Drinking water in the United States is among the safest in the world, but that does not mean it is contaminant-free. The U.S. Environmental Protection Agency (EPA) currently regulates more than 90 contaminants under the National Primary Drinking Water Regulations (NPDWRs), including microorganisms, disinfection byproducts, inorganic and organic chemicals, and radionuclides. In April 2024, the EPA added the first-ever federal limits for PFAS in drinking water, expanding protections to address emerging contaminants that have gone unregulated for decades.
This guide provides a comprehensive reference to the contaminants most likely to appear in U.S. tap water, organized by category. For each contaminant, we list the EPA Maximum Contaminant Level (MCL) or Treatment Technique (TT), common sources, documented health effects, and the filtration technologies with independent certifications for removal. All data is sourced from EPA regulations, NSF/ANSI certifications, and peer-reviewed toxicology research. No hands-on testing is claimed.
Table of Contents
1. Understanding the National Primary Drinking Water Regulations
The Safe Drinking Water Act (SDWA) of 1974 authorizes the EPA to set national health-based standards for drinking water contaminants. These standards are called the National Primary Drinking Water Regulations (NPDWRs). They set legally enforceable limits, known as Maximum Contaminant Levels (MCLs), for specific contaminants in public water systems. Where setting an MCL is not feasible, the EPA establishes a Treatment Technique (TT), which requires water systems to follow specific treatment processes.
As of 2025, the EPA regulates over 90 contaminants across six categories:
Microorganisms
Bacteria, viruses, and protozoa that can cause acute gastrointestinal illness. Regulated via Treatment Techniques requiring filtration and disinfection.
Disinfection Byproducts (DBPs)
Chemicals formed when disinfectants like chlorine react with organic matter. Includes trihalomethanes and haloacetic acids with established MCLs.
Inorganic Chemicals
Metals and minerals including lead, arsenic, nitrate, fluoride, and mercury. Most have numeric MCLs.
Organic Chemicals
Pesticides, herbicides, solvents, and volatile organic compounds (VOCs) from industrial and agricultural sources.
Radionuclides
Radioactive elements including radium-226/228, uranium, and gross alpha particles. Regulated in picoCuries per liter (pCi/L).
Emerging Contaminants
PFAS, pharmaceuticals, and microplastics. PFAS now has federal MCLs as of April 2024; others remain under study.
Key Terms:
- MCL (Maximum Contaminant Level): The highest level of a contaminant allowed in drinking water. MCLs are enforceable standards.
- MCLG (Maximum Contaminant Level Goal): The level below which there is no known health risk. MCLGs are non-enforceable health goals.
- Treatment Technique (TT): A required process or procedure when setting an MCL is not economically or technically feasible.
- Action Level: The concentration of a contaminant which, if exceeded, triggers treatment or other requirements. Used for lead and copper.
2. Six Major Categories of Drinking Water Contaminants
🦠 Microorganisms
Pathogenic microorganisms are the oldest and most immediate threat in drinking water. Before modern water treatment, cholera, typhoid, and dysentery killed thousands annually. Today, the EPA regulates these organisms through Treatment Techniques rather than numeric MCLs, requiring water systems to filter and disinfect surface water and groundwater at risk of contamination.
Regulated Microorganisms
| Contaminant | Type | Regulation | Health Effects | Removal Methods |
|---|---|---|---|---|
| Cryptosporidium | Protozoan | TT (99% removal) | Severe diarrhea, dehydration; dangerous for immunocompromised | NSF/ANSI 53-rated 1-micron filter, UV, boiling, reverse osmosis |
| Giardia lamblia | Protozoan | TT (99.9% removal) | Gastrointestinal illness, diarrhea, vomiting, cramps | Filtration (1 micron), UV, chlorination, boiling |
| Legionella | Bacterium | TT | Legionnaires' disease, a severe form of pneumonia | Chlorination, UV, maintaining hot water tanks above 140°F |
| Total Coliforms (incl. E. coli) | Bacterial indicator | 5.0% monthly limit | Indicates fecal contamination; pathogens cause GI illness | Chlorination, UV, ozone, proper well sealing |
| Enteric Viruses | Virus | TT (99.99% removal) | Gastroenteritis, hepatitis, meningitis | Chlorination, UV, reverse osmosis, distillation |
| Turbidity | Physical measure | TT (varies by tech) | Interferes with disinfection; indicates filtration failure | Sediment filtration, coagulation/flocculation |
Sources: EPA Surface Water Treatment Rules, Long Term 2 Enhanced Surface Water Treatment Rule. Treatment Technique (TT) requirements specify removal/inactivation percentages.
