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The average risk of Southern Californians developing air pollution-associated cancer has dropped by more than 50 percent, according to a new report from the South Coast Air Quality Management District, released online Friday.

The Multiple Air Toxics Exposure Study IV (MATES IV) looked at data collected between 2005 and 2012 and compared it to the previous report, which was released in 2005. The new report estimated that 418 people per million would develop some form of pollution-associated cancer in their lifetimes, a significant drop from the 1,194-per-million estimate found in MATES III, Philip Fine, assistant deputy executive officer for SCAQMD, told weather.com.

"All of our analyses show that we have more than a 50-percent reduction in cancer risk due to air toxic chemicals," he said. "The drop has mostly been driven by the reduction in diesel emissions." Diesel-fueled trucks, as well as cars and factories, release particulate air pollution. The other major type of pollution, ozone, was not considered in this report, Fine said.

California has the strictest air quality standards of any state and also the worst overall air quality in the nation. The Los Angeles and San Joaquin Valley regions have topped the American Lung Association's State of the Air list of the nation's most-polluted cities for the past 10 years, Bonnie Holmes-Gen, senior director of policy and advocacy for the American Lung Association in California, told weather.com.

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"Most of the state has issues, clearly, because all the major urban areas are basically noncompliant with [federal] ozone standards, and we still have many areas noncompliant with our particulate pollution standards," she said. Almost 80 percent of state residents live in an area with unhealthy air for at least part of the year, she added.

This particular report did not take into account the other well-known negative health outcomes associated with air pollution, including asthma and other respiratory conditions and heart disease.

The study methodology compares pollution data to future estimates of cancer risk from the California Office of Environmental Health Hazard Assessment. The agency is set to release new cancer-risk estimates in 2015 that are expected to increase estimated cancer risk by a factor of 2.7, Fine said, suggesting that air pollution has an even bigger impact on cancer risk than was once known. (Just last year, the World Health Organization officially labeled air pollution a carcinogen.)

"We're always excited to see data like this that demonstrates the effectiveness of our California air pollution control strategy," Holmes-Gen said of the new report, which she did not work on. "We want to celebrate the success of reducing toxic exposures, but recognize that we still have a long way to go. The numbers are still unacceptably high."

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