Metabolism of Hexavalent Chromium

Chromium is an element, the 24th element of the periodic table. Its ground-state electron configuration is [Ar]3d5 4s1. Compounds that contain chromium with an oxidation number of 6 are hexavalent chromium compounds. According to the U.S. Department of Labor Occupational Safety and Health Administration, or OSHA, hexavalent chromium is a nose, throat and lung irritant; it can cause dermatitis and skin ulcers on extended contact, and prolonged exposure to hexavalent chromium compounds over the course of many years may increase the risk of lung cancer.

Characteristics

• Hexavalent chromium is typically found in ionic compounds containing chromate or dichromate ions; the chemical formula for the chromate ion is CrO4 with a -2 negative charge, while the formula for the dichromate ion is Cr2O7--again with a -2 negative charge. Since oxygen typically has an oxidation state of -2 (except when bonded to fluorine), chromium in both these ions has an oxidation state of +6. Chromates and chromium compounds find widespread use in industry, especially in some pigments for dyes and paints, in the manufacture of stainless steel and in chrome plating.

Health Hazards

• According to the OSHA, hexavalent chromium, if present in high concentrations, acts as an irritant to the mucous membranes that line the nose and throat. The OSHA fact sheet for hexavalent chromium lists symptoms of irritation as runny nose, sneezing, coughing, itching and a burning sensation; moreover, in those exposed frequently and for a considerable length of time, hexavalent chromium may cause nasal sores, nosebleeds or even severe damage. In low concentrations, hexavalent chromium doesn't appear to have the same detrimental effects, except in those who become allergic. Skin contact can cause so-called "chrome ulcers" or small skin sores. In the long term, repeated exposure to high levels of hexavalent chromium can increase the risk of lung cancer.

Metabolism of Hexavalent Chromium

• Cells uptake hexavalent chromium via transmembrane proteins that typically transport sulfate ions; since sulfate ions and chromate ions share some similarities in their chemistry, the protein "recognizes" and transports the chromate ion--an innocent mistake with unpleasant consequences. Once inside, hexavalent chromium is reduced to a +3 oxidation state. (Reduction in chemical reactions refers to processes whereby the oxidation state of the chemical/species decreases). In the process, it generates free radicals and participates in reactions that can damage the cell's DNA or even lead to the death of the cell. According to the Environmental Protection Agency, past experiments found that hexavalent chromium caused mutations in bacteria while Cr(III) (chromium with a +3 oxidation state) did not.

Soluble and Insoluble Hexavalent Chromium compounds

• While chromate compounds with group I elements (e.g. sodium or potassium) are soluble, most other chromate compounds are not. Sodium chromate, for example, dissolves in water, unlike lead chromate. Most research on hexavalent chromium's toxicity has focused on soluble chromates; it's not entirely clear whether there is a distinct difference in toxicity between soluble and insoluble hexavalent chromium compounds. The National Institute for Occupational Safety and Health currently considers all Cr(VI) compounds to be carcinogens regardless of whether they are soluble in water or not.

How to Avoid Health Risks with Hexavalent Chromium

• OSHA requires employers working with these compounds to adhere to certain standards, including providing respiratory protection and limiting the length and level of exposure. A more complete description of OSHA standards on hexavalent chromium is available in the OSHA fact sheet listed in "Resources." As always, if you work with hexavalent chromium compounds and believe you're suffering an adverse reaction or may have developed an allergy, make sure you consult your doctor or other qualified health professional as soon as possible.