Key Facts About Microorganisms
- Cryptosporidium is resistant to standard chlorine disinfection. The 1993 Milwaukee outbreak infected over 400,000 people. Filtration and UV are the most reliable removal methods.
- Total coliforms are not harmful themselves but indicate that disease-causing organisms may be present. Any detection of fecal coliforms or E. coli requires immediate action by the water system.
- Groundwater was historically considered protected from pathogens, but the EPA's Ground Water Rule (2006) now requires disinfection of groundwater at risk of fecal contamination.
For homeowners: NSF/ANSI 53-certified filters rated for cyst reduction (tested against Cryptosporidium and Giardia) provide reliable point-of-use protection. UV purifiers add an extra layer of security against bacteria and viruses that may bypass filtration. Learn more in our Water Contaminants & Treatment Guide.
⚡ Disinfection Byproducts (DBPs)
Disinfection byproducts form when chemical disinfectants, primarily chlorine and chloramine, react with naturally occurring organic matter (decaying vegetation, algae) and bromide in source water. This is a classic public health tradeoff: disinfectants prevent waterborne disease, but the byproducts they create carry their own health risks, including cancer and reproductive effects.
The EPA has established MCLs for four major classes of DBPs under the Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules:
Regulated Disinfection Byproducts
| Contaminant / Group | EPA MCL | Health Effects | Sources / Formation |
|---|---|---|---|
| Total Trihalomethanes (TTHMs) Chloroform, bromoform, bromodichloromethane, dibromochloromethane |
80 ppb (0.080 mg/L) Running annual average |
Liver, kidney, and CNS problems; bladder cancer risk (EPA classified bromodichloromethane and bromoform as "likely carcinogenic") | Byproduct of chlorine + organic matter |
| Haloacetic Acids (HAA5) Mono-, di-, trichloroacetic acid; mono-, dibromoacetic acid |
60 ppb (0.060 mg/L) Running annual average |
Increased cancer risk; six HAAs classified by NTP as "reasonably anticipated human carcinogens" | Byproduct of chlorine/chloramine + organic matter |
| Bromate | 10 ppb (0.010 mg/L) | Increased cancer risk | Byproduct of ozone disinfection of bromide-containing water |
| Chlorite | 1,000 ppb (1.0 mg/L) | Anemia; nervous system effects in infants and young children | Byproduct of chlorine dioxide disinfection |
Sources: EPA Stage 1 and Stage 2 DBPRs; EWG Tap Water Database; National Toxicology Program (NTP) Report on Carcinogens. ppb = parts per billion. MCLs are running annual averages calculated from quarterly samples.
Important context: The EPA's DBP MCLs were set based on technical feasibility and treatment cost, not purely on health risk. The MCLG for bromodichloromethane and bromoform is zero, meaning no exposure level is considered without risk. A 2020 EWG peer-reviewed study estimated the lifetime cancer risk from DBPs at approximately 3 cases per 1,000 people exposed over a lifetime. Reducing DBP exposure through home filtration is a reasonable precautionary step.
Reducing DBP Exposure
Because DBPs form in the distribution system (not just at the treatment plant), point-of-use filtration is often the most practical reduction strategy for consumers:
- Activated carbon filters (GAC or carbon block) certified to NSF/ANSI 42 or 53 for chlorine/THM reduction are effective at lowering TTHM and HAA5 levels.
- Reverse osmosis systems remove virtually all DBPs.
- Letting tap water sit exposed to air (refrigerator pitcher) allows volatile THMs to off-gas, though this is less reliable than filtration.
