Heavy metals

WTR

0.004 ppbhealth-based level (arsenic, OEHHA PHG)

The federal limit for arsenic is 10 ppb. The health-based level is 2,500x lower.

Settled science

Stasis Home ResearchEvery claim on this page is graded against peer-reviewed health research and linked to its primary source.Updated 2026-06

Heavy metals are a class screen, and for most municipal homes the news is good: arsenic, cadmium, mercury, and the rest usually come back non-detect or low. The screen earns its place by confirming clean and by catching the rare real finding. Where the science is strongest, arsenic, the health-based goal sits far below the legal limit.

What it is, and where it comes from

Heavy metals and metalloids that can reach a home tap at health-relevant levels include arsenic, cadmium, mercury, antimony, barium, beryllium, selenium, thallium, nickel, and aluminum. Lead and hexavalent chromium have their own pages. They arrive by three routes: dissolution from the rock an aquifer sits in (arsenic, selenium, beryllium, barium), industrial or agricultural contamination of a water source (cadmium, mercury, nickel, antimony), and leaching from older in-home plumbing and fixtures (cadmium from pre-1960 galvanized pipe, nickel from some fittings). For most homes on treated municipal water, levels in this class run low. The reason to test is to confirm that and to interpret the exception when one shows up.

Why it matters

Arsenic carries the clearest evidence. At the high exposures seen in Taiwan, Bangladesh, Argentina, and Chile, inorganic arsenic causes bladder, lung, and skin cancer. It is also linked to cardiovascular disease, diabetes, and effects on child development. The World Health Organization holds that no exposure level is entirely safe and that concentrations should be kept as low as reasonably possible. Cadmium accumulates in the kidneys over a half-life measured in decades and, at the high exposures behind Japan's Itai-itai disease, damaged bone and renal function. Ingestion is the route that matters here. These ions are large and charged and pass poorly through intact skin, so showering is not the concern it is for THMs.

California's health-based goal for arsenic is 0.004 ppb, set at a one-in-a-million lifetime cancer risk from bladder and lung cancer. The legal limit is 10 ppb, a 2,500x gap, and the federal health goal is zero.CA OEHHA, Public Health Goal for Arsenic in Drinking Water (2004)
In a national survey of residential drinking water, arsenic averaged about 1.7 ppb and only 1.5% of samples exceeded the 10 ppb federal limit, with private wells running far higher than public supplies.Bradham et al., 2023 (J Expo Sci Environ Epidemiol), national survey of lead and other metal(loids) in US residential drinking water
The World Health Organization labels its 10 ppb arsenic guideline provisional because of the practical difficulty of removing arsenic, and holds that no exposure level is entirely safe.World Health Organization, Arsenic fact sheet
Arsenic, cadmium, beryllium, nickel compounds, and hexavalent chromium are all classified by IARC as Group 1, carcinogenic to humans, on strong human evidence at occupational and high environmental exposures.IARC Monographs, Group 1 carcinogen classifications

What we grade it against

Where the health research draws the line, versus the legal limit.
Contaminant	Health-based level	Legal limit	Source (health-based)
Arsenic (ppb)	0.004one-in-a-million cancer risk	10CA + federal MCL; federal MCLG is 0	CA OEHHA PHG, 2004
Cadmium (ppb)	0.04kidney + bone	5CA + federal MCL	CA Water Boards MCL/PHG table, 2025
Thallium (ppb)	0.1	2CA + federal MCL	CA Water Boards MCL/PHG table, 2025
Mercury, inorganic (ppb)	1.2	2CA + federal MCL	CA Water Boards MCL/PHG table, 2025
Nickel (ppb)	12	100CA MCL; federal limit remanded 1995, never replaced	CA Water Boards MCL/PHG table, 2025

Health-based levels come from peer-reviewed research and government risk scientists working without cost constraints. Legal limits are enforceable compromises. Your report grades to the health column.

What helps

Direct fixes

Reverse osmosis at the kitchen tap (NSF/ANSI 58)
Rejects most heavy metal ions by size and charge exclusion. Arsenic rejection is typically above 90% for both arsenite and arsenate, and the same membrane handles cadmium, mercury, nickel, antimony, beryllium, and the rest of the class.
Carbon block filter (NSF/ANSI 53)
Useful for chlorine, taste, and some organics, and some blocks carry a mercury-reduction certification.
Carbon block alone does not remove arsenic, cadmium, or the dissolved heavy metals as a class. For arsenic specifically, RO or an arsenic-rated adsorbent is the right tool, not carbon.

Bigger retrofits

Whole-house point-of-entry treatment
Treats every fixture in the home.
Not the right primary choice for heavy metals. The exposure route is the water you drink at the kitchen tap, and dermal absorption during bathing is poor, so point-of-use at the tap targets the actual exposure.

Free and behavioral

Test before you treat
A single kitchen-tap grab sample run on ICP-MS covers 15 to 25 metals at once and is already bundled into mid-tier residential panels, so the class is a check you already get rather than a paid add-on. Confirm what is actually present before installing anything.

Most of this class is a screen you want to come back boring. On a typical treated municipal supply it does, and confirming that is part of the value: knowing what is clean is as useful as knowing what is not. The screen exists for the exception, an older home with galvanized pipe, a source drawing on the wrong aquifer, so that a real finding gets read precisely instead of missed. For arsenic we anchor to the health-based goal, not the feasibility-set legal limit.

Common questions

Should I worry about heavy metals in my tap water?

For most homes on treated municipal water, no. Standard municipal treatment removes the dissolved forms reasonably well, so the class usually reads low or non-detect. Testing confirms that for your specific home and catches the rare exception, such as an older home with pre-1960 galvanized pipe.

Why is the health goal for arsenic so much lower than the legal limit?

California's health-based goal for arsenic is 0.004 ppb, set at a one-in-a-million lifetime cancer risk, while the legal limit is 10 ppb. The federal limit is also 10 ppb, a feasibility compromise, even though the federal health goal is zero. The World Health Organization calls its matching 10 ppb guideline provisional because arsenic is genuinely hard to remove at scale.

Does a carbon filter remove arsenic?

No. A standard carbon block leaves arsenic and the other dissolved metals in the water. Reverse osmosis at the kitchen tap is the effective point-of-use option, with arsenic rejection typically above 90%. Some carbon blocks add a mercury-reduction certification, but that does not extend to arsenic.

Is showering in this water a concern?

Not for heavy metals. These ions are large and charged and barely cross intact skin, so the dominant residential exposure route is drinking the water, not bathing in it. That is why point-of-use treatment belongs at the kitchen tap rather than on the whole house.

Do I need a separate paid test for heavy metals?

Usually not. Mid-tier and higher residential water panels already bundle the class: one kitchen-tap sample run on ICP-MS covers 15 to 25 metals at once. Choose a panel that includes it and there is nothing extra to pay.