Contaminant guide

Arsenic in drinking water

Arsenic is naturally occurring, invisible, and tasteless — which is exactly why it is worth understanding rather than fearing. Here is what it is, what the science says about health, and the treatment types that actually reduce it.

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A clear glass of tap water held to the light — arsenic is colorless, odorless, and tasteless, so it can only be confirmed by a laboratory test.

What arsenic is

Arsenic is a naturally occurring element found in rocks and soil. In many regions it dissolves slowly from that geology into groundwater, which is how it ends up in well water and some public supplies. The form that matters most for health is inorganic arsenic; it has no taste, color, or smell at the levels found in drinking water, so you cannot detect it without a test.

Because the source is usually natural geology rather than a spill, arsenic tends to be a regional and local issue — two wells on the same street can differ. That is also why the practical fix is almost always treating the water you drink with a method like reverse osmosis (RO), rather than assuming a problem everywhere.

Health effects, stated plainly

The concern with arsenic is long-term, repeated exposure rather than a single glass of water. Over many years, drinking water with elevated inorganic arsenic is associated with a higher risk of certain cancers (including skin, bladder, and lung) and with cardiovascular and skin effects. Public-health agencies treat it as a contaminant worth reducing where levels are elevated, which is why a health-based reference limit exists.

None of this means a low or trace detection is cause for alarm — it means arsenic is worth measuring and, if elevated, worth treating. For the specifics of exposure and risk, the CDC’s Agency for Toxic Substances and Disease Registry is a good primary source. CDC/ATSDR on arsenic →

Groundwater moving through rock and a residential well casing — the natural geologic pathway by which arsenic enters drinking water.

Where it comes from

Most arsenic in U.S. drinking water is natural, leaching from rock and sediment into groundwater. The U.S. Geological Survey, which has mapped this nationally, has found arsenic far more often and at higher levels in parts of the West and Southwest, where the underlying geology is the source.

Less commonly, arsenic can come from historic agricultural or industrial use. Either way, private wells are the highest-attention case, because no one monitors a private well but its owner. USGS: arsenic and drinking water →

How to know if you have it

You cannot taste, see, or smell arsenic, so the only reliable way to know is a laboratory test. If you are on a private well, this matters most: test specifically for arsenic, ideally through a certified lab rather than a quick strip. If you are on a public system, your utility’s annual water-quality report is a starting point, but a tap test still tells you what is actually reaching your kitchen.

Start here: a certified lab test is the honest first step before buying any equipment — it tells you whether you have a problem and how big it is. How to test your water →

What the current limit is

As of June 2026, the U.S. EPA enforceable limit — the Maximum Contaminant Level, or MCL — for arsenic in public drinking water is 0.010 mg/L (10 parts per billion). The EPA lowered it to that level from the older 50 ppb standard under the Arsenic Rule (adopted 2001), and set the health goal (MCLG) at zero. This limit legally applies to public water systems, not to private wells — a private well owner uses it as a health reference, not a requirement. EPA Chemical Contaminant Rules (verified June 2026) →

Reference levels for arsenic in drinking water. The EPA enforceable limit (MCL) is 10 ppb as of June 2026; the pre-2001 standard was 50 ppb; the non-enforceable health goal (MCLG) is zero. Source: U.S. EPA. This is a health reference, not a treatment mandate.

These figures can change. We verify them against the primary regulator and date the check; if you are reading this much later, confirm the current value at the EPA link above before relying on it.

What reduces it

Arsenic is removed by treatment types, not by any one brand. The well-established options for drinking water are reverse osmosis, adsorptive media designed for arsenic (such as iron-based media), anion exchange, and distillation. Reverse osmosis at the kitchen tap is the most common certified route for households, because arsenic is an ingestion-route contaminant — you mainly need to treat the water you drink and cook with, not every tap.

An honest cert nuance: certified arsenic reduction generally covers arsenic (V), the pentavalent form (commonly listed at a 50 ppb influent level). The other form, arsenic (III) / trivalent, is removed far less effectively and often needs pre-oxidation first. If your test shows arsenic, ask the lab which form is present, and confirm the exact unit’s certified arsenic claim before buying.

We deliberately do not name specific products on this page or attach certified claims to them here. When you are ready, the selector matches your inputs to options whose finished-product certification we have verified — and routes you to a point-of-use fix when that is the correct, cheaper answer.

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Tell the selector your source and what your test found. It will point you to the correct, certified treatment type — and the honest, lower-cost option when there is one.

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Educational information, not medical or regulatory advice. Regulatory limits are health references, not mandates to treat. For your specific water, consult a certified laboratory and your local water utility, and confirm any product’s current certification for your exact model in the NSF, IAPMO, or WQA database. Regulatory figures on this page were verified against the U.S. EPA in June 2026 and may change.