⚖ Inorganic Chemicals
Inorganic contaminants, primarily metals and mineral salts, enter water supplies through natural geologic deposits, mining and industrial discharge, agricultural runoff, and plumbing corrosion. They are regulated under the EPA's Inorganic Chemicals rule with specific numeric MCLs for each contaminant.
Major Regulated Inorganic Contaminants
| Contaminant | EPA MCL | Common Sources | Health Effects | Best Filtration |
|---|---|---|---|---|
| Lead | TT (Action Level: 15 ppb) 90th percentile |
Corroded lead pipes, solder, brass fixtures; pre-1986 plumbing | Developmental delays in children; kidney damage; hypertension in adults. No safe level identified. | NSF/ANSI 53 (carbon block), NSF/ANSI 62 (distillation), reverse osmosis |
| Copper | TT (Action Level: 1.3 ppm) | Corrosion of household plumbing; natural deposits | Gastrointestinal distress; liver/kidney damage at very high levels; Wilson's disease risk | Reverse osmosis, NSF/ANSI 62 distillation, ion exchange |
| Arsenic | 10 ppb (0.010 mg/L) | Natural deposits; runoff from orchards, glass, electronics production; coal ash | Skin damage, circulatory problems, increased cancer risk (lung, bladder, skin). MCLG = 0. | Reverse osmosis (NSF/ANSI 58), activated alumina, distillation |
| Nitrate (as N) | 10 mg/L | Fertilizer runoff; septic tank leaching; sewage; erosion of natural deposits | Blue baby syndrome (methemoglobinemia) in infants under 6 months; possible cancer risk | Reverse osmosis, ion exchange, distillation. Not removed by standard carbon filters. |
| Nitrite (as N) | 1 mg/L | Same as nitrate; more common in groundwater under reducing conditions | Same as nitrate; more potent cause of methemoglobinemia | Reverse osmosis, ion exchange, distillation |
| Fluoride | 4.0 mg/L | Water additive for dental health; natural deposits; fertilizer/phosphate mining | Dental fluorosis above 2 mg/L; skeletal fluorosis at prolonged high exposure. MCLG = 4.0 mg/L. | Activated alumina, reverse osmosis, bone char carbon, distillation. Standard carbon is ineffective. |
| Mercury (inorganic) | 2 ppb (0.002 mg/L) | Erosion of natural deposits; refinery and factory discharge; agricultural runoff | Kidney damage; neurological effects | Reverse osmosis, activated carbon (some formulations), distillation |
| Cadmium | 5 ppb (0.005 mg/L) | Corrosion of galvanized pipes; natural deposits; batteries and paint manufacturing | Kidney damage; bone fragility (Itai-Itai disease at extreme exposure) | Reverse osmosis, activated carbon (specific), distillation |
| Chromium (total) | 100 ppb (0.1 mg/L) | Natural deposits; mining, electroplating, pigment, chemical plant discharge | Allergic dermatitis; liver, kidney, circulatory disorders. Hexavalent chromium (Cr-6) is carcinogenic; no separate federal MCL. | Reverse osmosis, activated carbon (some), strong base anion exchange |
| Barium | 2 ppm (2 mg/L) | Natural deposits; drilling waste; pigment and rubber manufacturing | Increased blood pressure; neurological and cardiovascular effects | Reverse osmosis, cation exchange, distillation |
| Selenium | 50 ppb (0.05 mg/L) | Natural deposits; mining, petroleum refinery discharge | Hair/fingernail loss; numbness in fingers/toes; circulatory problems | Reverse osmosis, activated alumina, distillation |
| Antimony | 6 ppb (0.006 mg/L) | Fire retardant production; petroleum refining; ceramics; solder | Increases in blood cholesterol; decreases in blood sugar | Reverse osmosis, activated alumina, distillation |
Sources: EPA National Primary Drinking Water Regulations, 40 CFR Part 141. Lead and copper are regulated by Treatment Technique requiring corrosion control. Action Level = concentration exceeded in more than 10% of samples triggers additional treatment.
