Hexavalent chromium

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An example of a chromium(VI) compound: chromium trioxide
An example of a chromium(VI) compound: chromium trioxide

Hexavalent chromium (Cr(VI)) compounds are those which contain the element chromium in the +6 oxidation state.

Chromates, which are Chromium (VI) compounds, 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 high levels of toxicity they are being replaced by alternatives.

Hexavalent chromium is recognized as a human carcinogen via inhalation.[1] 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.

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[edit] 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.[1]

In some parts of Russia, pentavalent chromium was reported as one of the factors of incidence of premature senility. [2]

[edit] Permissible Exposure Limit (PEL or OSHA PEL)

The OSHA PEL for airborne exposures to hexavalent chromium is 5 µg/m3 (0.005 mg/m3).[3]

[edit] Reaction with Vitamin C

Researchers have recently reported discovering that vitamin C reacts inside human lung cells with chromium(VI), causing massive DNA damage. Low doses of chromium(VI), 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.[4]

[edit] Chromium(VI) and drinking water

Hexavalent chromium is the substance against which Erin Brockovich campaigned. It was found in drinking water in the Southern California town of Hinkley. Chromium(VI) is carcinogenic in groundwater,[5] and the 0.58 ppm in the groundwater in Hinkley exceeded the Maximum Contaminant Level of 0.10 ppm currently set by the United States Environmental Protection Agency.[6] A similar case was discovered in 2007 in Asopos River, near Oinofyta, Greece and Brockovich is again focusing on it. [7]

[edit] References

  1. ^ IARC [1990] (1999-11-05). Volume 49: Chromium, Nickel, and Welding (PDF). ISBN 92-832-1249-5. Retrieved on 2006-07-16. “There is sufficient evidence in humans for the carcinogenicity of chromium[VI] compounds as encountered in the chromate production, chromate pigment production and chromium plating industries.” 
  2. ^ Chromium Toxicity on the Corrosion Doctors Web site maintained by Canadian Physical Chemist, Pierre R. Roberge, PhD, P.Eng.
  3. ^ OSHA: Small Entity Compliance Guide for the Hexavalent Chromium Standards
  4. ^ Brown University (2007-03-12). "Chromium 6: A Killer Compound With An Improbable Trigger". Press release.
  5. ^ David Blowes, "Tracking Hexavalent Cr in Groundwater," Science 295: 2024 - 25 (Mar. 15, 2002).
  6. ^ U.S. Environmental Protection Agency, Consumer Factsheet on: CHROMIUM.
  7. ^ Pollution Flows in Asopos, by Erin Brockovich, published 25 August 2007 in The Brockovich Report, a blog

[edit] External links

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