Chromium-6 (Hexavalent Chromium) in Drinking Water

What Is Chromium-6?

Chromium-6, also known as hexavalent chromium or Cr(VI), is a toxic form of the metal chromium. It gained national attention through the Erin Brockovich case in the 1990s, when residents of Hinkley, California discovered that Pacific Gas & Electric had contaminated their groundwater with chromium-6 for decades — a story that became a legal landmark and a warning sign about what was flowing undetected through water supplies across the country.

Chromium exists in several forms, but two matter for drinking water. Chromium-3 (trivalent chromium) is a naturally occurring nutrient the human body needs in trace amounts for normal glucose metabolism. Chromium-6 is chemically distinct, far more water-soluble, and a recognized human carcinogen. Federal regulations currently lump both forms together under a single “total chromium” standard, leaving a significant regulatory gap for the more dangerous form. A utility can report full compliance while delivering water that contains substantial chromium-6 concentrations.

Chromium-6 is odorless, colorless, and tasteless. Without laboratory testing, there is no way to know whether it is present in your tap water. This invisibility is what makes it particularly concerning: millions of Americans have been drinking chromium-6 for years without knowing it.

How Chromium-6 Gets Into Drinking Water

Chromium-6 enters water supplies through both natural geology and industrial activity. Understanding the source matters because the concentration and geographic distribution of contamination follow different patterns depending on the pathway.

Natural sources account for baseline chromium-6 levels in many western and southwestern states. Chromium-bearing rock formations — primarily serpentinite and ultramafic rock — release chromium into groundwater as minerals weather over time. States including California, Arizona, and Nevada have elevated natural background levels in some aquifers as a direct result of local geology.

Industrial sources are the larger concern in much of the country. Chromium-6 is used as a corrosion inhibitor in cooling towers at power plants, as a component in chrome plating and stainless steel manufacturing, in leather tanning, and in textile dyeing. When industrial wastewater is improperly managed or stored, chromium-6 leaches into surrounding soil and groundwater.

Coal ash disposal is a significant and often underappreciated source. When ash from coal-fired power plants is stored in unlined surface impoundments — a common historical practice — chromium-6 and other heavy metals leach into groundwater beneath the site. A 2019 EPA assessment found evidence of contamination at the majority of coal ash sites that underwent groundwater monitoring.

Legacy contamination from decades of industrial use persists at thousands of sites across the country. Chromium-6 is chemically stable in oxidizing groundwater conditions and does not readily degrade, meaning contamination from industrial activity 30 or 40 years ago is still detectable today.

Health Effects

The National Toxicology Program (NTP) classifies chromium-6 as a known human carcinogen. A landmark 2008 NTP study provided some of the most direct evidence that ingestion — not just inhalation — is a route of harm: rats and mice given chromium-6 in drinking water developed cancers of the oral cavity and small intestine at statistically significant rates. That study changed the scientific consensus and put pressure on regulators to re-examine a standard written before oral carcinogenicity was well understood.

Cancer Risk

The primary cancer concern from chromium-6 in drinking water is gastrointestinal. Stomach cancer and small intestinal cancer are the most directly linked to ingestion exposure in animal studies, and epidemiological evidence from occupational cohorts and populations living near contaminated sites supports the association in humans. Oral cavity cancers have also been observed in animal models.

Inhalation of chromium-6 — the occupational exposure route — is associated with lung cancer. While ingestion is the relevant pathway for most drinking water consumers, workers at treatment plants or near spray irrigation sites with high chromium-6 levels may face some inhalation exposure.

The Environmental Working Group’s health guideline of 0.02 parts per billion (ppb) is derived from a one-in-a-million excess cancer risk model. At that level, one additional cancer case per million people exposed over a lifetime is the threshold the guideline tries to stay below. By comparison, the current federal MCL permits levels 5,000 times higher.

