> Muller and co-author Karen Clay looked at the substances hanging over U.S. cities and found evidence for three potential sources of the pollution rebound: increased economic activity, virulent wildfires and a smaller number of EPA enforcement actions. It is important to note that the pair have not made any causal links between the three suspects and the increase of air pollution.

>The U.S. economy has picked up steam overall since 2017. More economic activity means more vehicles on the roads, driving more miles and emitting more pollutants. It also means more factories producing emissions along with consumer goods.

>Soot is another contributor to the pollution jump. The economists traced a rise in airborne carbon particles in part to wildfire activity in the West since 2016. (Wildfire activity didn’t play a role in the pollution decline through 2016, they said.)

>Finally, they also looked at pollution and enforcement trends. If polluters for some reason fail to follow Clean Air Act rules, PM2.5 can pick up. Clay and Muller found that EPA enforcement activity has trended down since at least 2009.