KNOWN CARCINOGEN:
CHROMIUM AND CERTAIN CHROMIUM COMPOUNDS

CARCINOGENICITY

There is inadequate evidence for the carcinogenicity of chromium (7440-47-3) and most trivalent chromium compounds in experimental animals. There is sufficient evidence for the following hexavalent chromium compounds in experimental animals: calcium chromate (13765-19-0), chromium trioxide (1333-82-0), lead chromate (7758-97-6), strontium chromate (7789-06-2), and zinc chromate (13530-65-9) (IARC V.2, 1973; IARC V.23, 1980; IARC S.4, 1982; IARC S.7, 1987). Calcium chromate produced bronchial carcinomas after implantation of an intrabronchial pellet in rats and injection-site sarcomas after intramuscular implantation in rats and mice and after intrapleural injection in rats. Bronchial carcinomas were produced in rats after intrabronchial implantation of strontium chromate and zinc chromate. Injection-site sarcomas were produced in rats and mice after intramuscular, intrapleural, and subcutaneous injections of chromite ore, strontium chromate, chromium trioxide, lead chromate, and zinc chromate, but few or no sarcomas were induced by barium chromate (10294-40-3), sodium chromate (7775-11-3), sodium dichromate (10588-01-9), or chromic acetate (1066-30-4).

An IARC Working Group concluded that there were no adequate data available to evaluate the carcinogenicity of chromium and trivalent chromium compounds in humans; however, they also concluded that there was sufficient evidence for the carcinogenicity of hexavalent chromium compounds in humans (IARC S.7, 1987). An increased incidence of lung cancer has been observed among workers in both the bichromate-producing industry and chromate-pigment manufacturing. There is evidence of a similar risk among chromium platers and chromium-alloy workers. The incidences of cancers at other sites may also be increased in such populations. However, a clear distinction between the relative carcinogenicity of chromium compounds of different oxidation states or solubilities has been difficult to achieve. Recent studies of chromate-pigment makers and users, chrome platers, welders and chrome-alloy foundry workers have shed some light on this problem. For chromate-pigment makers and users, respiratory cancer excesses have usually been found. Chromium pigments are usually hexavalent and commonly include zinc, lead, or strontium chromate. Chrome platers have also been found to have excess lung cancer. Stainless steel welding involves the greatest exposure to hexavalent chromium, as well as to nickel, and one study of chromium-nickel alloy foundry workers showed a statistically significant excess of lung cancers. (For a discussion on the carcinogenicity of metals, see the Introduction, p. viii).

PROPERTIES

Chromium is an odorless, steel-to-semi-grey, lustrous metal available as crystals or powder (99.97% purity). It is insoluble in hot and cold water, nitric acid, and aqua regia, but reacts with dilute sulfuric acid and hydrochloric acid. Calcium chromate occurs in the form of yellow monoclinic prisms. It is soluble in cold and hot water and reacts with acids and ethanol. Chromium trioxide is odorless, dark-purplish-to-red-rhombus crystals that are deliquescent. It is soluble in alcohol, ethanol, sulfuric acid, and nitric acid. When heated to decomposition, chromium trioxide emits smoke and irritating fumes. Lead chromate occurs as yellow or orange monoclinic crystals that are insoluble in water, acetic acid, and ammonia but are soluble in acid and alkali. Lead chromate, when heated to decomposition, emits toxic fumes of lead. Basic lead chromate is a red amorphous or crystalline powder. It is insoluble in hot and cold water, reacts with most acid and alkali but not with acetic acid or ammonia, and emits very toxic fumes of lead when heated to decomposition. Strontium chromate occurs as monoclinic yellow crystals. It is soluble in cold and hot water and reacts with hydrochloric acid, nitric acid, acetic acid, and ammonium salts. Zinc chromate occurs as lemon yellow prisms. It is insoluble in cold water and acetone, dissolves in hot water, and is soluble in acid and liquid ammonia.

USE

In 1987 and 1989, estimated consumption of chromium ferro- alloys, metals, and other chromium-containing materials by end use was as follows: stainless and heat-resisting steel, 79%; full- alloy steel, 8%; super-alloys, 3%; and other alloys, 10% (USDOI, 1988, 1990). The steel industry is the major consumer of chromium. Chromium is used as an alloying and plating element on metal and plastic substrates for corrosion resistance in chromium- containing and stainless steels and in protective coatings for automotive and equipment accessories. It is also used in nuclear and high-temperature research. Similarly, barium chromate and calcium chromate find use in high-temperature applications, e.g., barium chromate in safety matches and pyrotechnics and both are used in high-temperature batteries. In 1985, 39% of the chromium trioxide produced was used for metal plating and treatment, 44% was used in wood treatment and preservatives, and 11% was exported (Chem. Profile, 1985). Chromium trioxide is used in chromium plating and in the manufacture of chromated copper arsenate (NCI DCE, 1985c). Chromium acetate, sodium chromate, and potassium chromate are used in the textile industry. Basic trivalent chromic sulfate is used in the tanning industry (Leather Industries of America, Inc., personal communication). Chromium compounds are also used as pigments (IARC V.23, 1980). Lead chromate is chrome yellow and a component of chrome orange and green; chromium trioxide is green cinnabar; and zinc chromate is zinc yellow (Kirk-Othmer V.6, 1979). Chromium phosphate, strontium chromate, calcium chromate, chromium acetate, and potassium chromate and dichromate are also used in pigments. Other uses for chromium and its compounds include organic chemical synthesis, photomechanical processing, and industrial water treatment. In medicine, chromium compounds are used in astringents and antiseptics (Sax, 1987).

