9 January 2008
South Coast Air Quality Management District (SCAQMD), the air quality agency in Southern California, published a draft report from the MATES III (Multiple Air Toxics Exposure Study) study on toxic air pollution. The study, a follow-up to the earlier MATES II study completed in 1999, concluded that the cancer risk from air pollution in Southern California decreased by 15%.
The focus of the MATES study is the carcinogenic risk from exposure to air toxics. The study included a monitoring program conducted from April 2004 through March 2006. The study also included an update of the toxics emissions inventories for the South Coast Air Basin and computer modeling to estimate toxics levels throughout the Basin. 33 air toxics were measured, including particulate matter (PM10, PM2.5), elemental and organic carbon, several hydrocarbon compounds (PAHs, toluene, styrene,...), heavy metals (Mn, Cu, Pb, Ni,...), and other compounds.
An important objective of the study was to estimate the cancer risk from exposure to diesel particulates. Since the ambient levels of diesel PM cannot be measured directly, they have to be estimated from other measurements. In the previous MATES II Study, elemental carbon (EC) was used as a surrogate for diesel particulate levels. For the present study, the chemical mass balance (CMB) source apportionment technique was used to estimate the contribution from diesel, as well as from other major source categories, to the measured particulate levels.
The monitored and modeled concentrations of air toxics were then used to estimate the cancer risks from exposure. Annual average concentrations were used to estimate a lifetime (70 years) risk from exposure to these levels, using cancer risk factors established by the Office of Environmental Health Hazard Assessment (OEHHA) of the California Environmental Protection Agency.
The study found that diesel exhaust was the major risk contributor, accounting for about 84% of the total cancer risk associated with the air pollutants that were investigated (up from 71% in MATES II). The overall cancer risk from air toxics was estimated at about 1,200 in every one million people (down from 1,400). The cancer risk more than doubled, to 2,900 per million, for those who live around the ports of Los Angeles and Long Beach, due to the significant diesel use at ports.
The MATES III estimate of the average ambient diesel PM concentration in Southern California is 3.2-3.49 µg/m3, compared to the 3.4 µg/m3 MATES II number.
According to health statistics, the lifetime risk of cancer from all causes (diet, genetic factors, pesticide residues on food, indoor air pollution,...) amounts in California to about 250,000 in a million. Therefore, outdoor toxic air pollution accounts for about 0.5% of the total cancer risk.
The study also included a non-cancer health effects assessment. It was found that almost all of the monitored air toxics were below their respective Chronic Reference Exposure Levels (RELs) established by OEHHA. The exceptions were formaldehyde and manganese (the latter only if a proposed more stringent REL for Mn is considered).
Two major uncertainties in the study include (1) the apportionment of diesel particulate matter, as the measured levels of pollutants come from a variety of sources, and (2) the cancer risk factors. The cancer risk factors for diesel particulates were adopted by OEHHA in 1998, in the process leading to the identification of diesel particulates as a toxic air contaminant. However, later scientific studies by the Health Effects Institute and the US EPA stated that the existing data is insufficient to derive quantitative cancer risk factors from exposure to diesel particulates.
The draft MATES III is available for a 90-day public review and comment period ending April 4, 2008.