1987-03-00 Air Toxics Update #3

This page last reviewed July 30, 2008


Air Toxics Update #3


The Air Resources Board (ARB) took action on five toxic compounds during 1986 as part of California's Air Toxics Program. Three substances -- hexavalent chromium, asbestos, and chlorinated dioxins / furans -- were identified as toxic air contaminants (TACs), and decisions were made about the need to control two previously identified air toxics -- benzene and ethylene dibromide. This Update is the third in a series of publications on California's air toxics program. It provides an overview of the air toxics decisions made by the Board during 1986, and summarizes the characteristics of the three newly named air toxic substances -- chromium, asbestos, and dioxins. In addition, the benzene control plan is described, and the decision not to develop control measures for ethylene dibromide is explained. For the reader 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 as air toxics in 1985 -- benzene, ethylene dibromide, and ethylene dichloride -- please see Update #2.


During 1986, there were 15 compounds at various stages of the identification and regulatory phase of the air toxics program. Those substances which had completed the identification phase and moved into or were already in the control measure phase were:

  • Benzene
  • Ethylene Dibromide
  • Ethylene Dichloride
  • Hexavalent Chromium
  • Asbestos
  • Chlorinated Dioxins / Furans

Those substances which were in the identification phase at the end of 1986 were:

  • Cadmium
  • Vinyl Chloride
  • Inorganic Arsenic
  • Carbon Tetrachloride
  • Chloroform
  • Ethylene Oxide
  • Methylene Chloride
  • Perchloroethylene
  • Trichlorethylene

Cadmium was considered and listed by the Board as a TAC in January 1987. Carbon tetrachloride, ethylene oxide, and methylene chloride are expected to follow later in the year.

Another Board action of general interest was the adoption in early 1987 of a revised compound ranking list. This list now contains 50 compounds of concern divided into three categories to reflect the status of the substances in the identification process. Category one contains those substances identified as TACs, category two includes those substances currently under review or soon to be scheduled for review, and category three contains those compounds lacking sufficient health effects information to support review at this time.

Because this list changes periodically as new health information becomes available, it is not included in this publication. A current list may be obtained by contacting ARB's Stationary Source Division in Sacramento as noted on the back page of this Update.


Hexavalent Chromium

In January, 1986, the ARB identified hexavalent chromium as a toxic air contaminant in California and listed it as a substance without any identifiable carcinogenic threshold. Chromium was selected for review because it is known to cause cancer in humans and animals and because it is emitted from many sources in California. Its presence in the air has been documented by ARB and the Environmental Protection Agency.

Chromium is a metallic element which occurs in nature primarily as chrome iron ore (chromite). This ore is not mined commercially in the U.S. but is imported principally from South Africa and the USSR. The ore is used to produce chromium metals and alloys, refractory materials (firebrick), and chromium chemicals.

Chromium in compounds exists in several diffferent chemical (oxidation) states, but only two of them, trivalent chromium and hexavalent chromium, are commercially or environmentally important. Trivalent chromium is the most stable oxidation state and is therefore the most common form. Hexavalent chromium is less stable and occurs less often in nature because it is very reactive, readily changing to trivalent chromium in the presence of organic matter.

The health impacts of trivalent and hexavalent chromium are very different. This is partially explained because hexavalent chromium readily penetrates biological membranes while trivalent chromium generally does not. Trivalent chromium is an essential trace element in our diets, helping reduce the buildup of glucose in the blood. The hexavalent form has been identified as a cancer causing substance. Thus, depending on the chemical state, it may either play a role in nutrition or may act as a carcinogen.

Stationary sources contribute most of the known chromium emissions in California. Chromium is emitted both directly in the use and production of chromium compounds -- mostly in the hexavalent form -- and secondarily, or inadvertently, through the combustion of chromium-containing fuels -- mostly in the trivalent form. Use of chromium in chrome plating and as a corrosion inhibitor in cooling towers accounts for most of the direct hexavalent chromium emissions in California. Secondary sources of chromium emissions -- combustion of coal and oil, cement production, waste incineration -- produce both trivalent and hexavalent chromium. Evidence suggests that chromium emitted from secondary sources is principally in the trivalent state.

The extent to which natural sources of chromium contribute to hexavalent chromium levels in the outdoor air in California is unknown. Trivalent chromium is a component of most soils and in this form is generally not easily absorbed by the body. Weathering and wind action can lift soil chromium into the air, but most of these particles are too large to be deposited in the lungs.