PRODUCTION

Two U.S. firms produced primary chromium chemicals, three firms produced chromium-ferro alloys, and four refractory firms produced chromite-containing refractories (USDOI, 1990). Chromite has not been mined in the United States since 1961. Domestic deposits are small or of low grade. The Bureau of Mines has estimated that in 1989 248.6 million lb of chromium were produced in the United States and 946 million lb were imported and 35.2 million lb were exported (USDOI, 1990). In 1988, 261.8 million lb were produced domestically, 990 million lb were imported, and 30.8 million lb were exported. In 1987, it was estimated that 213.4 million lb of chromium were produced, 708.4 million lb were imported, and 19.8 million lb were exported. Of the 184.8 million lb of chromium produced and 767.8 million lb of chromium imported in 1986, 77 million lb were exported. In 1985, the United States produced 187 million lb, imported 640.2 million lb, and exported 83.6 million lb of chromium. Production of chromium in 1984 was 176 million lb and exports were 72 million lb. Imports of chromium rebounded in 1984 after a 55% decrease from 1981 to 1983. The amount imported in 1984 was 694 million lb while in 1983, 462 million lb were imported. In 1983, 154 million lb of chromium were produced of which 26 million lb were exported. Of the 96 million lb of chromium produced in 1982, 20 million lb were exported (USDOI, 1988; USDOI, 1987). In 1982, 478 million lb of chromium were imported, which was over 50% less than the 1.0 billion lb imported in 1981. Exports of chromium in 1981 were 104 million lb while in 1980, 80 million lb were exported. The amount of chromium imported in 1980 was 922 million lb (USDOI, 1987; USDOI, 1985). The 1979 TSCA Inventory identified a total of 11 producers producing 91 million lb of chromium and 12 importers importing 6.2 million lb of chromium in 1977 (TSCA, 1979).

In 1986, 3.9 million lb of zinc chromate, reported as zinc yellow, and 39.2 million lb of lead chromate, reported as chrome yellow and orange, were produced. In 1985, 4.42 million lb of zinc yellow, and 41.4 million lb of chrome yellow and orange were produced (USDOC Inorganic, 1987). In 1985, the following chemicals were imported: 9.8 million lb of chromic acid, 403,447 lb of chrome green (lead chromate), 6.4 million lb of chrome yellow, 3.0 million lb of chromium oxide green, 862,465 lb of strontium chromate, and 3.5 million lb of zinc yellow (USDOC Imports, 1986). In 1985, 731,941 lb of chromic acid were exported (USDOC Exports, 1986). In 1984, the production of chrome yellow and orange was 46.8 million lb, and of lead chromate, as chrome green, was 16.5 million lb (USDOC Inorganic, 1987). In 1984, the United States imported 105,569 lb of chrome green, 5.1 million lb of chrome yellow, 393,529 lb of strontium chromate, 2.4 million lb of zinc yellow, and 4.9 million lb of chromic acid (USDOC Imports, 1985). In 1983, production of chrome yellow and orange was 43.1 million lb and production of chrome green was 10.4 million lb. In 1982, 40.8 million lb of chrome yellow and orange and 8.6 million lb of chrome green were produced (USDOC Inorganic, 1987; Chem. Prod., 1983a). The 1979 TSCA Inventory reported that in 1977, 10 companies produced 74,000 lb of calcium chromate and 2 companies imported 6,000 lb; 10 companies produced 11.6 million lb of lead chromate and there were 3 importers; 6 companies produced 555,000 lb of strontium chromate and 9 firms imported 124,500 lb. Also reported in the TSCA Inventory was that 199,000 lb of zinc chromate and 6 companies imported 7.2 million lb. The CBI Aggregate was less than 1 million lb barium chromate and between 1 million and 100 million lb for zinc chromate (TSCA, 1979).

EXPOSURE

The primary routes of potential human exposure to chromium and certain chromium compounds are inhalation, ingestion, and dermal contact. Chromium (in the form of unidentified chromium compounds) is widely distributed in air, water, soil, and food. In trace amounts, its trivalent form may be an essential ingredient in the diet. The entire population is possibly exposed to some of these compounds, but the levels of exposure vary. Hexavalent chromium (chromium (VI)) compounds are of greater health concern than trivalent chromium compounds (chromium (III)), but hexavalent compounds are readily reduced to trivalent forms in the presence of organic matter.

