November 3, 2014
On October 3, 2014 California’s South Coast Air Quality Management District (SCAQMD) released a Draft Report and presentation from the Multiple Air Toxics Exposure Study IV (MATES IV)—a comprehensive air quality study focused on the carcinogenic risk from exposure to air toxics in the California South Coast Air Basin. Based on the exposure levels from the 10 monitoring sites used in the study, the average cancer risk from air toxics in the South Coast Air Basin is 67% lower than the estimated risk from the 2004-2006 time period.
The MATES IV Study included a monitoring program, an updated emissions inventory of toxic air contaminants, and a modeling effort to characterize risk across the South Coast Air Basin. The study is a follow-up to the MATES III study completed in 2008.
The study measured 37 substances, including PM10, PM2.5, several organic compounds (formaldehyde, benzene, toluene, PAHs,), heavy metals (Mn, Cu, Pb, Ni,…), and other compounds. Two new substances—black carbon (BC) and ultrafine particles (UFP)—not monitored in previous MATES studies were included in the MATES IV measurement program.
Concentrations of diesel particulates, singled out as a major contributor to the cancer risk, were calculated based on the measured elemental carbon (EC) exposures which were adjusted by the ratio of emissions of EC and diesel PM from the emissions inventory estimates. This indirect method of estimation of diesel PM exposures is one of the major sources of uncertainty in the study.
Although the estimated Basin-wide risks declined from the MATES III period, areas near the ports and near transportation corridors continue to show the highest air toxics risk, emphasized the authors of the MATES IV study. Some of the key findings of the study are:
- According to the regional modeling analysis, the highest risks from air toxics exists in areas surrounding the ports, with the highest risk about 1,000 per million, followed by Central Los Angeles, where there is a major transportation corridor, with modeled carcinogenic risks ranging from about 700 to 750 per million.
- Diesel exhaust was the key driver for air toxics risk, accounting for 68% of the total estimated air toxics risk estimated from monitoring.
- Ultrafine particle measurements at the ten fixed sites revealed that regional ultrafine levels are higher in western areas of the Basin with greater population and traffic density.
- Consistent with previous studies, short-term, local-scale measurements near a rail yard, an airport, and a busy freeway intersection showed higher diesel PM and ultrafine concentrations than the nearest fixed site monitor.
SCAQMD has released the Draft Report for a 90-day review period and will accept comments until January 5, 2015.
What this means to you
Highest concentrations of diesel particulate matter, especially PM 2.5, in southern California exist near the ports of Los Angeles and Long Beach. Sources operating in these areas may be a target for future regulation.