Arsenic

Arsenic is a naturally occurring element found in rock, soil, and groundwater throughout the world. It is one of the most significant drinking water contaminants because of its prevalence and toxicity — the EPA classifies arsenic as a known human carcinogen. Like lead, arsenic in water is completely invisible: it has no taste, no smell, and no color, even at dangerous concentrations.

The EPA's Maximum Contaminant Level for arsenic is 10 parts per billion (ppb), set in 2001. The MCLG (health goal) is zero, reflecting the fact that any level of arsenic exposure carries some risk.

That 10 ppb limit is a regulatory compromise between health protection and treatment costs — not a safety guarantee. The EPA itself estimates that lifetime exposure to arsenic at 10 ppb could cause up to 600 cancers per million people. California's Public Health Goal for arsenic is 0.004 ppb (4 parts per trillion) — roughly 2,500 times stricter than the federal limit. The gap between what's legal and what's considered truly safe is unusually wide for arsenic.

Why Is Arsenic Dangerous?

Arsenic is a potent carcinogen even at low levels. According to the EPA, WHO, and ATSDR, long-term health effects include:

• Skin, bladder, and lung cancer
• Cardiovascular disease and heart problems
• Type 2 diabetes
• Skin changes including darkening and thickening
• Nervous system effects including numbness
• Developmental effects in children, including reduced IQ

How Does Arsenic Get Into Water?

Arsenic enters water primarily through natural geological processes — it dissolves from rock and soil into groundwater. It is particularly common in the western United States, parts of New England, and areas with volcanic rock. Industrial sources include mining, smelting, and agricultural pesticide runoff. Groundwater sources are far more likely to contain arsenic than surface water (lakes and rivers).

Who Is Most at Risk?

Private well owners. Roughly 23 million US households get their drinking water from private wells, which are not regulated by the EPA. The well owner is solely responsible for testing and treatment. Arsenic in groundwater is especially common in areas with volcanic rock and certain aquifer types — and many well owners have never tested for it.

Western US and New England residents. Arsenic concentrations in groundwater are geologically driven and vary dramatically by region. The most affected areas include Arizona, Nevada, California, Oregon, parts of the Midwest (particularly Iowa and Wisconsin), and portions of New England (Maine, New Hampshire). The reason: bedrock in these regions — volcanic deposits, certain sedimentary formations — contains naturally occurring arsenic that dissolves into groundwater over time.

Children and pregnant women. Children are particularly vulnerable because they consume more water per kilogram of body weight than adults. Pregnant women should minimize exposure, as arsenic crosses the placenta and has been linked to low birth weight and developmental effects. Long-term exposure at even low levels can cause cumulative damage across all age groups.

How to Test Your Water for Arsenic

Arsenic has no taste, no smell, and no color — even at concentrations far above the EPA limit. A laboratory test is the only way to detect it.

If you're on a public water system: Your utility tests regularly and publishes results in the annual Consumer Confidence Report (CCR). You can look up your city's arsenic data on TapWater.org. If your utility reports arsenic below the EPA limit, that's a good sign — but the system-wide average doesn't tell you exactly what arrives at your specific tap.

If you have a private well: No government agency tests your water for you — it's entirely on you. The recommendation: test at least once for arsenic through an ELAP-certified laboratory. If your well was recently drilled, retest after 6 months, as arsenic levels can fluctuate during the initial period. If the first test shows no arsenic, routine retesting is generally not necessary.

Which tests work: Standard home test strips are not accurate enough for arsenic at the levels that matter. A lab-based test kit using EPA Method 200.8 (ICP-MS) is the standard — it can detect arsenic down to 1 ppb. We've reviewed the best options in our water test kits guide.

How to Remove Arsenic From Your Water

Understanding arsenic III vs. arsenic V: There are two forms of arsenic in water. Arsenate (arsenic V) is relatively easy to filter. Arsenite (arsenic III) is much harder to remove and must first be oxidized to arsenic V — typically using chlorine or hydrogen peroxide — before a filter can be effective. A lab test can determine which form is present, and this directly impacts which treatment system you need.

Effective treatment methods:

• Reverse Osmosis (RO): The most effective and practical home treatment, removing 90–95% of arsenic. An under-sink RO system typically costs $150–$500 and protects your drinking water tap.
• Activated Alumina: Effective especially for arsenate (arsenic V). Requires periodic media replacement.
• Iron-Based Adsorption: Newer technology using iron oxide media, very effective for both forms of arsenic.
• Distillation: Effective but slow and energy-intensive — impractical for daily use.

What does NOT work: Standard activated carbon filters — including most Brita pitchers, refrigerator filters, and basic faucet filters — do not remove arsenic. Water softeners do not remove it either. And boiling makes it worse: water evaporates, but arsenic stays behind, increasing the concentration.

Certification matters: Any filter marketed for arsenic removal should be certified to NSF/ANSI Standard 53 (health-related contaminants) or NSF/ANSI Standard 58 (reverse osmosis). You can verify certifications at nsf.org or through the Water Quality Association.

Point-of-use vs. whole-house: An under-sink RO system ($150–$500) protects your drinking water faucet. For whole-house treatment (every tap), arsenic-specific adsorption or oxidation-based systems are available but significantly more expensive ($1,000+) and typically require professional installation.

What to Do If Your Water Has Arsenic

Your response should match the level detected:

Under 10 ppb (within EPA limit): Your water meets the legal standard. Health risks at these levels are low but not zero — the EPA's own health goal is zero. An RO filter at your drinking water tap is a sensible precaution, especially if children or pregnant women are in the household.

Over 10 ppb (exceeds EPA limit): Switch to an alternative drinking water source or install a certified filter immediately. If you're on public water, contact your utility — exceedances must be reported and addressed. If you're on well water, a professional filtration system is strongly recommended.

Over 35 ppb: At these levels, children and infants face elevated short-term risk. Switch to filtered or bottled water immediately, including for preparing baby formula and cooking.

Over 500 ppb: Do not use the water for bathing or showering. Secure an alternative water supply and contact your local health department.

Important: Arsenic is not absorbed through skin in meaningful amounts during normal use. Bathing, showering, laundry, and dishwashing are safe at levels below 500 ppb. The risk is exclusively from ingestion — drinking and cooking.