There is very little information available on the reactivity of chromium compounds in the atmosphere. How, and to what extent, chromium reacts in the atmosphere is not well understood, and the persistence of hexavalent chromium in the atmosphere has not been determined. Physical removal of chromium from the air occurs both by fallout (dry deposition) and by washout (wet deposition). Measurements have shown that most chromium is removed from the atmosphere through wet deposition. Small chromium particles, however, may remain airborne for extended periods of time, allowing long distance transport. Because of this, weather conditions can play a significant role in the dispersion of chromium.

Hexavalent chromium has been measured in the air at sites in many populated areas of California. To assess the public exposure around the State the ARB collected samples in eight cities during the last four months of 1985. Over 80 percent of the samples had measureable levels of hexavalent chromium and the average 24-hour concentration for all samples was 1.0 nanogram per cubic meter. This suggests that there is general population exposure to hexavalent chromium in California outside the immediate vicinity of major hexavalent chromium sources.

Many workplace studies have shown a high association between hexavalent chromium and respiratory cancer. The Department of Health Services (DHS) therefore agreed with the findings of the International Agency for Research on Cancer that hexavalent chromium is carcinogenic to both animals and humans. In contrast, DHS found that workplace studies of trivalent chromium do not provide adequate evidence to confirm or refute whether this form is carcinogenic.

Cancer, however, is not the only health effect of concern associated with hexavalent chromium. Exposure to hexavalent chromium in occupational settings has resulted in irritation of the respiratory tract (ulcerations, sore throat, irritation of the mucous membranes) and other skin reactions. Hexavalent chromium exposure has also been associated with kidney, liver, and lung damage. Because it has strong oxidizing powers, it can readily be absorbed through the abdomen or through the skin, explaining much of its irritating and toxic properties.

Another area of concern from exposure to hexavalent chromium is adverse reproductive effects. Animal studies have demonstrated adverse effects on male reproductive systems and fetal development. Epidemiological studies on humans are needed to verify whether humans can also be affected in this way.

In summary DHS has estimated that the maximum excess cancer risk from 70-year exposure to 1.0 nanogram per cubic meter of atmospheric hexavalent chromium is 146 cases for each million people exposed. DHS also concluded that based on the current ambient chromium exposure levels, no adverse health effects other than cancer are expected to occur with the possible exception of reproductive effects.

Nanogram = One billionth of a gram
Picogram   = One trillionth of a gram


Asbestos was the fifth substance to be identified as a toxic air contaminant in California by the Air Resources Board. In March 1986, the Air Resources Board listed asbestos as a TAC having no identifiable carcinogenic threshold. Asbestos was selected for review because it is known to cause cancer in humans and its presence in the atmosphere has been documented in California. Another significant consideration is that EPA has identified asbestos as a hazardous air pollutant and has promulgated national emission standards.

The word asbestos is not a mineral name but is a term applied to a group of naturally occurring mineral silicate fibers. There are a number of asbestiform varieties of asbestos that have been associated with cancer; chrysotile, actinolite, amosite, anthophyllite, crocidolite, and tremolite.

As a fibrous mineral that can be woven, asbestos is unique. The industrial usefulness of asbestos is based on it strength, its flexibility, and its resistance to heat and chemicals. Asbestos has therefore been used in countless ways throughout the years -- in making fireproof materials, electrical insulation, roofing, flooring, pipe, brake linings, etc. People are exposed to asbestos throughout the life cycle of the substance -- when it is mined, milled, processed, fabricated into industrial and consumer products, and when these products are used, repaired, and disposed of.

The sources of asbestos emissions in California currently being investigated are asbestos mines and mills, the manufacturing of asbestos products, its use in automobile brakes, serpentine rock quarrying operations, and roads surfaced with gravel containing asbestos. Mines and mills account for over 90 percent of the estimated emissions. California has two companies that mine, mill, and market chrysotile asbestos.

Other potential sources of asbestos emissions are the demolition and renovation of buildings containing asbestos products, landfills, and natural weathering or human disturbance of asbestos deposits. The amount to which these sources contribute to the level of asbestos in the ambient air has not been established, but as more information becomes available, these emission sources will be added to the inventory.

As a substance, asbestos is relatively inert and does not normally undergo change in the ambient environment. Winds can disperse asbestos fibers and once in the air, the fine fibers can remain airborne for long periods of time. Asbestos fibers eventually settle out of the air because of gravitational pull or because of rain or fog. Once in the soil, they can easily re-enter the ambient air because of natural or manmade disturbances.

