Hexavalent chromium

Overview
Hexavalent chromium or Cr(VI) compounds are those which contain the element chromium in the +6 oxidation state. Chromates are often used as pigments for photography, and in pyrotechnics, dyes, paints, inks, and plastics. They can also be used for stainless steel production, textile dyes, wood preservation, leather tanning, and as anti-corrosion and conversion coatings. They are used as corrosion inhibitors, but due to their toxicity they are being replaced by alternatives.

Hexavalent chromium is recognized as a human carcinogen via inhalation. Workers in many different occupations are exposed to hexavalent chromium. Occupational exposures occur mainly among workers who:
 * handle dry chromate-containing pigments
 * spray chromate-containing paints and coatings
 * operate chrome plating baths
 * weld, cut or grind chromium-containing metals such as stainless steel.

Toxicity
In an organism's cells, hexavalent chromium undergoes reduction, first to metastable pentavalent chromium, then to trivalent chromium. Trivalent chromium binds to proteins and creates haptens which trigger immune system reaction. Once developed, chrome sensitivity becomes fairly persistent; in such cases, even contact with chromate-dyed textiles or wearing of chromate-tanned leather shoes can cause or exacerbate contact dermatitis.

Hexavalent chromium compounds are genotoxic carcinogens. Chronic inhalation of hexavalent chromium compounds increases risk of lung cancer (lungs are especially vulnerable, followed by fine capillaries in kidneys and intestine). It appears that the mechanism of genotoxicity relies on pentavalent or trivalent chromium. According to some researchers, the damage is caused by hydroxyl radicals, produced during reoxidation of pentavalent chromium by hydrogen peroxide molecules present in the cell. Zinc chromate is the strongest carcinogen of the chromates used in industry. Soluble compounds, like chromic acid, are much weaker carcinogens.

In some parts of Russia, pentavalent chromium was reported as one of the factors of incidence of premature senility. 

Permissible Exposure Limit (PEL or OSHA PEL)
The OSHA PEL for airborne exposures to hexavalent chromium is 5 µg/m3 (0.005 mg/m3).

Vitamin C
Researchers have recently reported discovering that vitamin C reacts inside human lung cells with chromium 6, causing massive DNA damage. Low doses of chromium 6, combined with vitamin C, produce up to 15 times as many chromosomal breaks and up to 10 times more mutations, compared with cells lacking vitamin C. Outside cells, vitamin C actually protects against the cellular damage caused by hexavalent chromium. 

Erin Brockovich
Hexavalent chromium is the substance against which Erin Brockovich campaigned. It was found in drinking water in the Southern California town of Hinkley.