At a glance
| Chemical family | Heavy metal, oxidized oxidation state. Chromium(VI) — the chromate (CrO42−) and dichromate anions — is the toxic, carcinogenic form. Sharply distinct from chromium(III), the essential trace nutrient. The two convert under different chemical conditions; in the body, chromium-6 is partly reduced to chromium-3, but not before it can damage cells. |
| CAS number | 18540-29-9 (hexavalent chromium ion); 7440-47-3 (chromium element); 7789-12-0 (sodium dichromate dihydrate, the NTP study compound); 1333-82-0 (chromium trioxide) |
| Classification | IARC Group 1 — Chromium(VI) compounds, carcinogenic to humans (lung cancer, inhalation; Monograph Vol 100C, 2012). NTP known human carcinogen. EPA total-chromium drinking-water MCL 0.1 mg/L (100 ppb) — no federal Cr-6-specific limit. California Cr-6 MCL 10 µg/L (effective Oct 1, 2024); California Public Health Goal 0.02 µg/L. |
| Where you encounter it | Drinking water — from industrial discharge (chrome plating, stainless-steel and pigment manufacture, leather tanning, cooling-tower corrosion inhibitors) and from natural weathering of chromium-bearing rock in some aquifers (parts of the US Southwest, California). Also an occupational inhalation hazard for welders and platers. Legacy contamination near industrial sites is the classic drinking-water scenario. |
| Sleep micro environment relevance | Indirect — chromium-6 in drinking water is an ingestion exposure, not a bedroom-air one. It appears in the Atlas as part of "the chemistry of where you live," alongside the other tap-water contaminants; the occupational inhalation route is a separate, workplace exposure. |
| Activated carbon capture | Not the relevant technology. For water, reverse osmosis and strong-base anion exchange are the validated methods for chromium-6; standard carbon does not reliably remove the chromate anion. Inferred from standard chromium water-treatment engineering; carbon adsorption is not the chromate-removal mechanism |
Regulatory & certification status
Where chromium-6 stands across the major systems. The defining feature is the mismatch between federal total-chromium regulation and the chromium-6-specific standards a few jurisdictions have begun to set.
| United States (federal) | EPA regulates total chromium (chromium-3 plus chromium-6 combined) at a Maximum Contaminant Level of 0.1 mg/L (100 ppb) under the Safe Drinking Water Act. There is no federal MCL specific to hexavalent chromium; EPA has studied one for years without finalizing it. This means federal compliance can be met while chromium-6 specifically goes unmeasured as its own value. Regulatory — US EPA |
| California | The first chromium-6-specific drinking-water standard in the US: a Maximum Contaminant Level of 10 µg/L, adopted by the State Water Resources Control Board in April 2024 and effective October 1, 2024. This reinstated a 2014 standard that a court had invalidated in 2017 over the economic-feasibility analysis; the 2024 version was re-adopted after a new economic study. Regulatory — CA State Water Board |
| California Public Health Goal | OEHHA set a Public Health Goal of 0.02 µg/L in 2011 — the level associated with negligible lifetime added cancer risk. It is not enforceable; it is the health target the enforceable MCL is measured against. The 500-fold gap between the 0.02 µg/L goal and the 10 µg/L MCL is the visible distance between "ideal for health" and "feasible at scale." Regulatory — OEHHA |
| European Union | The recast EU Drinking Water Directive (2020/2184) introduced a chromium parameter of 25 µg/L (total chromium), tightening from the previous 50 µg/L, with the value scheduled to apply for chromium from 2036 — a step toward the chromium-6 concern, though still expressed as total chromium. Chromium(VI) compounds are also REACH-Authorisation-listed (Annex XIV) for industrial use. Regulatory — European Commission |
| Canada / WHO | Health Canada sets a maximum acceptable concentration of 0.05 mg/L (50 µg/L) for total chromium, with guidance treating it as effectively hexavalent for the health basis. The WHO provisional guideline is 50 µg/L for total chromium, flagged as provisional because of database and analytical limitations for chromium-6 specifically. Regulatory — Government of Canada |
| International | IARC classifies Chromium(VI) compounds in Group 1 (carcinogenic to humans), with sufficient evidence for lung cancer via inhalation (Monograph Vol 100C, 2012). The US NTP lists hexavalent chromium compounds as known human carcinogens. The oral (drinking-water) carcinogenicity rests on the 2008 NTP rodent bioassay plus the Chinese population studies. Regulatory — IARC Monographs Vol. 100C |
