LONG BEACH (CBSLA.com) — For the second year in a row, Southern California has topped the list for the most polluted air in the country, according to a new report released Wednesday.

The American Lung Association’s State of the Air 2015 reveals that although California has made steady progress, drought weather conditions have caused an uptick in particle pollution levels for much of the state.

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“The State of the Air 2015 report shows that California’s clean air laws and initiatives are working,” said Olivia Gertz, President and CEO of the American Lung Association. “However, over 70 percent of Californians — 28 million residents — live in counties plagued with unhealthy air during certain parts of the year.”

California’s pollution problems are primarily caused by emissions from transportation sources including: cars, diesel trucks and buses, locomotives, ships and agricultural equipment, and other sources, which include: oil refineries, manufacturing plants, residential wood burning and wildfires. The weather conditions, heat and geography in California facilitate smog and soot formation.

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Residents in high pollution areas are at a higher risk of suffering from lung cancer, asthma attacks, heart attacks and premature deaths.

“We must increase our efforts to cut pollution so all Californians can breathe clean and healthy air,” Gertz said.

Here is a list of the top five ozone-polluted cities in the Southland:

  1. Los Angeles, Long Beach
  2. Visalia, Porterville
  3. Bakersfield
  4. Fresno-Madera
  5. Sacramento-Roseville
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Despite the rankings, the State of the Air Report also shows that many of the state’s cities made significant improvements in reducing unhealthy ozone and particle pollution over the history of the report. Major regions have seen reductions in unhealthy ozone days of approximately 40 percent or more since the 2000 report.