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The most common contaminants in US tap water are disinfection byproducts (TTHM and HAA5) formed when chlorine meets organic matter, lead picked up from service lines and household plumbing, and PFAS “forever chemicals” now detected in roughly half of the systems EPA has sampled. Regional groundwater problems — arsenic, nitrate, radium and uranium — affect fewer systems but at levels that more often exceed federal limits. The table below lists every contaminant EPA regulates or monitors that matters at the tap, its legal limit, and which certified filter type removes it; the section after it shows how often each one actually appears in EPA records for 756 US water systems.
Limits: EPA National Primary Drinking Water Regulations. Prevalence: EPA SDWIS/ECHO 2026Q2 · UCMR5 (2023–2026, July 2026 release) · SYR4 six-year review 2012–2019 · Methodology
Across the 756 EPA water systems behind our graded cities, the most frequently measured problems are disinfection byproducts (TTHM and HAA5 — 286 systems had at least one TTHM sample over 80 ppb in 2012–2019), PFAS (345 of 749 sampled systems had a detection in 2023–2025), and lead from household plumbing (85 systems reported a 90th-percentile lead result of 5 ppb or more). Arsenic, nitrate and radium are regional groundwater problems rather than national ones.
Across the 756 EPA water systems behind our graded cities, the most frequently measured problems are disinfection byproducts (TTHM and HAA5 — 286 systems had at least one TTHM sample over 80 ppb in 2012–2019), PFAS (345 of 749 sampled systems had a detection in 2023–2025), and lead from household plumbing (85 systems reported a 90th-percentile lead result of 5 ppb or more). Arsenic, nitrate and radium are regional groundwater problems rather than national ones.
Legally, yes. EPA Maximum Contaminant Levels are set as close as feasible to the health goal (MCLG) while accounting for treatment cost and detectability, so some limits — lead, arsenic, TTHM, PFAS — sit above the level at which no health effect is expected. Meeting every limit means the water is compliant, not that it is contaminant-free.
An MCL is an enforceable maximum concentration. An action level (used for lead and copper) is a 90th-percentile tap-sampling trigger that forces corrosion control and public notice rather than a hard cap. A treatment technique replaces a number with a required process — for example, 99.9% Giardia removal or a turbidity ceiling — when a contaminant is impractical to measure directly.
Standard carbon pitchers are certified under NSF/ANSI 42 for chlorine taste and odor only. Specific models (ZeroWater, Brita Elite, Clearly Filtered, some Waterdrop) add NSF/ANSI 53 claims for lead, chromium, mercury and PFOA/PFOS. No pitcher removes nitrate, fluoride, arsenic or radium — those need reverse osmosis or specialty media.
Start with your city page on this site (EPA violation, lead and PFAS records for your utility), then read your utility's annual Consumer Confidence Report, published by July 1. For lead, copper or anything from your own plumbing, only a test at your tap tells you — a certified mail-in lab test covers 40+ contaminants.
Under-sink reverse osmosis (NSF/ANSI 58). Its membrane rejects lead, arsenic, nitrate, fluoride, chromium-6, radium, uranium, sodium and PFAS, and its carbon stages handle chlorine and disinfection byproducts. It costs more, needs a drain connection and wastes some water, which is why a certified carbon filter is the better fit when chlorine byproducts or lead are the only issue.
23 contaminant groups. Limit types: MCL Maximum Contaminant Level · AL Action Level · MRDL Maximum Residual Disinfectant Level · SMCL Secondary standard · TT Treatment technique · none No federal limit. “NSF/ANSI” numbers are the certification to look for on the cartridge — the standard alone is not enough; the listing must name the contaminant.
