An Air Resources Board study, published Monday in the prestigious scientific journal “Environmental Science & Technology,” shows that the cancer risk from exposure to the state’s most significant air toxics declined 76 percent over a 23-year period in California, a direct result of regulations targeting unhealthful emissions from these air pollutants.

The study quantifies emission trends for the period from 1990 through 2012 for seven toxic air contaminants (TACs) that are responsible for most of the known cancer risk associated with airborne exposure in California.

“These impressive reductions in California’s most hazardous toxic contaminants in our air took place against a backdrop of more than two decades of steady growth in California, with a growing population, and increasing numbers of cars and trucks that used ever larger quantities of gas and diesel,” Air Resources Board Chair Mary D. Nichols said. “There is no way these improvements in public health would have occurred without a strong, well designed program to reduce public exposure to toxic air pollution.”

Significant findings of the study, “Ambient and Emission Trends of Toxic Air Contaminants in California,” include:

* Thanks to state regulations, emissions from perchloroethylene from dry cleaners and hexavalent chromium from chrome plating, each dropped by more than 90 percent, and regulations already in place are expected to eliminate the remaining emissions of perchloroethylene and greatly reduce hexavalent chromium.

* Diesel particulate matter, which is emitted mainly from trucks and buses and is responsible for most of the airborne cancer risk in California, declined 68 percent, as a result of the State’s regulatory efforts to clean up diesel exhaust. This reduction took place even while the state’s population increased 31 percent, diesel vehicle-miles-traveled increased 81 percent and the gross state product increased 74 percent. The implementation of ARB’s recent diesel engine retrofit and replacement requirements has accelerated fleet turnover to cleaner trucks, and significant additional reductions are projected statewide.

* Two other toxic air contaminants emitted mainly from mobile sources, benzene and 1,3-butadiene, declined by nearly 90 percent. This was largely the result of California gasoline reformulation in 1996.

* The aggregated collective cancer risk from exposure to these seven air toxics declined 76 percent over the 23-year period.

The paper makes clear that further significant reduction in cancer risk to California residents is expected to continue as a result of continued implementation of air toxic controls. Such controls are part of broader statewide transportation initiatives, including the Truck and Bus Rule and more than a dozen rules focused on diesel equipment serving ports and railyards. Neighborhoods in freight corridors, including those near ports, will especially benefit.

The nearly 70 percent drop in harmful diesel particle pollution coincided with actions taken over the years, beginning in the 1990s, to reduce diesel emissions. In the 1990s, California adopted a reformulated diesel fuel program, started a heavy-duty diesel truck roadside inspection program, implemented particle pollution standards for urban transit buses and established standards for off-road diesel engines. In 2006, California began requiring ultra-low-sulfur diesel fuel. And following the establishment of California’s statewide Truck and Bus Rule in 2008, California began requiring diesel particulate filters on trucks, dramatically reducing diesel particulate matter, or soot, from the exhaust gas of diesel engines.

ARB regulations have reduced air toxics emissions from vehicles and their fuels, from stationary sources and from consumer products since the mid-1980s. In response to public concern, the California Legislature passed the Toxic Air Contaminant Identification and Control Act in 1984. Since then, ARB has implemented regulations to limit TAC emissions. In 1987, the California Legislature passed the Air Toxics “Hot Spots” Information and Assessment Act, which requires businesses to reduce risks from exposure to emitted TACs.