The Environmental Protection Agency says more than 100 million people living in 46 metro areas are sometimes breathing air with too much soot in it.
Updating a list made in 2004, the EPA has added 15 cities to the sooty air list, mostly in states like Alaska, Utah, Idaho and Wisconsin.
Those cities have to come up with a plan to clean the air by 2012, and then have to do it by 2014.
A top EPA official says the prevalence of wood stoves in those regions could be why those unlikely areas were added.
But environmentalists say the agency isn't going far enough. The group Clean Air Watch says the list only looks at cities that violated the EPA's daily soot standards, while those running afoul of yearly soot standards aren't being held accountable.