For each contaminant: what the agencies say it does, who is most at risk, how strong the evidence is, what is uncertain, the legal limit versus the health goal, where it usually comes from, and which treatment and certification reduce it. 20 contaminants. Quoted from EPA, ATSDR, WHO, CDC and IARC pages; nothing here is medical advice.
Our ranking by likelihood × severity for a typical US household: lead, Legionella, fecal pathogens (mainly private wells), PFOA/PFOS, nitrate, arsenic, disinfection byproducts, then the rest. This is our judgement from the sources, not a published score.
The legal limit and the health goal are often different numbers. Where they differ, both are shown.
Lead
Health effects (as the agencies state them): EPA: 'Infants and children: Delays in physical or mental development'; 'Adults: Kidney problems; high blood pressure'. Neurodevelopmental (IQ, attention) effects in children; cardiovascular and renal effects in adults.
Exposure level: EPA MCLG zero (no known safe level). CDC uses a blood lead reference value rather than a safe threshold. WHO provisional guideline 10 ug/L. Water lead is episodic (particulate release), so single samples can miss spikes.
Most at risk: Fetuses/pregnant women, formula-fed infants, children <6 yr
Strength of evidence: strong. Key uncertainties: Contribution of water vs paint/dust/soil to blood lead in a given home; dose-response at very low blood lead; sampling protocols under-detect particulate lead.
Legal limit (EPA MCL): Treatment technique; action level 0.015 mg/L, becoming 0.010 mg/L from 1 Nov 2027 (LCRI, per EPA table footnote; LCRI under AWWA litigation as of late 2025) · health goal (MCLG): zero · IARC: Inorganic lead compounds: Group 2A (IARC list; not re-fetched this scan)
Where it usually comes from: Home plumbing (lead service lines, leaded solder, brass fixtures); corrosivity set by utility treatment
What reduces it, and which certification proves it: NSF/ANSI 53 (lead reduction claim); also NSF/ANSI 58 (RO) and 62 (distillation) with lead claim. Utility: corrosion control (orthophosphate), full lead service line replacement.
Health effects (as the agencies state them): EPA: 'Short term exposure: Gastrointestinal distress'; 'Long term exposure: Liver or kidney damage'. People with Wilson's disease should consult a doctor above the action level.
Exposure level: EPA action level 1.3 mg/L; WHO guideline 2 mg/L. MMWR 1993 case: first-draw kitchen tap 7.8 mg/L fell to 0.2 mg/L after flushing.
Most at risk: Infants, people with Wilson's disease or other copper-metabolism disorders
Strength of evidence: moderate. Key uncertainties: Acute GI threshold well characterized; chronic liver effects in genetically non-susceptible people less clear.
Legal limit (EPA MCL): Treatment technique; action level 1.3 mg/L · health goal (MCLG): 1.3 mg/L · IARC: Not classified (none found in this scan)
Where it usually comes from: Home plumbing (copper pipe + corrosive/low-pH or stagnant water)
What reduces it, and which certification proves it: NSF/ANSI 58 and 62 commonly carry copper claims (verify product listing); flush stagnant lines; utility corrosion control.
E. coli / fecal bacteria (incl. E. coli O157:H7, Campylobacter)
Health effects (as the agencies state them): Acute gastroenteritis; E. coli O157:H7 can cause hemolytic uremic syndrome (kidney failure) and death (Walkerton 2000: 7 deaths). Total coliforms per EPA are 'Not a health threat in itself' but indicate possible pathogens.
Exposure level: Any detection of E. coli indicates fecal contamination; infectious dose for O157:H7 is low.
Most at risk: Young children, elderly, immunocompromised, private-well households
Strength of evidence: strong. Key uncertainties: Indicator bacteria do not reliably predict viruses or protozoa; private wells are untested unless owner tests.