Other Health Effects

Cancer is not the only concern. Chronic exposure to chromium-6 has been associated with:

• Liver toxicity: Elevated liver enzymes and structural liver damage in animal studies at moderate exposure levels
• Reproductive and developmental effects: Reduced fertility, early pregnancy loss, and developmental abnormalities observed in animal models; the implications for human reproductive health are still being studied
• Kidney damage: High-dose exposure can cause tubular damage in the kidneys, which are responsible for filtering chromium from the bloodstream

Children are more vulnerable than adults because their gastrointestinal tracts absorb a higher fraction of ingested chromium, and their developing tissues are more sensitive to genotoxic compounds. Pregnant women face the additional concern of potential transfer to the fetus. Immunocompromised individuals, including cancer patients undergoing chemotherapy, have reduced capacity to repair cellular damage caused by oxidative stress — a key mechanism through which chromium-6 causes harm.

Because chromium-6 can accumulate in tissues over time, chronic low-level exposure is not the same as acute high-dose exposure, and the risks compound with duration.

August 2024 EPA IRIS Assessment

In August 2024, the EPA finalized its Integrated Risk Information System (IRIS) assessment of chromium-6, the agency’s most comprehensive scientific review of the contaminant to date. The assessment formally classified chromium-6 as a “likely human carcinogen” via oral ingestion — updating language that had previously been more equivocal about the ingestion route. The IRIS review incorporated the full body of NTP evidence, mechanistic data, and epidemiological studies, and concluded that the carcinogenic hazard is not limited to inhalation.

The finalized IRIS assessment is expected to inform a revision to the Maximum Contaminant Level for total chromium and potentially a separate MCL for chromium-6 specifically. As of early 2026, EPA has indicated it is reviewing regulatory options, but no new MCL has been proposed. The IRIS assessment provides the scientific foundation; the regulatory process that follows involves economic feasibility analysis, public comment, and agency rulemaking — a process that can take several years.

EPA Regulation and Limits

The current regulatory framework for chromium in drinking water was established under the 1991 National Primary Drinking Water Regulations. The standard covers total chromium, not chromium-6 specifically, and was set before evidence of oral carcinogenicity was established in animal models.

Standard	Value	Notes
EPA MCL (Total Chromium)	100 ppb	Covers all chromium forms combined; no Cr(VI)-specific limit
California Proposed MCL (Cr(VI))	10 ppb	Proposed 2014; court-ordered withdrawal 2017; revised rulemaking ongoing
EWG Health Guideline (Cr(VI))	0.02 ppb	Based on 1-in-1,000,000 lifetime cancer risk
EPA IRIS Oral Slope Factor (2024)	Under review	Finalized assessment expected to inform MCL revision

California was the first state to attempt a chromium-6-specific standard. The California Office of Environmental Health Hazard Assessment (OEHHA) established a public health goal of 0.02 ppb in 2011 — consistent with the EWG guideline. The California Division of Drinking Water then proposed an enforceable MCL of 10 ppb in 2014, which would have been the first chromium-6-specific standard in the country. A court challenge citing inadequate cost-benefit analysis required the state to withdraw the proposed rule in 2017. California’s revised rulemaking process has been ongoing, but no finalized chromium-6 MCL was in place as of early 2026.

The gap between 100 ppb (federal MCL) and 0.02 ppb (health guideline) represents one of the widest disparities between regulatory compliance thresholds and health-based recommendations for any drinking water contaminant — wider, for example, than the gap for arsenic, where the MCL of 10 ppb is itself considered inadequate by many health researchers.

How Widespread Is Chromium-6?

Chromium-6 is not a niche or regional contaminant. The EPA’s Unregulated Contaminant Monitoring Rule Round 3 (UCMR 3), which required water systems serving 10,000 or more people to test for chromium-6 between 2013 and 2015, produced the most comprehensive national dataset available.

The results were striking: chromium-6 was detected in more than 75% of sampled water systems, at measurable concentrations in systems across all regions of the country. An Environmental Working Group analysis of the UCMR 3 data estimated that over 200 million Americans — more than 60% of the US population — may be drinking water with chromium-6 at levels above the EWG’s 0.02 ppb health guideline.

Geographic distribution is uneven. Systems with the highest reported concentrations tend to cluster in:

• The western United States, particularly California, Arizona, and Nevada, where natural geology contributes background levels
• Industrial corridors in the Midwest and Southeast, where legacy manufacturing contamination affects groundwater
• Communities near coal ash disposal sites, which are distributed across more than 30 states

Even regions without known industrial sources show detectable chromium-6 in many cases, reflecting the broad distribution of chromium-bearing geology and the pervasive historical use of chromium compounds in manufacturing.

