1986-04-00 Air Toxics Update #2

This page last reviewed July 30, 2008


Air Toxics Update #2




This Program Update is the second publication in a series on California's Air Toxics Program and describes the progress made during the first two years of the program. In this Update information is presented on the start up of the program, the substances in the evaluation process, and the characteristics of the compounds identified as toxic air contaminants during 1985.

The first Air Toxics Program Update is recommended reading for anyone wanting a general overview of the Califoria Air Toxics Program. It provides information on the legal requirements of the program and describes its two major processes -- risk assessment, where toxic air contaminants are identified and risk management, where regulatory decisions are made.


The first action the Air Resources Board (ARB) took to establish the program was to create a ranking scheme for prioritizing possible toxic substances. Based on research studies and a review of compounds either listed or under consideration by the Environmental Protection Agency (EPA), the ARB narrowed the field to approximately 50 compounds of concern using the criteria set out in the law:

  • Risk of Harm to Public Health
  • Amount of Emissions
  • Manner of Usage
  • Persistence in the Atmosphere
  • Ambient Concentrations in California

A current listing of the compounds of concern is available from ARB's Stationary Source Division. Because the list changes as new information is received, it is updated periodically and is therefore available as a separate publication. Early in 1986 ARB revised the ranking system to clarify the groupings and added a category for those substances now listed as toxic air contaminants.


At its January 1984 meeting, the ARB approved the initial ranking of compounds and endorsed a schedule for starting compounds through the reivew process. Since then, review of a new substance has been initiated roughly every two months. The eleven substances that formally entered the risk assessment phase of the air toxics program during 1984 and 1985 are:

  benzene ethylene dibromide  
  asbestos ethylene dichloride  
  chromium ethylene oxide  
  cadmium carbon tetrachloride  
  dioxins vinyl chloride  
  inorganic arsenic    


Once a substance has been selected for evaluation, the ARB requests the Department of Health Services (DHS) to review the available health effects information on the substance. As a part of this review, DHS evaluates the health effects data, examines the biological characteristics of the substance, and estimates the probable incidence of an adverse health effect to humans at a given exposure level. In addition, DHS determines whether the substance has a threshold exposure level below which there will be no adverse human health effects. In cases where there is no threshold, DHS provides the range of risk to humans which can be expected from current or anticipated exposure.

At the same time that DHS is doing its health evaluation, the ARB staff compiles information on exposure levels. This includes information on uses, sources, and emissions, as well as concentrations of the substance in the ambient air, the locations where concentrations are greatest, its persistence in the atmosphere, and what is the present or potential public exposure to the substance.

These two segments -- the DHS health effects evaluation and the ARB public exposure assessment -- are published by ARB as the risk assessment report. Because these reports provide a comprehensive analysis of the estimated risk to public health, they are ultimately used by the Board as the technical basis for making its decision whether the compound should be listed as a toxic air contaminant.

During the first two years of the program, the ARB requested health evaluations for the 11 substances listed on the previous page. The risk assessment reports for the candidate compounds are made available to the public for comment before being submitted to the Scientific Review Panel (SRP) composed of nine highly qualified professionals engaged in Scientific Researh (Health & Safety Code Section 39670). Copies of these reports are available from the ARB's Stationary Source Division.


During the 1984 and 1985, reports for six compounds were submitted to the SRP for evaluation. The SRP is responsible for reviewing the scientific procedures and methods used in the report, the health and exposure data, and the report's conclusions. During its approximate two month review, the Panel must make a finding regarding the report's acceptability or deficiency. Where a report is found to be deficient, it is returned to ARB and DHS for revision.

During 1985, the SRP reviewed five reports and found that two of the reports were deficient -- the chromium report and the dioxins report. In finding the reports deficient, the SRP asked that additional information and clarification be provided.


In January 1985, the Board held its first public hearing to consider listing a substance as a toxic air contaminant. At this meeting, the Board decided to list benzene as a toxic air contaminant. The listing appears in Title 17, Section 93000, of the California Administrative Code. Approximately six months later, in July 1985, the Board listed ethylene dibromide (EDB) as a toxic air contaminant and in September 1985 added ethylene dichloride (EDC) to the list.

Having been identified as toxic air contaminants, these three compounds are now in the second part of the ARB program -- the risk management or control decision phase. Working with local air districts and the interested public, ARB is studying the need and appropriate degree of control for each of these substances.

The first of the regulatory needs reports (benzene) is currently available to the public in preliminary draft form and scheduled for discussion by the Board in 1986. The report will be used as the basis for the Board's identification of control measures to reduce benzene emissions. The report provides information on present and future benzene emissions and exposure, identifies available and potential benzene control measures, and discusses respective costs and effectiveness. For copies of the regulatory needs reports on specific toxic air contaminants, contact the ARB's Stationary Source Division.


