8 November 1999

South Coast Air Quality Management District (SCAQMD), the air quality control agency in Southern California, published a draft study on urban toxic air pollution, known as the Multiple Air Toxics Exposure Study II (MATES II). AQMD will accept public comment on the draft report for 90 days before publishing a final version.

In MATES II, AQMD monitored more than 30 toxic air pollutants at 24 sites over a one-year period ending last spring. Based on the collected data, AQMD estimated cancer risk assuming 70 years of continuous exposure to monitored levels of pollutants. MATES II included an effort to analyze the cancer risk from exposure to diesel particulates, which were listed as a toxic air contaminant by the California Air Resources Board (ARB) in 1998. Diesel particulates were not included in earlier air quality studies by AQMD.

The study identified particulate emissions from diesel engines as an important cancer risk factor. According to the draft MATES II, diesel soot accounted for 71% of the total cancer risk associated with air pollutants which were investigated. Other cancer risk contributors, primarily from gasoline engines and other non-diesel sources, included 1,3-butadiene at 8% of the risk, benzene at 7%, carbonyls (including formaldehyde and acetaldehyde) at 3%, and other pollutants (primarily from stationary sources) at 11%. Overall, the study showed that motor vehicles and other mobile sources accounted for about 90% of the cancer risk and industries and other stationary sources the remaining 10%. MATES II analyzed only cancer risk, neglecting other health effects of air pollution.

The study also confirmed that the cancer risk from some air toxics in Southern California has declined by as much as 75% over the last decade. MATES II estimated that the current cancer risk from toxic air pollution averages about 1,400 in a million in the region. According to California health statistics, the lifetime risk of cancer from all causes, including diet, genetic factors, lifestyle choices and other forms of environmental contamination, including pesticide residues on food, water contaminants and indoor air pollution, amounts to 250,000 in a million. A comparison of the above numbers shows that outdoor toxic air pollution accounts for less than 0.6% of the total cancer risk in California.

AQMD announced that, based on the preliminary finding that diesel particulates are the primary toxic air pollutant in the region, AQMD is developing a rule, to be considered early next year, to regulate diesel emissions from fleets of trucks and buses in the region.

The validity of the MATES II findings has been challenged by the Engine Manufacturers Association (EMA). According to EMA, the study is "rushed, not ready for public review, and contains unreliable information". Two major weaknesses of the MATES II, highlighted by the EMA, include flawed diesel particulate inventories and invalid unit risk factors for cancer risk estimates.

EMA said, the relative contribution of diesel engines to the total inventory of particulates reported in the MATES II study is in conflict with results from more thorough, state-of-the-art studies (e.g., the Denver Northern Front Range Air Quality Study) which indicate higher contributions of non-diesel sources to emissions of fine particulate matter.

AQMD calculated the cancer risk of diesel particulate based upon methods outlined by the state of California. These risk factors were adopted by the California ARB in the process leading to the identification of diesel particulates as a toxic air contaminant. Later scientific studies by the Health Effects Institute and the federal EPA’s Clean Air Scientific Advisory Committee stated that there is insufficient data linking diesel exhaust particulates to cancer to derive a quantitative unit risk factor. Therefore, current scientific knowledge does not corroborate SCAQMD’s use of that risk factor to calculate increased cancer incidence reported in the draft MATES II study.

MATES II study