Hexavalent chromium is a toxic form of the metal chromium in which each atom has lost six electrons, giving it a +6 oxidation state. It’s a known human carcinogen, primarily dangerous to workers in industries like electroplating and welding, and a persistent groundwater contaminant that has drawn regulatory attention across the United States. Unlike trivalent chromium (+3), which is naturally abundant and relatively harmless, hexavalent chromium is highly reactive, dissolves easily in water, and can penetrate living cells to damage DNA.
Hexavalent vs. Trivalent Chromium
Chromium exists in several forms, but only two matter in practice: trivalent chromium (Cr III) and hexavalent chromium (Cr VI). The trivalent form is the most stable under natural conditions and is found in common ores like ferrochromite. It’s generally insoluble in water, which limits how far it travels in the environment and how easily your body absorbs it.
Hexavalent chromium is a different story. Compounds like chromic acid and sodium chromate dissolve readily in water, making them far more mobile in soil and groundwater. This solubility is also what makes them biologically dangerous: dissolved hexavalent chromium passes through cell membranes easily. Once inside, the body’s own antioxidants reduce it back toward the trivalent state, but this conversion generates free radicals and creates bulky DNA damage that cells struggle to repair. The trivalent form is the destination, but the journey through the cell is where the harm occurs.
Hexavalent chromium rarely appears in nature. It shows up in a handful of rare minerals like crocoite, but the overwhelming majority of environmental contamination comes from industrial activity.
Where It’s Used and Who’s Exposed
Hexavalent chromium compounds are used across a range of industries: electroplating (chrome plating on metal parts), stainless steel production, leather tanning, textile manufacturing, and wood preservation. Workers most at risk include those who handle chromate-containing pigments, spray paints, or coatings, those who operate chrome plating baths, and anyone who welds or cuts stainless steel or other chromium-containing metals.
These processes release hexavalent chromium as dust, fumes, or mist. Breathing contaminated air is the primary occupational hazard. For the general public, the main exposure route is contaminated drinking water, where industrial discharge or natural weathering of chromium-bearing rock has allowed hexavalent chromium to leach into groundwater supplies.
How It Damages the Body
The harm hexavalent chromium causes depends largely on how it enters the body. Inhalation is the most dangerous route and is strongly linked to lung cancer, based on decades of occupational studies. Workers with elevated lung cancer risk also commonly experience nasal irritation and perforation of the nasal septum. Beyond cancer, inhaled chromium can trigger pneumonia, pulmonary fibrosis, and disruption of the immune system.
Ingestion through contaminated water or food carries a different risk profile. Swallowed hexavalent chromium is associated with gastrointestinal tumors, kidney disease, and male infertility. Both routes cause DNA damage and can promote the spread of abnormal cells, but the specific organs affected differ.
Skin contact produces its own set of problems. Repeated exposure to chromium dust can cause severe eczema with swelling. When a chromate solution touches broken skin, it creates deep, slow-healing lesions called chrome holes or chrome ulcers. Chromate dust also irritates the eyes, gums, and mucous membranes.
What Happens Inside Your Cells
Once hexavalent chromium crosses a cell membrane, the cell’s natural antioxidants (primarily vitamin C and a compound called glutathione) begin breaking it down through a series of chemical reductions. This process generates hydrogen peroxide and other free radicals that damage proteins, fats, and DNA. The end product, trivalent chromium, doesn’t simply wash away. It bonds directly to DNA strands, cross-linking them with other molecules to form bulky structures the cell’s repair machinery can’t easily fix.
Hexavalent chromium also disrupts the cell’s ability to repair itself at a deeper level. It increases activity of enzymes that alter how tightly DNA is packaged, effectively silencing genes responsible for catching and fixing errors. One key repair gene, MLH1, gets shut down this way. With its error-correction system compromised and its DNA physically damaged, the cell becomes far more likely to accumulate mutations and eventually turn cancerous.
Workplace Exposure Limits
OSHA sets the permissible exposure limit for airborne hexavalent chromium at 5 micrograms per cubic meter of air, calculated as an 8-hour average. The action level, the threshold that triggers required monitoring and medical surveillance, is half that: 2.5 micrograms per cubic meter. Employers are required to keep workers below the permissible limit through engineering controls, work practices, and respiratory protection when other measures aren’t sufficient.
Drinking Water Standards
The federal drinking water standard, set by the EPA in 1991, allows up to 0.1 milligrams per liter (100 parts per billion) of total chromium. This standard covers all forms of chromium combined, but the EPA treats any total chromium measurement as though it were 100 percent hexavalent chromium, the more toxic form. The agency is currently developing a comprehensive health assessment that could lead to a revised standard specifically for hexavalent chromium.
California has moved ahead on its own. After years of legal challenges and rulemaking, the state adopted a hexavalent chromium-specific limit of 10 micrograms per liter, effective October 1, 2024. That’s ten times stricter than the federal total chromium standard and reflects growing evidence that even low-level ingestion carries health risks. Before this rule took effect, hexavalent chromium in California was regulated only under the state’s total chromium limit of 50 micrograms per liter.
Cleaning Up Contaminated Water and Soil
Remediation strategies for hexavalent chromium all revolve around one core idea: converting the soluble, toxic hexavalent form back into the insoluble, far less harmful trivalent form. Chemical treatment typically involves adding a reducing agent, such as iron-based materials or sulfur compounds, that donates electrons to the hexavalent chromium, dropping it to the +3 state. Once converted, trivalent chromium precipitates out of the water as a solid that can be filtered or settled out.
Biological approaches use microorganisms or plants to accomplish the same conversion. Certain bacteria naturally reduce hexavalent chromium as part of their metabolism, and phytoremediation uses plants to absorb and stabilize the contaminant in soil. Newer technologies include injecting nanoscale iron particles directly into contaminated groundwater, which can treat contamination in place rather than pumping water to the surface for processing. Each method has trade-offs in cost, speed, and suitability depending on whether the contamination sits in soil, groundwater, or surface water.