A new study finds that smoke from larger wildfires is reversing more than a decade of American improvements in smog. Thursday’s study finds that between 2003 and 2015, the US national average smog level dropped by 11%, but since 2015 it’s increased by 4%. Scientists fear that a dozen years of progress will be wiped out in about 20 years as a warmer world makes larger wildfires. The study says the extra smog, mostly in the Northern Rockies and Midwest, means smog deaths have increased by 318 deaths per year since 2013.