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Hexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via conversion to sodium dichromate. Approximately 136,000,000 kilograms (300,000,000 lb) of hexavalent chromium were produced in 1985.[1] Other hexavalent chromium compounds are chromium trioxide and various salts of chromate and dichromate. Hexavalent chromium is used for the production of stainless steel, textile dyes, wood preservation, leather tanning, and as anti-corrosion and conversion coatings as well as a variety of niche uses.
Hexavalent chromium is recognized as a human carcinogen via inhalation.[2] Workers in many different occupations are exposed to hexavalent chromium. Problematic exposure is known to occur among workers who handle chromate-containing products as well as those who arc weld stainless steel.[2] Within the European Union, the use of hexavalent chromium in electronic equipment is largely prohibited by the Restriction of Hazardous Substances Directive.
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Hexavalent chromium is transported into cells via the sulfate transport mechanisms, taking advantage of the similarity of sulfate and chromate with respect to their structure and charge. Trivalent chromium, which is the more common variety of chromium compounds, is not transported into cells.
Inside the cell, Cr(VI) is reduced first to metastable pentavalent chromium (Cr(V)), then to trivalent chromium (Cr(III)). Trivalent chromium binds to proteins and creates haptens that trigger immune response. Once developed, chrome sensitivity can be persistent. In such cases, contact with chromate-dyed textiles or wearing of chromate-tanned leather shoes can cause or exacerbate contact dermatitis. Vitamin C and other reducing agents combine with chromate to give Cr(III) products inside the cell.[3]
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. Strontium chromate is the strongest carcinogen of the chromates used in industry. Soluble compounds, like chromic acid, are much weaker carcinogens.[3]
In the U.S., the OSHA PEL for airborne exposures to hexavalent chromium is 5 µg/m3 (0.005 mg/m3).[4][5]
Hexavalent chromium was found in drinking water in the Southern California town of Hinkley and was brought to wide popular attention by the involvement of Erin Brockovich. 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 occurred in 2007 in Asopos River, near Oinofyta, Greece and Brockovich was again involved.[7] In June 2009, the ground water in Midland, Texas (US) was found to be contaminated with chromium. Erin Brockovich was involved again. The Midland groundwater reached higher levels of contamination than in Hinkley with 5250 ppb or 5.250 ppm.[8]
The Unified Air Pollution Control District reported high airborne levels of chromium(VI) at an elementary school and fire department in Davenport, California. The substance apparently originated from a local Cemex cement plant. The levels of chromium(VI) were eight times the air district's acceptable level at Pacific Elementary School and ten times at the Davenport Fire Department.[9] The levels detected did not exceed EPA limits. However, the air samples taken by the air district from June to August at the elementary school and fire department in Davenport registered measurements of hexavalent chromium that were up to ten times higher than allowed by California environmental standards. [10] The case highlights the possible release of chromium(VI) as a previously unrecognized byproduct of the cement-making.[11] [12]
In 2008, the defense contractor KBR was alleged to have exposed 16 members of the Indiana National Guard, as well as its own workers, to hexavalent chromium in Iraq in 2003.[13] Later, 433 members of the Oregon National Guard's 162nd Infantry Battalion were informed of possible exposure to hexavalent chromium while escorting KBR contractors.[14] One of the National Guard Soldiers, David Moore died in February 2008. The cause was lung disease at age 42. His death was ruled service-related. His brother believes it was hexavalent chromium.[15]
Trivalent chromium (Cr(III)) is considered to be essential for the human metabolism, whilst hexavalent (Cr(VI)) is very toxic and soluble in water and can be migrated into the direction of the groundwater. Using the GFAAS and ICP-MS for total chromium, diphenylcarbazide-Cr(VI)-complex colorimetric method for hexavalent chromium, and flame-AAS and ICP-MS for other elements, their concentrations were determined in 63 surface and groundwater samples from the Thiva – Tanagra – Malakasa basin, Eastern Sterea Hellas, Greece. [1]. In that area, which is notorious for the industrial activities during the last 40 years, significant chromium concentrations were determined in the urban water supply of Oropos (up to 80 μg/L Cr(VI)) and Inofyta (up to 53 μg/L Cr(VI)). High concentrations of Cr(VI), ranging from 5 to 33 μg/L Cr(VI), were also found in the groundwater that is used for the urban water supply of the town of Thiva (NW). High As content (up to 34 μg/L As) among with Cr(VI) (up to 40 μg/L) were detected in the urban water supply of Schimatari. In the Asopos River, although total chromium values are up to 13 μg/L, hexavalent chromium less than 5 mg/L and other toxic elements were relatively low, during our research, their values suggest a connection with the industrial activity in the area. The pollution of groundwater by Cr(VI) in the majority of water wells in the Thiva – Tanagra – Malakasa basin, is related to the widespread industrial activity, the usage of hexavalent chromium in various processes and the discharges of Cr-bearing wastes[2].
   | Australian National Pollutant Inventory Chromium VI fact sheet - NPI: Chromium (VI) compounds fact sheet |
| | ''Environmental Health:'' "Selected science: an industry campaign to undermine an OSHA hexavalent chromium standard" - Environmental Health | Full text | Selected science: an industry campaign to undermine an OSHA hexavalent chromium standard |
| | US OSHA Health and Safety Topics: Hexavalent Chromium" - Safety and Health Topics: Hexavalent Chromium |
| | Chromium 6: A Killer Compound With An Improbable Trigger - 06-115 (Chromium 6) |
| | 3M US: OSHA Hexavalent Chromium Standard - An overview of the Chromium Six (CrVI) standard - 3M US: OSHA Hexavalent Chromium Standard - An overview of the Chromium Six (CrVI) standard; Impacts of the New Hex Chrome standard |
| | ''Pollution Flows in Asopos'' - Pollution Flows in Asopos :: The Brockovich Report |
| | National Institute for Occupational Safety and Health - Hexavalent Chromium - NIOSH Topic: Hexavalent Chromium | CDC/NIOSH |
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