1988-07-00 Air Toxics Update #4

This page last reviewed July 30, 2008


Air Toxics Update #4




The Air Resources Board (ARB) took action on five toxic compounds during 1987 as part of California's Air Toxics Program. Three substances - cadmium, carbon tetrachloride, and ethylene oxide - were identified as toxic air contaminants (TACs). Two previously identified TACs, benzene and ethylene dichloride were evaluated by the Board for potential control measures.

This Update is the fourth in a series of publications on California's air toxics program and provides an overview of the decisions made by the Board during 1987. It summarizes the characteristics of the three newly named TACS, cadmium, carbon tetrachloride, and ethylene oxide, and explains decisions made about benzene and ethylene dichloride control strategies.

For anyone wanting a general description of California's air toxics law and how the program works, Air Toxics Program Update #1 is recommended. For a discussion of the start-up of the program and a description of the three substances identified in 1985 - benzene, ethylene dibromide, and ethylene dichloride - please see Update #2. For those TACs identified in 1986 - hexavalent chromium, asbestos, and chlorinated dioxins / furans - please see Update #3.


During 1987 there were 17 compounds at various stages in the review process. Substances identified as toxic air contaminants that moved into or were already in the control measure phase are:

  • asbestos
  • benzene
  • cadmium
  • carbon tetrachloride
  • chlorinated dioxins/furans
  • hexavalent chromium
  • ethylene dichloride
  • ethylene dibromide
  • ethylene oxide

Those substances which were in the identification phase at the end of 1987 are:

  • methylene chloride
  • inorganic arsenic
  • vinyl chloride
  • chloroform
  • formaldehyde
  • nickel
  • perchloroethylene
  • trichloroethylene



In January 1987, the ARB identified cadmium as a toxic air contaminant with no identifiable threshold exposure level. Cadmium was selected for review because it is a proven animal carcinogen and may cause cancer in humans, and because it is emitted from many sources in California.

Cadmium is a rare element, making up on the average between one and two parts in ten million of the earth's crust. It is found in oil and coal at higher concentrations than are normally found in the earth's crust. It is a contaminant of zinc and copper ores, from which it is recovered commercially. Cadmium is used in a variety of industrial applications, being a component of certain alloys, a corrosion inhibiting coating, and in some electrical batteries. Compounds containing cadmium are used as pigments and stabilizers, and in semiconductor manufacturing.

Cadmium is emitted from both stationary and mobile sources. The metal's wide usage, its presence as a natural contaminant in fossil fuels, other metals, and industrial raw materials, along with its high volatility compared to other metals, combine to create a high potential for its release into the atmosphere. Stationary sources include secondary smelters, cement manufacturing plants, cadmium electroplating facilities, plants burning oil or coal, and sewage sludge incinerators. Mobile sources which emit cadmium include diesel and gasoline vehicles and particles resulting from tire wear. Persons smoking tobacco or breathing second-hand tobacco smoke are exposed to cadmium at concentrations several times or orders of magnitude greater than people not experiencing these exposures. Approximately 17 tons of cadmium are emitted in California yearly with 80 percent or more coming from stationary sources.

Atmospheric cadmium occurs principally on the surface of breathable particles. Cadmium is removed from the atmosphere through both wet and dry deposition. The mechanisms and rates of trace metal deposition are believed to depend on meteorology, vegetation (canopy) characteristics, and differences in local or regional emissions.

The atmospheric concentrations of cadmium are much lower than the concentrations causing either the adverse health effects in occupational settings or the acute effects seen in animal experiments. Cancer is the primary potential health effect of atmospheric cadmium exposure. Cadmium is thought to exhibit a threshold effect for non-cancer effects, so adverse health effects other than cancer are not expected to occur from inhalation of cadmium at current or future atmospheric levels.

In summary, DHS has estimated that the excess cancer risk associated with a 24-hour per day exposure to atmospheric concentrations of cadmium ( 1 to 2.5 ng/m ) is between 2 and 30 excess lifetime cancer cases per million persons exposed. For those living or working near sources of cadmium emissions, this may rise to 80 to 480 excess lifetime cancer cases per million persons exposed.