| Certifications | For point-of-use treatment, NSF/ANSI 58 (reverse osmosis) carries a hexavalent-chromium reduction claim, and some NSF/ANSI 53 filters are specifically certified for chromium-6 reduction — verify the "hexavalent chromium" language, as a "chromium" claim may cover only chromium-3. Industry — NSF |
| The 72-hour test window | Not applicable. Chromium-6 in drinking water is an ingestion exposure detected by a specialized water lab test (distinct from the total-chromium test), with no relationship to the short VOC-emission chamber tests used for mattresses and foam. Inferred — from the exposure route (ingestion via water) versus the VOC/air focus of product emissions testing |
What it is
Chromium is a metal that occurs in several oxidation states, but two dominate the health conversation. Chromium(III) — trivalent, or chromium-3 — is an essential trace nutrient involved in glucose metabolism; it is the form in food and dietary supplements, and it is poorly absorbed and low in toxicity. Chromium(VI) — hexavalent, or chromium-6 — is the oxidized, industrially-produced (and sometimes naturally-occurring) form that exists in water as the chromate and dichromate anions. It is a strong oxidizer, is readily taken up by cells, and is the form responsible for chromium's carcinogenicity. The entire public-health story of "chromium" turns on which of these two you are talking about.
In drinking water, chromium-6 arrives by two routes. The first is industrial: chrome plating, stainless-steel and chromate-pigment manufacture, leather tanning, and the use of chromium as a corrosion inhibitor in cooling-tower and industrial water systems — the Hinkley, California contamination that inspired the Erin Brockovich case came from PG&E using hexavalent chromium as a cooling-tower rust inhibitor. The second is natural: in some aquifers, chromium-6 forms from the weathering of chromium-bearing rock, so it can appear in groundwater with no industrial source at all, notably in parts of California and the Southwest.
Once ingested, chromium-6 is partly reduced to chromium-3 by stomach acid and cellular processes — which is protective, but incomplete, especially at higher doses that overwhelm the reducing capacity. The fraction that reaches cells in the hexavalent form generates reactive intermediates that damage DNA. This is the mechanistic basis for treating chromium-6 and chromium-3 as fundamentally different exposures despite being the same element.
Where you encounter it
From industrial contamination of groundwater
The classic scenario. Facilities that plate chrome, manufacture stainless steel or pigments, tan leather, or historically used chromium corrosion inhibitors in cooling water can leave hexavalent chromium in the underlying aquifer, where it migrates to drinking-water wells. Legacy contamination persists for decades because chromium-6 is mobile in groundwater. Homes on wells downgradient of such sites are the priority-testing group.
From naturally chromium-bearing geology
In parts of California (the Central Valley, the Mojave), the Southwest, and other regions with chromium-rich bedrock, oxidation of naturally-occurring chromium produces chromium-6 in groundwater without any industrial input. This is why some rural wells with no nearby industry still show detectable chromium-6 — and why California's dedicated standard mattered statewide, not just near factories.
From occupational inhalation (a separate route)
Welders (especially on stainless steel), electroplaters, and pigment and chromate workers inhale hexavalent chromium fumes and dust — the exposure route on which the IARC Group 1 lung-cancer classification is primarily built. This is a workplace exposure governed by occupational limits (OSHA), distinct from the drinking-water route, but it is why chromium-6's carcinogenicity was established long before the water debate.
What the research says
Carcinogenicity — the IARC Group 1 classification
The IARC Monograph Vol 100C (2012) classifies Chromium(VI) compounds as Group 1, carcinogenic to humans, with sufficient evidence for lung cancer from occupational inhalation. Regulatory This inhalation classification is long-settled and is the anchor of chromium-6's carcinogen status. The more recent and more contested question has been the drinking-water (oral) route.
The oral route — the 2008 NTP rodent bioassay
The National Toxicology Program's 2008 two-year study of sodium dichromate dihydrate in drinking water found that oral chromium-6 exposure produced tumors of the small intestine in mice and of the oral cavity in rats. Regulatory This was the pivotal evidence extending chromium-6's carcinogenicity from inhalation to ingestion, and it underpins California's decision to set a drinking-water standard specifically for the hexavalent form.
