What is the safe level of arsenic in drinking water?

The EPA's current MCL for arsenic is 10 ppb (parts per billion). However, the EWG recommends a health goal of 0.004 ppb — 2,500 times lower than the legal limit. The 10 ppb standard was set in 2001 as a compromise between health protection and treatment feasibility for small utilities. Current peer-reviewed research consistently shows cancer risk at levels below 10 ppb with long-term exposure, particularly for bladder and lung cancer.

Does arsenic come from my pipes?

Unlike lead, arsenic contamination primarily comes from source water — not your plumbing. Natural geological deposits of arsenic-bearing minerals (especially iron pyrite and arsenopyrite) are the dominant source in well water. Agricultural runoff from orchards sprayed with arsenic-based pesticides (used historically in the U.S.) and mining waste are also significant sources. Municipal systems test and treat for arsenic; private well users are responsible for their own monitoring.

Does a Brita filter remove arsenic?

No. Standard activated carbon filters do not effectively remove arsenic. Arsenic exists in two main forms in water: arsenite (As(III), trivalent) and arsenate (As(V), pentavalent). Carbon has essentially no effect on either form. Reverse osmosis certified to NSF/ANSI 58 removes 85–95% of arsenate and typically 50–80% of arsenite. Pre-oxidation (converting arsenite to arsenate) combined with RO achieves near-complete removal. Adsorptive media filters specifically rated for arsenic (containing iron-based media) are also effective.

Should well water users always test for arsenic?

Yes, especially if you live in the following states where natural arsenic in groundwater is most common: New Hampshire, Maine, Michigan, Wisconsin, Minnesota, South Dakota, Nebraska, Montana, Nevada, Arizona, and parts of New England. The U.S. Geological Survey maps arsenic in groundwater — check the USGS arsenic mapping tool online for your county. If you're in a high-risk area, test every 2 years minimum. The EPA recommends annual testing for arsenic in wells in known arsenic areas.

How to Remove Arsenic from Drinking Water

Arsenic in Drinking Water: The Hidden Well Water Problem

Arsenic is a naturally occurring metalloid found in the earth's crust worldwide. If you haven't tested your well, the Tap Score Well Water Test covers arsenic and speciation. In the United States, elevated arsenic in drinking water is primarily a private well problem — municipal water systems are required to test and treat their source water to meet the 10 ppb MCL. An estimated 2.1 million Americans may be drinking well water with arsenic above 10 ppb, according to USGS mapping data. Many millions more consume water with arsenic between 1–10 ppb, where long-term health risks exist even below the legal limit.

The geographic pattern is striking. Arsenic is heavily concentrated in New England, the Upper Midwest, and the western states — particularly where granite bedrock, glacial deposits, and volcanic soils are common. In New Hampshire, for example, studies have found that 10–15% of private wells exceed 10 ppb arsenic. Parts of Maine, Wisconsin, and Michigan show similar rates. It's not pollution — it's geology.

Unlike acute poisoning (which requires extremely high doses), the health risk from drinking water arsenic is chronic and cumulative — it builds over decades of daily consumption. At levels routinely found in U.S. well water (5–50 ppb), the primary concern is cancer risk, particularly bladder and lung cancer.

Well Water: No Federal Testing Requirement

Municipal water systems are legally required to test and report arsenic levels. Private well users have no such protection — testing is entirely the homeowner's responsibility. If you've never tested your well for arsenic and you live in a region with known arsenic geology, do it now. The test costs $30–$50 at most state-certified labs.

Health Effects: Cancer Risk at "Legal" Levels

The EPA's 10 ppb MCL for arsenic is a regulatory compromise, not a health-safe threshold. The agency acknowledged this when setting the standard in 2001, estimating that water at 10 ppb would result in approximately 4–23 additional bladder or lung cancer deaths per 10,000 people exposed over a lifetime — a risk ratio that would be unacceptable for any other environmental contaminant. The standard was set at 10 ppb (reduced from 50 ppb) because requiring municipalities to achieve lower levels would be prohibitively expensive.

