Agency for Toxic Substances and Disease Registry
Case Studies in Environmental Medicine (CSEM) 

Chromium Toxicity
Exposure Pathways

Chromium is a hard, steel-gray metal that is highly resistant to oxidation, even at high temperatures. It is the sixth most abundant element in the earth's crust, where it is combined with iron and oxygen in the form of chromite ore. Russia, South Africa, Albania, and Zimbabwe together account for 75% of world chromite production. Chromite ore has not been mined in the United States since 1961; by 1985 the United States was completely dependent on importation for its primary chromium supply.

• Chromium is released to air primarily by combustion processes and metallurgical industries.

Chromium is used in three basic industries: metallurgical, chemical, and refractory (heat-resistant applications), and these industries are the second largest source of ambient chromium. In the metallurgical industry, chromium is an important component of stainless steels and various metal alloys. Metal joint prostheses made of chromium alloys are widely used in clinical orthopedics. In the chemical industry, chromium is used primarily in paint pigments (chromium compounds can be red, yellow, orange, and green), chrome plating, leather tanning, and wood treatment. Smaller amounts are used in drilling muds, water treatment, catalysts, safety matches, copy machine toner, corrosion inhibitors, photographic chemicals, and magnetic tapes. Refractory uses of chromium include magnesite-chrome firebrick for metallurgical furnace linings and granular chromite for various other heat-resistant applications.

• Chromium exists in three common stable valence states; in order of generally increasing toxicity, these states are chromium (0), (III), and (VI).

Chromium exists in a series of oxidation states from -2 to +6 valence; the most important stable states are 0 (elemental metal), +3 (trivalent), and +6 (hexavalent). Chromium in chromite ore is in the trivalent state; industrial processes also produce the elemental metal and hexavalent chromium. The health effects of chromium are at least partially related to the valence state of the metal at the time of exposure. Trivalent (Cr [III]) and hexavalent (Cr [VI]) compounds are thought to be the most biologically significant. Cr (III) is an essential dietary mineral in low doses. Certain compounds of Cr (VI) appear to be carcinogenic, but insufficient evidence exists to determine whether Cr (III) or chromium metal can be human carcinogens. Cr (VI) is generally considered 1,000 times more toxic than Cr (III).

Cr (III) is an essential dietary nutrient. It is required to potentiate insulin and for normal glucose metabolism. Chromium deficiency has been associated with impaired glucose tolerance, fasting hyperglycemia, glucosuria, elevated percent body fat, decreased lean body mass, maturity-onset diabetes, cardiovascular disease, decreased sperm count, and impaired fertility.

The National Academy of Science has established a safe and adequate daily intake for chromium in adults of 50200 micrograms per day (µg/day). Cr (III) is found in most fresh foods and drinking water. Dietary sources rich in chromium include breads, cereals, spices, fresh vegetables, meats, and fish. Other significant sources of chromium are mineral supplements, brewer's yeast, and beer. The daily dietary intake of chromium for a typical American adult is approximately half the minimum suggested safe and adequate daily intake of 50 µg/day.

Cr (III) and Cr (VI) are released to the environment primarily from stationary point sources resulting from human activities. Of the total atmospheric chromium emissions in the United States, approximately 64% are due to Cr (III) from fuel combustion (residential, commercial, and industrial) and from steel production; about 32% are due to Cr (VI) from chemical manufacture, chrome plating, and industrial cooling towers that used chromate chemicals as rust inhibitors in the past. A U.S. Environmental Protection Agency (EPA) report estimates that in the United States, about 2,840 metric tons of total chromium are emitted annually into the atmosphere (compared to approximately 110,000 tons of chromium metal produced each year).

• Nonoccupational sources of chromium include contaminated soil, air, and water.

Electroplating, leather tanning, and textile industries release relatively large amounts of chromium in surface waters. Leaching from topsoil and rocks is the most important natural source of chromium entry into bodies of water. Solid wastes from chromate-processing facilities, when disposed of improperly in landfills, can be sources of contamination for groundwater, where the chromium residence time might be several years. The content of chromium in tap water in U.S. households is from 0.4 to 8.0 micrograms per liter (µg/L). (EPA's maximum contaminant level for chromium in drinking water is 100 µg/L.)

In the 1960s and 1970s, chromium-containing slag was used as landfill in residential, commercial, and recreational settings in over 100 locations in Hudson County, New Jersey. This fill contained chromium in carcinogenic forms and in concentrations that are acutely toxic in certain circumstances. Community exposure from this fill occurred in a variety of ways. Wind erosion of the soil could have made slag particles airborne, increasing the opportunity for inhalation of chromium. Chromium compounds leached by rainwater could have migrated through cracks in soil, asphalt roadways, and masonry walls, forming high-content chromium crystals on their surfaces. In soil and roadways, these particles might have been eroded by wind and foot traffic and carried as chromium-laden dust into homes and workplaces. Children playing in areas where the slag was used as fill might also have been exposed through skin contact with chromium-contaminated dust, dirt, and puddles.

Other environmental sources of chromium include road dust contaminated by emissions of chromium-based catalytic converters or erosion products of asbestos brake linings, cement dust, tobacco smoke, and foodstuffs. Chromium picolinate is now heavily marketed as a dietary supplement, and dietary supplements are virtually unregulated by the Food and Drug Administration.

Cigarettes contain 0.24 to 14.6 milligrams (mg) chromium per kilogram (kg). It is plausible that cigarette smoking might constitute a significant source of chromium intake.

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