Human drinking-water evidence — the Chinese cohorts
Beaumont and colleagues (2008) in Epidemiology re-analyzed mortality in a Liaoning Province, China population whose drinking water was contaminated by hexavalent chromium from a ferrochromium smelter, and reported elevated stomach-cancer mortality associated with the exposure. Peer-reviewed The human evidence for the oral route is thinner than for inhalation and has been debated, but combined with the NTP bioassay it forms the basis for regulating ingested chromium-6 as a carcinogen.
Regulatory reference values
The federal EPA MCL for total chromium is 0.1 mg/L (100 ppb) — there is no federal chromium-6-specific limit. Regulatory California's chromium-6-specific MCL is 10 µg/L (effective October 1, 2024), and its Public Health Goal is 0.02 µg/L (OEHHA, 2011). Regulatory The ATSDR Toxicological Profile for Chromium details the oral and inhalation toxicology. Regulatory
What helps reduce exposure
Look up your water's chromium number. Public systems publish total chromium (and, in California, chromium-6) in the annual Consumer Confidence Report. This is a knowable figure for your address — start by reading it rather than guessing.
For private wells near industry or in chromium-bearing geology: test for hexavalent chromium specifically. This is a different, more specialized lab test than total chromium. Wells downgradient of plating, tanning, pigment, or metal facilities, and wells in known chromium-geology regions, are the priority.
For water with elevated chromium-6: install reverse osmosis or strong-base anion exchange. Point-of-use reverse osmosis (NSF/ANSI 58 with a hexavalent-chromium claim) at the kitchen sink reduces both chromium forms. Strong-base anion exchange captures the chromate anion specifically. Verify the certification names hexavalent chromium.
For occupational exposure: this is a workplace-controls problem, not a home-water one. Welders and platers rely on ventilation, respiratory protection, and OSHA exposure limits — a different regime from the drinking-water measures above.
What does NOT help
- Reasoning from chromium supplements. "Chromium is an essential nutrient" is true of chromium-3 and irrelevant to chromium-6 in water. The nutrient and the carcinogen are different oxidation states of the same element.
- Boiling. Chromium-6 is a dissolved, non-volatile ion; boiling concentrates it as water evaporates rather than removing it.
- Standard carbon pitcher filters. Not reliable for the chromate anion unless a filter is specifically certified for hexavalent chromium reduction; a generic "chromium" or "contaminant" claim is not sufficient.
- Assuming the federal total-chromium number covers you. Meeting the 100 ppb total-chromium MCL does not mean chromium-6 specifically is low — the two are not the same measurement.
- Assuming "no nearby factory" means no chromium-6. Natural chromium geology can produce chromium-6 in wells with no industrial source at all.
Open research questions
- The oral cancer-potency slope for low-level drinking-water chromium-6 — the reducing capacity of stomach acid complicates extrapolation from the high-dose NTP bioassay to typical water concentrations, and the low-dose shape is actively contested. Speculation re: low-dose extrapolation; the high-dose carcinogenicity is established
- Whether and when EPA will finalize a federal chromium-6-specific MCL — a long-pending regulatory and cost-benefit question rather than a purely scientific one. Inferred from the federal drinking-water rulemaking record
- Non-cancer endpoints of oral chromium-6 (reproductive, developmental, hepatic) at environmentally relevant doses remain less characterized than the cancer endpoint. Speculation
- Real-world performance and cost of chromium-6 treatment at the community-water-system scale — the feasibility question that repeatedly shapes where the enforceable limit lands. Inferred from the California MCL economic-feasibility record
Where you meet Chromium 6 across your home
The same compound turns up in more than one place you live. Here's where it shows up in Embr — each links to the full breakdown for that part of your home.