Small water systems — those serving fewer than 10,000 people — were not required to participate in UCMR 3, which means the national exposure picture is likely incomplete. Rural systems and private wells in chromium-bearing geology have received less systematic monitoring than large utilities.

How WaterVerge Tracks Chromium-6

WaterVerge pulls water quality data from the EPA’s Safe Drinking Water Information System (SDWIS) to track total chromium violations and identify systems with documented compliance issues. SDWIS contains the official record of every MCL violation, monitoring violation, and treatment technique violation reported to the EPA since the early 1990s.

Because most systems test for total chromium rather than chromium-6 specifically, WaterVerge also incorporates UCMR 3 monitoring data, which contains the most granular publicly available information on chromium-6 concentrations at the utility level. Where UCMR 3 data exists for a system, city pages on WaterVerge display the reported chromium-6 concentration alongside the applicable standards and health benchmarks, so users can see exactly where their system’s levels fall relative to both federal compliance thresholds and health-based guidelines.

When a system exceeds the federal total chromium MCL or shows elevated chromium-6 in UCMR 3 data, we flag it prominently in the city water quality profile. Systems with chromium-6 levels above the EWG guideline but below the federal MCL are also identified, since regulatory compliance and health risk are not the same thing.

How to Remove Chromium-6

Standard activated carbon filters — including most pitcher filters, refrigerator filters, and basic faucet-mount units — are not effective against chromium-6. Carbon filtration is designed primarily to remove organic compounds and chlorine taste; it does not have an adsorptive affinity for chromium-6 in its ionic form. Buying a carbon filter because of chromium-6 concerns is money spent without protection.

Proven removal methods include the following:

Method	Removal Rate	Certification	Best For
Reverse osmosis (RO)	90—97%	NSF/ANSI 58	Household point-of-use (under-sink)
Strong-base anion exchange	85—95%	NSF/ANSI 61	Whole-house; municipal scale
Iron/titanium adsorptive media	80—95%	NSF/ANSI 61 (varies)	Point-of-use; emerging whole-house products
Reduction-coagulation-filtration (RCF)	95%+	Municipal-scale treatment	Large-scale municipal treatment
Standard activated carbon	<10%	N/A	Not suitable for Cr(VI)

Reverse osmosis is the most accessible option for households. Point-of-use under-sink RO systems reduce chromium-6 by 90% or more and are available for $150—$400. Look specifically for NSF/ANSI 58 certification that covers hexavalent chromium, not just total chromium — the certification language matters.

Strong-base anion exchange uses specialized resin beads that carry a positive charge and selectively attract chromate ions (the dissolved form of chromium-6). These systems are more common in commercial and municipal contexts but are available as whole-house units for larger budgets.

Specialized adsorptive media based on iron hydroxide or titanium dioxide have shown strong chromium-6 removal in peer-reviewed studies and are increasingly available in point-of-use cartridges. These are worth evaluating as an alternative or supplement to RO, particularly where water pressure or waste-water ratio is a concern.

Reduction-coagulation-filtration is the primary municipal-scale approach. It works by adding a reducing agent (typically ferrous sulfate) that converts chromium-6 to chromium-3, which then coagulates and is removed by filtration. Several California utilities have piloted or implemented RCF in response to elevated chromium-6 levels.

When shopping for a home filter, look for products from NSF International’s certified products database and confirm that the listing specifically addresses hexavalent chromium at the concentration relevant to your water. For more guidance on certified pitcher and countertop options, see our guide to the best water filter pitchers, which notes which models are tested for Cr(VI) specifically.

Check Your City

Chromium-6 is widespread but unevenly distributed. Industrial areas, communities near coal ash impoundments, and regions with chromium-rich geology carry the highest risk — but given that three in four sampled utilities showed detectable levels in UCMR 3, no community should assume it is unaffected without data.

Search your city on WaterVerge to look up your water system’s chromium data, review any historical violations, and see how your reported levels compare to both the federal MCL and health-based benchmarks. If your utility has not tested for chromium-6 specifically — as is the case for many smaller systems — accredited labs offer private tap water tests for chromium-6 starting at around $30 to $75. Knowing your actual concentration is the first step toward deciding whether a point-of-use treatment system is a worthwhile investment for your household.