Carbon Tetrachloride

In September 1987, the ARB identified carbon tetrachloride as a toxic air contaminant with no identifiable threshold exposure level. Carbon tetrachloride was selected for review because the International Agency for Research on Cancer (IARC) has identified it as an animal carcinogen, and has recommended that it be regarded as a potential human carcinogen. It is emitted from many sources in California and its presence in the atmosphere has been documented by the ARB and other researchers.

In 1984, the major identified sources of carbon tetrachloride emissions fell into four main categories that have not changed significantly. They are: 1) carbon tetrachloride production; 2) pesticide / grain fumigant usage; 3) chlorinated paraffin wax production; and 4) fluorocarbon production. The first two source categories account for over 80 percent of these emissions. Other potential sources include oil companies, scientific laboratories, organic chemical manufacturers, and publicly owned water treatment facilities. However, the emissions from these sources are believed to be less significant. The Environmental Protection Agency has cancelled the use of grain fumigants containing carbon tetrachloride so the emissions from this source should decrease.

Carbon tetrachloride is a stable, gaseous compound in the lower atmosphere with an estimated atmospheric lifetime of fifty years. Since there are no rapid removal mechanisms, carbon tetrachloride accumulates in the lower atmosphere. This accumulation has caused the build-up of a global background concentration which has been estimated to be between 0.11 and 0.15 parts per billion (ppb).

Some of the removal mechanisms for carbon tetrachloride are ultraviolet photolysis in the stratosphere, gas phase reactions with hydroxyl radicals, and absorption into the oceans, with ultraviolet photolysis recognized as the major removal mechanism. However, one of the photolysis products, chlorine, is a major concern because it reacts with ozone in the stratosphere, thereby, depleting the ozone layer which protects us from harmful ultraviolet radiation.

Based on ARB monitoring data, the statewide annual average concentration of carbon tetrachloride is around 0.13 ppb. The global background concentration is, therefore, the major source of exposure for Californians. Based on modeling analysis, people living or working near sources of carbon tetrachloride emissions may be exposed to elevated levels which have been estimated to be as high as 0.62 ppb.

At current ambient carbon tetrachloride concentrations, no chronic adverse health effects are expected to occur. The carcinogenic effects of carbon tetrachloride have been demonstrated in three animal species. The IARC has determined that carbon tetrachloride is a probable human carcinogen based on the animal data.

Department of Health Services staff estimated that continuous exposure to 0.16 ppb of carbon tetrachloride has an upper limit risk of 10 to 42 excess lifetime cancer cases per million persons exposed.

Ethylene Oxide

Ethylene oxide was identified as a toxic air contaminant with no identifiable threshold exposure level in November of 1987. Ethylene oxide was chosen for evaluation because it is identified by the IARC and the EPA as a probable human carcinogen, it is emitted from several sources in the state, and it will not break down in the atmosphere at a rate that would significantly reduce the resulting public exposure.

A colorless gas (formula C H O) at room temperature and pressure, ethylene oxide is one of the 25 chemicals of highest production volume in the U.S. Natural sources are believed to be insignificant. Although it is not produced in California, it is distributed in the state for use in manufacturing surfactants (detergents), and for sterilization and fumigation. Air Resources Board staff estimates that about 360 tons of ethylene oxide were emitted into the state's atmosphere in 1985, with no significant change since then. Because it is a registered economic poison, ethylene oxide is also subject to regulation by the California Department of Food and Agriculture.

The sources of ethylene oxide emissions in California in 1985 were, in descending order: sterilization, fumigation, surfactant manufacture, and distribution. About three-fifths of the total come from sterilization, principally of medical devices and health care products. Most of the emissions result from industrial sterilizers. Virtually all of the ethylene oxide used in sterilization and fumigation is assumed to be emitted into the atmosphere. Approximately one-third of the ethylene oxide is from fumigation of spices and related botanical products.

Ethylene oxide can be removed from the atmosphere by reactions with hydroxyl radicals, hydrolysis, and wet deposition (rain). The first mechanism would significantly reduce ambient concentrations over a period of several weeks. Where acidic fog occurs, ambient concentrations could be reduced by up to 5 percent. No significant pathways exist for ethylene oxide formation in the atmosphere.

When medical equipment is sterilized or spices are fumigated, ethylene oxide is released both to the atmosphere and to waste water. The ethylene oxide in the waste water is probably emitted into the atmosphere before hydrolysis can occur. Most of the ethylene oxide dissolved in waste water is expected to evaporate within two miles of the point of introduction.