| Contaminant | Federal limit | Health effect | Main sources | Filters that remove it | Certification |
|---|---|---|---|---|---|
| Lead Metals | AL15 ppb action level (0.015 mg/L); health goal 0. Drops to 10 ppb under the 2024 Lead and Copper Rule Improvements (compliance from 2027) | Lowered IQ and developmental delay in children; kidney and blood-pressure effects in adults; no known safe level | Lead service lines, pre-1986 solder, brass fixtures — picked up inside the distribution system and home plumbing, not at the plant | Certified pitcher, faucet-mount, under-sink carbon block, reverse osmosis | NSF/ANSI 53 (lead); NSF/ANSI 58 (RO) |
| Copper Metals | AL1.3 mg/L action level (90th percentile at the tap) | Stomach upset short term; liver or kidney damage with long exposure; risk for people with Wilson's disease | Corrosion of copper household plumbing, especially with acidic or soft water | Reverse osmosis, certified carbon block, distillation | NSF/ANSI 53 (copper claim on some cartridges); NSF/ANSI 58 |
| Arsenic Metals | MCL10 ppb (0.010 mg/L) | Skin lesions, circulatory problems and increased risk of bladder, lung and skin cancer | Natural erosion of rock into groundwater (Southwest, Upper Midwest, New England); mining and orchard-pesticide legacy | Reverse osmosis, iron-oxide or activated-alumina arsenic media, distillation; standard carbon does not remove it | NSF/ANSI 58 (RO); NSF/ANSI 53 (pentavalent arsenic on specific media) |
| Chromium (total) / chromium-6 Metals | MCL100 ppb (0.1 mg/L) total chromium; no separate federal limit for hexavalent chromium (California sets 10 ppb) | Hexavalent chromium is a carcinogen when ingested (stomach cancer in animal studies); allergic dermatitis | Natural deposits; steel, plating, leather-tanning and cooling-tower discharge | Reverse osmosis, ion-exchange pitchers/cartridges certified for chromium | NSF/ANSI 53 (hexavalent + trivalent chromium); NSF/ANSI 58 |
| Nitrate / nitrite Nitrogen & minerals | MCLNitrate 10 mg/L as N; nitrite 1 mg/L as N | Methemoglobinemia ("blue baby syndrome") in infants under 6 months; possible thyroid and cancer effects with long exposure | Fertilizer and manure runoff, septic systems, sewage — worst in shallow agricultural groundwater | Reverse osmosis, anion exchange, distillation; carbon and pitchers do not remove it; boiling concentrates it | NSF/ANSI 58 (RO); NSF/ANSI 62 (distillation) |
| Fluoride Nitrogen & minerals | MCL4.0 mg/L enforceable; 2.0 mg/L secondary (cosmetic) limit | Dental fluorosis (mottled enamel) above 2 mg/L; skeletal fluorosis and bone pain with long exposure above 4 mg/L | Natural deposits (high in parts of TX, CO, NM); intentional fluoridation targets 0.7 mg/L | Reverse osmosis, activated alumina, bone-char, distillation; carbon pitchers do not remove it | NSF/ANSI 58 (RO); NSF/ANSI 62 |
| Sodium Nitrogen & minerals | noneNone enforceable; EPA advisory of 20 mg/L for people on very low-sodium diets | Only relevant for sodium-restricted diets (heart failure, hypertension); no general-population limit | Road-salt runoff, seawater intrusion, water softeners, natural brines | Reverse osmosis or distillation (softeners add sodium) | NSF/ANSI 58 (TDS reduction) |
| PFAS — PFOA and PFOS PFAS | MCL4.0 ppt each (April 2024 rule; EPA announced in 2025 it would keep these limits and extend compliance to 2031) | Kidney and testicular cancer, high cholesterol, reduced vaccine response, low birth weight, thyroid disease | Firefighting foam (airports, bases), industrial discharge, landfill leachate, biosolids; persists indefinitely | Reverse osmosis, certified activated-carbon block, anion exchange | NSF/ANSI 53 or 58 with a PFOA/PFOS (or expanded PFAS) reduction claim |
| PFAS — PFHxS, PFNA, HFPO-DA (GenX), PFBS PFAS | MCL10 ppt each for PFHxS, PFNA and HFPO-DA plus a mixture hazard index of 1 including PFBS (2024 rule; EPA said in 2025 it intends to rescind and reconsider these) | Immune, thyroid, liver and developmental effects; less studied than PFOA/PFOS | Same as PFOA/PFOS; GenX from fluoropolymer manufacturing; PFBS as a PFOS replacement | Reverse osmosis is the most reliable for short-chain PFAS; carbon block works but exhausts faster | NSF/ANSI 58; NSF/ANSI 53 with expanded PFAS claim |
| Total trihalomethanes (TTHM) Disinfection | MCL80 ppb (0.080 mg/L) locational running annual average | Liver, kidney and central-nervous-system effects; increased bladder-cancer risk with decades of exposure | Formed when chlorine reacts with natural organic matter — highest in surface-water systems in warm months | Activated-carbon block (pitcher, faucet, under-sink), reverse osmosis with carbon stages | NSF/ANSI 53 (TTHM / VOC claim); NSF/ANSI 42 for taste |
| Haloacetic acids (HAA5) Disinfection | MCL60 ppb (0.060 mg/L) locational running annual average | Increased cancer risk; reproductive and developmental effects in animal studies | Chlorination byproduct alongside TTHM | Activated-carbon block, reverse osmosis | NSF/ANSI 53 (VOC claim covers most); NSF/ANSI 58 |
