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Christopher Helman, Forbes.com

California has gone to extremes to improve the state’s air quality, pushing out coal-fired power plants and implementing the strictest auto emissions standards in the nation. L.A.’s persistent smog layer may be a shadow of its former self, but it hasn’t been enough. Lots of people and too many cars means California still has seven big cities that rank among the 20 most polluted in the nation.

L.A. ranks No. 2 on our list of America’s Dirtiest Cities, and San Diego is no. 9, but some of the worst air in the country is in smaller cities in the San Joaquin Valley, where a ring of mountains traps a stagnant stew of ozone and particulate matter. According to data that Forbes crunched from The American Lung Association’s State of the Air 2011 report, the most hazardous breathing in America is in Bakersfield. Hot, dusty, adjacent to California’s biggest oil fields, Bakersfield has 60 days a year of unhealthy air, 10 times a level considered acceptable. Its ozone levels are better than at any time in the past 15 years, but still unhealthy for 100 days out of the year.

By contrast, Houston (No. 18) has 25 bad ozone days a year while New York (No. 14) suffers just 17, down from 40 a decade ago).

The Lung Association figures that half of the U.S. population lives in places where the air is sometimes unfit to breathe, contributing to asthma and lung cancer. And death. The data show that more people die of respiratory ailments on bad-air days.

Here are the top 10 dirtiest cities:

#10 Sacramento-Arden-Arcade-Yuba City, CA-NV

Population: 2.4 million
Year-round particulate pollution rank: 45 (approx.)
Short-term particulate pollution rank: 9
Ozone pollution rank: 5

Sacramento, CA Photo: Getty Images

A little better than Bakersfield and Fresno, but Sacramento still suffers stagnant air stuck in the San Joaquin Valley.

#9 San Diego-Carlsbad-San Marcos, CA

Population: 3 million
Year-round particulate pollution rank: 30 (approx.)
Short-term particulate pollution rank: 15
Ozone pollution rank: 7

San Diego, CA Photo: iStock

There's no dirty coal plants here, and you'd think the ocean breeze would keep the air clean, but San Diego has a big port and busy highways that lead to and from Mexico.

#8 Phoenix-Mesa-Glendale, AZ

Population: 4.4 million
Year-round particulate pollution rank: 2
Short-term particulate pollution rank: 24
Ozone pollution rank: 19

Phoenix, AZ Photo: iStock

Heat plus cars equals ozone. Epic dust storms also regularly engulf Phoenix.

#7 Birmingham-Hoover-Cullman, AL

Population: 1.2 million
Year-round particulate pollution rank: 8
Short-term particulate pollution rank: 8
Ozone pollution rank: 21

Birmingham, AL Photo: iStock

Pollution levels have improved a lot, but Birmingham suffers from being located in a valley that tends to trap stagnant summer air. Two interstate highways don't help either.

#6 Modesto, CA

Population: 500,000
Year-round particulate pollution rank: 10
Short-term particulate pollution rank: 12
Ozone pollution rank: 14

Photo: Forbes

Home to E&J Gallo Winery, which also operates the largest wine bottle manufacturing plant in the world.

#5 Pittsburgh-New Castle, PA

Population: 2.4 million
Year-round particulate pollution rank: 7
Short-term particulate pollution rank: 3
Ozone pollution rank: 24

Pittsburgh, PA Photo: iStock

This old steel town is a new boom town for natural gas drilling, and downwind of coal-fired power plants in Ohio and West Virigina.

#4 Fresno-Madera, CA

Population: 1.1 million
Year-round particulate pollution rank: 6
Short-term particulate pollution rank: 2
Ozone pollution rank: 4

Fresno, CA Photo: Forbes

San Joaquin Valley air might be getting better; last August, for the first time in recorded history, Fresno had no days with unhealthy ozone levels.

#3 Visalia-Porterville, CA

Population: 430,000
Year-round particulate pollution rank: 2
Short-term particulate pollution rank: 7
Ozone pollution rank: 3

Visalia, CA Photo: Forbes

Proximity to the giant trees of Sequoia National Park isn't enough to clean Visalia's smoggy San Joaquin Valley air.

#2 Los Angeles-Long Beach-Riverside, CA

Population: 17.8 million
Year-round particulate pollution rank: 2
Short-term particulate pollution rank: 4
Ozone pollution rank: 1

Los Angeles, CA Photo: iStock

Millions of cars, a huge port in Long Beach, busy airports -- at least L.A.'s smog is better than it used to be.

#1 Bakersfield-Delano, CA

Population: 800,000
Year-round particulate pollution rank: 1
Short-term particulate pollution rank: 1
Ozone pollution rank: 2

Bakersfield, CA Photo: iStock

Hot, dusty and surrounded by California's biggest oil fields, Bakersfield has all the ingredients for the worst air in the nation.