Where are people choking on smog, and where can they stay healthy? The American Lung Association has answers.

The sun reflects on downtown skyscrapers as it sets through the Los Angeles smog and haze. Lucy Nicholson/Reuters

Since the passage of the Clean Air Act in 1970, American air quality has steadily improved. Emissions of air pollutants have decreased by around 1 to 3 percent each year, which may not seem like much, but over a 40-year period it has led to a decrease in harmful emissions of more than 50 percent.

To track progress, each year the the American Lung Association monitors the country's air pollution with its State of the Air report. This year's report, released yesterday, shows improvements nationwide, but as you can see, some areas (many of them in California) could still use some help. Here, we've ranked the 10 most polluted and 10 cleanest cities in the U.S. on the basis of long-term particle pollution (such as smog).

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Specifically, the report tracks levels of ozone and particle pollution found across the country using the following methodology:

For particle pollution, the report examines fine particulate matter (PM2.5) in two different ways: averaged year-round (annual average) and over short-term levels (24-hour). For both ozone and short-term particle pollution, the analysis uses a weighted average number of days that allows recognition of places with higher levels of pollution. For the year-round particle pollution rankings, the report uses averages calculated and reported by the U.S. Environmental Protection Agency. For comparison, the State of the Air 2010 report covered data from 2006, 2007 and 2008.

Here are some other interesting findings—some uplifting, others worrisome—from the 2011 report:



Roughly half the people in the United States (50.3 percent) live in counties that have unhealthy levels of either ozone or particle pollution.



Roughly one in 17 people—more than 18.5 million in the United States—live in 10 counties with unhealthful levels of all three measures covered in the report.



More than half of the country's most-smog-polluted cities experienced their best year yet.

All metro areas in the 25 cities most-polluted by ozone showed improvement over last year's report.



All but two of the 25 cities most polluted by year-round levels of particle pollution (sometimes called soot) improved over last year's report.



Nineteen of those cities reported their best-ever particle pollution levels.

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