Legal limit (EPA MCL): Total coliform: no more than 5.0% positive samples/month (legacy TCR row in EPA table); E. coli MCL under Revised Total Coliform Rule · health goal (MCLG): zero · IARC: Not applicable
Where it usually comes from: Private wells (septic, manure, flooding); utility treatment/distribution failures
What reduces it, and which certification proves it: NSF/ANSI 55 Class A (UV) for disinfection of clear water; NSF/ANSI 62 distillation. For wells: shock chlorination, wellhead protection, annual testing.
Health effects (as the agencies state them): EPA: 'Gastrointestinal illness (such as diarrhea, vomiting, and cramps)'. Prolonged/fatal in severely immunocompromised (Milwaukee 1993 deaths mostly elderly or immunocompromised).
Exposure level: Oocysts resist chlorine; control relies on filtration, UV or ozone. EPA TT requires at least 99% (2-log) removal for filtered systems, with additional LT2 requirements by source risk.
Most at risk: People with advanced HIV/AIDS, transplant/chemotherapy patients, young children, elderly
Strength of evidence: strong. Key uncertainties: Low-level endemic waterborne contribution in US is poorly quantified; oocyst viability/infectivity measurement.
Legal limit (EPA MCL): Treatment technique (Surface Water Treatment Rules, IESWTR, LT2ESWTR) · health goal (MCLG): zero · IARC: Not applicable
Where it usually comes from: Utility surface water (treatment failure); recreational water; some wells under surface influence
What reduces it, and which certification proves it: NSF/ANSI 53 cyst reduction (≤1 um absolute), NSF/ANSI 58 cyst claim, NSF/ANSI 55 Class A UV, NSF/ANSI 62; boiling.
Health effects (as the agencies state them): EPA: 'Gastrointestinal illness (such as diarrhea, vomiting, and cramps)'. Can become chronic with malabsorption.
Exposure level: EPA TT: 99.9% (3-log) removal/inactivation for surface water systems.
Most at risk: Children, immunocompromised, campers drinking untreated surface water, private surface-water users
Strength of evidence: strong. Key uncertainties: Endemic burden attributable to tap water vs person-to-person spread.
Legal limit (EPA MCL): Treatment technique · health goal (MCLG): zero · IARC: Not applicable
Where it usually comes from: Surface water without adequate filtration/disinfection; shallow wells
What reduces it, and which certification proves it: NSF/ANSI 53 cyst reduction, NSF/ANSI 58, NSF/ANSI 55 Class A, NSF/ANSI 62; boiling.
Health effects (as the agencies state them): EPA: 'Gastrointestinal illness (such as diarrhea, vomiting, and cramps)'. Wisconsin 2007 well outbreak: 229 ill, 6 hospitalized.
Exposure level: EPA TT: 99.99% (4-log) removal/inactivation of viruses (Surface Water/Ground Water Rules).
Most at risk: Elderly, young children, immunocompromised; private well users in karst/fractured rock
Strength of evidence: strong. Key uncertainties: Viruses pass through soil faster than assumed (6 days septic-to-well in Wisconsin case); coliform tests do not detect viruses.
Legal limit (EPA MCL): Treatment technique (Ground Water Rule; SWTR) · health goal (MCLG): zero · IARC: Not applicable
Where it usually comes from: Private and small non-community wells near septic systems; utility treatment failures
What reduces it, and which certification proves it: NSF/ANSI 55 Class A UV (clear water), NSF/ANSI 62 distillation; carbon/cyst filters (NSF 53) are NOT virus barriers. Well disinfection (chlorination).
Health effects (as the agencies state them): EPA: 'Legionnaire's Disease, a type of pneumonia'. Transmitted by inhaling aerosols (showers, cooling towers), not by drinking. CDC 2015-2020: Legionella caused 184/214 (86%) drinking-water outbreaks and 86/88 (98%) deaths.
Exposure level: Grows in warm, stagnant premise plumbing with low disinfectant residual; no household exposure threshold defined.
Most at risk: Adults >=50, smokers, chronic lung disease, immunocompromised; residents of hospitals/long-term care (53% of outbreaks were in health care settings)
Strength of evidence: strong. Key uncertainties: Underdiagnosis (testing rarely ordered); household-level risk from home water heaters poorly quantified.