Ethylene oxide has been found to be carcinogenic in animals following subcutaneous, oral, and inhalation administration. An increase in rat brain tumors was observed after exposure by inhalation. In humans there is some evidence of carcinogenicity. The IARC has concluded that ethylene oxide is probably carcinogenic in humans. There is no evidence of a threshold exposure level.

Dose-response data from a two-year inhalation study of female rats were used to estimate the carcinogenic risk from ambient air exposure to ethylene oxide. Using a multistage model, DHS staff estimates that the excess lifetime cancer risk from continuous exposure to an average ambient concentration of 50 parts per trillion (ppt) is from 6 to 8 cases per million persons exposed.

Based on the findings of carcinogenicity and the results of the risk assessment, DHS staff finds that ambient ethylene oxide may cause or contribute to an increase in mortality or an increase in serious illness. ARB staff estimated that seven million people in the Los Angeles area were exposed to a population-weighted annual average concentration of 50 ppt in 1985.


Benzene Control Plan and Airborne Toxic Control Measure

In 1985, the Air Resources Board identified benzene as a toxic air contaminant without an identifiable safe threshold exposure level. Once identified, the second phase of the air toxics program began - control measure development. As described in Air Toxics Update #3, the risk management or control part of California's toxics program begins with a "control plan" which may be followed by development of one or more control measures. The benzene control plan is described in Air Toxics Update #3.

In reviewing emission levels of benzene, the ARB staff found that vehicular exhaust and evaporative emissions account for over 90 percent of the benzene emissions in California. Other sources such as gasoline marketing, refineries, and detergent alkylate producers make up most of the balance.

In July of 1987, the Board passed a motion to require Phase I and Phase II vapor recovery systems on all new retail service stations when gasoline sales commence, and on existing stations selling more than 480,000 gallons yearly within two years after district adoption of the measure. Phase I vapor recovery systems recover losses from transferring gasoline from gasoline delivery trucks to underground storage tanks. Phase II vapor recovery systems recover losses from transferring gasoline from underground storage tanks to vehicles (cars, trucks, etc.). In October of 1987, the Board passed an additional requirement that all existing retail service stations selling 480,000 gallons or less yearly be equipped with Phase I and Phase II controls when underground storage tanks are replaced or when major modifications to the underground piping are made.

This measure will reduce the individual risk from vehicle fueling by 85 percent and will ultimately reduce the overall cancer incidence from uncontrolled service station benzene emissions by more than 80 percent. This measure will also help achieve and maintain the State and Federal ozone standards in those areas that have not yet attained them.

Ethylene Dichloride

Ethylene dichloride (EDC) was identified as a toxic air contaminant in September 1985. In March 1987, the Board considered and approved a control plan for EDC.

EDC is primarily used in California as an additive to leaded gasoline. However, due to the stringent limits on the lead content of gasoline adopted by the Environmental Protection Agency, this use of EDC has declined greatly. The other major category which the ARB has the authority to control is the use of EDC as a solvent. Most large users of EDC in this category have applied recovery or control equipment to prevent emissions or are ceasing use of EDC. Ethylene dichloride is used as a pesticide against wood borers in landscape and structural applications and is registered under the California Department of Food and Agriculture which is authorized to control its pesticidal use.

Based on these factors, the ARB decided not to develop EDC control measures. The Board did approve continued ambient monitoring of EDC to ensure that levels decrease and directed ARB staff to return to the Board with proposed control measures if ambient levels of EDC increase.


The ARB continues to investigate and review compounds for which information indicates there may be a significant public health risk. During 1988, the Board is expected to consider the identification of methylene chloride and inorganic arsenic as toxic air contaminants. For more information about the current status of these compounds or others in the identification process, please contact:

Chief, Air Quality Measures Branch
Stationary Source Division
Air Resources Board
P.O. Box 2815
Sacramento, CA 95812
(916) 322-7072

In addition, the Board will consider control plans and / or measures for benzene, hexavalent chromium, carbon tetrachloride, and dioxins in 1988. For more information about the current status of compounds in the control development process, please contact:

Chief, Emissions Assessment Branch
Stationary Source Division
Air Resources Board
P.O. Box 2815
Sacramento, CA 95812
(916) 322-6023

Air Toxics Updates