After four decades of improving air quality, the U.S. has started to take a step backwards, as the number of polluted days has ticked upwards over the last two years, the Associated Press reported.

Federal data shows there were 15 percent more days with unhealthy air over the U.S. in 2017 and 2018 than there were on average from 2013 through 2016, according to the AP. In that four-year span, Americans had the fewest number of polluted days since 1980.

President Trump has claimed that air quality is improving under his watch, saying earlier this month, "We have the cleanest air in the world, in the United States, and it's gotten better since I'm president." Yet, the facts tell a different story. World Health Organization data says otherwise. When looking at fine particulate matter, a key to air pollution, the U.S. ranks tenth in the world, well below several European countries, Australia, Canada and New Zealand, as the New York Times reported. "We do not have the cleanest air and we have not crossed the finish line when it comes to pollution," said former Obama U.S. Environmental Protection Agency (EPA) Chief Gina McCarthy, now director of the Center for Climate, Health and the Global Environment at the Harvard School of Public Health, as the AP reported. Fine particulate matter is especially relevant after the EPA rolled back the Obama-era Clean Power Plan Wednesday, as EcoWatch reported. Fine particulate matter is a byproduct of the burning that frequently comes from power plants, engine exhaust and wildfires. It is also hazardous to human health, leading to asthma and respiratory inflammation, and increasing the risk for lung cancer, heart attack and stroke, according to the New York Times.