Carcinogenicity

Chromium hexavalent (VI) compounds are known to be human carcinogens based on sufficient evidence of carcinogenicity from studies in humans.

Properties

Elemental chromium is a transition-group metal belonging to group VIB of the periodic table and has oxidation states ranging from −2 to +6, of which the divalent (+2, II), trivalent (+3, III), and hexavalent (+6, VI) forms are the most important. Elemental chromium does not occur naturally in the environment. The divalent (chromous) state is readily oxidized to the more stable trivalent (chromic) state. Although the hexavalent state (including chromates) is more stable than the divalent state, it is rarely found in nature. Chromium(VI) compounds are strong oxidizing agents and are highly corrosive. In the environment, they generally are reduced to chromium(III) compounds. The chromium(VI) compounds most commonly encountered in industry are calcium chromate, chromium trioxide, sodium chromate and dichromate, potassium chromate and dichromate, lead chromate, strontium chromate, and zinc chromate (Costa 1997; IARC 1990). However, this listing applies to all hexavalent chromium compounds, not just to those specified above.

Calcium chromate occurs as yellow crystals or a bright-yellow powder. It is slightly soluble in water and soluble in dilute acids, and it reacts with acids and ethanol. Although calcium chromate is not flammable, toxic chromium fumes may be formed in fires, and mixtures with boron burn violently when ignited. Chromium trioxide (also known as chromic trioxide) occurs as dark-red or brown crystals, flakes, or granular powder and is soluble in water, ethyl alcohol, ethyl ether, sulfuric acid, and nitric acid. Contact of chromium trioxide with organic chemicals may result in violent or explosive reactions, and fires with chromium trioxide may produce irritating, corrosive, and toxic gases (ATSDR 2000; HSDB 2009). Lead chromate occurs as yellow, orange, or red crystals or a yellow or orange-yellow powder that is insoluble in water, acetic acid, and ammonia but soluble in dilute nitric acid. When heated, it emits highly toxic fumes, and it may react explosively with azo dyes. The term “lead chromate” is also used to refer to various commercial lead chromate pigments (HSDB 2009; IARC 1980; 1990). Potassium chromate occurs as yellow crystals and is soluble in water but insoluble in ethanol. Potassium dichromate occurs as red or orange-red crystals and is soluble in water but insoluble in ethanol and acetone. It poses a dangerous fire risk when in contact with organic materials or finely divided combustible materials, such as sawdust (ATSDR 2000; HSDB 2009).

Sodium chromate occurs as yellow crystals and is soluble in water and slightly soluble in methanol. Although it is not flammable, toxic chromium oxide fumes may be formed in fires with sodium chromate (ATSDR 2000; HSDB 2009). Sodium dichromate occurs as bright orange-red or red hygroscopic crystals and is soluble in water and methanol. It reacts explosively with hydrazine, acetic anhydride, boron, silicon, and other materials (HSDB 2009; IARC 1980). Strontium chromate occurs as yellow monoclinic crystals or a yellow powder. It is slightly soluble in water and soluble in dilute hydrochloric acid, nitric acid, and acetic acid. It is not flammable but reacts explosively with hydrazine (HSDB 2009). Zinc chromate occurs as lemon-yellow crystals or powder. It is insoluble in cold water and acetone, sparingly soluble in hot water, and soluble in acid and liquid ammonia. Zinc chromate reacts explosively with hydrazine. The term “zinc chromate” is also used to refer to various commercial zinc and zinc potassium chromates (HSDB 2009; IARC 1990). Physical and chemical properties of these chromium(VI) compounds are listed in Table 1, along with their chemical formulas.

Table 1

Properties of Chromium(VI) Compounds.