The evidence showing asbestos to be carcinogenic via inhalation is undisputed, although the evidence for ingestion is not conclusive. DHS in its review of the health effects found that asbestos has been linked to cancer in humans in both occupational and non-occupational settings. Asbestos is commonly associated with lung cancer and mesothelioma, a cancer of the lining of the chest cavity or abdomen. Other human cancers including larynx and gastrointestinal tract cancer have also been seen.

Although the method by which asbestos causes cancer is unknown, asbestos fibers can enter the body by inhalation or ingestion. These fibers are needle-like in shape and can penetrate body tissues and be preserved intact in the body for decades after exposure. The persistence of these fibers may be the essential reason for their chronic toxicity. In exerting the carcinogenic effect, asbestos may exhibit a significant latency period between the time of initial exposure to asbestos and the time that the cancer appears. This latency period may be as short as five years for mesotheliomas to as long as 40 years for lung cancers.

Other than cancer, the most serious adverse health effects associated from exposure to asbestos is asbestosis which is a disabling lung disease caused by asbestos fibers in the lung. The buildup of asbestos fibers can also result in pulmonary changes varying in severity from mild impairment to progressive lung failure. The DHS concluded, however, that ambient levels of asbestos in California are not expected to cause asbestosis, or any other serious chronic health effects, other than cancer.

From a 1981 study where asbestos was monitored at ten locations in California, generally, the highest concentrations of asbestos was found near localized and industrial sources of asbestos. Asbestos concentrations that were measured range from below the detection limit to 140,000 fibers per cubic meter. The lowest concentrations were generally found at sites isolated from asbestos emission sources. These samples were analyzed by transmission electron microscopy.

The excess cancer risks from exposure to asbestos were estimated by DHS to be no more than 110 cases of lung cancer and 190 cases of mesotheliomia for each million people exposed to 100 fibers of asbestos per cubic meter over a lifetime. In making these estimates, DHS found no compelling reason to differentiate between fiber types, thus all fiber types listed above have been identified as toxic air contaminants with the same associated risks.

Chlorinated Dioxins / Furans

In July 1986, the Air Resources Board voted to list 15 compounds from the chlorinated dioxin and dibenzofuran family of substances as toxic air contaminants. Dioxins and furans were selected for identification as TAC's because several of these compounds are proven to cause cancer in animals and have the potential to be carcinogenic to humans. Another significant reason for their consideration is that dioxins and furans have been detected in the emissions from a number of different sources which are currently operating or are proposed for construction in California.

Americans first began to hear about dioxins after Vietnam veterans attributed adverse health effects to exposure to the defoliant Agent Orange, an herbicide which contains dioxins. Dioxins, however, are not a single substance but a generic term used to denote a family of chemicals. There are in all 75 chlorinated dioxins and 135 chlorinated dibenzofurans in this family, differing from each other by the number and position of chlorine atoms on the molecule. These compounds are grouped together (dioxins / furans) for identification as toxic air contaminants because of their similarities in chemical, physical, and toxic properties, as well as their environmental origins.

Out of this group of 210 compounds, the 15 that were identified as toxic air contaminants were those with a chlorine atom attached at the 2, 3, 7 and 8 positions and containing between four and seven chlorine atoms. No cancer threshold was identified because there was not sufficient scientific evidence available to identify a safe exposure level.

These diagrams of dioxin and furan molecules show in red the four positions where, if chlorine atoms are attached, the substances are toxic air contaminants.

Information about emissions of dioxins and furans is limited because they occur as unwanted by-products of certain chemical manufacturing and combustion processes. These substances are produced in very small quantities and are difficult to detect.

Recent evidence suggests that combustion processes are the major environmental source of dioxins, but few of these possible sources of emissions have been extensively tested. Emission rates appear to be related to the chemical composition of the fuel and the efficiency with which the burning takes place. Combustion temperatures seem to play a role in the formation and destruction of dioxins and furans. Although conflicting results from several studies make it unclear as to the exact role temperature plays in dioxin formation and emissions, tests of existing sources do give a strong indication that high temperatures, long resident time, and efficient mixing in an incinerator can reduce the amount of dioxins emitted.

From the tests conducted thus far, it appears that waste incineration may be a major source of dioxin and furan emissions, along with facilities burning wastes containing pentachlorophenol (a wood preservative) and wire recovery furnaces. Other potential sources in California are incinerators burning hazardous waste, sewage sludge, and hospital waste. However, because the composition of the wastes burned in these incinerators varies, it has been difficult to estimate emissions. Waste oil derived fuel is also a potential source of dioxins and furans.