Legal limit (EPA MCL): Treatment technique · health goal (MCLG): zero · IARC: Not applicable
Where it usually comes from: Building/home plumbing (water heaters, dead legs), cooling towers; utility residual loss contributes (Flint)
What reduces it, and which certification proves it: No household NSF claim is a full solution; building water management programs (ASHRAE 188), hot water temperature control, residual maintenance; point-of-use 0.2 um filters used in healthcare.
Health effects (as the agencies state them): EPA: infants below six months who drink water above the MCL 'could become seriously ill' (methemoglobinemia, 'blue baby'). Epidemiology also studies cancer, thyroid and birth-defect associations.
Exposure level: MCL 10 mg/L nitrate-N; 1 mg/L nitrite-N. WHO: 50 mg/L as nitrate, 3 mg/L as nitrite. Documented infant cases at 22.9, 27.4 and 58 mg/L nitrate-N (well water).
Most at risk: Formula-fed infants <6 months, pregnant women; rural well households
Strength of evidence: strong. Key uncertainties: Methemoglobinemia: role of co-factors (gastroenteritis, copper, bacterial contamination) debated; chronic effects (colorectal cancer, birth defects) below the MCL are suggestive but not established.
Legal limit (EPA MCL): 10 mg/L (nitrate as N); 1 mg/L (nitrite as N) · health goal (MCLG): 10 mg/L (nitrate-N); 1 mg/L (nitrite-N) · IARC: Ingested nitrate or nitrite under conditions resulting in endogenous nitrosation: Group 2A (IARC Vol. 94; not re-fetched)
Where it usually comes from: Private wells in agricultural areas (fertilizer, manure, septic); some utilities
What reduces it, and which certification proves it: NSF/ANSI 58 (RO with nitrate/nitrite claim), NSF/ANSI 62; anion exchange. Carbon filters do not remove nitrate. Boiling concentrates nitrate.
Health effects (as the agencies state them): EPA: 'Skin damage or problems with circulatory systems, and may have increased risk of getting cancer'. EPA IRIS final (13 Jan 2025): cancer slope factor based on bladder and lung cancer; RfD based on diabetes and ischemic heart disease.
Exposure level: MCL 0.010 mg/L; WHO provisional 10 ug/L. IRIS 2025 raised oral slope factor from 1.5 to 32 per mg/kg-day (~21x) and lowered RfD from 0.0003 to 0.00006 mg/kg-day, implying non-trivial lifetime cancer risk at the current MCL.
Most at risk: Long-term well users in As-rich geology (e.g. parts of New England, Southwest, Upper Midwest), fetuses/children
Strength of evidence: strong. Key uncertainties: Shape of dose-response below ~50 ug/L; whether MCL will be revisited after IRIS 2025.
Legal limit (EPA MCL): 0.010 mg/L · health goal (MCLG): zero · IARC: Group 1 (IARC list; not re-fetched)
Where it usually comes from: Private wells (natural geology); small utilities
What reduces it, and which certification proves it: NSF/ANSI 58 (arsenic V claim; arsenic III needs pre-oxidation), NSF/ANSI 53 adsorptive media with arsenic claim, NSF/ANSI 62.
Health effects (as the agencies state them): EPA: 'Cardiovascular, immune and liver effects; increased incidence of certain types of cancers including kidney and testicular' (PFOA) / '...including liver' (PFOS). C8 Science Panel (2012) probable links for PFOA: kidney cancer, testicular cancer, pregnancy-induced hypertension, thyroid disease, high cholesterol, ulcerative colitis.
Exposure level: MCL 4.0 ng/L (ppt) each, MCLG zero. Compliance date 2029; EPA proposal of 20 May 2026 would allow exemptions to 26 Apr 2031 (interim controls for systems >=12 ppt). WHO: no final guideline; 2022 draft withdrawn into broader PFAS review. UCMR5 (lab summary of EPA data): ~8% of systems above 4 ppt PFOA, 8.9% above PFOS.
