Common Drinking Water Contaminants
The EPA regulates over 90 contaminants in US drinking water, from bacteria and heavy metals to synthetic chemicals and radioactive elements. Here is what you need to know about the most common ones, their health effects, and what the legal limits actually mean.
Check your water by ZIP code2024 EPA PFAS Rule
In April 2024, the EPA finalised the first-ever national drinking water standards for PFAS (forever chemicals). The new rule sets maximum contaminant levels at 4 parts per trillion (ppt) for PFOA and PFOS individually, and limits for three other PFAS compounds. Water systems have until 2029 to comply. This is the most significant drinking water regulation in decades.
Microorganisms
Bacteria, viruses, and parasites that can enter water supplies through sewage overflows, animal waste, or treatment failures.
E. coli
Health-basedHealth Effects
Gastrointestinal illness, diarrhoea, vomiting. Can be severe in children and the elderly.
EPA Limit
0 (zero tolerance)
Sources
Human and animal faecal waste, sewage overflows
Giardia
Health-basedHealth Effects
Giardiasis, causing prolonged diarrhoea, cramps, nausea, and dehydration.
EPA Limit
99.9% removal required
Sources
Contaminated surface water, animal waste, infected individuals
Cryptosporidium
Health-basedHealth Effects
Cryptosporidiosis, causing watery diarrhoea and stomach cramps. Particularly dangerous for immunocompromised individuals.
EPA Limit
99% removal required
Sources
Contaminated surface water, agricultural runoff. Resistant to chlorine disinfection.
Legionella
Health-basedHealth Effects
Legionnaires' disease, a serious form of pneumonia. Can be fatal, especially in older adults and smokers.
EPA Limit
No MCL (treatment technique required)
Sources
Grows in warm water systems, cooling towers, plumbing biofilms
Total Coliforms
SecondaryHealth Effects
Not necessarily harmful on their own, but their presence indicates that other pathogens may be in the water.
EPA Limit
No more than 5% positive samples/month
Sources
Naturally present in soil and vegetation; indicates potential contamination pathway
Inorganic Chemicals
Metals and minerals that can enter water from natural deposits, industrial discharge, or corrosion of household plumbing.
Lead
Health-basedHealth Effects
Developmental delays and learning difficulties in children. Kidney damage and high blood pressure in adults. No safe level of exposure.
EPA Limit
Action level: 15 ppb
Sources
Corrosion of old lead pipes, solder, and fixtures. Most common in homes built before 1986.
Arsenic
Health-basedHealth Effects
Linked to skin, bladder, and lung cancer with long-term exposure. Also causes skin damage and circulatory problems.
EPA Limit
MCL: 10 ppb
Sources
Natural geological deposits, mining and smelting operations, industrial discharge
Copper
Health-basedHealth Effects
Short-term: nausea, vomiting, diarrhoea. Long-term: liver and kidney damage. Wilson's disease patients are especially sensitive.
EPA Limit
Action level: 1.3 ppm
Sources
Corrosion of copper plumbing, natural deposits, industrial discharge
Nitrates
Health-basedHealth Effects
Blue baby syndrome (methemoglobinemia) in infants under 6 months. Potentially linked to certain cancers in adults.
EPA Limit
MCL: 10 ppm
Sources
Agricultural fertilizer runoff, septic systems, animal feedlots, natural deposits
Fluoride
SecondaryHealth Effects
At recommended levels (0.7 ppm), helps prevent tooth decay. Excess causes dental fluorosis in children and skeletal fluorosis in adults.
EPA Limit
MCL: 4 ppm
Sources
Added intentionally by many water systems; also found naturally in groundwater
Mercury
Health-basedHealth Effects
Damages the nervous system, kidneys, and developing fetuses. Chronic exposure causes tremors, mood changes, and cognitive impairment.
EPA Limit
MCL: 2 ppb
Sources
Industrial discharge, coal-fired power plants, natural deposits, landfill runoff
Chromium-6
Health-basedHealth Effects
Probable human carcinogen linked to stomach and intestinal cancers. Made famous by the Erin Brockovich case.