Use

The steel industry is the major consumer of chromium. In 2007, estimated consumption of chromium in the United States by end use was 78% in stainless and heat-resisting steel, 13.8% for other steel uses, 3.7% in superalloys, and 4.5% in other alloys and end uses (Papp 2009). Alloys of stainless steel and chromium typically contain between 11.5% and 30% chromium (ATSDR 2000). Chromium(VI) compounds are widely used as corrosion inhibitors, in the manufacture of pigments, in metal finishing and chrome plating, in stainless steel production, in leather tanning, and in wood preservatives (ATSDR 2000; Costa 1997). In 1996, about 52% of all chromium compounds used in the U.S. chemical industry were used in production of wood preservatives; the rest were used in leather tanning (13%), metals finishing (13%), pigments (12%), refractories (linings for high-temperature industrial furnaces) (3%), and other uses (7%) (ATSDR 2000). The use of chromium(VI) compounds in wood preservatives increased dramatically from the late 1970s to the early 2000s; however, this use is expected to decrease because of a voluntary phase-out of all residential uses of wood treated with chromated copper arsenate (pressure-treated wood) that went into effect December 31, 2003 (Brooks 2009). Chromium(VI) compounds are also used in textile-dyeing processes, printing inks, drilling muds, pyrotechnics, water treatment, and chemical synthesis (HSDB 2009).

Calcium chromate is used primarily as a corrosion inhibitor and as a depolarizer in batteries (HSDB 2009; IARC 1973; 1990). Chromium trioxide is used primarily in chrome plating and other metal finishing (particularly in the production of automobiles and military aircraft), in production of wood preservatives, as a corrosion inhibitor, and in production of organic chemicals and catalysts. Lead chromate has been used in paints and printing inks and as a colorant in vinyl, rubber, and paper. Potassium chromate is used in production of dyes and in textile-dyeing processes. Potassium dichromate has largely been replaced by sodium dichromate in many applications; however, it is still used in photomechanical processes and production of pigments and wood preservatives. Sodium chromate is used as a corrosion inhibitor and in textile dyeing processes, inks, paints, leather tanning, wood preservatives, drilling muds, cutting oils, water treatment, and production of other chromium compounds. Sodium dichromate is the primary base material for the production of chromium compounds and is used as a corrosion inhibitor, in metal treatments, in drilling muds, and in the production of dyes, wood preservatives, synthetic organic chemicals, and catalysts. Strontium chromate is used as a corrosion inhibitor and metal conditioner, in aluminum flake coatings, as a colorant in polyvinyl chloride, in pyrotechnics, in chrome plating, and for sulfate ion control in electrochemical processes. Zinc chromates are used as corrosion inhibitors and metal conditioners and in paints, varnishes, and oil colors.

Production

The United States is one of the world’s leading producers of chromium compounds. U.S. primary production levels of chromium (i.e., mine production of chromite ore) have not been reported since 1961 (Papp 2007). One surface mine was developed in the United States in the mid to late 2000s (Papp 2009), but production levels from that mine were not reported. Other domestic sources of chromium include recycled stainless-steel scrap, industry stocks, and the Defense National Stockpile. Overall U.S. production of chromium metal decreased between 2000 and 2008 (Papp 2009), but production in 2015 (shown in Table 2) was similar to that in 2000. Apparent chromium consumption in 2017 (about 1.2 billion pounds) also was similar to that in 2000. In contrast, U.S. imports and exports of chromium metal were lower in 2017 than in 2000 (when they each totaled about 1 billion pounds) (USGS 2005; 2019).

Table 2

Production, Imports, and Exports of Chromium(VI) Compounds.

Table 2 also shows the production, import, and export data available for specific hexavalent chromium compounds. The United States has produced or imported several chromium compounds in quantities of at least 1 million pounds for decades, and some compounds have also been exported in large quantities. Production of sodium chromate and dichromate combined was about 280 million pounds in the late 1990s (HSDB 2009), similar to the figure reported in 2015 for production plus imports of sodium dichromate. In contrast, chromium trioxide production increased from around 66 million pounds in the late 1970s through the 1980s (HSDB 2009; IARC 1990) to over 100 million pounds in 2015. U.S. production of potassium chromate and dichromate combined or separately and of strontium chromate was generally less than 10 million pounds in the 1960s to 1970s (HSDB 2009; IARC 1990) and remained below 10 million pounds in 2015.

