MNN Galleries

7 U.S. cities with the worst air pollution

By: chief on Aug. 4, 2010, 12:20 p.m.
Interstate highway traffic

Photo: ZUMA Press

2 of 11


A new area moved to the top of the most-polluted list when judged by year-round particle levels: Phoenix-Mesa-Scottsdale, Ariz. Particle pollution is a combination of soot, dust and aerosols that are suspended in the air. Cities with year-round particle pollution face unhealthy levels day-in and day-out, while those with problems with short-term levels have "spikes" in particle pollution that last anywhere from hours to days.