From Pollution

“I think moving forward, the key action is at the local level, with cities and states addressing their own emissions,” he said.

“For ozone in particular, there’s need for regional action, because emissions from a nearby state affect another state’s ozone levels,” he added.

Cromar added that the most effective action is replacing old technologies, vehicles, and equipment, with modern versions that release less emissions into the atmosphere.

“Any effort to transition to newer technology will really deliver a lot of bang for their buck,” he said.

Read More: White People Cause Most Air Pollution in US, but Black and Hispanic Communities Suffer: Report

He noted that while local and state action is crucial, federal action is irreplaceable.

“The regulations at the federal level are extremely cost effective,” he said. “When they elect not to do it, all it does it push those costs to the city and states.”

The researchers created a tool for citizens and policymakers to monitor air pollution levels and come up with solutions.

The also ranked the area’s in the country with the highest levels of air pollution. 

Here are the 10 US cities with the worst air quality:

1. Los Angeles-Long Beach-Glendale, CA | Annual premature deaths: 1,322

2. Riverside-San Bernardino-Ontario, CA | Annual premature deaths: 940

3. Bakersfield, CA | Annual premature deaths: 293

4. Pittsburgh, PA | Annual premature deaths: 232

5. Fresno, CA | Annual premature deaths: 225

6. New York-Jersey City-White Plains, NY-NJ | Annual premature deaths:188

7. Phoenix-Mesa-Scottsdale, AZ | Annual premature deaths: 152

8. Visalia-Porterville, CA | Annual premature deaths: 131

9. Cleveland-Elyria, OH | Annual premature deaths: 116

10. Chicago-Naperville-Arlington Heights, IL | Annual premature deaths: 122