NIOSH has found that certain forms of chromium (VI) are noncarcinogenic; they are the monochromates and dichromates of hydrogen, lithium, sodium, potassium, rubidium, cesium, and ammonium, and chromium (VI) oxide (chromic acid anhydride) (NIOSHb, 1979b). The National Occupational Hazard Survey, conducted by NIOSH from 1972 to 1974, estimated that 2.5 million workers were possibly exposed to chromium and its compounds in the workplace (NIOSH, 1976). NIOSH estimated that 175,000 workers were potentially exposed to chromium (VI), which is produced principally from chromite ore (NIOSHb, 1979b). The National Occupational Exposure Survey (NOES) (1981-1983) estimated that a total of 115,788 workers, including 3,101 women, were potentially exposed to chromium; 6,339 total workers were potentially exposed to chromite ore (NIOSH, 1984). The NOES also estimated that a total of 196,725 workers, including 31,444 women, were potentially exposed to hexavalent chromium (VI) compounds (barium chromate, calcium chromate, chromium trioxide, lead chromate, strontium chromate, and zinc chromate). Occupational airbone chromium concentrations have declined significantly during the past decades because of improved emission controls. Occupational exposure occurs mainly from stainless steel production and welding, chromate production, chrome plating, ferrochrome alloys, chrome pigment, and tanning industries. Occupational exposure is due to the soluble and insoluble fractions of chromium (III) and chromium (VI), depending upon the industry. The typical concentration ranges of chromium (VI) in these industries are: stainless steel welding, 50-400 µg/m3; chromate production, 100-500 µg/m3; chrome plating, 5-25 µg/m3; ferrochrome alloys, 10-140 µg/m3; and chrome pigment, 60-600 µg/m3. In the tanning industry, exposure is almost exclusively to soluble chromium (III), typically in the range of 10-50 µg/m3 (ATSDR, 1989h). ACGIH has designated the following threshold limit values (TLVs) as 8-hr time-weighted averages (TWAs) for chromium compounds, as chromium: 0.5 mg/m3 for chromium (VI) compounds, and 0.05 mg/m3 for lead chromate, water soluble chromium (VI) compounds, and certain water insoluble chromium (VI) compounds (ACGIH, 1986).

The general population can be exposed to chromium through the air, water, soils and food. Chromium has been detected in at least 386/1,777 sites on the National Priorities List (ATSDR, 1989h). The Toxic Chemical Release Inventory (EPA) listed 929 industrial facilities that produced, processed, or otherwise used chromium in 1988 (TRI, 1990). In compliance with the Community Right-to-Know Program, the facilities reported releases of chromium to the environmental which were estimated to total 9.9 million lb. The atmospheric chromium concentration in the United States is typically < 0.01 µg/m3 in rural areas and 0.01-0.03 µg/m3 in urban areas. In the United States it was calculated that 64% of the atmospheric chromium emissions, which originate from coal, contain 1.5-54 ppm chromium (Merian, 1984). It is been reported that tap water contains 0.4-0.8 µg chromium/l (ATSDR, 1989h). The chromium concentration in rivers and lakes is usually between 1 and 10 µg/l (Merian, 1984). The earth's crust and rocks contain about 100 ppm chromium; soils contain, on the average, about 400 ppm (Merian, 1984). Typical chromium levels in most fresh foods are low. Chromium has been detected in vegetables, fruits, grains, and cereals at concentrations between 20 and 50 µg/kg (ATSDR, 1989h). CPSC investigated the potential hazard to consumers from chromium-containing inks, printed products, and nonprinted consumer products. Although chromium was present in some inks used in printed products, the levels found in the final products did not warrant further investigation.

REGULATIONS

In FY 1982, an evaluation was made of exposure to chromium from nonprinted consumer products. EPA regulates chromium and its compounds under the Clean Water Act (CWA), Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), Resource Conservation and Recovery Act (RCRA), Superfund Amendments and Reauthorization Act (SARA), and Toxic Substances Control Act (TSCA). CERCLA establishes reportable quantities (RQs) for chromium and some chromium compounds. RCRA and SARA subject chromium and its compounds to report/recordkeeping requirements. The EPA has proposed a maximum contaminant level goal (MCLG) and maximum contaminant level (MCL) of 0.1 mg/l for total chromium because no evidence suggests the hexavalent chromium is carcinogenic by ingestion. EPA's Carcinogen Assessment Group includes 10 chromium compounds on its list of potential carcinogens. FDA regulates the use of chromium as an indirect food additive and the use of chromium oxide in drugs and cosmetics. NIOSH recommends an exposure limit of 1 µg/m3 for carcinogenic hexavalent chromium compounds. For noncarcinogenic chromium compounds, it recommends an exposure limit of 50 µg/m3 as a 10-hr time-weighted average (TWA). OSHA adopted an 8-hr TWA permissible exposure limit (PEL) of 0.5 mg/m3 for chromium (II) and chromium (III) compounds and l mg/m3 for chromium metal; for chromic acid and chromate, the ceiling is 0.1 ppm. OSHA regulates chromium and certain chromium compounds under the Hazard Communication Standard, and hexavalent chromium compounds as chemical hazards in laboratories.