| Chlorine / chloramine (residual disinfectant) Disinfection | MRDL4.0 mg/L as Cl₂ (both); chlorine dioxide 0.8 mg/L | Eye and nose irritation, stomach discomfort; the residual itself is intentional and keeps water safe in the pipes | Added at the treatment plant; chloramine used by many large systems for a longer-lasting residual | Any activated-carbon filter for chlorine; catalytic carbon for chloramine | NSF/ANSI 42 (chlorine or chloramine reduction) |
| Radium-226 + radium-228 Radionuclides | MCL5 pCi/L combined | Bone cancer with long exposure; radium behaves like calcium in the body | Natural decay of uranium/thorium in deep bedrock aquifers (Midwest, Texas, Mid-Atlantic) | Reverse osmosis, cation-exchange softener, lime softening at the plant | NSF/ANSI 58; NSF/ANSI 44 (softeners with radium claim) |
| Uranium Radionuclides | MCL30 µg/L | Kidney toxicity (chemical) plus increased cancer risk (radiological) | Natural granite and sandstone aquifers, mining legacy — Colorado Plateau, Great Plains, New England wells | Reverse osmosis, anion exchange | NSF/ANSI 58 |
| Gross alpha particle activity Radionuclides | MCL15 pCi/L (excluding radon and uranium) | Increased cancer risk from alpha-emitting isotopes | Natural radionuclides in groundwater | Reverse osmosis, ion exchange (depends on the isotope) | NSF/ANSI 58 |
| Total coliform / E. coli Microbial | MCLE. coli: no confirmed positive allowed (MCL); total coliform: treatment technique — repeat positives trigger a system assessment (Revised Total Coliform Rule) | E. coli signals fecal contamination — diarrhea, cramps, kidney complications in children and the elderly | Sewage or animal-waste intrusion, main breaks, low chlorine residual, unsealed wells and tanks | UV disinfection, RO with a certified cyst/bacteria stage, boiling during a notice; standard carbon does not disinfect | NSF/ANSI 55 Class A (UV); NSF P231 microbiological purifiers |
| Cryptosporidium / Giardia Microbial | TTTreatment technique: 99% (2-log) Cryptosporidium and 99.9% (3-log) Giardia removal or inactivation (Surface Water Treatment Rules) | Prolonged diarrhea; life-threatening in immunocompromised people — Milwaukee 1993 outbreak sickened 400,000 | Surface water contaminated by animal or human waste; filtration failures at the plant | Any filter certified for cyst reduction (1 µm absolute), reverse osmosis, UV | NSF/ANSI 53 or 58 (cyst reduction); NSF/ANSI 55 Class A |
| Turbidity Microbial | TTTreatment technique: at or below 0.3 NTU in 95% of monthly samples, never above 1 NTU (conventional/direct filtration) | Not harmful itself, but cloudiness shelters microbes and interferes with disinfection | Soil runoff, algae, filter breakthrough, main breaks | Sediment pre-filter, any certified particulate filter | NSF/ANSI 42 (particulate Class I); NSF/ANSI 53 (turbidity claim) |
| Manganese Aesthetic | SMCL0.05 mg/L secondary (staining); EPA lifetime health advisory 0.3 mg/L | Neurological effects at high doses, infants most sensitive; black staining and metallic taste at far lower levels | Natural groundwater minerals, iron/steel and mining discharge | Oxidizing/greensand media, softener, reverse osmosis | NSF/ANSI 42 (iron/manganese aesthetic claims); NSF/ANSI 58 |
| Iron Aesthetic | SMCL0.3 mg/L secondary (taste, rust staining) | Not a health concern at tap levels; rust color, metallic taste, laundry staining | Natural groundwater, corroding iron mains | Oxidizing media, softener, sediment + carbon; RO for dissolved iron | NSF/ANSI 42 |
| Atrazine and other pesticides Emerging | MCLAtrazine 3 ppb; alachlor 2 ppb; glyphosate 700 ppb; many pesticides have no federal limit | Endocrine disruption, cardiovascular and reproductive effects (atrazine); varies by compound | Corn-belt herbicide runoff peaking in spring; lawn and golf-course applications | Activated-carbon block, reverse osmosis | NSF/ANSI 53 (atrazine / VOC claims); NSF/ANSI 58 |
| Pharmaceuticals and personal-care compounds Emerging | noneNone — unregulated; some monitored under UCMR | Trace levels (parts per trillion) far below therapeutic doses; long-term mixture effects not established | Excreted medications, flushed drugs, wastewater effluent upstream of intakes | Activated carbon with an emerging-compound claim, reverse osmosis | NSF/ANSI 401 (15 incidental compounds incl. ibuprofen, BPA, DEET); NSF/ANSI 58 |
| Microplastics Emerging | noneNone — unregulated federally; no health-based value yet | Under study; concerns are particle inflammation and chemicals adsorbed to plastics | Tire wear, synthetic textiles, plastic breakdown; also shed by bottled water | Sub-micron carbon block, reverse osmosis, ceramic | NSF/ANSI 401 (microplastics claim) or NSF/ANSI 42 Class I particulate |
Values are EPA's National Primary and Secondary Drinking Water Regulations as of 2026; where a rule is under revision the row says so. Contaminants EPA regulates but which rarely reach a US tap (e.g. benzene, mercury, asbestos, cyanide) are omitted — see the full EPA table.