Once emitted into the atmosphere, the chemistry of dioxins is largely unknown. Several research studies do indicate, however, that dioxins can be transported over long distances by the wind and therefore could have effects both in the immediate vicinity of the source and at areas distant from the source.

Dioxins and furans attached to airborne particles are eventually deposited on soil or water opening a secondary exposure route via ingestion and skin exposure. These substances are highly persistent in soils and can be present years after the introduction of dioxins occurred. The chemical properties of dioxins also allow it to be accumulated in fatty tissue. This has led to concerns that dioxins could accumulate up the food chain resulting in an effective dose of dioxins greater than that indicated by the levels in the air.

Thus, secondary exposure may be as significant as atmospheric exposure and could substantially increase the total public health risk of dioxin and furan emissions. However, in the absence of data on exposure for California residents, the DHS did not include effects of these secondary exposures in its risk assessment.

A number of polychlorinated dioxins and furans have been tested for their toxicity in animal studies and have been found to be highly toxic. The toxic effects of these substances include severe weight loss, liver disease, skin lesions, reproductive toxicity, suppression of the immune system, cancer, and death.

The one dioxin that has been studied the most extensively and is considered to be the most toxic is 2, 3, 7, 8 tetrachlorodibenzodioxin or TCDD, a dioxin with four chlorine atoms attached at the 2, 3, 7, and 8 positions. Based on animal studies DHS believes TCDD may act both as an initiator and as a promoter of cancer. The other chlorinated dioxins and furans identified by ARB as TAC's are thought to be carcinogens with similar toxicities but decreasing potency as the number of the chlorine atoms increases. All other dioxins and furans are not thought to be carcinogenic and were not included for identification at this time.

The epidemiological studies of persons exposed to dioxins have not been adequate to prove that these substances are carcinogenic to humans. Short term exposure of humans to high concentrations of dioxins has caused chloracne, a skin lesion which resembles mild to very severe acne which may last many years. Acute and chronic human exposure has also been associated with liver toxicity. However, no toxic effects other than cancer are expected to occur in humans at the ambient levels of chlorinated dioxins and furans predicted by the ARB staff.

Evaluating exposure levels to dioxins is difficult. Ambient concentrations of these compounds are so low that until recently they could not be detected in ambient air. Another complicating factor is that dioxins and furans are emitted from emission sources as a mixture which varies depending on the source, thereby making determination of the presence of its toxic forms more difficult. To expand our information on background levels of dioxins and furans, ARB is currently funding an air monitoring study in Southern California. Monitoring will be done at a variety of locations both near and away from potential dioxin sources. In addition, ARB staff is surveying and evaluating the degree to which various stationary sources emit dioxins and furans.  

Once the background exposure levels are determined, the overall public health risk from exposure to airborne dioxins and furans can be calculated using the DHS estimate of excess cancers -- a maximum of 38 cancer cases per million people exposed to one picogram per cubic meter of air. In addition, the development of emission information about specific sources will be used in the regulatory review process where specific sources will be considered for regulation.


Benzene Control Plan

Early in 1985, the Air Resources Board identified benzene as a toxic air contaminant in California and listed it as a TAC without an identifiable safe threshold. Once identified, the second phase of the air toxics program began -- control measure development. During this risk management phase, the ARB prepares a report on the need and appropriate degree of regulation for the toxic substance.

The regulatory needs report, according to the state's air toxics legislation (California Health and Safety Code 39665), must contain information on:

  1. Sources and Emission Levels;

  2. The Physical and Chemical Characterisics of Benzene;

  3. Its Public Health Effects; and

  4. Control Measures, Their Availability, Feasibility, Costs and Risk Reduction.

Additionally, where there is no identified threshold, the law requires that toxic control measures reduce emissions to the lowest achievable level by using best available control technology or a more effective method unless another level is determined by a risk assessment to be adequate.

At the July, 1986 Board meeting, the ARB approved the Benzene Control Plan which describes an overall course of action for developing benzene control measure but does not adopt any specific measures. The plan identifies potential benzene control measures that reflect the use of either presently available control technology or technology which is expected to be feasible in the near future.

In reviewing emission levels, the ARB staff found that vehicular exhaust and fuel systems create 91 percent of the benzene emissions in California. The other non-vehicular sources -- primarily gasoline marketing and refineries -- combine to make up the remaining 9 percent of the benzene emissions.

Benzene emission levels have varied in California over the last 30 years with 1964 being the estimated high at around 40,000 tons per year. Since the introduction of smog controls on cars in 1964, benzene emissions have dropped over 50 percent. Without further controls, total benzene emissions in the year 2000 are expected to drop another 29 percent primarily because older cars will be replaced with newer, better controlled vehicles.