Lead is the most dangerous regulated contaminant for most U.S. households. Unlike contaminants with MCLs measured at the treatment plant, lead enters water inside your home through corroded service lines and plumbing. The EPA Lead and Copper Rule (LCR) uses a 90th percentile action level of 15 ppb, but the MCLG is zero. No level of lead exposure is considered safe for children. If you live in a pre-1986 home, have your water tested. Read our Lead Water Filter Guide for certified removal options.
Other Regulated Inorganics
The EPA also regulates: Beryllium (MCL: 4 ppb), Thallium (MCL: 2 ppb), Asbestos (MCL: 7 million fibers/L), Cyanide (MCL: 0.2 mg/L), and Nickel (no current federal MCL; remanded to EPA for further regulation).
☢ Organic Chemicals
Organic chemicals in drinking water include pesticides, herbicides, industrial solvents, petroleum products, and volatile organic compounds (VOCs). They enter water supplies through agricultural runoff, industrial discharge, leaking storage tanks, and improper disposal. The EPA regulates approximately 60 organic contaminants with individual MCLs.
Major Classes of Organic Contaminants
| Contaminant / Class | EPA MCL | Common Sources | Health Effects | Best Filtration |
|---|---|---|---|---|
| Atrazine (herbicide) | 3 ppb | Agricultural runoff from corn, sorghum, sugarcane | Cardiovascular and reproductive effects; possible endocrine disruptor | Activated carbon (NSF/ANSI 53), reverse osmosis |
| 2,4-D (herbicide) | 70 ppb | Agricultural and residential herbicide runoff | Liver, kidney damage; eye irritation | Activated carbon, reverse osmosis |
| Glyphosate (herbicide) | 700 ppb | Agricultural herbicide (Roundup) runoff | Kidney damage; reproductive effects (IARC: "probably carcinogenic") | Reverse osmosis; carbon has limited effectiveness |
| Benzene (VOC) | 5 ppb | Petroleum refining, coal mining, gas storage tanks | Anemia; increased leukemia risk; immune system depression | Activated carbon (NSF/ANSI 53), aeration, reverse osmosis |
| TCE / PCE Trichloroethylene / Tetrachloroethylene |
TCE: 5 ppb PCE: 5 ppb |
Dry cleaning, metal degreasing, textile processing | Liver, kidney, CNS effects; carcinogenic (IARC Group 1 for TCE) | Activated carbon (NSF/ANSI 53), reverse osmosis, aeration |
| Vinyl Chloride | 2 ppb | PVC pipe manufacturing; plastic degradation | Liver damage; angiosarcoma (liver cancer); carcinogenic | Activated carbon, reverse osmosis, aeration |
| MTBE Methyl tert-butyl ether |
None (monitoring) | Gasoline additive; leaking underground storage tanks | Potential carcinogen; affects taste/odor at low ppb levels | Activated carbon, reverse osmosis, aeration |
| 1,4-Dioxane | None (HA: 0.35 ppb) | Industrial solvent; byproduct of cosmetics/detergents manufacturing | Liver, kidney damage; likely carcinogen (EPA) | Advanced oxidation, reverse osmosis (limited). Carbon alone is ineffective. |
Sources: EPA National Primary Drinking Water Regulations; IARC Monographs; EPA Integrated Risk Information System (IRIS). HA = Health Advisory (non-enforceable). VOCs are tested under the EPA's VOC Rule (1987).
VOCs and carbon filtration: Most volatile organic compounds are effectively adsorbed by activated carbon. NSF/ANSI Standard 53 certification specifically tests for VOC reduction including benzene, TCE, PCE, and toluene. For comprehensive VOC protection, look for filters certified to both NSF/ANSI 42 (aesthetic reduction) and NSF/ANSI 53 (health effects reduction). Reverse osmosis (NSF/ANSI 58) provides additional protection against carbon-resistant compounds like 1,4-dioxane.
☣ Radionuclides
Radionuclides are unstable atoms that emit ionizing radiation. They occur naturally in geologic formations, particularly in granite, shale, and phosphate deposits, and can also enter water from mining, nuclear power, and medical waste. The EPA's Radionuclides Rule (2000) established MCLs for four groups.