Most at risk: Fetuses, infants (immune/vaccine response, birth weight), pregnant women
Strength of evidence: moderate. Key uncertainties: Steenland 2020 review: cholesterol association consistent; thyroid evidence 'has gotten weaker'; kidney cancer suggestive but inconsistent. Causality and low-dose shape debated; WHO and EPA reached very different numeric values.
Legal limit (EPA MCL): 0.0000040 mg/L (4.0 ppt) each — final 2024 rule retained in 2026 proposal · health goal (MCLG): zero · IARC: PFOA Group 1; PFOS Group 2B (IARC Vol. 135, 2023)
Where it usually comes from: Utility source water near manufacturing, airports/military (firefighting foam), landfills; private wells near sources
What reduces it, and which certification proves it: NSF/ANSI 53 and 58 (2022 editions: PFOA/PFOS claims revised and 'Total PFAS' claim covering 7 PFAS). Utility GAC, ion exchange, RO.
Health effects (as the agencies state them): EPA: GenX 'Immune, liver and kidney effects; potential concern for cancer'; PFHxS 'Immune, thyroid, and liver effects'; PFNA 'Elevated cholesterol levels and immune and liver effects'.
Exposure level: 2024 rule: 10 ng/L each for PFHxS, PFNA, GenX; Hazard Index of 1 for mixtures incl. PFBS (HBWC 2,000 ng/L). EPA proposed (20 May 2026) to RESCIND these four MCLs; comment closed 20 Jul 2026; still proposals.
Most at risk: Fetuses, infants
Strength of evidence: weak. Key uncertainties: Human epidemiology much thinner than for PFOA/PFOS; mostly animal data; regulatory status in flux.
Legal limit (EPA MCL): 0.00001 mg/L each (PFHxS, PFNA, GenX); HI = 1 — proposed for rescission · health goal (MCLG): Same as MCL (PFBS: none individual) · IARC: Not classified (none found in this scan)
Where it usually comes from: Utility source water near fluorochemical plants (e.g. GenX) and foam use
What reduces it, and which certification proves it: NSF/ANSI 53/58 include PFHxS, PFNA, PFBS claims (2022); GenX claim not confirmed in this scan.
Health effects (as the agencies state them): EPA: TTHM 'Liver, kidney or central nervous system problems; increased risk of cancer'; HAA5 'Increased risk of cancer'. Epidemiology associates long-term chlorinated-water/THM exposure with bladder cancer; reproductive outcomes studied.
Exposure level: MCLs are locational running annual averages. WHO sets individual values (e.g. chloroform 0.3 mg/L) rather than TTHM sum.
Most at risk: Pregnant women (studied for reproductive outcomes), long-term residents of high-DBP systems
Strength of evidence: moderate. Key uncertainties: Which specific DBPs drive risk (many unregulated, e.g. iodinated, nitrosamines); exposure via showering/inhalation; risk trade-off: disinfection must not be reduced to cut DBPs.
Legal limit (EPA MCL): TTHM 0.080 mg/L; HAA5 0.060 mg/L · health goal (MCLG): n/a for group (individual MCLGs for some compounds) · IARC: Chloroform Group 2B; chlorinated drinking water Group 3 (IARC list; not re-fetched)
Where it usually comes from: Utility (chlorination of water with organic matter); highest at end of distribution
What reduces it, and which certification proves it: NSF/ANSI 53 (VOC/TTHM reduction claim), NSF/ANSI 58 with VOC claim; utility precursor removal.
Health effects (as the agencies state them): EPA: 'Bone disease (pain and tenderness of the bones); Children may get mottled teeth'. NTP 2024: 'moderate confidence' that exposures above 1.5 mg/L 'are consistently associated with lower IQ in children'. Benefit side: caries prevention is the basis of community fluoridation (US PHS recommends 0.7 mg/L).
Exposure level: MCL/MCLG 4.0 mg/L; secondary standard 2.0 mg/L (dental fluorosis). WHO guideline 1.5 mg/L. NTP: 'More studies are needed' for lower exposures; NTP did not conclude on 0.7 mg/L. Acute poisoning at 150 mg/L (Hooper Bay 1992).
