WASHINGTON — Environmental Protection Agency chief Scott Pruitt on Wednesday praised significant improvements in the nation’s air quality, even as he moves to roll back regulations aimed at making further gains.

An EPA report released Wednesday shows that in the 45 years since passage of the Clean Air Act, emissions of six harmful pollutants declined by a combined 73 percent even as U.S. economic output tripled.

The biggest gains were made cutting emissions of lead and sulfur dioxide, which declined 99 percent and 85 percent, respectively, between 1990 and 2016.

“Despite this success, there is more work to be done,” Pruitt said in a statement. “Nearly 40 percent of Americans are still living in areas classified as ‘non-attainment’ for failing to achieve national standards. EPA will continue to work with states, tribes, and local air agencies to help more areas of the country come into compliance.”

According to the report, the least progress has been made in reducing ground-level ozone, which is down by 22 percent since 1990.

Ground-level ozone is created when common pollutants emitted by cars, power plants, oil refineries, chemical plants and other sources react in the atmosphere to sunlight. It can cause serious breathing problems among sensitive groups of people, contributing to thousands of premature deaths each year.

Since his appointment by President Donald Trump earlier this year, Pruitt has moved to block or delay several Obama-era regulations aimed at reducing pollutants caused by burning fossil fuels.


Follow Associated Press environmental writer Michael Biesecker at www.Twitter.com/mbieseck