1990-05-00 Air Toxics Update #6

This page last reviewed July 30, 2008


Air Toxics Update #6




This Update is the sixth in a series of publications on California's Air Toxics Program. It provides an update of the current list of substances in the Toxic Air Contaminant Program, status of the compounds under review, a description of air toxic control actions considered by the Air Resources Board (ARB) during 1989, landfill gas testing results, and the status of air toxics activities for the future.

The previous publications, Program Updates #1 thru #5, are available from the ARB public information office. The first Update #1 includes a general description of California's air toxics law and how the program works. Updates #2, #3, and #4 include descriptions of substances identified as toxic air contaminants (TACs) during the years 1985 to 1987. To date, ten compounds have been identified as TACs - benzene, ethylene dibromide, ethylene dichloride, hexavalent chromium, asbestos, chlorinated dioxins / furans, cadmium, carbon tetrachloride, ethylene oxide, and methylene chloride. Update #3 includes descriptions of control decisions for benzene and ethylene dibromide, and Update #4 provides control decisions for benzene and ethylene dichloride. Update #5 describes the control decisions for carbon tetrachloride and hexavalent chromium and the adoption of further control measures for benzene. Descriptions of the Air Toxic "Hot Spots" Information and Assessment Act and Integrated Motor Vehicle / Fuels Control Strategy are also included in Update #5.


During 1989 there were 17 substances at various stages in the review process. Substances identified as toxic air contaminants that moved into or were already in the control measure phase in 1989 are:

  • asbestos
  • benzene
  • cadmium
  • chlorinated dioxins / furans
  • hexavalent chromium
  • ethylene oxide
  • methylene chloride

Those substances in the identification phase at the end of 1989 are:

  • inorganic arsenic
  • trichloroethylene
  • vinyl chloride
  • perchloroethylene
  • nickel
  • chloroform
  • 1,3-butadiene
  • formaldehyde
  • benzo(a)pyrene
  • diesel exhaust

The 1989 Update to the List of TAC

In February 1989, the ARB approved a staff recommendation to update the TAC Program List of Substances (see Update #1 for descriptions of list compound ranking categories). Based on: 1) the health effects of the substance, 2) the amount or potential amount of emissions of the substance, 3) the manner of usage of the substance in California, 4) the persistence of the substance in the atmosphere, and 5) the ambient concentrations of the substances in the community, ARB decided to make the following changes or additions to the TAC list.

  • 1,3-butadiene was moved from Category II-B to Category II-A.

  • Dimethyl sulfate, ethyl acrylate, 4,4'-methylene- dianiline, and toluene diisocyanates were added to Category II-B.     

  • The glycol ether class was added to Category III.


Methylene Chloride

In July 1989, the Board identified methylene chloride as a toxic air contaminant in California and determined there is not sufficient available scientific evidence to support the identification of a threshold exposure level. Methylene chloride was selected for review because of its presence in the atmosphere and its potential adverse effects on public health. The California Department of Health Services (DHS) concluded that methylene chloride meets the definition of a toxic air contaminant based on findings of carcinogenicity.

Carcinogenicity - The ability of a chemical or physical agent to induce cancerous tumors.

Methylene chloride, also known as dichloromethane, is a non-flammable volatile liquid that is completely miscible with a variety of solvents. It is a colorless liquid with a slight odor and volatilizes readily when mixed in water.

Methylene chloride is widely used (24,000 tons in California in 1989) because of its excellent solvent characteristics, low flammability, low reactivity, and low boiling point. It is used in both industrial and consumer products. Ninety percent of the estimated emissions of methylene chloride comes from use of paint removers, degreasing operations, polyurethane foam manufacturing, and aerosols. Methylene chloride is used in pharmaceuticals, electronics, chemical production and processing, and pesticide manufacturing. About 80 percent of the methylene chloride used in California is emitted directly to the atmosphere because many sources of methylene chloride are not currently equipped with air pollution control equipment.

Methylene chloride has been measured in the ambient air at sites in many populated areas of California. To assess public exposure to methylene chloride around the State, the ARB collected samples at 20 monitoring sites. Data was collected from these monitoring sites from 1985 through March 1987. The mean concentration for the methylene chloride samples taken at the monitoring stations ranged from 0.4 ppb at the Merced and Stockton sites to 2.5 ppb at the El Monte site. Peak concentrations ranged from 2.0 ppb at the Concord, Richmond, and Stockton stations to 21.0 ppb at the Santa Barbara station.

