California has made tremendous progress cleaning its once-notorious air pollution over the last generation, with Los Angeles smog easing in response to the state's ever-stricter emissions standards.
On Monday, there was more good news. The Air Resources Board reported that Californians' cancer risk from toxic air pollution has declined 76% over more than two decades, a trend the agency attributes to the state's array of regulations targeting everything from diesel trucks to dry cleaners.
State scientists measured the drop from 1990 to 2012 by tracking airborne concentrations of the seven toxic air contaminants that are most responsible for increasing cancer risks. They include the particulate matter in diesel exhaust, benzene from gasoline, perchloroethylene emitted by dry cleaners and hexavalent chromium from chrome plating operations.
The authors of the study, published in the journal Environmental Science & Technology, said they were able to link declines in toxic compounds to specific policies, including rules targeting exhaust from diesel trucks, gasoline vapors and emissions from dry cleaners.
When California required reformulated gasoline in the mid-1990s, for instance, levels of benzene in the air dropped immediately, said Álvaro Alvarado, a toxicologist at the Air Resources Board and an author of the study.
Concentrations of diesel particulate matter — the largest contributor to airborne cancer risk in the state — declined more than 68% in California over the 23-year study period, the agency found, largely because of state requirements for cleaner fuels and strict emissions-control rules for diesel trucks adopted in 2008.
"This is real-world proof that the regulations are having the kind of impact that we had hoped for," Alvarado said.
The reductions took place even as California's population and economy grew and the number of driving miles increased.
California has fought to curb smog for more than 50 years, but the state did not begin targeting toxic air contaminants until the 1980s.
The new findings mirror other recent studies that have found big drops in levels of cancer-causing air pollutants and show Californians are reaping the benefits of the state's pollution regulations with stronger lungs and better health.
A study last year by the South Coast Air Quality Management District found a 65% drop in cancer risk from toxic air pollution since 2005 across that agency's four-county jurisdiction.
Despite that progress, California still has the nation's worst air pollution and significant obstacles to clean air remain.
In one recent development, state environmental officials now estimate the cancer risk from toxic air contaminants is nearly three times what experts had previously thought.
Alvarado and other officials say they also remain concerned about unacceptably high pollution levels in many air toxics "hot spots," especially in neighborhoods near some of Southern California's biggest pollution sources: freeways, rail yards, ports and other busy transportation corridors.
"Our regulations are still phasing in," Alvarado said. "So while it has gone down a lot, we expect it to continue."