Exposure to benzene varies in California between urban and rural areas. Based on the 1984 ambient air monitoring data from around the state, ARB found that urban areas registered higher levels with the greater Los Angeles area showing the highest annual average at about 4 parts per billion (ppb). The statewide annual average was estimated to be above 3 ppb.

To estimate the current and future risk levels, ARB used the DHS health assessment range of risk for benzene -- 22 to 170 excess cancers for each million people exposed to 1 ppb over a lifetime. When this risk factor is used with the statewide annual average exposure to benzene of 3.3 ppb, the individual risk of cancer is estimated to be between 72 and 560 for each million people in California. Applying this same DHS range of risk to the benzene emissions inventory for the year 2000, the individual risk of cancer was reduced 30 percent to a level between 52 and 400 excess cancers.

The Board found these risks to be significant and directed staff to implement the Benzene Control Plan over the next three to four years. The major purpose of the plan is to identify and prioritize control measure strategies from which further reductions can be achieved. Individual measures will be developed and brought back to the Board to be considered for adoption as airborne toxic control measures.

In approving the plan, the Board directed staff to take the following major steps toward developing additional benzene control measures:

  • Step one is to continue to work on the control technologies which are being developed as a part of ARB's efforts to reduce hydrocarbon emissions. These measures include motor vehicle controls which are being developed for hydrocarbon control purposes. Although not specifically aimed at controlling benzene, these measures are expected to lower the benzene levels and thereby the excess cancer risk by 8 percent.

  • The second approach approved and given the highest priority by the Board is for staff to develop and bring back benzene-specific measures for motor vehicles and motor vehicle fuels. These measures are technologically feasible but require additional time for development. This group includes setting a gasoline specification which would limit the amount of benzene in gasoline and setting an exhaust limit for benzene from motor vehicles. These measures are expected to reduce the individual risk of cancer by another 12 percent and are scheduled to be brought to the Board for consideration between 1987 and 1989.        

  • The third group of measures the staff will be working on are gasoline marketing and refinery stationary source control measures. The Board directed ARB staff to work with local air pollution control agencies and the affected industries to analyze further sources related to gasoline marketing and refineries. Those measures warranting further consideration will be brought back to the Board. This group of strategies has less potential for reducing risk, with staff estimating the risk reduction to be approximately 1 percent.   

ARB's current hydrocarbon control program coupled with the implementation of the measures identified in the three groups above are expected to reduce the statewide risk from benzene in the year 2000 by 50 percent from 1984 levels. These new benzene controls would be accomplished at an annual cost of approximately $425 to $450 million.

Ethylene Dibromide

Ethylene dibromide (EDB) was identified as a toxic air contaminant in mid-1985. In September 1986, the Board considered the need for adoption of EDB control measures.

EDB is a commercial chemical used primarily in leaded gasoline to remove lead deposits from automobile engines. Prior to 1984, EDB was also used in significant quantities as a pesticide.

The use of EDB declined about 65 percent between 1983 and 1985 and will continue to decline as lead in gasoline is phased out. Nearly all pesticidal applications are now banned except for special circumstances. Because of these two control actions, EDB emissions are expected to be reduced to 1 percent of their 1983 levels by 1990.

Based on these factors, and the lack of other identified significant sources of EDB, ARB estimates that the lifetime cancer risk from exposure to ambient EDB will be one or less excess cancer per million persons exposed by 1990. Therefore, the Board decided not to develop specific EDB control measures at this time. The Board did approve continued ambient monitoring of EDB to ensure that levels decrease and directed ARB staff to return to the Board with a proposal for control measures if EDB ambient levels increase.

Plan for 1987

The 15 substances that have entered the Air Toxics Program thus far were selected because available information indicates they pose the most significant potential adverse health risk to the greatest number of people in California. These substances are compounds with documented exposure information and substantial health effects data. Many of the substances that ARB will be reviewing in the future are going to be more difficult to evaluate. Extensive background work will need to be done on some of the candidate compounds to determine if sufficient health effects and exposure information are available before they can formally enter the toxics identification process.

During 1987, the Air Resources Board plans to hold air toxics identification hearings on the following four substances: cadmium, carbon tetrachloride, ethylene oxide, and methylene chloride. In addition, eight control reports are anticipated to be under development during the year for those substances identified as TACs, and at least one control measure will be proposed for adoption.

For more information about the current status of the California Air Toxics Program or for copies of documents on specific substances, please contact:

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

March 1987

Air Toxics Updates