What are America's Most Polluted Cities?

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Remember the fog that lingered over Beijing during the 2008 Summer Olympics? It wasn't fog. It was dirty air. Yes, China faces a world-class air quality problem. But before you get too smug about its smog, consider this: 154.5 million Americans, just over half the nation's population, live in areas where air pollution levels are often dangerous to breathe. That's according to the American Lung Association's (ALA) just-released report 2011 State of the Air.

The cities ranked worst in air quality? California metros like Los Angeles, Visalia, Hanford and Fresno. Bakersfield, Calif., ranked the worst in terms of short-term and year-round particle pollution. It also ranked second-worst, behind Los Angeles, for smog levels, known formally as ozone pollution. California metros "really have made great strides to try to clean up, but they have a lot of sources--a lot of people driving, a lot of ships coming into the port, especially ocean-going vessels," explains Janice Nolen, lead author of the report and an assistant vice president of national policy and advocacy at the ALA. "Plus, California has the warm, sunny climate that encourages pollutants to form and the geography that helps trap them."

Behind The Numbers The American Lung Association's study ranks the air quality of 900 counties and more than 200 U.S. Census-defined Metropolitan Statistical Areas and Combined Statistical Areas--cities and their surrounding suburbs--where an air pollution monitor is present.

The ALA analyzed data from 2007 to 2009 that had been collected daily through monitors operated by federal agencies, states and Native American tribes. The data was reviewed by the Environmental Protection Agency. The ALA analyzed the data based on ozone pollution, short-term particle pollution and year-round particle pollution. Its report grades cities using, in part, the EPA's color-coded Air Quality Index, as well as its own weighting system that ranks the severity of pollution on a daily basis.

The EPA defines particle pollution as "a complex mixture of extremely small particles and liquid droplets ... made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles." Year-round air pollution is an average of the daily levels of particle pollution on all 365 days. Short-term particle pollution measures the number of times in a year when levels spike above healthy standards for several days to several weeks. Ozone pollution has adverse effects on health as well, but is not as dangerous in the short-term.

"All three can kill people, and all three can cause asthma attacks," asserts Nolen. "But short-term and year-round particle pollution are more deadly than ozone."

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