Regulated Radionuclides
| Contaminant / Group | EPA MCL | Common Sources | Health Effects | Best Treatment |
|---|---|---|---|---|
| Combined Radium-226/228 | 5 pCi/L | Natural erosion of uranium/thorium deposits; coal and nuclear industries | Bone cancer; lymphoma; anemia. Radium deposits in bone. | Ion exchange, reverse osmosis, lime softening |
| Gross Alpha Particle Activity | 15 pCi/L (excluding uranium and radon) |
Erosion of natural deposits of uranium and other radioactive minerals | Cancer risk; tissue damage from internal radiation exposure | Reverse osmosis, ion exchange, distillation |
| Uranium | 30 μg/L | Erosion of natural deposits; mining and milling operations | Kidney toxicity (heavy metal); cancer risk from radiation. Chemical toxicity drives the MCL. | Reverse osmosis, anion exchange, activated alumina, distillation |
| Beta / Photon Emitters | 4 mrem/year (for man-made isotopes) |
Nuclear power plants, research labs, medical waste; fallout | Cancer risk; tissue damage dependent on isotope type | Reverse osmosis, ion exchange |
Sources: EPA Radionuclides Rule (2000); NRC Background Information. pCi/L = picoCuries per liter; μg/L = micrograms per liter; mrem = millirem (radiation dose unit).
Where radionuclides occur: Elevated radium and uranium levels are most common in the Midwestern and Southwestern U.S., particularly in aquifers drawing from granite, shale, or sandstone formations. The EPA estimates that about 1 in 10,000 people drinking water at the radium MCL of 5 pCi/L for a lifetime will develop cancer. This risk increases linearly with concentration.
Radon is a radioactive gas that occurs in groundwater (primarily from radium decay) and can volatilize into indoor air during showering and washing. The EPA has proposed but not finalized an MCL for radon in water. The proposed alternative standard is 4,000 pCi/L, but because radon in indoor air poses a far greater risk than radon in water, the EPA recommends that states focus on indoor air radon mitigation programs.
📢 Emerging Contaminants
Emerging contaminants are substances detected in water supplies that are not yet federally regulated, or that have only recently received regulatory attention. They include industrial chemicals, pharmaceuticals, personal care products, and microplastics. While many occur at trace levels, their potential health effects, interactions, and persistence are active areas of research.
PFAS: Per- and Polyfluoroalkyl Substances
On April 10, 2024, the EPA finalized the first-ever National Primary Drinking Water Regulation for PFAS, establishing enforceable MCLs for six specific compounds:
| PFAS Compound | EPA MCL (enforceable) | MCLG (health goal) | Common Sources | Best Filtration |
|---|---|---|---|---|
| PFOA Perfluorooctanoic acid |
4.0 ppt | 0 (zero) | Non-stick cookware (Teflon), waterproofing, stain repellents, firefighting foam | Reverse osmosis (NSF/ANSI 58), activated carbon (NSF/ANSI 53 P473), anion exchange |
| PFOS Perfluorooctane sulfonic acid |
4.0 ppt | 0 (zero) | Firefighting foam (AFFF), stain repellents (Scotchgard), chrome plating | Same as PFOA |
| PFHxS | 10 ppt | 10 ppt | Firefighting foam, metal plating, fluoropolymer manufacturing | Same as PFOA |
| PFNA | 10 ppt | 10 ppt | Fluoropolymer processing, chemical manufacturing | Same as PFOA |
| HFPO-DA (GenX) | 10 ppt | 10 ppt | Replacement for PFOA in fluoropolymer manufacturing; discharged from chemical plants into rivers | Same as PFOA |
| PFAS Mixture (Hazard Index) PFHxS + PFNA + HFPO-DA + PFBS |
HI = 1.0 (unitless) |
HI = 1.0 | Various industrial and consumer product sources | Same as PFOA |
Sources: EPA PFAS National Primary Drinking Water Regulation (April 2024); EPA Health Advisories. ppt = parts per trillion (also ng/L). MCLGs for PFOA and PFOS are zero, reflecting the EPA's determination that there is no level of exposure without risk of health impacts. Compliance deadline: 2029. Monitoring began 2027.