Most at risk: Fetuses and infants (formula reconstituted with fluoridated water), children <8 (fluorosis), people with kidney disease
Strength of evidence: contested. Key uncertainties: Whether neurodevelopmental effects occur at 0.7 mg/L; size of modern caries benefit; NASEM/NRC 2006 advised EPA to lower the 4 mg/L MCLG; EPA announced review 7 Apr 2025; Utah and Florida banned fluoridation in 2025; dental and pediatric bodies continue to support fluoridation.
Legal limit (EPA MCL): 4.0 mg/L · health goal (MCLG): 4.0 mg/L · IARC: Group 3 (fluorides, inorganic, used in drinking-water; IARC Suppl. 7)
Where it usually comes from: Utility (added at ~0.7 mg/L) and natural geology (some wells >2-4 mg/L)
What reduces it, and which certification proves it: NSF/ANSI 58 (RO) and NSF/ANSI 62 (distillation) with fluoride claim; activated alumina. Standard carbon pitchers generally do not remove fluoride.
Health effects (as the agencies state them): Total chromium EPA effect: 'Allergic dermatitis'. Cr(VI) ingested: carcinogenic in rodents (GI tumors); EPA final IRIS Cr(VI) review released Aug 2024. California: at 10 ppb about 1-in-2,000 lifetime cancer chance (2 L/day, 70 yr).
Exposure level: No federal Cr(VI) MCL; total Cr 0.1 mg/L. California MCL 10 ug/L (adopted Apr 2024; 2-4 yr compliance by system size). WHO provisional 0.05 mg/L total chromium.
Most at risk: Long-term consumers of high-Cr(VI) groundwater
Strength of evidence: contested. Key uncertainties: Human oral cancer evidence limited (IARC Group 1 rests largely on inhalation/occupational data); mode of action (mutagenic vs threshold) disputed by industry vs EPA.
Legal limit (EPA MCL): 0.1 mg/L total chromium; no Cr(VI)-specific MCL · health goal (MCLG): 0.1 mg/L (total chromium) · IARC: Chromium (VI) compounds: Group 1 (IARC list; not re-fetched)
Where it usually comes from: Utility and private groundwater (natural geology and industrial releases)
What reduces it, and which certification proves it: NSF/ANSI 58 (RO) with Cr(VI) claim (verify listing); utility reduction-coagulation-filtration or ion exchange.
Health effects (as the agencies state them): EPA screening basis: effects in patients on therapeutic lithium — kidney (urine-concentrating defects; severe kidney disease with long-term high dose) and endocrine (thyroid, parathyroid).
Exposure level: No MCL or health advisory. EPA Health Reference Level 10 ug/L (screening only), from p-RfD 2 ug/kg-day. UCMR5 (lab summary): ~26.6% of systems exceeded the HRL. EPA: risk between 10 ug/L and therapeutic-equivalent levels 'not yet well characterized'.
Most at risk: Pregnant women, people with kidney or thyroid disease (inferred from therapeutic data)
Strength of evidence: weak. Key uncertainties: Extrapolation from therapeutic doses; no drinking-water-level outcome data; ecological studies also report possible beneficial associations (not established).
Legal limit (EPA MCL): None · health goal (MCLG): None · IARC: Not classified (none found in this scan)
Where it usually comes from: Natural groundwater (mainly utility and well groundwater in arid West)
What reduces it, and which certification proves it: No specific NSF claim found; RO generally reduces dissolved salts (verify).
Most at risk: Long-term users of deep sandstone aquifers (regional)
Strength of evidence: strong. Key uncertainties: Risk at levels near MCL extrapolated from high-dose cohorts.
Legal limit (EPA MCL): 5 pCi/L (Ra-226+228); 15 pCi/L gross alpha · health goal (MCLG): zero · IARC: Radium-224/226/228: Group 1 (IARC list; not re-fetched)
Where it usually comes from: Natural geology; utility groundwater and private wells
What reduces it, and which certification proves it: Cation-exchange softening or NSF/ANSI 58 RO with radium claim (verify listing); utility softening/IX.