Counts below are computed at build time from EPA records for the 756 EPA public water systems behind our graded cities, 2012–2026, sources as noted per table. They describe the systems we cover (mostly larger utilities), not a random sample of all 50,000 US community systems.
| Systems with UCMR5 PFAS results | 749 | 99.1% |
| Any PFAS detected (≥ reporting limit) | 345 | 46.1% |
| PFOA or PFOS at or above the 4 ppt MCL (any single sample) | 216 | 28.8% |
| PFHxS, PFNA or HFPO-DA at or above 10 ppt | 29 | 3.9% |
Single-sample maxima; the MCL is enforced on a running annual average, so “at or above 4 ppt” is broader than a violation. PFAS by city →
| Systems with a 2020-or-later 90th-percentile result | 742 | 98.1% |
| ≥ 5 ppb | 85 | 11.5% |
| ≥ 10 ppb (2027 action level) | 14 | 1.9% |
| ≥ 15 ppb (current action level) | 3 | 0.4% |
System-wide 90th percentiles understate individual homes with lead service lines. Lead by city → · Lead service line lookup →
| Analyte | Federal limit | Systems with samples | ≥1 sample over the limit | Share | Any sample or mean above 80% of limit |
|---|---|---|---|---|---|
| TTHM | 80 µg/L | 688 | 286 | 41.6% | 407 |
| HAA5 | 60 µg/L | 684 | 165 | 24.1% | 254 |
| Arsenic | 10 µg/L | 516 | 32 | 6.2% | 55 |
| Nitrate | 10 mg/L | 518 | 23 | 4.4% | 52 |
| Fluoride | 4 mg/L | 513 | 2 | 0.4% | 6 |
| Chromium (total) | 100 µg/L | 506 | 1 | 0.2% | 2 |
| Combined radium | 5 pCi/L | 251 | 13 | 5.2% | 21 |
| Uranium | 30 µg/L | 248 | 6 | 2.4% | 9 |
| Gross alpha | 15 pCi/L | 232 | 8 | 3.4% | 9 |
| Copper (individual tap samples vs 1.3 mg/L AL) | 1300 µg/L | 532 | 45 | 8.5% | 68 |
SYR4 rows count individual finished-water samples, so a TTHM or HAA5 exceedance here is one sample over the MCL — compliance is judged on a locational running annual average, and most of these systems were never in violation. Arsenic (32 systems) and nitrate (23) exceedances are almost entirely groundwater systems.
70 of 756 systems (9.3%) had at least one health-based violation (MCL, MRDL or treatment technique); 158 violations in total. Monitoring and reporting violations are excluded.
| Rule family | Systems | Violations |
|---|---|---|
| Disinfection byproducts (TTHM / HAA5 / Stage 1–2 DBPR) | 23 | 62 |
| Lead and Copper Rule (treatment technique) | 19 | 21 |
| Surface water treatment rules (filtration / turbidity / Crypto) | 15 | 39 |
| Coliform / E. coli (Total Coliform Rule) | 11 | 11 |
| Groundwater Rule (fecal indicators / disinfection) | 7 | 12 |
| Nitrate / nitrite MCL | 5 | 5 |
| Disinfectant residual (MRDL) | 2 | 7 |
| Other chemical MCL / TT | 1 | 1 |
Every violation is listed with dates and status on the utility's city page. Cities with the most violations →
Match the filter to the contaminant, then check the cartridge's NSF/ANSI listing for that exact claim. Our per-contaminant guides compare pitcher, faucet, under-sink and reverse-osmosis options with verified products and replacement costs.
Cities with the cleanest EPA records
Cities with the most violations and highest results
Surface water vs groundwater systems
State-by-state water quality
Side-by-side EPA data
How to decode a Consumer Confidence Report