Chromium compounds that have been imported in quantities of at least 1 million pounds since the 1970s include sodium chromate and dichromate (over 40 million pounds in 2002 and 2008), chromium trioxide (over 35 million pounds in 2002), and potassium chromate and dichromate (about 1 to 2 million pounds in the mid 1980s) (HSDB 2009; IARC 1990), but only chromium trioxide imports exceeded 1 million pounds in 2017.

U.S. exports of sodium chromate and dichromate exceeded 50 million pounds in 1999, but no export data specific for either of these compounds were reported in 2017 (HSDB 2009; USITC 2018). U.S. exports of chromium trioxide were less than 10 million pounds in 1977, but approached 40 million pounds by 2008 (HSDB 2009); however, no export data specific for chromium trioxide were reported in 2017. U.S. exports of potassium dichromate were only 170,000 lb in 2008 (USITC 2009), but approached 5 million pounds in 2017 (as shown in Table 2).

Exposure

Chromium, in the form of unidentified chromium compounds, occurs naturally in the earth’s crust and is widely distributed in air, water, soil, and food. Chromium(III) is an essential trace element in humans. The general population is exposed to some chromium(VI) compounds, but the levels of exposure vary. Environmental exposure specifically to chromium(VI) compounds is difficult to quantify, because specific forms of chromium seldom are identified in exposure studies. Although chromium(VI) compounds in the environment may be reduced to chromium(III) compounds, hexavalent forms can persist under some conditions. The general population may be exposed to chromium(VI) compounds through inhalation of ambient air, ingestion of water, or dermal contact with products that contain chromium(VI) compounds, such as pressure-treated wood. People who live near industrial facilities that use chromium(VI) compounds or near chromium waste disposal sites have the greatest potential for exposure (ATSDR 2000).

A 1990 study reported the average concentration of chromium(VI) to be 0.0012 μg/m³ (range = <0.001 to 3 μg/m³) in indoor air samples collected from residences in Hudson County, New Jersey. Other reports of exposure to chromium were not specific for chromium(VI) compounds, but provide general information on exposure to chromium and chromium compounds. Between 1977 and 1984, typical total chromium concentrations in ambient air in the United States were less than 0.01 μg/m³ in rural areas and 0.01 to 0.03 μg/m³ in urban areas. Average atmospheric concentrations of chromium from more than 2,100 monitoring stations ranged from 0.005 to 0.525 μg/m³. A survey of more than 3,800 tap water samples in 1974 and 1975 found chromium concentrations ranging from 0.4 to 8.0 μg/L, with a mean of 1.8 μg/L. In surveys of U.S. surface waters, chromium concentrations in rivers ranged from less than 1 to 30 μg/L, and concentrations in lakes typically were less than 5 μg/L. Typical chromium levels in most fresh foods are low; chromium was detected in vegetables, fruits, grains, cereals, eggs, meat, and fish at concentrations of between 20 and 520 μg/kg. The mean daily dietary intake of chromium was estimated to be less than 0.2 to 0.4 μg from air, 2.0 μg from water, and 60 μg from food (ATSDR 2000).

Evidence that people in the United States are exposed to some form of chromium is provided by the National Health and Nutrition Examination Survey for 2015–2016, which found that the 95th-percentile concentration of chromium in whole blood for the U.S. general population was 1.08 µg/L, based on samples from 3,442 individuals of all ages, both genders, and all race and ethnicity groups (CDC 2018).

