Chromium exists in three oxidation states, with hexavalent chromium (chromium VI) being toxic and a carcinogen. It is commonly found in industrial sources and can cause serious health effects through inhalation and skin contact. A lawsuit in 1993 brought attention to the contamination of groundwater with chromium VI, resulting in a settlement paid to affected residents. The evidence that ingesting chromium VI can cause cancer in humans is inconclusive. The California EPA has set a limit for drinking water, and there are methods to remove these compounds from tap water.
The metallic element chromium can exist in three oxidation states: +2, +3 and +6, in which it forms compounds by donating two, three and six electrons respectively to atoms of other elements. Chromium in its +6 oxidation state is known as hexavalent chromium or chromium VI. The element is most commonly found in its +3 oxidation state, known as trivalent chromium or chromium III, and in this form it is an essential trace element for humans. Hexavalent chromium, however, is toxic and is classified as a carcinogen by the Environmental Protection Agency (EPA) in the United States. The most commonly encountered hexavalent chromium compounds are chromates (Cr2O42-) and dichromates (Cr2O72-).
Some chromium VI compounds occur naturally, such as the mineral crocoite or lead chromate (PbCrO4); however, the main health concern is chromium VI from industrial sources. People can be exposed to it in the workplace or in the environment. Chromates and dichromates are used industrially in chromium plating, pigments and anti-corrosion treatments. They are also produced as unintended by-products in other processes, such as the welding of chromium steel. In the laboratory they are sometimes used as oxidizing agents and for cleaning glassware.
Exposure to chromium VI through inhalation of dust and aerosols and through skin contact has serious adverse health effects. Acute toxic effects include respiratory distress, gastrointestinal symptoms, and skin burns, while chronic exposure can cause bronchitis, asthma, and dermatitis. Numerous laboratory animal studies have provided compelling evidence that inhaling chromium VI significantly increases the risk of lung cancer, and workplace studies indicate an increased incidence of lung cancer in workers exposed to these compounds by inhalation.
Hexavalent chromium first came to public attention through a lawsuit related to contaminated groundwater filed against a well-known energy company in 1993, in which environmental activist Erin Brockovich – then working for a local law firm – played a major role. . Sewage containing chromium VI, used to protect a cooling tower from corrosion, had been discharged into unlined pools near the city of Hinkley, California, and subsequently filtered into groundwater, which was the city’s primary source of drinking water. Levels of hexavalent chromium reached 580 micrograms per liter, about 10 times the limit set by the EPA. It was successfully claimed that the company’s negligence had resulted in a number of serious health problems, including cancer, caused by hexavalent chromium. The company eventually paid a $333 million US Dollar (USD) settlement to 648 Hinkley residents, and the case was the subject of a blockbuster film starring Julia Roberts as Brockovich.
Although it is generally accepted that hexavalent chromium is toxic and carcinogenic by inhalation, the evidence that ingesting chromium VI at levels found in contaminated groundwater can cause cancer in humans is inconclusive. When ingested at low levels, chromium VI appears to be converted by stomach acid to relatively non-toxic chromium III. One study reported elevated levels of gastrointestinal cancer in mice fed high doses of chromium VI, but, due to differences in the digestive tract, it may not be valid to conclude from this that there is a similar risk to humans. A survey of cancer rates in Hinkley between 1988 and 2008 concluded that the number of cancer cases was not above the norm; however, it was argued that many cancer cases may have been ruled out due to some patients dying and others being relocated prior to the study.
In the United States, the California EPA has set a limit of 50 micrograms – 50 millionths of a gram – of hexavalent chromium per gallon for drinking water. Chromium VI levels exceed this limit in a number of areas, in some cases due to industrial pollution, in others due to natural sources. There are a number of steps that can be taken to remove these compounds from tap water, such as ion exchange, reverse osmosis, and lime softening.
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