PFAS health effects: The EPA classifies PFOA and PFOS as "likely to be carcinogenic to humans." Epidemiological studies link PFAS exposure to developmental effects in fetuses (low birth weight, accelerated puberty), liver damage, immune suppression (reduced vaccine response), thyroid disease, testicular and kidney cancer, and elevated cholesterol. PFAS are called "forever chemicals" because they resist environmental degradation and can persist in the human body for years (half-life of PFOA: 2-3 years; PFOS: 4-5 years).
For a detailed breakdown of PFAS filtration options, see our PFAS Water Filter Guide.
Other Emerging Contaminants (Currently Unregulated)
| Contaminant | Status | Sources | Potential Concerns |
|---|---|---|---|
| Pharmaceuticals Ibuprofen, acetaminophen, antibiotics, hormones, antidepressants |
Unregulated Monitored under UCMR |
Human and veterinary pharmaceutical excretion; improper disposal; hospital discharge | Endocrine disruption (hormones); antibiotic resistance; ecological effects. Human health risks from drinking water are considered low at current levels, but long-term exposure is unknown. |
| Microplastics Particles <5mm |
Unregulated No EPA method |
Plastic degradation, synthetic textile fibers, tire particles, cosmetic microbeads | Physical tissue irritation; potential chemical transfer (plasticizers, flame retardants). WHO (2019) concluded risk from microplastics in drinking water is low based on available evidence, but research is ongoing. |
| Chromium-6 (Hexavalent) | Regulated as total Cr (MCL: 100 ppb); no federal Cr-6 specific MCL | Steel production, electroplating, leather tanning, natural deposits | Inhalation is known to cause lung cancer (IARC Group 1). California has set an MCL of 10 ppb for Cr-6 specifically. NSF/ANSI Standard 53 includes Cr-6 reduction testing. |
| 1,4-Dioxane | Unregulated EPA HA: 35 ppb (lifetime) |
Industrial solvent; byproduct of surfactant manufacturing | Likely carcinogen; present in many groundwater supplies. Carbon filters have limited effectiveness; advanced oxidation processes (AOP) are most effective. |
| Manganese | Secondary standard: 0.05 mg/L (aesthetic); health advisory for infants | Natural deposits; mining and industrial discharge | Neurotoxic at high exposure; EPA issued a health advisory for infants in 2004. Aesthetic issues: staining, metallic taste. |
| Perchlorate | Regulatory determination made; final rule pending | Rocket fuel, fireworks, munitions, Chilean fertilizer | Interferes with iodine uptake in the thyroid gland; can affect fetal and infant brain development. |
Sources: EPA Contaminant Candidate List (CCL) 5; EPA Unregulated Contaminant Monitoring Rule (UCMR) 4 & 5; WHO Microplastics Report (2019); California MCLs (where stricter than federal). HA = Health Advisory.
3. Master Contaminant Reference Table
The following table consolidates the most common and controversial drinking water contaminants into a single quick-reference format. Use this to identify contaminants of concern in your area and match them to certified filtration technologies.
