Southern California's air is getting healthier, and with that the chances of getting cancer from inhaling toxins has fallen significantly.

The risk of cancer from airborne pollutants has dropped by more than 50 percent on average since 2005, according to a study released Thursday by the region's air quality regulators.

Concerted efforts to reduce emissions from diesel trucks and other vehicles account for much of the drop.

The findings may not surprise residents of the region long cited for poor air quality. Unlike previous decades, it's now uncommon for smog to brown out the mountains that crisscross the region.

Still, risks persist from toxic pollutants such as diesel particulate matter and benzene.

Those risks are still some of the highest in the nation, said Philip Fine, assistant deputy executive officer with the South Coast Air Quality Management District, which conducted the study.

Exact comparisons between Southern California and other metropolitan areas are not possible because other areas measure risk differently, according to officials with the air district.

Their study was conducted in 2012-2013 and was the fourth of its kind, with the first in 1987. Each time, the region's air quality has gotten better, though the improvement this time was greater than in the past.

However, air management district officials also noted that calculations of cancer risk from the particles have been underestimated.

The California Office of Environmental Health Hazard Assessment found that actual cancer risk due to the toxins tracked in the study is about three times greater than current state guidelines suggest.

Those findings are being reviewed and the guidelines could be updated in 2015, which would increase the cancer risk figures the air district has reported over the years.

The expected revision "does not change the fact that actual emissions and risk have declined by more than 50 percent since 2005," the air management board said in a statement.

For every 1 million people who live in Southern California for 70 years, 418 would be expected to develop cancer due directly to the current levels of toxic air - compared to 1,194 extra cases based on 2005 air quality, according to the study.

That reduction is "probably larger than anyone expected," said Scott Fruin, a professor of preventive medicine at the University of Southern California who has studied the region's air quality.

The area of greatest concern remains around the massive ports of Los Angeles and Long Beach, where thousands of trucks and huge ocean-going ships carry goods near neighborhoods. Air-associated cancer risk there is at least double that of other urban areas of Los Angeles, Orange, Riverside and San Bernardino counties.

Changes to diesel trucks starting in 2007 had a major impact on reducing emissions. Both of the ports also have made efforts to reduce pollution from cargo ships.

In one sign of how effective those efforts have been, the study showed that cancer risk in areas near the ports had dropped to risk levels that were about the same as regulators calculated in 2005 for areas far from the ports.

Overall, based on measurements from 10 monitoring stations spread across the four counties, cancer risk declined 65 percent. Computer models for areas elsewhere in the region concluded that risk fell 57 percent.