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 …