According to the U.S. Environmental Protection Agency’s Toxics Release Inventory, environmental releases of chromium compounds since reporting began in 1988 were lowest in 2001 (about half the average from 1988 to 2000). In 2007, 1,384 facilities released 12 million pounds of chromium, and 1,147 facilities released 51 million pounds of chromium compounds. The 100 facilities with the largest releases accounted most of the total amounts released (TRI 2008).

Most occupational exposure to chromium(VI) compounds is through inhalation or dermal contact. Exposure to specific chromium compounds varies by industry. Chromate production workers are exposed to a variety of chromium compounds, including chromium(VI) and chromium(III) compounds. Chromate pigment workers are exposed to chromates in the pigment and to soluble chromium(VI) compounds used in pigment production. Chrome platers are exposed to soluble chromium(VI) compounds and possibly to nickel. Ferrochromium workers are exposed mainly to chromium(III) compounds and possibly to chromium(VI) compounds.

Occupational exposure to chromium generally exceeds non-occupational exposure. However, concentrations of airborne chromium in workplaces have declined significantly since the 1980s because of improved emission controls. Typical concentration ranges for airborne chromium(VI) in industries that use chromium(VI) compounds are as follows: stainless-steel welding, 50 to 400 μg/m³; chromate production, 100 to 500 μg/m³; chrome plating, 5 to 25 μg/m³; ferrochrome alloy production, 10 to 140 μg/m³; and chromate pigment production, 60 to 600 μg/m³ (ATSDR 2000; IARC 1990). In the tanning industry, hides are soaked with chromium(VI) compounds in the presence of other chemicals that reduce them to chromium(III) compounds (Costa 1997); therefore, exposure in the tanning industry is almost exclusively to soluble chromium(III) (ATSDR 2000). In a study assessing chromium exposure among stainless-steel welders and mild-steel welders, chromium levels in blood, plasma, and urine were higher among the stainless-steel welders, particularly those engaged in manual metal arc welding, which produces fumes with high concentrations of total water-soluble chromium, mainly chromium(VI) (which constituted up to 61% of total soluble chromium) (Edmé et al. 1997).

The National Occupational Hazard Survey (conducted from 1972 to 1974) estimated that 16,576 workers potentially were exposed to chromium (types and compounds not specified), 42,043 to potassium dichromate, and 3,519 to calcium chromate (NIOSH 1976). The National Occupational Exposure Survey (conducted 1981 to 1983) estimated that 386,142 workers, including 10,433 women, potentially were exposed to chromium; 61,073, including 19,198 women, to potassium dichromate; 32,129, including 5,565 women, to calcium chromate; and 30,784, including 8,856 women, to lead chromate (NIOSH 1990).