| Contaminant | EPA MCL | Primary Sources | Health Effects | Best Filtration Technology |
|---|---|---|---|---|
| Lead | 15 ppb (Action Level) | Corroded pipes, solder, brass fixtures | Childhood cognitive impairment; kidney damage; hypertension | RO NSF 53 Carbon Distillation |
| Arsenic | 10 ppb | Natural deposits; agricultural/industrial runoff | Skin damage; lung, bladder, skin cancer risk | RO Activated Alumina Distillation |
| Nitrate | 10 mg/L | Fertilizer; septic systems; livestock waste | Blue baby syndrome (infants); cancer risk | RO Anion Exchange Distillation |
| Fluoride | 4.0 mg/L | Water additive; natural deposits; phosphate mining | Dental/skeletal fluorosis at high levels | Activated Alumina RO Distillation |
| Copper | 1.3 ppm (Action Level) | Plumbing corrosion; natural deposits | GI distress; liver/kidney damage (Wilson's disease) | RO Distillation |
| Mercury | 2 ppb | Natural deposits; industrial/refinery discharge | Kidney damage; neurological effects | RO Carbon Distillation |
| Cadmium | 5 ppb | Galvanized pipe corrosion; batteries; paint | Kidney damage; bone fragility | RO Carbon Distillation |
| Chromium (total) | 100 ppb | Electroplating; mining; natural deposits | Dermatitis; liver/kidney damage (Cr-6: cancer) | RO Carbon |
| TTHMs | 80 ppb | Chlorine + organic matter byproduct | Liver/kidney/CNS damage; bladder cancer | GAC / Carbon Block RO Distillation |
| HAA5 | 60 ppb | Chlorine + organic matter byproduct | Increased cancer risk | GAC / Carbon Block RO Distillation |
| Bromate | 10 ppb | Ozone disinfection byproduct | Cancer risk | RO GAC |
| PFOA | 4 ppt (2024 rule) | Non-stick coatings; firefighting foam; waterproofing | Cancer; immune suppression; developmental effects | RO NSF 53 P473 Anion Exchange |
| PFOS | 4 ppt (2024 rule) | Firefighting foam; stain repellents; chrome plating | Cancer; thyroid disease; cholesterol elevation | RO NSF 53 P473 Anion Exchange |
| GenX (HFPO-DA) | 10 ppt (2024 rule) | Replacement for PFOA; fluoropolymer manufacturing | Liver effects; cancer risk (limited data) | RO NSF 53 Anion Exchange |
| Benzene | 5 ppb | Petroleum; coal; gas storage | Anemia; leukemia risk; immune depression | GAC RO Aeration |
| TCE | 5 ppb | Dry cleaning; metal degreasing | Liver/kidney/CNS damage; carcinogenic | GAC RO |
| Atrazine | 3 ppb | Agricultural herbicide runoff | Cardiovascular; endocrine disruption | GAC RO |
| Radium-226/228 | 5 pCi/L | Natural radioactive deposits | Bone cancer; lymphoma | RO Ion Exchange |
| Uranium | 30 μg/L | Natural deposits; mining | Kidney toxicity; cancer | RO Anion Exchange Distillation |
| Cryptosporidium | TT (99% removal) | Animal/human fecal waste | Severe diarrhea; dehydration (immunocompromised) | 1μm Filter UV RO |
| Giardia | TT (99.9% removal) | Animal/human fecal waste | Gastroenteritis; cramps; vomiting | 1μm Filter UV RO |
| Chlorine (residual) | MRDL: 4.0 mg/L | Disinfectant added at treatment plant | Eye/nose irritation; stomach discomfort; forms DBPs | GAC / Carbon Block RO KDF |
Legend: RO = Reverse Osmosis (NSF/ANSI 58); GAC = Granular Activated Carbon; NSF 53 = Carbon block certified to NSF/ANSI Standard 53; NSF 53 P473 = Carbon certified for PFOA/PFOS; 1μm Filter = Absolute 1-micron or smaller rated filter; UV = Ultraviolet purification; KDF = KDF copper-zinc media. Filter technologies listed in approximate order of effectiveness.
Key insight: Reverse osmosis is the only technology that effectively removes all categories of contaminants listed above, including inorganics (lead, nitrate, fluoride), organics (pesticides, VOCs), DBPs, PFAS, and radionuclides. However, RO systems waste water (typically 3-4 gallons per gallon filtered) and remove beneficial minerals. Activated carbon filters are more practical for targeted concerns like chlorine taste, THMs, and VOCs. For a full analysis of treatment options, see our Water Contaminants & Treatment Guide.
4. City Water vs. Well Water: Different Contamination Profiles
The contaminants you are most likely to encounter depend heavily on whether your water comes from a municipal (public) water system or a private well. Federal regulations apply very differently to each.
City Water (Public Water Systems)
Regulated by EPA under SDWA: Yes. Over 90 contaminants monitored. Annual Consumer Confidence Reports required.
