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Arsenic

A naturally occurring carcinogen. Highest in private wells and the rural Southwest.

Federal legal limit (MCL)
MCL: 10 ppb (lowered from 50 ppb in 2001)
Federal health goal
MCLG: 0 ppb
EWG health guideline
0.004 ppb
What it is

The science, plainly.

Arsenic is a semi-metal that occurs naturally in bedrock across much of the U.S. It leaches into groundwater, which is why private wells and small rural systems are the highest-risk source. Industrial sources include mining, smelting, and historical agricultural pesticides. Arsenic is a known human carcinogen (IARC Group 1).

Where it comes from

The pathways into the tap.

  • Naturally occurring in bedrock and groundwater
  • Mining and smelting operations
  • Historical pesticide use (especially on cotton and orchards)
  • Coal-fired power plant ash
Health effects

What the evidence shows.

Long-term exposure to even moderate arsenic levels is linked to bladder, lung, and skin cancer; cardiovascular disease; diabetes; and developmental effects in children. Acute toxicity is rare in U.S. tap water, but chronic low-dose exposure is the concern.

Cancer

Established human carcinogen. Bladder, lung, and skin cancers show the clearest dose-response relationship.

Cardiovascular

Chronic exposure linked to elevated blood pressure and heart disease.

Children

Prenatal and early-life exposure linked to reduced cognitive function and immune effects.

Regulation

What the law allows vs. what's actually safe.

Federal legal limit (MCL)
MCL: 10 ppb (lowered from 50 ppb in 2001)
Federal health goal (MCLG)
MCLG: 0 ppb
EWG health guideline
0.004 ppb

EWG's health guideline of 0.004 ppb reflects a 1-in-a-million cancer risk.

Regions most affected

Where exposure is highest.

Highest in the Southwest (especially New Mexico, Arizona, Nevada), parts of the Upper Midwest, and New England. Private wells nationwide are at elevated risk because they are not federally regulated.

How to remove it

Filtration that actually works.

Effective filtration
  • Reverse osmosis (most effective)
  • Activated alumina
  • Anion exchange
  • Distillation

We don't recommend brands. The certification on the box matters more than the brand printed on it. Look for the actual NSF/ANSI standard number specific to the contaminant you're removing.

Check your tap

Is arsenic a problem at your address?

Enter your ZIP and we'll pull every contaminant your utility has reported — measured against EWG's health-protective guidelines.

Sources

  1. National Primary Drinking Water Regulations — U.S. Environmental Protection Agency
  2. Guidelines for Drinking-water Quality, Fourth Edition — World Health Organization