Each of the three compounds listed as toxic air contaminants in 1985 is a component in leaded gasoline. These compounds are emittted in the exhaust and evaporation of fuel from motor vehicles, and from the production and marketing of gasoline. These uses contribute to the occurrence of the three compounds in the air throughout California. However, control measures adopted to reduce other pollutants related to gasoline use have also had the effect of reducing benzene, EDB, and EDC emissions. More detail about each of these compounds is provided below.


Benzene was chosen as the first substance to enter the toxic review process because it is known to be a human and animal carcinogen, and because it has been identified as a hazardous air polluant in the federal hazardous air pollutant program (Section 112 of the Clean Air Act). Benzene emissions are widespread and result in significant concentrations in the outdoor air in California. Because of its relative persistence in the atmosphere, it is expected to be present throughout an urban area.

Benzene is a hydrocarbon which occurs naturally in crude oil and is formed as the oil is refined into products such as gasoline. It is emitted into the air from the marketing and burning of petroleum based fuels and other combustion processes such as waste, agricultural, and forest management burning. With the shutdown of benzene production at Chevron's El Segundo refinery, benzene is no longer specifically manufactured in California as a commercial product.

ARB staff estimates that mobile sources create over 90 percent of the benzene emissions statewide and that non-vehicular sources contribute the remaining amount. The major sources where the public is most likely to be exposed are busy roadways and service stations. Benzene is emitted from vehicles as a part of the exhaust gases as the fuel is burned and with the evaporation of fuels from the fuel system.

At the present time benzene makes up approximately two percent of the gasoline we buy. However, benzene does not have to be in the fuel to be emitted; it can be formed from other aromatic compounds in the fuel through the combustion process. An additional concern is that as lead is phased out over the next few years, the aromatic content of gasoline, which includes benzene, is expected to rise to replace the lost octane value.

Although a relatively minor source, waste burning is the second largest source of benzene emissions followed by gasoline marketing where vapors escape during the transfer of fuels. In the past, benzene has been used in industrial and consumer products such as adhesives, and solvent and paint removers, but these uses have recently been reduced. It has also been used in the making of lacquers, varnishes, and dyes and other organic compounds, but the amounts used now for these purposes are believed to be relatively small and are decreasing.

Documented human health effects of benzene have occurred mainly as a result of exposure in occupational settings. Deaths have occurred when workers entered enclosed spaces such as tanks where there were high levels of benzene. For humans, relatively high doses cause intoxication, and respiratory and circulatory collapse. It can also act as a central nervous system depressant.

In smaller doses, benzene has been associated epidemiologically with increased incidence of leukemia, and recently animal studies have shown it causes a variety of other cancers as well. Staff of the Department of Health Services and the ARB therefore agree with the International Agency for Research on Cancer (IARC) that there is sufficient evidence to consider benzene a human carcinogen and recommend that benzene be treated as a substance without a threshold below which it can be considered safe.

People living in California were exposed to an annual average statewide benzene concentration of about 3 parts per billion (ppb) in community air during 1984. In the South Coast Air Basin -- the four county area around Los Angeles -- the average concentration of about 4 ppb is the highest of California's seven air basins where toxics are monitored. The DHS estimates that the risk to health from continuous exposure to 1 ppb of benzene is probably below the range of 22 to 170 additional cancers per million people exposed during a 70-year lifetime. Using this range and the average exposure of 4 ppm, the added lifetime cancer risk for residents of the South Coast Air Basin is estimated to be no more than 680 for each million people exposed for a lifetime. This risk must be viewed in the context of the overall probability of developing cancer which is on the order of 237,000 cases for each million people over a 70-year lifetime.


Ethylene dibromide, the second compound to enter the toxics review process, is a known animal carcinogen, and has been identified as a potential human carcinogen. EDB is widespread in the ambient air in California and its presence in the atmosphere is documented. EDB reacts slowly in the atmosphere and is a persistent pollutant that can be transported long distances.

EDB is a clear and colorless liquid that is produced commercially by reacting ethylene gas with liquid bromine. There are no natural sources of EDB nor is it manufactured in this state. Its primary use in California is as a scavenger in leaded gasoline to prevent the buildup on lead deposits in automobile engines. EDB can also be used as a pesticide, as an intermediate in the synthesis of dyes and pharmaceuticals, and as a solvent for resins, gums, and waxes.

The major sources of EDB in California at this time are gasoline marketing, gasoline blending facilities, and individual motor vehicles. Until recently, EDB was used to fumigate soil, stored grains and fruits, grain mills, and termite-infested areas. However, in September 1983, EPA cancelled major pesticidal uses of EDB.

Further reductions in EDB are occurring as EPA reduces the allowable levels of lead in gasoline. There are, however, unanswered questions about the observed levels of EDB measured in the South Coast. The estimated emissions from known sources of EDB in the South Coast do not account for measured concentrations, which are four to six times higher than expected. This discrepancy could be caused by underestimations of auto emissions or unknown sources in the basin. When the EDB regulatory needs report is published late in 1986, current information on exposure levels and sources of EDB will be provided.

