Editor’s note: SCAQMD is the air pollution control agency for Orange County and major portions of Los Angeles, San Bernardino and Riverside counties. This report is from an SCAQMD press release.

A major study by Southland air quality officials has found that reductions in toxic emissions have cut residents’ cancer risk from air pollution on average by more than 50 percent since 2005.

“Air pollution controls on everything from cars to trucks to industrial plants have dramatically reduced toxic emissions in our region,” said South Coast Air Quality Management District Executive Officer Barry Wallerstein. “However, remaining risks are still unacceptably high in some areas,” Wallerstein said. “We need to maintain our commitment to reducing toxic emissions so that everyone can breathe healthful air.”

SCAQMD’s Multiple Air Toxics Exposure Study IV found that the average cancer risk from air pollution across the region declined from 1,194 in 1 million during MATES III in 2005 to 418 in 1 million in 2012-13 using similar methods of analysis. The risk reduction follows a trend of declining toxic emissions in the region since the first MATES study in 1987.

MATES IV found that diesel particulate emitted from diesel trucks and other diesel-powered vehicles and equipment was responsible for 68 percent of the total cancer risk. Fully 90 percent of the risk is due to mobile sources, which includes everything from cars and trucks to ocean-going ships, locomotives, aircraft and construction equipment.

The highest cancer risk, about 1,050 in 1 million, was found in and around the ports of Los Angeles, the region’s hub of goods movement activity powered by ships, trucks and locomotives. Central Los Angeles and transportation corridors including freeways and rail lines also had some of the highest risks. The lowest cancer risks were found in central and south Orange County, southwest Riverside County and the Coachella Valley. Since MATES III, the greatest reductions in cancer risk have occurred in areas with the highest overall risk, including the ports.

MATES IV measured levels of some 37 gaseous and particulate air toxics, from benzene to lead to diesel particulate, at 10 permanent monitoring stations across Los Angeles, Orange, Riverside and San Bernardino counties. Toxics were monitored every sixth day from July 1, 2012 to June 30, 2013.

In addition to the permanent monitoring sites, SCAQMD used mobile monitoring platforms to assess local impacts in several locations including near airports, rail yards and warehouses.

Air quality data and economic trends between 2004 and 2013 indicate that the reduction in toxic emissions and cancer risk was not due solely to the recession. From 2010 to 2013, for instance, the number of containers shipped through the ports of Los Angeles and Long Beach –— a key indicator of goods movement activity — has rebounded, and yet toxic emissions during that period continued to decline.

Although toxic emissions and the resulting cancer risk have been significantly reduced over time, a recent assessment by the California Office of Environmental Health Hazard Assessment has found that the actual magnitude of cancer risk has been significantly underestimated by current state guidelines.

During the past decade, scientists have found that children’s sensitivity to air toxics has been underestimated. When OEHHA approves and implements its revised cancer risk guidelines, expected during the first quarter of 2015, cancer risk estimates will increase by an average factor of 2.7. SCAQMD’s risk numbers for MATES IV, and risks posed by mobile sources, businesses and industrial plants, will be revised upward accordingly.

This revision in the cancer risk calculation methodology does not change the fact that actual emissions and risk have declined by more than 50 percent since 2005.

SCAQMD’s landmark MATES studies combine monitoring of ambient air toxics, toxic emissions inventories, and complex computer modeling to present a detailed picture of the sources of cancer risk from air pollution and the magnitude of those risks across the entire Los Angeles Basin.

MATES I measured a limited number of air toxics in 1987. MATES II in 1998-99 and MATES III in 2004-06 monitored more than 30 toxic air pollutants at the same 10 locations used in MATES IV.

Estimating cancer risk from air pollution is not an exact science. There are some uncertainties in the cancer potencies of substances, the estimated population exposure and the estimates for diesel particulate pollution. However, SCAQMD staff, aided by a technical advisory panel of outside experts, used the best-available science to assess risk in MATES IV.

Toxic air pollutants are specific hazardous chemicals known or suspected to cause cancer or other serious health effects. Health effects from other pollutants, including ozone and fine particulates (PM2.5), are not addressed in the MATES studies and are assessed in the agency’s AQM Plans.

The complete report is online at www.aqmd.gov.