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Inorganic

Chromium-6 (hexavalent chromium) in drinking water

Hexavalent chromium (Cr(VI), often called chromium-6) is the carcinogenic form of chromium, distinct from the nutritionally essential trivalent form. EPA regulates total chromium at 0.1 mg/L but has not set a federal MCL for chromium-6 specifically. California has set a state MCL of 0.010 mg/L; California's non-binding Public Health Goal is 0.00002 mg/L.

Two chromiums, not one

Chromium exists in nature in two oxidation states that behave very differently in the body:

  • Chromium-3 (trivalent Cr(III)) is a trace nutrient found in food. Toxicity at dietary levels is minimal.
  • Chromium-6 (hexavalent Cr(VI)) is the form made famous by the Erin Brockovich case (Hinkley, California). It is a known human carcinogen via inhalation and a likely carcinogen via ingestion.

EPA’s drinking-water rule regulates “total chromium” at 0.1 mg/L (100 ppb). That total can be made up of either form. The rule does not distinguish between them, which is the central reason public-health advocates and many state regulators have called the federal standard inadequate.

Where chromium-6 in drinking water comes from

  • Industrial discharge: electroplating, leather tanning, stainless steel production, pigment manufacturing. Documented contamination plumes near major industrial sites (Hinkley CA, Midland MI, Hudson River NY, parts of New Jersey).
  • Cooling-tower corrosion inhibitors: a historic source for some industrial and commercial systems; phased out in the U.S. but still present in legacy plumes.
  • Natural rock weathering: chromium-bearing serpentine rocks can produce naturally elevated Cr(VI) in groundwater, notably in parts of California.

The largest documented exposure events in the U.S. are industrial, but naturally occurring Cr(VI) is more widespread than once assumed.

Why it matters for health

Cr(VI) inhalation is unambiguously carcinogenic: it causes lung cancer in workers with occupational exposure, and is regulated as such by OSHA.

Cr(VI) ingestion via drinking water has been the active scientific question for two decades. The clearest evidence is from rodent studies (stomach and intestinal tumors at high doses in drinking water). Human epidemiology is limited and contested, partly because stomach acid partially reduces Cr(VI) to Cr(III) (a detoxification step), so low ingested doses may be less hazardous than the inhalation evidence would predict.

California’s Office of Environmental Health Hazard Assessment set a Public Health Goal of 0.00002 mg/L (0.02 ppb) for Cr(VI) based on the rodent cancer data. California’s enforceable MCL is 0.010 mg/L, 500 times higher. The gap reflects what is achievable with current treatment technology versus what would be health-protective at the PHG level.

What you can do

  1. Ask your utility for Cr(VI)-specific sampling data if you live near a documented industrial site or in California (which routinely monitors). The federal total-chromium number does not separate the two forms.
  2. Treatment that works: reverse osmosis, ion exchange (strong- base anion-exchange resin), and stannous chloride reduction.
  3. Treatment that does NOT work: carbon filters do not remove Cr(VI).

This page is general information, not medical advice.

Sources

Editorial review: reviewed 2026-05-11 by RK 2026-05-11. Editorial standards.