Controlling Air Toxics

Toxic air contaminant (TAC) levels in the Bay Area continue to drop, according to the Toxic Air Contaminant Control Program Annual Report for 1997, prepared by the Bay Area Air Quality Management District (Air District). The program, adopted in 1990, is designed to control both the ambient levels of TACs and local high concentrations ("Hot Spots"). The program has three regulatory elements:

~ New and modified sources of air pollution are reviewed during permit applications for potential health risks from emissions of TACs. If the predicted health risks are significant, the Best Available Control Technology must be used to minimize TAC emissions.

~ The Air Toxics "Hot Spots" Information and Assessment Act, enacted in 1987, requires preparation of a TAC emissions inventory for each facility permitted by the Air District. The Air District updates its TAC inventory annually and brings any new facilities into the program. Based on the quantity and potency of the emissions and the proximity of people living and working nearby, the facilities are assigned to three priority categories. High priority facilities are required to prepare health risk assessments estimating the probability of adverse health effects from exposure to routine emissions of TACs. Of the 123 facilities that prepared health risk assessments in 1991, 30 were judged by the Air District to pose sufficient risks that their neighbors should be notified of the results. Since then, all but two facilities—Dow Chemical Company in Pittsburg and Martinez Refining Company in Martinez—have reduced their TAC emissions so that their estimated cancer risk is less than 10 cancer cases per million individuals.

The Air District evaluated small emitters such as dry cleaners and gasoline stations in industry-wide risk assessments. Some gas stations posed cancer risks greater than 10 per million, and many dry cleaners using perchloroethylene as a cleaning solvent presented risks of 100 per million or greater. Air District regulations mandate risk reduction and emission controls of facilities whose emission levels of TACs are this high. About 95 percent of the dry cleaners have complied. Amendments to the "Hot Spots" Act specify that these industry-wide risk assessments and regulations must now be adopted on a state-wide basis.

~ The State Toxic Air Contaminant Act (AB 1807) and the federal Clean Air Act are the basis of control measures to reduce emissions from categories of sources of TACs. The state Air Resources Board (ARB), as well as air districts, adopts, implements and enforces control measures. To date, Airborne Toxic Control Measures (ATCMs) have been adopted for the following 8 source categories: chrome plating and anodizing, cooling towers, commercial and hospital sterilizers, medical waste incinerators, paving operations that use serpentine rock and gravel, gasoline stations, non-ferrous metal melting operations, and perchloroethylene dry cleaners.

The Air District's air toxics program also contains two support elements:

~ The TAC inventory contains the most recent available information on TAC emissions from permitted stationary sources in the Bay Area. The data, plus similar inventories for mobile and area sources compiled by the ARB, are used to plan strategies to reduce public exposure to TAC emissions. The resulting data are sorted and reported by pollutant and by city and county. If the emission level of a TAC exceeds its reporting level, that TAC is reported individually. The threshold for reporting is based on the possibility of increasing the cancer risk by one in a million.

~ The Air District has 17 monitoring sites throughout the Bay Area. The following compounds are sampled at these sites: benzene, 1,3-butadiene, carbon tetrachloride, chloroform, ethylene dibromide, ethylene dichloride, methyl tertiary butyl ether (MTBE), methylene chloride, perchloroethylene, toluene, trichloroethane, trichloroethylene, and vinyl chloride.

The ARB conducts monitoring at 5 of these sites. In addition to compounds monitored by the Air District, information is collected on emissions of formaldehyde, acetaldehyde, dichlorobenzene, and particulate-based TACs.

The emissions of some TACs have decreased markedly. The TAC emissions inventory shows that the emissions of the solvent 1,1,1-trichloroethane (TCA), the production of which was banned in 1996 as a stratospheric ozone depleting compound, and 1,4-dioxane, a stabilizer in TCA, both decreased. So has perchloroethylene, as a result of the Air District's perchloroethylene dry cleaning rule. Most other TAC emissions have been reduced less dramatically.

Monitoring data show that emissions of 1,3-butadiene and benzene have decreased, as a result of the use of California's Phase 2 reformulated gasoline. These two compounds are responsible for more than half of the average calculated cancer risk in the Bay Area.

Areport prepared for Bay Area Congressmembers by Democratic staff of the House Government Reform Committee assesses the health risk presented to Bay Area residents by TACs (or hazardous air pollutants in the federal terminology). To determine the level of exposure, data for 10 TACs collected from 1995 to 1998 at 6 Bay Area TAC monitoring sites were used. The California Environmental Protection Agency (CalEPA) has determined the benchmark concentrations for each TAC, as the concentration of each TAC that could cause l additional cancer case per million exposed individuals.

The average concentration from the monitoring data for each TAC was compared with its benchmark concentration to determine the cancer risk of each to Bay Area residents. To determine the total cancer risk from the 10 TACs, the 10 values thus obtained were added, a method consistent with the National Academy of Science's recommendations for risk assessment.

Of the 10 TACs, the greatest health risks are from 1,3-butadiene and benzene. Mobile sources—cars, trucks, and off-road vehicles—account for two-thirds of the emissions of these chemicals, and stationary sources for the remainder. In order of their cancer risks, the remaining 8 TACs for which data was collected are carbon tetrachloride, hexavalent chromium, formaldehyde, 1,4-dichlorobenzene, methylene chloride, perchloroethylene, chloroform, and trichloroethylene. The findings of this report were consistent with the Air District's annual report, allowing for the differences in monitoring sites and the time periods during which the data were collected.

BAAQMD, 425-771-6000; the Congressional report, "Exposure to Hazardous Air Pollutants in the San Francisco Bay Area", may be obtained from Congressional offices in the Bay Area.