Citations
- International Agency for Research on Cancer (2012). IARC Monographs Volume 100C — Chromium(VI) compounds Group 1 (lung cancer). Lyon: IARC. NCBI Bookshelf Regulatory
- US Environmental Protection Agency. National Primary Drinking Water Regulations — total chromium MCL 0.1 mg/L (100 ppb). epa.gov Regulatory
- California State Water Resources Control Board (2024). Hexavalent Chromium MCL — 10 µg/L, adopted April 2024, effective October 1, 2024. waterboards.ca.gov Regulatory
- California Office of Environmental Health Hazard Assessment (2011). Public Health Goal for Hexavalent Chromium in Drinking Water — 0.02 µg/L. oehha.ca.gov Regulatory
- US National Toxicology Program (2008). Toxicology and Carcinogenesis Studies of Sodium Dichromate Dihydrate in Rats and Mice (Drinking Water Studies), NTP TR-546 — tumors of the small intestine and oral cavity. ntp.niehs.nih.gov Regulatory
- Agency for Toxic Substances and Disease Registry (2012). Toxicological Profile for Chromium. atsdr.cdc.gov Regulatory
- Beaumont JJ, Sedman RM, Reynolds SD, Sherman CD, Li LH, Howd RA, Sandy MS, Zeise L, Alexeeff GV (2008). Cancer mortality in a Chinese population exposed to hexavalent chromium in drinking water. Epidemiology, 19(1):12-23. DOI 10.1097/EDE.0b013e31815cea4c Peer-reviewed
Frequently asked questions
Is chromium-6 the same chromium that's in supplements?
No — and the distinction is the whole story. Chromium exists in two main forms with opposite health profiles. Trivalent chromium (chromium-3) is an essential trace nutrient — the form in supplements and a normal part of the diet. Hexavalent chromium (chromium-6) is an industrial and sometimes naturally-occurring oxidized form that is a known carcinogen. They are the same element in different oxidation states, and conflating them is the most common error in this topic. When people say "chromium is good for you," they mean chromium-3; when toxicologists say "chromium causes cancer," they mean chromium-6.
Is there a federal limit for chromium-6 in drinking water?
Not specifically. The US EPA sets a federal limit of 0.1 mg/L (100 ppb) for total chromium — that number lumps chromium-3 and chromium-6 together, and it was set based on chromium-6's effects but does not measure chromium-6 on its own. There is no separate federal chromium-6 standard. California became the first state to set a chromium-6-specific limit, at 10 µg/L (10 ppb), adopted in April 2024 and effective October 1, 2024. So depending on where you live, chromium-6 may be regulated as its own contaminant or only as part of the much looser total-chromium number.
Is this the Erin Brockovich chemical?
Yes. The 2000 film dramatized the real case in Hinkley, California, where hexavalent chromium from a Pacific Gas & Electric compressor station contaminated the town's groundwater, and residents alleged a cluster of illnesses. The case put chromium-6 into public consciousness and is a large part of why California pursued a dedicated standard. The science on chromium-6's carcinogenicity by inhalation was already established; the Hinkley case and later rodent studies pushed attention onto the ingestion (drinking-water) route specifically.
How do I remove chromium-6 from my water?
Reverse osmosis is the most reliable point-of-use option and reduces both chromium-6 and chromium-3. Strong-base anion exchange is effective for chromium-6 specifically because the chromate anion is captured by the resin. Standard carbon pitcher filters are not reliable for chromium-6 unless specifically certified. If you are on a public system, your utility's Consumer Confidence Report lists chromium results; if you are on a private well near industrial activity or in a naturally chromium-bearing area, a certified lab test for hexavalent chromium (a different, more specialized test than total chromium) is the starting point.
Why is California's health goal so much lower than the limit?
Because the two numbers answer different questions. California's Public Health Goal for chromium-6 is 0.02 µg/L — the level associated with negligible added cancer risk over a lifetime. The enforceable limit (MCL) is 10 µg/L, five hundred times higher, because the MCL is set as low as is considered technically and economically feasible for water systems to actually achieve, not at the point of zero risk. The gap between 0.02 and 10 is not a contradiction; it is the visible distance between "what would be ideal for health" and "what is achievable at scale" — exactly the kind of gap Embr exists to make legible.
Should I worry about chromium-6 if my water is from a big city system?
Check the number rather than assuming. Large systems test for total chromium and, in states like California, for chromium-6 specifically, and publish results in an annual Consumer Confidence Report. Many systems are well below any limit; some — particularly in the Southwest and areas with naturally chromium-bearing geology or legacy industry — carry detectable chromium-6. The point is not blanket alarm; it is that this is a knowable number for your address, and worth looking up rather than guessing.
Related compounds
Embr researches the chemistry of where you live — including the water you drink. See the methodology page for how this Atlas tags claims by evidence strength, and the tap-water source hub for how chromium-6 fits alongside the other things in your water.
Last reviewed 2026-07-12. If you find a factual error, contact us.