4. Los Angeles
most polluted cities los angeles
Population: 18.1 million
Cardiovascular cases: 4 million
Rank among ozone-polluted cities: 1

Los Angeles is still one of the worst cities for all types of pollution, but the air quality is much better than it was in the first State of the Air report released 14 years ago. Days with unhealthy ozone levels have fallen by a third since then, to about 125 days a year.

Improvements in auto engines and clean-burning gasoline have made most of the difference, according to Carsten Warneke, a scientist with the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder.

"The main emissions source [in Los Angeles] is vehicle exhaust, but the cars we're driving are getting better every year," he said.

With those improvements, many of the volatile organic compounds that used to fill the skies of Tinseltown, such as benzene and toluene, have been reduced by about 98%, according to Warneke.


Source: American Lung Association's State of the Air 2013 report
1. Ranking is based on year-round particle pollution. This is in the air we breathe and consists of such things as dust, metals, smoke, exhaust, pollen and acids, like nitrates and sulfates. 2. The data for cardiovascular disease includes all cases in a given city whether they were caused by air pollution or not. Air pollution puts those with cardiovascular disease at a higher risk of health issues. 3. Ozone is created when a chemical or fossil fuel, like coal or gasoline, is partially burned and the unburned hydrocarbons, when combined with ultraviolet light, form a gas that is very corrosive to lungs.
- Last updated April 24 2013 12:33 PM ET
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