One expert was not surprised by the findings.
"Being from California, I knew we had a few pretty bad areas, but this report really drives it home," said Irva Hertz-Picciotto, a professor of environmental and occupational health at the University of California, Davis.
The cleanest cities, which did not have a single day of unhealthy levels of ozone or particle pollution from 2009 to 2011, were Bismarck, N.D.; Rapid City, S.D.; and the Fort Meyers and Palm Beach areas of Florida.
A lot of it has to do with geography, Nolen said. Ozone, also known as smog, forms when gases from car exhaust, coal-fired power plants and other sources react with sunlight. In valley areas, like around Los Angeles, these gases get trapped, whereas they are dispersed quickly in elevated areas, she said.
The current report also listed more cities with unhealthy ozone levels than the last report, which is probably explained by hotter and sunnier weather, especially in the middle regions of the United States, Nolen said.
In addition to geography and climate, "a lot of transportation-related sources -- cars and trucks and shipping ports -- contribute to [the ozone] problem in the California area, and a lot of the most polluted cities in the eastern part of the country have coal-fired power plants," Nolen said.
To reduce the level of smog-causing vehicle emissions, the Environmental Protection Agency (EPA) proposed rules in March that would require less sulfur in gasoline and stricter vehicle emissions standards. Similar requirements went into effect in California in 2012.
Although health and environmental groups and the automobile industry support the proposal, oil companies oppose it, Nolen said. "We are encouraging people to support the EPA rule," she added.
Nolen recommended that people keep track of the air quality in their area through the AIRNow.gov websit
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