Drinking Water Contaminants
A guide to the most common contaminants found in US tap water — what they are, where they come from, their health effects, and how to filter them.
PFAS "Forever Chemicals"
High ConcernMan-made "forever chemicals" that persist in the environment and the human body. The EPA established the first federal PFAS drinking water limits in 2024.
GenX (HFPO-DA)
A replacement chemical for PFOA, marketed as safer but also persistent in the environment.
EPA Limit: 10 ppt (parts per trillion)
PFHxS (Perfluorohexane Sulfonate)
A "forever chemical" used as a replacement for PFOS in firefighting foams and industrial applications.
EPA Limit: 10 ppt (parts per trillion)
PFNA (Perfluorononanoic Acid)
A long-chain "forever chemical" found in some food packaging and industrial processes.
EPA Limit: 10 ppt (parts per trillion)
PFOA (Perfluorooctanoic Acid)
A man-made "forever chemical" historically used in non-stick coatings (Teflon), food packaging, and firefighting foam.
EPA Limit: 4 ppt (parts per trillion)
PFOS (Perfluorooctane Sulfonate)
A man-made "forever chemical" formerly used in Scotchgard, firefighting foam, and industrial processes.
EPA Limit: 4 ppt (parts per trillion)
Metals
Moderate–HighMetals that can enter water from natural deposits, industrial pollution, or corroding plumbing. Some, like lead, have no safe level of exposure.
Arsenic
A naturally occurring element found in rock and soil that can dissolve into groundwater.
EPA Limit: 10 ppb (parts per billion)
Chromium-6 (Hexavalent Chromium)
A toxic form of chromium made famous by the Erin Brockovich case. Can occur naturally or from industrial pollution.
EPA Limit: 100 ppb for total chromium (no federal limit for chromium-6 alone)
Copper
A metal that enters water primarily through corrosion of household copper plumbing.
EPA Limit: 1.3 ppm (action level) / 1.0 ppm MCLG
Lead
A toxic heavy metal that typically enters drinking water through corroding pipes, faucets, and solder — not from the water source itself.
EPA Limit: 15 ppb action level (no safe level of exposure)
Disinfection Byproducts
ModerateChemicals that form when disinfectants like chlorine react with organic matter in water. Found in virtually all chlorinated water systems.
Haloacetic Acids (HAA5)
A group of five chemicals that form as byproducts when chlorine or other disinfectants react with organic matter in water.
EPA Limit: 60 ppb (parts per billion)
Trihalomethanes (TTHM)
A group of chemicals (chloroform, bromoform, and others) that form when chlorine used to disinfect water reacts with natural organic matter.
EPA Limit: 80 ppb (parts per billion)
Radioactive Contaminants
ModerateNaturally occurring radioactive elements that can dissolve into groundwater from rock and soil.
Inorganic Chemicals
VariesNaturally occurring and man-made inorganic compounds regulated in drinking water.
Fluoride
A mineral added to most US water systems to prevent tooth decay. Also occurs naturally in some groundwater.
EPA Limit: 4 ppm (MCL) / 0.7 ppm recommended level
Nitrate
A compound found naturally in soil and water, but elevated levels usually indicate contamination from fertilizers or sewage.
EPA Limit: 10 ppm (mg/L) as nitrogen
Disinfectants
Low ConcernChemicals added to water to kill harmful bacteria and viruses. Generally safe at regulated levels, but some groups need to be aware.