Chromium-6, also called hexavalent chromium, is a toxic form of the metal chromium that poses serious health risks, particularly as a carcinogen linked to lung cancer. It exists in water, air, and soil, most often as a result of industrial contamination. While chromium naturally occurs in rocks and soil, the dangerous hexavalent form is relatively rare in nature and primarily enters the environment through manufacturing and improper waste disposal.
How Chromium-6 Differs From Other Forms of Chromium
Chromium is a metallic element that can exist in several different electrical states, but two matter most: trivalent chromium (chromium-3) and hexavalent chromium (chromium-6). The number refers to the element’s oxidation state, which determines how it behaves chemically and biologically.
Chromium-3 is the more common and stable form. It occurs naturally in ores, is generally insoluble in water, and is actually an essential trace nutrient that helps your body process sugar and fat. Chromium-6 is a different story. It dissolves readily in water (especially as sodium or potassium salts), moves easily through soil and groundwater, and is far more toxic. In the presence of organic matter, chromium-6 tends to convert back to the safer trivalent form. But in clean water with little organic material, it remains stable and can persist for long periods.
Where Chromium-6 Comes From
Small amounts of chromium-6 occur naturally in rare minerals like crocoite, but the vast majority in the environment comes from industrial activity. It is widely used in electroplating, stainless steel production, leather tanning, textile manufacturing, and wood preservation. Workers who handle chromate-containing pigments, spray paints, or coatings, operate chrome plating baths, or weld and cut stainless steel face the highest exposure levels.
Contamination of soil and groundwater typically happens through leakage, poor storage, or improper disposal of industrial waste. Once chromium-6 enters the ground, it is relatively mobile. It can travel through soil and dissolve into groundwater, displacing clean water as it spreads. The soil itself can slowly neutralize chromium-6 by converting it to chromium-3 through chemical reactions with minerals and organic matter, but this natural cleanup process is often too slow to prevent contamination from reaching drinking water supplies.
Health Effects of Exposure
Chromium-6 can enter the body through inhalation, skin contact, or ingestion in contaminated water. The health effects depend on the route and duration of exposure, but the compound is harmful in all three cases.
Breathing It In
Inhaling chromium-6 dust or fumes irritates the nose, throat, and lungs. Repeated or prolonged exposure can ulcerate the lining of the nasal passages and, in severe cases, perforate the nasal septum (the wall between the nostrils). Some workers develop occupational asthma after becoming sensitized, experiencing wheezing and shortness of breath even at low concentrations. The most serious long-term risk is cancer: occupational exposure is associated with increased rates of lung cancer and cancer of the nasal sinuses.
Skin and Eye Contact
Prolonged skin contact causes dermatitis and a distinctive type of wound called a chrome ulcer, a painless, pitted lesion covered with fluid and a crust. Some people develop allergic contact dermatitis, a long-lasting and increasingly severe skin reaction that can flare from even small amounts of chromium-6 after sensitization occurs. Handling chromate-containing materials like portland cement is a common trigger. Direct eye contact with chromium-6 dust or solutions can cause permanent eye damage.
Drinking Contaminated Water
Ingesting chromium-6 over time has been linked to kidney and liver damage. The compound can also cause upper abdominal pain and damage to the gastrointestinal tract. The cancer risk from drinking contaminated water has been a major public concern, most famously in the Hinkley, California case that became the basis for the film “Erin Brockovich.”
Drinking Water Standards
The EPA set a drinking water standard of 0.1 milligrams per liter (100 parts per billion) for total chromium in 1991. This standard covers both chromium-3 and chromium-6 together, because the two forms can convert back and forth in water and in the body depending on conditions. There is no separate federal limit specifically for chromium-6.
The World Health Organization recommends a lower provisional guideline of 0.05 milligrams per liter (50 parts per billion) for total chromium in drinking water, noting this level is unlikely to pose significant health risks. Some U.S. states, notably California, have pursued their own stricter standards specifically targeting chromium-6, recognizing that even levels well below the federal total chromium limit could carry risk over a lifetime of exposure.
Workplace Exposure Limits
For workers who encounter airborne chromium-6 on the job, OSHA sets a permissible exposure limit of 5 micrograms per cubic meter of air, calculated as an 8-hour average. The action level, the concentration at which employers must begin monitoring and taking protective steps, is half that: 2.5 micrograms per cubic meter. These limits apply across industries including welding, plating, painting, and manufacturing.
How Chromium-6 Is Removed From Water
Several proven technologies can remove chromium-6 from drinking water, each with different levels of effectiveness.
- Ion exchange uses specialized resins to swap chromium-6 ions for harmless ones. Both weak-base and strong-base anion exchange resins can reduce chromium-6 to below 1 microgram per liter, which is essentially undetectable in practical terms.
- Reduction, coagulation, and filtration (RCF) works by chemically converting chromium-6 back into insoluble chromium-3, which then forms solid particles that can be filtered out. With standard sand-type filters, this approach achieves levels below 5 micrograms per liter. Switching to membrane filtration pushes that below 1 microgram per liter.
- Reverse osmosis forces water through a membrane that blocks chromium-6 along with most other dissolved contaminants. Full-scale plants in California have demonstrated reduction from about 5 micrograms per liter to below 1.
For individual households concerned about chromium-6, reverse osmosis systems installed at the tap or under the sink use the same principle as the large-scale plants, though performance varies by product and water chemistry. Ion exchange filters designed for chromium removal are also available for home use. Standard carbon filters, the type found in most pitcher-style filters, are not effective at removing chromium-6.