California's Central Valley retains its title as home to the worst air in America, according to the American Lung Association's "State of the Air" assessment for 2013 — but, like other cities and counties, has still shown improvement in air quality.

For 13 years, the Lung Association has been compiling data on two types of air pollution in cities and counties across the country. Particulate pollution, measured over the short-term and year-round, is comprised of exactly what you might think: small particles of soot and dust that float through the air and which when inhaled can lead to breathing difficulties or even heart attacks. Ozone pollution, a key component of smog, can similarly cause respiratory damage and premature death. The report summarizes the risks Americans face from existing levels of air pollution:

  • More than 4 in 10 people (42%) in the United States live in counties that have unhealthful levels of either ozone or particle pollution.
  • Nearly 4 in 10 people in the United States (38%) live in areas with unhealthful levels of ozone.
  • Fifteen percent (15%) of people in the United States live in an area with too many days of unhealthful levels of particle pollution.
  • Over 44.3 million people (14%) in the United States live in an area with unhealthful year-round levels of particle pollution.

So how risky is your area? The State of the Air website allows you to search for your city or county and see its assigned grade. But we've gone ahead and plotted the counties identified in the report as the best and worst for pollution.

Best and worst areas for particulate pollution

The worst cities in America unsurprisingly correlate to the worst counties. The top ten worst metropolitan areas for short-term particle pollution:

  • Bakersfield-Delano, CA
  • Fresno-Madera, CA
  • Hanford-Corcoran, CA
  • Los Angeles-Long Beach-Riverside, CA
  • Modesto, CA
  • Salt Lake City-Ogden-Clearfield, UT
  • Pittsburgh-New Castle, PA
  • Merced, CA
  • Fairbanks, AK
  • Logan, UT-ID

And long-term:

  • Bakersfield-Delano, CA
  • Merced, CA
  • Fresno-Madera, CA
  • Hanford-Corcoran, CA
  • Los Angeles-Long Beach-Riverside, CA
  • Modesto, CA
  • Visalia-Porterville, CA
  • Pittsburgh-New Castle, PA
  • El Centro, CA
  • Cincinnati-Middletown-Wilmington, OH-KY-IN

Worst areas for ozone pollution

The list of cities with the worst air for ozone will look familiar:

  • Los Angeles-Long Beach-Riverside, CA
  • Visalia-Porterville, CA
  • Bakersfield-Delano, CA
  • Fresno-Madera, CA
  • Hanford-Corcoran, CA
  • Sacramento—Arden-Arcade—Yuba City, CA-NV
  • Houston-Baytown-Huntsville, TX
  • Dallas-Fort Worth, TX
  • Washington-Baltimore-Northern Virginia, DC-MD-VA-WV
  • El Centro, CA

Why does California's Central Valley — home to Visalia and Bakersfield and Fresno — have such bad air? In part, because it's a valley, meaning that it's easy for pollution to stagnate in place. Also, it's home to an enormous amount of agriculture, kicking up a lot of dust and resulting in a lot of fuel usage. But it's getting better, at least in parts: the report notes that a number of cities in the valley reported their fewest days with unhealthy levels of ozone.

The Lung Association does have some indirect recommendations on where you might want to live if you want clean air. For the first time, four cities made the top ten list for low pollution levels in all three categories. Two are in the Dakotas, two in Florida.

  • Bismarck, ND
  • Cape Coral, FL
  • Palm Bay, FL
  • Rapid City, SD

Before you make plans to move, though, we recommend you look at other amenities provided in those cities besides the cleanliness of the air.

Photo: Los Angeles, covered by smog in 2003.