Regulations

Guidelines

References

1. ATSDR. 2000. Toxicological Profile for Chromium (Final Report). Atlanta, GA: Agency for Toxic Substances and Disease Registry. [PubMed: 24049864]
2. Brooks WE. 2009. 2008 Minerals Yearbook: Arsenic. U.S. Geological Survey; Last accessed: 2/16/10. https://minerals​.usgs​.gov/minerals/pubs/commodity​/arsenic/myb1-2008-arsen.pdf
3. CDC. 2018. Chromium. In Fourth National Report on Human Exposure to Environmental Chemicals, Updated Tables, March 2018, vol. 1. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention. p. 298.
4. Costa M. 1997. Toxicity and carcinogenicity of Cr(VI) in animal models and humans. Crit Rev Toxicol. 27(5):431–442. 10.3109/10408449709078442 [PubMed: 9347224] [CrossRef]
5. Edmé JL, Shirali P, Mereau M, Sobaszek A, Boulenguez C, Diebold F, Haguenoer JM. 1997. Assessment of biological chromium among stainless steel and mild steel welders in relation to welding processes. Int Arch Occup Environ Health. 70(4):237–242. 10.1007/s004200050213 [PubMed: 9342623] [CrossRef]
6. EPA. 2016. Chemical Data Reporting Summary: Chromium Hexavalent Compounds. U.S. Environmental Protection Agency; https://chemview​.epa.gov/chemview and search on CAS number or substance name and select Manufacturing, Processing, Use, and Release Data Maintained by EPA and select Chemical Data Reporting Details.
7. HSDB. 2009. Hazardous Substances Data Bank. National Library of Medicine; Last accessed: 12/17/09. http://toxnet​.nlm.nih​.gov/cgi-bin/sis/htmlgen?HSDB and search on CAS numbers.
8. IARC. 1973. Chromium and chromium compounds. In Some Inorganic and Organometallic Compounds. IARC Monographs on the Evaluation of Carcinogenic Risk of Chemicals to Humans, vol. 2. Lyon, France: International Agency for Research on Cancer. pp. 100–125.
9. IARC. 1979. Chromium and certain chromium compounds. In Chemicals and Industrial Processes Associated with Cancer in Humans. IARC Monographs on the Evaluation of the Carcinogenic Risks to Humans, suppl. 1. Lyon, France: International Agency for Research on Cancer. pp. 29–30.
10. IARC. 1980. Chromium and chromium compounds. In Some Metals and Metallic Compounds. IARC Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans, vol. 23. Lyon, France: International Agency for Research on Cancer. pp. 205–323. [PubMed: 7000666]
11. IARC. 1990. Chromium and chromium compounds. In Chromium, Nickel and Welding. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, vol. 49. Lyon, France: International Agency for Research on Cancer. pp. 49–256.
12. NIOSH. 1976. National Occupational Hazard Survey (1972-74). Cincinnati, OH: National Institute for Occupational Safety and Health. DHEW (NIOSH) Publication No. 78-114.
13. NIOSH. 1990. National Occupational Exposure Survey (1981-83). National Institute for Occupational Safety and Health; https://www​.cdc.gov/noes/noes1/19395sic​.html, https://www​.cdc.gov/noes/noes1/80064sic​.html, https://www​.cdc.gov/noes/noes1/83496sic​.html, https://www​.cdc.gov/noes/noes1/x5638sic​.html
14. NTP. 2008. NTP Technical Report on the Toxicology and Carcinogenesis Studies of Sodium Dichromate Dihydrate (CAS No. 7789-12-0) in F344/N Rats and B6C3F1 Mice (Drinking Water Studies). Research Triangle Park, NC: National Toxicology Program. NTP Technical Report Series 546, NIH Publication No. 08-5887. [PubMed: 18716633]
15. Papp JF. 2007. Chromium — A National Mineral Commodity Perspective. Reston, VA: U.S. Geological Survey. Open-File Report 2007-1167. https://citeseerx​.ist​.psu.edu/viewdoc/download?doi=10​.1.1.550​.16&rep=rep1&type=pdf
16. Papp JF. 2009. 2007 Minerals Yearbook: Chromium. U.S. Geological Survey; https://minerals​.usgs​.gov/minerals/pubs/commodity​/chromium/myb1-2007-chrom.pdf
17. TRI. 2008. TRI Explorer Chemical Report. U.S. Environmental Protection Agency; https://www​.epa.gov/triexplorer/ and select Chromium Compounds (Except Chromite Ore Mined in the Transvaal Region).
18. USGS. 2005. Chromium. In Historical Statistics for Mineral and Material Commodities in the United States. U.S. Geological Survey; Last updated: 9/1/05. https://minerals​.usgs​.gov/minerals/pubs/historical-statistics/chromium-use.pdf
19. USGS. 2019. Chromium. In Mineral Commodity Summaries, February 2019. U.S. Geological Survey; https://minerals​.usgs​.gov/minerals/pubs/commodity​/chromium/mcs-2019-chrom.pdf
20. USITC. 2018. USITC Interactive Tariff and Trade DataWeb. United States International Trade Commission; Accessed on: 9/24/18. https://dataweb​.usitc​.gov/scripts/user_set.asp and search on HTS nos. 8112210000.