Health effects (as the agencies state them): EPA: 'Increased risk of cancer, kidney toxicity'. Chemical kidney toxicity is the main concern at drinking-water levels.
Exposure level: MCL 30 ug/L; WHO provisional 30 ug/L. Navajo Nation unregulated sources: 12.8% exceeded the uranium standard (arsenic 15.1%).
Most at risk: Rural well users in mining/uranium-bearing regions (e.g. Navajo Nation), people with kidney disease
Strength of evidence: moderate. Key uncertainties: Long-term low-dose renal outcomes; cancer risk at MCL is radiologically low.
Legal limit (EPA MCL): 30 ug/L · health goal (MCLG): zero · IARC: Internalized alpha emitters: Group 1 (applies via radioactivity; not re-fetched)
Where it usually comes from: Private/unregulated wells (natural, mining legacy)
What reduces it, and which certification proves it: NSF/ANSI 58 RO or anion exchange (verify specific uranium claim); NSF/ANSI 62.
Health effects (as the agencies state them): Some epidemiology links elevated Mn in drinking water with 'poorer memory, attention and motor function' and hyperactivity in children. WHO protects against neurological effects in bottle-fed infants.
Exposure level: No MCL. EPA secondary (aesthetic) 0.05 mg/L; EPA 2004 lifetime health advisory 0.3 mg/L, also used as short-term guide for infants <6 months. WHO provisional guideline 0.08 mg/L (high uncertainty, UF 1000).
Most at risk: Bottle-fed infants, young children
Strength of evidence: weak. Key uncertainties: WHO calls value provisional due to high uncertainty; human dose-response mostly cross-sectional; EPA HA dates from 2004 and is lower-protective than WHO.
Legal limit (EPA MCL): None (SMCL 0.05 mg/L, non-enforceable) · health goal (MCLG): None · IARC: Not classified (none found in this scan)
Where it usually comes from: Private wells and small groundwater utilities (natural geology)
What reduces it, and which certification proves it: Oxidation-filtration, greensand, softeners for low levels; RO. Check listing for manganese claim.
Health effects (as the agencies state them): ATSDR: ingestion/skin contact 'can damage the liver and kidneys'; animal studies show liver/nasal tumors. HHS: 'reasonably anticipated to be a human carcinogen'. EPA: 'likely to be carcinogenic to humans' (2013).
Exposure level: No federal MCL. EPA reference: 0.35 ug/L at 1-in-a-million (10^-6) lifetime cancer risk; 35 ug/L at 10^-4 (ITRC citing EPA). SOURCES DIFFER: an older EPA 2016 fact sheet cites 3.0 ug/L at 10^-6. New York set a 1 ug/L MCL (rulemaking from 2019).
Most at risk: Long-term consumers of contaminated groundwater
Strength of evidence: moderate. Key uncertainties: Human cancer data limited; animal-to-human relevance debated (ATSDR).
Legal limit (EPA MCL): None · health goal (MCLG): None · IARC: Group 2B (IARC list; not re-fetched)
Where it usually comes from: Utility groundwater/surface water near industrial solvent sites and wastewater discharges
What reduces it, and which certification proves it: Not removed well by GAC or standard filters; utility advanced oxidation (UV/H2O2). No common household NSF claim found.
Health effects (as the agencies state them): WHO 2022: 'the limited data provide little evidence that MNPs have adverse effects in humans'; data 'of only very limited use for assessing the risk'.
Exposure level: No MCL or health-based value anywhere found. NSF/ANSI 401 microplastics claim: >=85% reduction of 0.5-1 um particles.
Most at risk: Unknown
Strength of evidence: weak. Key uncertainties: No standardized measurement; nanoplastics largely unmeasured; toxicology of particles and additives unresolved.
Legal limit (EPA MCL): None · health goal (MCLG): None · IARC: Not evaluated
Where it usually comes from: Ubiquitous (tap and bottled water, food, air)
What reduces it, and which certification proves it: NSF/ANSI 401 (microplastics claim).