California Has Most 'Dirty Air’ Counties in the U.S., American Lung Association Says

A new report released by the American Lung Association on Wednesday said that most Americans are now breathing cleaner air; however, 42 percent are still living in cities that are more polluted than they were a decade ago.

According to the "State of the Air 2013″ report, more than 131.8 million people live in areas with unhealthful levels of ozone, putting them at risk for premature death, aggravated asthma, difficulty breathing, cardiovascular harm and lower birth weight, the report revealed.

The state of California proved to have the worst ratings. Los Angeles once again tops the list for the worst ozone pollution in the country for the 13th time in the 14 years the report has been issued. Overall, seven of the top-10 most air polluted cities on the list are in California. Houston, Dallas and Washington, D.C., round out the top 10 for dirtiest air.

The actual number who breathe unhealthy levels of ozone is likely much larger, since this number doesn't factor people who live in adjacent counties in metropolitan areas where no monitors exist.

Despite improvements made by many cities in the country to diminish average daily pollution levels, the American Lung Association said much remains to be done on a broad scale to clean up the nation's air.

The researchers examine the most recent pollution data - collected by the Environmental Protection Agency - to analyze the two most common types of pollution: ozone (smog) and particle pollution (soot). Then they ranked cities and gave counties grades based on their scores for year-round particle pollution, short-term particle pollution, and ozone.

In a separate report, researchers at the University of Michigan School of Public Health found that higher concentrations of fine particulate air pollution  were linked to a faster thickening of the inner two layers of the common carotid artery, an important blood vessel that provides blood to the head, neck, and brain.