Hexavalent chromium is an established human carcinogen in certain occupational settings as a result of inhalation exposure. However, hexavalent chromium compounds have been found in drinking water. There was uncertainty regarding the long-term consequences of exposure to these compounds in drinking water sources.
Chromium is a naturally occurring element present in various states. Trivalent chromium is most commonly found in nature and proposed to be an essential nutrient. Hexavalent chromium compounds are the next most stable forms of chromium. However, hexavalent chromium rarely occurs naturally and is typically associated with industrial sources.
In 2000 and 2001, NTP received nominations from California officials to study the toxicity and carcinogenicity of hexavalent chromium in drinking water.
As a result of these nominations, NTP studied the short-term toxicity and long-term carcinogenicity of sodium dichromate dihydrate, a hexavalent chromium compound, administered to rodents in drinking water. NTP found that sodium dichromate dihydrate in the drinking water caused oral cancers in rats and small intestine cancer in mice.
NTP’s work on hexavalent chromium has informed research and decision making for various stakeholders, including federal agencies, state agencies, and academia. The results of NTP’s two-year toxicity and carcinogenicity studies on a hexavalent chromium compound (TR-546) were cited in the initial statement to adopt a maximum contaminant level for hexavalent chromium in drinking water in California. The 0.010-milligram per liter MCL, equivalent to 10 parts per billion, became effective on July 1, 2014.