US air is getting cleaner, but not as fast as was thought George Frey / Bloomberg via Getty

Air pollution levels are falling in the US – but not as rapidly as the US Environmental Protection Agency thinks they are.

The US has been reducing its emissions of nitrogen oxides (NO x ) and carbon monoxide for about 50 years. The EPA keeps tabs on the progress, in part by calculating how technological improvements should change the emissions from vehicles and factories.

But those calculations seem to be overestimating the progress being made, according to Zhe Jiang at the University of …