Real — worth checking
Lead — from your pipes, not the source
Lead almost never comes from the water supply — it leaches from old pipes, solder, and brass fixtures on the way to your tap. There's no established safe level, and the risk is highest for infants and young children.
What matters: if your home predates ~1986 or has a lead service line, run the cold tap ~30 seconds before drinking and use a filter certified to NSF/ANSI 53 for lead.
Evidence: Regulatory EPA action level & "no safe level" for children
Read the full Lead page
Real, newly regulated
PFAS — "forever chemicals," but it varies by utility
PFOA and PFOS show up in many US water systems — but levels vary enormously between utilities, and 2024 brought the first enforceable federal limits (4 ng/L). It's a real concern where present, not a universal emergency.
What matters: check your utility's PFAS results in its annual report. If elevated, reverse osmosis or a certified activated-carbon filter is what actually removes them — most pitchers don't.
Evidence: Peer-reviewed health data · Regulatory 2024 EPA limits
Read PFOA
Depends on your source
Arsenic — a well problem more than a city problem
Arsenic occurs naturally in groundwater and is a genuine, well-documented health risk at elevated levels — but public systems test and treat for it. The people actually exposed are usually on private wells in certain geologies.
What matters: on a private well? Test for arsenic — nobody does it for you. On city water, it's already monitored.
Evidence: Peer-reviewed · Regulatory 10 ppb standard
Read the full Arsenic page
It's complicated — a real tradeoff
Chloroform & disinfection byproducts (THMs)
These form when chlorine disinfects your water — and here's the honest tension: the disinfection that creates them is also what keeps your water free of the pathogens that used to kill people. They're regulated, and for most people sit within limits.
What matters: a carbon filter reduces them if you want — but do not fear-drop disinfection. The cure is worse than the tradeoff.
Evidence: Peer-reviewed bladder-cancer signal · Regulatory 80 µg/L total-THM limit
Read the full Trihalomethanes page
Mostly plumbing-dependent
Copper — usually low, occasionally not
Copper leaches from copper pipes, usually at harmless levels. It becomes worth attention mainly with corrosive water, brand-new plumbing, or for infants on reconstituted formula.
What matters: run the tap in the morning before use; if you have corrosive or well water and an infant, test it. Otherwise, low priority.
Evidence: Regulatory EPA action level 1.3 mg/L
Read the full Copper page
Real for wells & infants
Nitrate — a farm-country well problem
Nitrate leaches into groundwater from fertilizer and septic systems, so it's mainly a private-well issue. It causes "blue-baby syndrome" in infants — and newer evidence links colorectal cancer to levels below the legal limit. It's colorless and tasteless.
What matters: on a well, test for nitrate — and again before mixing infant formula. Never boil to fix it; boiling concentrates nitrate. Reverse osmosis removes it; carbon filters don't.
Evidence: Regulatory 10 mg/L limit (blue-baby) · Peer-reviewed cancer signal below it
Read the full Nitrate page
Real — patchily regulated
Chromium-6 — the "Erin Brockovich" contaminant
Hexavalent chromium is a known carcinogen — and not the same as the chromium-3 in supplements. The federal limit only covers total chromium; there's no national chromium-6-specific standard. California set the first one in 2024 (10 µg/L), a value its own health goal puts 500× lower.
What matters: read your utility's chromium number. On a well near industry or chromium-bearing rock, test for hexavalent chromium specifically. Reverse osmosis or anion exchange removes it.
Evidence: Regulatory IARC Group 1 · CA 10 µg/L (2024)
Read the full Chromium-6 page
Every verdict above links to its full, cited compound page, and carries an evidence tier — how strong the science behind the verdict actually is. That tagging is the whole method. Detection of a chemical is not proof of harm; we say so, every time.