PPB - Parts per billion. For comparison, one part per billion is like four people in the earth's total population of approximately four billion.

The atmospheric lifetime of methylene chloride is estimated to range from 80 to 250 days. The primary mechanism for removing methylene chloride from the atmosphere involves reactions with hydroxyl radicals.     





.CH C1


    H O








The health impacts associated with exposure to methylene chloride are well documented. Methylene chloride is rapidly absorbed through the lungs and is well distributed throughout the body. Methylene chloride crosses the blood brain barrier and the placental barrier to the fetus. It is known to accumulate in body fats in higher concentrations than in other tissues. The principal target organ is the central nervous system (CNS). CNS depression results at inhalation concentrations greater than 1000 ppm.

PPM - Parts per million. For comparison, one part per million is like in 30 people in California's total population of approximately 30 million.

The U. S. Environmental Protection Agency (EPA) has concluded that methylene chloride is capable of inducing mutations in exposed human cells. The International Agency for Research on Cancer (IARC) has concluded there is sufficient evidence of carcinogenicity in animals after exposure to methylene chloride. The IARC reviewed short-term tests of DNA damage for methylene chloride and concluded there was sufficient evidence to classify methylene chloride as genetically active. The IARC also concluded that methylene chloride causes cell transformation in mammalian cells cultured in vitro. Staff of the DHS concur with the IARC's and EPA's evaluations but stress that the high concentrations of methylene chloride necessary to induce mutagenic and cellular transformations demonstrates that methylene chloride should be considered weakly genotoxic.

Mutagenic - The ability of a chemical or physical agent to produce inheritable changes in the genetic information stored in the DNA of living cells.

Genotoxic- The ability of a chemical or physical agent to adversely affect the genetic material of a living cell.

Based on methylene chloride's presence in the ambient air and its potential to cause cancer to the general public, the DHS established that an overall lifetime exposure to 1 ppb of methylene chloride results in a carcinogenic potency ranging from 1 x 10 to 10 x 10 . This estimate was based on the upper-bound of the 95 percent confidence interval of the models used. Using this range of potency estimates, exposure to the range of annual mean ambient concentrations (weighted by population) of 1.1 to 2.4 ppb, for an exposed population of 20.3 million people, could result in up to 20 to 500 excess lifetime cancers. The most plausible estimate for methylene chloride's carcinogenic potency as suggested by the California Scientific Review Panel on Toxic Air Contaminants and the DHS is 4 x 10 per ppb. Applying the most plausible estimate of upper limit risk to the annual mean range of ambient concentrations (weighted by population), for a population of 20.3 million, could result in up to 90 to 200 excess lifetime cancers.

1 x 10 - One in one million.

Potency - A quantitative estimate of the ability of a substance to cause physiological effects.

The next step for methylene chloride after identification as a toxic air contaminant is to evaluate more carefully specific emission sources and resulting public exposures, and then to develop control measures as needed to reduce the public health risk from these exposures.


Hexavalent Chromium

Hexavalent chromium is a cancer-causing substance identified by the ARB as a toxic air contaminant in January 1986. As mentioned in Update #5, the hexavalent chromium control plan and the first control measure (for chrome plating and chromic acid anodizing) were approved by the ARB in February 1988. A control technology demonstration project for chrome plating facilities was completed in 1989. The staff presented the findings of this project to the Board in October showing that the most stringent control requirements were achievable (99.8 percent control or .006 mg/amp-hour emission rate).

Elimination of the use of chromates in cooling towers is the second toxic control measure for hexavalent chromium. A control measure for this source was approved by the Board in March 1989. Implementation of the chromate-treated cooling tower measure will prevent an estimated 70 - 900 potential cancers over 70 years statewide. Within six months of local district rule adoption, the control measure prohibits the use of hexavalent chromium in the circulating water of a cooling tower and requires the concentration of hexavalent chromium in cooling tower circulating water to be 0.15 parts per million or less. Up to six months additional time may be allowed for towers of wooden construction. The control measure does not require a district permit for cooling towers; however, the cooling tower owner or operator must provide information about the cooling tower to the district so that the district will have the information necessary to enforce the control measure.