In evaluating the health effects, the Department of Health Services found the EDB is a potent carcinogen in more than one animal species and recommended that it be considered potentially carcinogenic in humans. When administered to animals, EDB caused malignancies both at the site of the first contact (skin, forestomach, and nasal cavity), as well as at other sites (circulatory system, lung, and pituitary gland among others.

Adverse health effects such as reproductive effects and systemic toxic reactions have occurred with animals at levels much higher than the measured outdoor levels in California. Exposure to EDB vapor can cause respiratory inflammation, anorexia, headache, and throat and eye irritation. Chemical plant workers have reported respiratory irritation at levels of 75 parts per million. At slightly higher levels, or longer exposures, gastrointestinal discomfort and vomiting have been reported. However, DHS concluded that health effects, other than cancer, are not expected to occur in humans at current community exposure

The DHS analysis estimates the added lifetime risk from exposure to EDB is no more than 5.5 additional cases of cancer for each million people exposed to 10 parts per trillion (ppt). In the South Coast Air Basin between 1983 and mid-1984, the ARB found average concentrations of EDB ranging from 5 to 9 ppt in the outdoor air. These levels are less than urban measurements made before 1982 when average concentrations ranged from 5 to 34 ppt. Continued reductions in EDB concentrations are expected in 1986 as the EPA proposed lead standard for gasoline takes effect.


The third substance to be listed as a toxic air contaminant in the California program is ethylene dichloride or EDC. EDC is a known animal carcinogen and has been identified as a potential human carcinogen. It is known to be emitted into the air in California and it persists in the atmosphere with a half life of about 42 days. Because of its long residence time, EDC can be transported throughout an air basin before it is removed from the air. EDC is used in California in leaded gasoline causing it to be emitted with vehicle exhaust and with the evaporation of any leaded gasoline.

There are no known natural sources of EDC. The compound is synthetically produced by the chlorination of ethylene gas. The dominant use of EDC nationwide is as a reactant in the production of vinyl chloride and other chemical products. Because of increased demand for vinyl chloride, the production and use of EDC has steadily increased nationally over the past 12 years. However, in California, EDC is no longer produced or used to produce vinyl chloride since the 1982 closure of the Stauffer Chemical plant in Carson.

The primary use of EDC in this state has been in leaded gasoline to scavenge the lead and to keep it from forming deposits in the engine cylinders. EDC is also used as a solvent and as a pesticide. In 1983, the use of gasoline and pesticides accounted for most EDC pollution in California. However, actions taken by the Environmental Protection Agency have largely eliminated EDC's use as a pesticide and have reduced gasoline-related emissions of EDC by 88 percent with the reduction of the allowable lead content in gasoline. In the future the major source of EDC will be its use as a solvent and related industrial uses.

Other sources where EDC can be found are hazardous waste landfills and sewage treatment plants. Wherever EDC contaminated waste is buried, EDC emissions may evaporate into the air for several years. New gas recovery systems installed at landfills to reduce the emissions of other pollutants are having the effect of reducing EDC emissions as well. Large sewage treatment plants can also emit significant amounts of EDC where aeration basins or other treatment operations promote evaporation. However, because relatively little EDC is used in California its presence in sewage effluent is expected to be less than is typical in other parts of the country.

The primary short-term health effects of high levels of EDC are liver toxicity and central nervous system depression. Immediate symptoms such as narcosis or unconsciousness following acute inhalation exposure are caused by central nervous system depression. The severity of the effects on humans is dependent upon the duration and concentration of exposure.

EDC is known to cause cancer in laboratory animals when ingested and may be carcinogenic when inhaled. In addition to causing cancer, long term exposure can also produce kidney and liver damage. However, DHS concluded from its study that adverse effects other than cancer are not expected to occur from exposure to EDC at the levels currently found in the ambient air in California.

The DHS estimate of the range of added lifetime risk of cancer from exposure to EDC is no more than 88 cases per million people exposed to 1 ppb over 70 years. The average concentrations of EDC found in the South Coast Air Basin in 1983 were estimated to be between .019 and .11 ppb. However, establishing accurate measurements of EDC has proven to be a difficult task because in most air samples, EDC concentrations are below the current instrument detection limit of 0.1 ppb. The average of the areawide data collected in Northern California during 1985 is estimated to be less than 0.1 ppb with 11 out of 137 of the collected samples reflecting measurable amounts of EDC. The ARB expects the levels of EDC to continue to decline as the amount of lead in gasoline used statewide is reduced.


ARB is continuing to implement the toxic air contaminant program with a new substance entering the identification process approximately every two months. As many as six substances could be considered by the Board for listing as toxic air contaminants during 1986. The Board will also be considering the regulatory needs reports for the three substances discussed in this Update.

For more information about the current status of the toxic air contaminant program or for copies of risk assessment or regulatory needs reports, please contact:

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

April 1986

Air Toxics Updates