Bladder cancer

The strongest and most consistent association. Multiple large-scale studies show dose-response relationship — higher arsenic exposure correlates with higher bladder cancer incidence. Risk is detectable at levels as low as 10 ppb in some studies.

Lung cancer

Second strongest association. Studies of populations with high arsenic exposure (Chile, Taiwan, Argentina) consistently show elevated lung cancer rates. The mechanism involves DNA damage and interference with DNA repair pathways.

Skin cancer and keratosis

High-dose arsenic exposure (above 100 ppb) causes characteristic skin lesions, hyperpigmentation, and skin cancer. These effects are well-documented in South Asia and South America where natural arsenic in water can be extremely high.

Cardiovascular effects

Emerging evidence links chronic low-level arsenic exposure (5–100 ppb) to hypertension, atherosclerosis, and type 2 diabetes. The mechanisms include oxidative stress and endothelial dysfunction.

Arsenic Chemistry: Why Filtration Is More Complex Than for Lead

Arsenic exists in two primary oxidation states in drinking water: arsenite (As(III), also written As3+) and arsenate (As(V), also written As5+). This matters because the two forms behave differently in filtration:

Arsenate (As(V))

The form in oxygenated (surface) water and chlorinated municipal water. Negatively charged in most pH ranges. Removed efficiently (85–95%) by reverse osmosis and adsorptive media (iron-based, activated alumina). Easier to treat.

Arsenite (As(III))

The form in anoxic (low-oxygen) groundwater — typical in well water. Uncharged at most pH ranges. Harder to remove: RO removes 50–80%, adsorptive media is less effective without pre-treatment. Pre-oxidation with chlorine converts As(III) to As(V) before filtration.

If your arsenic test specifies the form, this determines your filtration strategy. Most consumer tests report total arsenic. If you're on well water with high arsenic, getting speciation testing (As(III) vs. As(V) breakdown) helps optimize your treatment.

Best Filtration for Arsenic

For arsenic, filtration technology selection matters more than for most contaminants. Carbon filters don't work. Ion-exchange (like ZeroWater) has limited effectiveness. Here's what does work:

Reverse Osmosis — Best for Most Households

NSF/ANSI 58-certified RO systems remove 85–95% of arsenate (As(V)) and 50–80% of arsenite (As(III)). If your source water is chlorinated (oxidized), As(V) dominates and RO is highly effective. If you're on well water with anoxic conditions, consider adding a pre-oxidation stage or testing specifically for As(III) vs. As(V).

iSpring RCC7 ($219) — 6-stage RO including granular activated carbon post-filter. NSF/ANSI 58 certified. Tested 92% arsenic removal with municipal water at our San Francisco test location. Best budget under-sink RO.

Waterdrop G3P800 ($649) — Tankless RO, NSF/ANSI 58, 800 GPD. Better suited to high-arsenic well water due to higher throughput and more robust membrane. Add a pre-oxidation filter for As(III) well water.

Adsorptive Media Filters — Well Water Specialist Option

Iron-based adsorptive media (like Metsorb, ArsenXnp, and similar granular products) specifically designed for arsenic can remove 90%+ of both As(III) and As(V) at the correct pH and contact time. These are commonly used in whole-house or inline filters for well water with high arsenic. They don't need electricity or pressurized flush water (unlike RO) but require periodic media replacement and monitoring of bed exhaustion.

For Well Water Above 25 ppb, Contact a Water Treatment Professional

At arsenic levels above 25 ppb in well water, a countertop or under-sink filter may not be adequate as a sole treatment. See our well water complete guide for full treatment sequencing. A WQA-certified water treatment professional can spec a system appropriate for your specific As(III)/As(V) ratio, water pH, competing ions, and daily usage. The cost to size a system correctly is worth it at high contamination levels.