The cost associated with implementation of this control measure will affect both large and small businesses. The increases in tower operating costs could range from $400 per year for small comfort cooling towers to over $100,000 per year for large industrial cooling towers. The ARB has determined that this control measure will not have a significant adverse economic impact on small businesses.


As mentioned in Update #3, the ARB formally identified 15 compounds from the chlorinated dioxin and dibenzofuran family of substances as toxic air contaminants. Currently, the ARB, with the participation of the local districts, is developing a control measure for dioxin emissions from medical waste incinerators. Medical waste incinerators have a high priority for control because they are generally uncontrolled and are located in residential areas. The control measure for medical waste incinerators is scheduled to be presented to the Board for approval in 1990.

The draft regulation in its current form would require operational modifications and application of best available control technology to reduce dioxin emissions from these sources. The draft regulation provides the option of either achieving specific emission limits or a specified control efficiency. The need for control of other dioxin sources is currently under study through research, source tests, and air quality modeling.


In March 1986, the ARB listed asbestos as a toxic air contaminant (Update #3) having no identifiable threshold exposure level. Currently, the ARB staff, with the participation of the districts, is evaluating controls for airborne emissions of asbestos associated with the disturbance of asbestos-containing serpentine rock and from asbestos dust emitted from industrial activities in serpentine areas. Currently, asbestos emissions associated with the use of serpentine rock as a surface material are not uniformly regulated in California. The ARB staff is analyzing data showing there may be potentially high cancer risks to people residing in areas of serpentine rock disturbance.

The control measures will be evaluated in two phases. In the first part of Phase I, ARB staff is currently proposing the use of asbestos-containing serpentine rock as surfacing material be eliminated. This draft control measure is scheduled to be presented to the Board for consideration in 1990. In the second part of Phase I, ARB staff and the local districts will evaluate the need for reasonable dust controls for industrial activities (i.e., quarries and mines) and construction sites that occur within serpentine areas. In Phase II, the ARB staff plans to conduct additional monitoring and evaluation of asbestos exposure near existing unpaved roads surfaced with asbestos- containing serpentine material. After the Phase II research is completed, a decision will be made whether mitigation measures are needed for existing serpentine-surfaced roads.


The ARB staff plan to present control measures for ethylene oxide and benzene to the Board during 1990. The control measure for ethylene oxide will focus on control of its use as a sterilant. The proposed control measure for benzene is a part of the Integrated Motor Vehicle / Fuels Control Strategy discussed in Update #5. In December 1989, the staff presented a status report to the Board proposing a short-term program for conventional gasoline and a long-term program for phasing in low-emission vehicles and clean fuels. The short-term program is a set of specific compositional regulations proposed for gasoline which includes requirements for a lower benzene content.

In June of 1989, the staff presented to the Board a report which addressed the nature, severity, and extent of toxic emissions from motor vehicles. The report contained a list of available or projected control measures to reduce public exposure to toxic emissions. The report stated that 11 of the 25 candidates and identified toxic substances known or suspected to be emitted by vehicular sources have sufficient information for estimating the number of statewide potential excess lifetime cancer cases. Five of the 11 substances account for about 99 percent of the known cumulative statewide number of potential cancer cases attributable to motor vehicles. This report complied with Health and Safety Code section 39663, mentioned in Update #5, and is to be followed in June 1990 by a plan for maximizing the control of motor vehicle toxics.


In June, 1989, the Board approved the second report to the California Legislature concerning landfill gas testing. The report summarizes the testing results for 356 landfill sites throughout California. The report recommends that districts become more involved in the permitting of landfills and should consider adopting rules to require gas collection and disposal systems. The report recommends that districts continue to evaluate the test data and, by the end of 1990, identify sites needing further testing or other remedial action.


Compounds expected to be considered for identification by the Board over the next 10-20 months include inorganic arsenic, trichloroethylene, vinyl chloride, chloroform, perchloroethylene, nickel, and formaldehyde. During this same period, control measures for asbestos, ethylene oxide, and dioxins are scheduled to be considered by the Board. For further information about ARB's TAC program, please contact:

Stationary Source Division
Air Resources Board
P.O. Box 2815
Sacramento, CA 95812
(916) 445-0650


Air Toxics Updates