All six types of harmful air pollutants tracked by the Environmental Protection Agency have declined since the agency was founded in 1970, but some year-to-year gains were reduced by last year's wildfires out West.
Under the Clean Air Act, the EPA tracks levels of lead, carbon monoxide, ozone and smog. The agency said levels of pollutants like lead and smog are decreasing over the long term but that there are year-to-year changes that still need to be addressed.
"This report details a remarkable achievement that should be recognized, celebrated and replicated around the world," Andrew Wheeler, the acting administrator, said in a statement.
One of the changes from last year's report is a small increase in smog -- technically particulate matter that contains dust, dirt or smoke and can cause health problems for people with asthma or heart or lung conditions. Levels of particulate matter are still below the national standard, according to the report, but there was an uptick nationally.
"We think a lot of that is attributable to the wildfires out West, and we think that has enough of an effect that it actually influences the national average numbers," Bill Wehrum, assistant administrator for the EPA air office, told reporters.
The EPA under the Trump administration has proposed rolling back some parts of the Clean Power Plan, which was proposed under the Clean Air Act to regulate pollution from power plants. Environmental advocates pointed out that some of the gains in reducing air pollution happened under previous administrations but the current EPA has proposed more limited policies.
"Amazing what happens when political leaders at EPA enforce the Clean Air Act instead of undercutting it," David Doniger, senior strategic director of the Climate and Clean Energy program at the Natural Resources Defense Council, said in a statement.
Wehrum acknowledged that the Clean Air Act has been passed through multiple administrations and said the Trump administration is still "aggressively" implementing the core parts of the law that set maximum levels of pollutants that are then regulated at the state level.
"There certainly are areas where we disagree with what the Obama administration did and we think they got it wrong," Wehrum said. "But that doesn't mean that we're taking a step backward with regard to air pollution control and air quality. We very definitely continue to move forward in that regard."