The most comprehensive study of urban toxic air pollution ever undertaken shows that motor vehicles and other mobile sources of air pollution are the predominant source of cancer-causing air pollutants in the Southland.
The study confirms that the cancer risk from some air toxics in the Southland has declined by as much as 75% over the last decade. However, it also shows that based upon more extensive monitoring of the variety of toxic compounds in the air, the current cancer risk from toxic air pollution averages about 1,400 in a million in the region.
Air quality scientists expect this risk will decline as pollution control programs progress in the years ahead.
These findings are outlined in a draft final report presented today to the Governing Board of the South Coast Air Quality Management District known as the Multiple Air Toxics Exposure Study II. AQMD will accept public comment on the draft report for 90 days before publishing a final version.
"Clearly we’re on the right path to reducing toxic air pollution and the associated cancer risk in our area," said Barry Wallerstein, AQMD executive officer. "Regulations now on the books will make cars and industries cleaner in the future and assure continued progress. But the study highlights the need to do more to reduce pollution from mobile and stationary sources."
AQMD found that the risk of cancer from breathing toxic compounds is down substantially throughout the region. For instance, the risk from pollutants monitored continuously in Burbank has declined from about 1,000 in a million in 1990 to 500 in a million today, presuming 70 years of continuous exposure at the monitored levels. Likewise, cancer risk is down by two-thirds from the same pollutants monitored at three sites in both a 1987 study and today’s analysis—namely, downtown Los Angeles, Long Beach and Rubidoux in Riverside County. Cancer risk from diesel exhaust was not included in the earlier study.
MATES II included a ground-breaking effort by AQMD to analyze the cancer risk from exposure to diesel particulate, or soot, which the California Air Resources Board in 1998 listed as a carcinogenic air pollutant.
AQMD’s MATES II shows that the average cancer risk in the region from carcinogenic air pollutants, excluding diesel particulate, ranges between 300 in a million and almost 600 in a million, with a regional average of 420 in a million. When diesel particulate is included, the cancer risk ranges from about 1,100 in a million to 1,750 in a million, with an average regional risk of about 1,400 in a million. The higher risk levels were found in the urban core areas in central Los Angeles County.
AQMD calculated the cancer risk of diesel particulate based upon a method for estimating diesel particulate in air samples outlined by the state of California and a cancer potency factor determined by the state. Diesel particulate is emitted by diesel-fueled trucks, buses, trains, ships, tugboats, construction equipment, power generators and stationary engines used throughout the region for tasks such as pumping water.
"While diesel particulate has been in the air, perhaps at even higher levels in the past, our study clearly shows that programs to clean up diesel emissions should be accelerated," said Wallerstein.
AQMD found that that diesel soot accounted for 71% of the cancer risk, 1,3 butadiene (a byproduct of incomplete combustion in engines) 8% of the risk, benzene (mostly from motor vehicles) 7%, carbonyls (including formaldehyde and acetaldehydes from both mobile and stationary sources) 3%, and other pollutants (primarily from stationary sources) 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%. The study analyzed only cancer risk, though some of the pollutants cause other health problems too.
Based on the preliminary finding that diesel particulate is the primary toxic air pollutant in the region, AQMD is developing a rule, to be considered early next year, to clean up diesel exhaust emitted by fleets of trucks and buses in the region. That rule will be one of many measures outlined in a comprehensive plan to reduce toxic air pollution from both mobile and stationary sources.
Other programs to reduce diesel emissions include AQMD grant programs that cover conversion of diesel equipment to clean fuels.
In MATES II, AQMD monitored more than 30 toxic air pollutants at 24 sites over a one-year period ending last spring. AQMD collected more than 4,500 air samples and together with the California Air Resources Board performed more than 45,000 separate laboratory analyses of these samples. A similar study known as MATES I was conducted in 1986 and 1987. In each study, AQMD calculated cancer risk assuming 70 years of continuous exposure to monitored levels of pollutants.
Toxic air pollutants consist of a variety of compounds, including metals, minerals, soot and hydrocarbon-based chemicals. These pollutants are heavily controlled, both through regulations aimed a reducing smog, like California’s requirement for cleaner burning gasoline, and regulations aimed at specific sources of toxic pollutants, such as chrome plating plants that emit hexavalent chromium.
Health statistics show that one in four people will contract cancer over their lifetime, or 250,000 in a million, 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. Outdoor toxic air pollution accounts for less than 1% of all these factors.
The monitoring study was accompanied by a computer modeling study in which AQMD estimated the risk of cancer from breathing toxic air pollution throughout the region based on emissions and weather data.
The modeling shows that the highest risk is in the urban areas where there is heavy traffic and high concentrations of population and industry. Monitoring data closely tracked what the modeling shows.
AQMD’s Governing Board ordered the study as part of the agency’s environmental justice initiatives adopted in late 1997. A panel of scientists from universities, an environmental group, businesses and other government agencies helped design and guide the study.
The study was aimed at determining the cancer risk from toxic air pollution throughout the area by monitoring toxics continually for one year at 10 monitoring sites.
Another goal was to determine if there were any sites where concentrations of industry were causing a disproportionate cancer burden on surrounding communities. To do so, AQMD monitored toxic pollutants at 14 sites for one month each with three mobile monitors specially designed for the study. Although no such sites were identified, models show that elevated levels can occur very close to facilities emitting toxic pollutants. Monitoring platforms were placed in or near residential areas adjacent to clusters of facilities.
MATES II covered the South Coast Air Basin, which includes all of Orange County and the parts of Los Angeles, Riverside and San Bernardino counties seaward of the mountains. AQMD is the air pollution control agency for this area, plus the Coachella Valley.
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