EPA Limit
No federal MCL (California set 10 ppb, later withdrawn)
Sources
Industrial discharge, natural erosion, coal ash, steel manufacturing
Organic Chemicals (PFAS / Forever Chemicals)
Per- and polyfluoroalkyl substances are synthetic chemicals that persist in the environment and the human body. Found in the drinking water of an estimated 176 million Americans.
PFOA
Health-basedHealth Effects
Linked to kidney and testicular cancer, thyroid disease, liver damage, immune system suppression, and developmental effects in fetuses.
EPA Limit
MCL: 4 ppt (2024 EPA rule)
Sources
Manufacturing of non-stick coatings (Teflon), stain-resistant fabrics, food packaging
PFOS
Health-basedHealth Effects
Similar to PFOA. Linked to cancer, thyroid disease, immune system effects, and elevated cholesterol. Accumulates in the body over decades.
EPA Limit
MCL: 4 ppt (2024 EPA rule)
Sources
Firefighting foam (AFFF), industrial facilities, military bases, wastewater treatment plants
GenX (HFPO-DA)
Health-basedHealth Effects
Liver damage, kidney effects, developmental toxicity, and potential cancer risk. Intended as a safer PFOA replacement but still raises serious concerns.
EPA Limit
MCL: 10 ppt (2024 EPA rule)
Sources
Industrial discharge, replacement chemical for PFOA in manufacturing
Disinfection Byproducts
Formed when chlorine and other disinfectants react with naturally occurring organic matter in water. A trade-off between killing pathogens and creating new chemical risks.
Trihalomethanes (THMs)
Health-basedHealth Effects
Long-term exposure linked to bladder cancer, liver and kidney problems, and adverse reproductive outcomes including miscarriage.
EPA Limit
MCL: 80 ppb (annual average)
Sources
Formed when chlorine reacts with organic matter (leaves, soil, algae) during water treatment
Haloacetic Acids (HAAs)
Health-basedHealth Effects
Similar cancer risks to THMs. Also linked to liver toxicity and developmental effects in animal studies.
EPA Limit
MCL: 60 ppb (annual average)
Sources
Same formation process as THMs; higher in systems with more organic matter in source water
Chloramine
SecondaryHealth Effects
Generally less irritating than chlorine, but can leach lead from pipes. Harmful to aquarium fish and dialysis patients.
EPA Limit
MCL: 4 ppm (as chlorine residual)
Sources
Added intentionally as an alternative disinfectant to chlorine; used by many large water systems
Radionuclides
Radioactive elements that occur naturally in rock and soil. They dissolve into groundwater and are more common in certain geological regions.
Radium (226 & 228)
Health-basedHealth Effects
Long-term exposure linked to bone cancer, lymphoma, and leukaemia. Accumulates in bones over time.
EPA Limit
MCL: 5 pCi/L (combined 226+228)
Sources
Natural radioactive decay of uranium and thorium in rock; more common in deep wells
Uranium
Health-basedHealth Effects
Kidney damage and toxicity with chronic exposure. Also a radioactive cancer risk at elevated levels.
EPA Limit
MCL: 30 ppb
Sources
Natural geological deposits, mining operations, phosphate fertilizers
Radon
Health-basedHealth Effects
Increases cancer risk when inhaled (released from water during showering). Second leading cause of lung cancer after smoking.
EPA Limit
No federal MCL (proposed: 300 pCi/L)
Sources
Dissolves into groundwater from natural radioactive decay in rock and soil
The right filter depends on what's in your water
Different contaminants require different filtration technologies. A carbon filter that removes chlorine taste will not remove lead or PFAS. Start by finding out what is in your water, then choose a filter that targets those specific contaminants.
Check your water now
Enter any US ZIP code to see which contaminants have been found in your local water system, how they compare to EPA limits, and what you can do about it.
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