5. Dallas-Fort Worth, Texas. Smog covers this cityscape in the South, where heat year-round adds to the problem of air quality.
The Environmental Protection Agency says the Dallas-Fort Worth area has some of the nation's most polluted air.
EPA says the area on Tuesday will be designated a "serious" violator of federal air-quality standards.
EPA spokesman David Bary says the Texas Commission on Environmental Quality will be required to develop a new clean-air plan for DFW by July 2012.
David Brymer, the state's air quality division director, says the new EPA label will probably have little effect on North Texas since a plan to clean the air is already underway because of previous orders by the federal government to do so.