Washington, D.C. ? More than 100 million people living in 46 metro areas are breathing air that has gotten too full of soot on some days, and now those cities have to clean up their air, the Environmental Protection Agency said Monday.

The EPA added 15 cities to the sooty air list, mostly in states not usually thought of as pollution-prone, such as Alaska, Utah, Idaho and Wisconsin. That’s probably because of the prevalence of wood stoves in western and northern regions, a top EPA official said.

But environmentalists said the EPA was only doing half its job on soot-laden areas, letting some southern cities with long-term soot problems — such as Houston — off the hook.

The EPA notified elected officials in 211 counties in 25 states that their air violated newly tightened daily standards for fine particles of pollution from diesel-burning trucks, power plants, wood-burning stoves and other sources. Those particles, often called soot, can cause breathing and heart problems.

These lists of what EPA calls “nonattainment areas” are important because regions that have air that is too sooty must come up with plans by 2012 on how to clean it and then do it by 2014. When old power plants and factories in these areas expand or do major refurbishing, they have to show EPA that it would not further pollute the air. It could mean also controls on vehicle emissions and regions having to take pollution into effect when they build new roads.