The metallic element chromium can exist in three oxidation states: +2, +3 and +6, in which it forms compounds by giving 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 USA. The most commonly encountered hexavalent chromium compounds are chromates (Cr2O42-) and dichromates (Cr2O72-).
Some chromium VI compounds occur naturally — for example, the mineral crocoite or lead chromate (PbCrO4); however, the main health concern is with chromium VI from industrial sources. People may be exposed to it in the workplace or in the environment. Chromates and dichromates are used industrially in chrome plating, pigments and anti-corrosion treatments. They are also produced as unintentional by-products in other processes, such as welding of chrome 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 breathing difficulties, gastrointestinal symptoms and burns from skin contact, while chronic exposure can result in bronchitis, asthma and dermatitis. A number of laboratory studies on animals have provided convincing evidence that inhalation of chromium VI significantly increases the risk of lung cancer, and workplace studies indicate increased incidence of lung cancer in workers exposed to these compounds through inhalation.
Hexavalent chromium came to the attention of the general public through a lawsuit relating to contaminated groundwater filed against a well known energy company in 1993, in which the environmental campaigner Erin Brockovich — then working for a local law firm — played a major role. Wastewater containing chromium VI, used to protect a cooling tower from corrosion, had been discharged into unlined pools near the town of Hinkley, California and subsequently seeped into the groundwater that constituted the town’s main drinking water supply. Levels of hexavalent chromium reached 580 micrograms per liter — approximately 10 times the limit set by the EPA. It was successfully argued that the company’s negligence had resulted in a variety of serious health problems, including cancer, caused by hexavalent chromium. The company eventually paid out a $333 million US dollar (USD) settlement to 648 residents of Hinkley and the case was the subject of a highly successful film starring Julia Roberts in the role of Brockovich.
While there is general acceptance that hexavalent chromium is toxic and carcinogenic by inhalation, evidence that ingestion of chromium VI at the levels found in contaminated groundwater can cause cancer in humans is inconclusive. When ingested at low levels, it appears that chromium VI is converted by stomach acid to the 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 investigation into cancer rates in Hinkley between 1988 and 2008 concluded that the number of cancer cases was not above normal; however, it has been argued that many cancer cases may have been excluded due to the deaths of some sufferers and to others relocating prior to the study.
In the USA, the California EPA has set a limit of 50 micrograms — 50 millionths of a gram — of hexavalent chromium per liter for drinking water. Levels of chromium VI 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.