Arsenic

Arsenic is classified by the EPA and the International Agency for Research on Cancer (IARC Group 1) as a known human carcinogen. Long-term exposure through drinking water is linked to cancers of the skin, lung, and bladder, plus kidney and liver tumors at higher exposure levels.

Beyond cancer, chronic low-dose arsenic exposure damages the cardiovascular system: large epidemiological studies tie it to elevated rates of heart disease, hypertension, and stroke. Skin effects include hyperpigmentation, thickening on the palms and soles (hyperkeratosis), and characteristic lesions. Arsenic also weakens bone density over decades of exposure.

Acute high-dose exposure causes nausea, abdominal pain, and peripheral neuropathy, but the bigger public health concern in the United States is the chronic, low-level exposure that millions of well users absorb daily without symptoms.

If you're on city water, your utility's annual Consumer Confidence Report (CCR) lists the arsenic level. The federal limit is 10 ppb, but health researchers note that effects appear well below that, so a "compliant" reading is not the same as a safe one.

Private well users should test for arsenic at least once: it's geological, so if your aquifer has it, you have it. State health departments often publish arsenic risk maps, and the USGS has nationwide groundwater data showing which counties exceed 10 ppb. Wells in known hot spots (parts of Maine, New Hampshire, Nevada, Arizona, the Dakotas) should be tested every few years.