Typical concerns:
- Disinfection byproducts (TTHMs, HAA5) from chlorine/chloramine treatment
- Lead from aging service lines and building plumbing
- Chlorine taste and odor (aesthetic, not a health risk at regulated levels)
- PFAS from industrial sources upstream of water intake
- Copper from plumbing corrosion
Advantage: Continuous monitoring, mandatory treatment, public notification of violations.
Blind spot: Contaminants not on EPA's regulated list (many emerging contaminants) may go undetected until UCMR monitoring cycles catch them.
Private Well Water
Regulated by EPA under SDWA: No. Private wells serving fewer than 25 people or 15 connections are not federally regulated. The owner is responsible for all testing and treatment.
Typical concerns:
- Bacteria and pathogens (coliforms, E. coli, Giardia) from septic systems, livestock, or surface water intrusion
- Nitrate from agricultural fertilizer and septic leaching
- Arsenic from natural geologic deposits (especially in Western U.S., New England, Upper Midwest)
- Radon and radium from granite, shale, or sandstone formations
- Iron, manganese, hardness minerals (aesthetic and health concerns)
- VOCs from nearby industrial sites, gas stations, or landfills
Advantage: No chlorine or disinfection byproducts if untreated; often excellent mineral content.
Blind spot: No mandatory testing. The EPA recommends annual testing for total coliforms, nitrates, pH, and TDS, plus testing every 3 years for heavy metals and organics.
Well water owners should test for: At minimum, test annually for total coliform bacteria, nitrates, total dissolved solids (TDS), and pH. Test every 2-3 years for arsenic, lead, radon, radium, uranium, VOCs, and pesticides if you live near agricultural or industrial areas. If you have infants in the home, test for nitrate immediately, as levels can fluctuate seasonally. See our guide to water quality testing for specific test kits and laboratories.
5. How to Read Your Consumer Confidence Report (CCR)
Every community water system serving more than 15 connections or 25 people year-round must provide an annual Consumer Confidence Report (CCR), also known as a Water Quality Report, to its customers by July 1 of each year. This report tells you where your water comes from, what contaminants were detected, and how those levels compare to EPA standards.
How to Find Your CCR
- Check your water utility's website (search "[your city] water quality report")
- Use the EPA's CCR search tool at epa.gov/ccr
- Contact your water utility directly by phone
Key Sections to Read
| Section | What It Tells You | What to Look For |
|---|---|---|
| Water Source | Surface water (river, lake, reservoir) or groundwater (well) | Surface water is more vulnerable to pathogens, agricultural runoff, and seasonal quality changes. |
| Detected Contaminants Table | Lists every regulated contaminant tested, its detected level, the EPA MCL, and typical sources | Any detected level approaching 50-80% of the MCL warrants attention. Levels exceeding the MCL trigger a violation notice. |
| Detected Unregulated Contaminants | Lists contaminants monitored under the UCMR that have no federal MCL | Presence of PFAS, 1,4-dioxane, or chromium-6 here signals potential future concern. Compare to state or health advisory levels. |
| Violations | Any MCL exceedances, treatment technique failures, or monitoring violations | Even minor violations should prompt follow-up. Chronic violations may indicate systemic infrastructure problems. |
| Lead & Copper | 90th percentile levels from the most recent Lead and Copper Rule sampling | If the 90th percentile lead value exceeds 15 ppb, the system must take corrective action. Your individual home may still be higher. |
Red Flags in Your CCR
- Any MCL violation: Public notification is legally required, but the report may bury it. Look for bold text or footnotes.
- Lead 90th percentile above 10 ppb: Even if below the 15 ppb action level, this indicates significant lead release in the distribution system.
- TTHMs or HAA5 above 60 ppb / 40 ppb respectively: Approaching the MCL suggests inadequate DBP control. Request information on Stage 2 DBPR compliance.
- Nitrate above 5 mg/L: Half the MCL is a warning threshold. Agricultural areas may see seasonal spikes.
- Any PFAS detection above 4 ppt: Even without an MCL violation, any detectable PFOA or PFOS warrants point-of-use filtration given the zero MCLG.
6. When to Test Your Water
Your water utility tests at the treatment plant and at select points in the distribution system, but the water reaching your specific tap can differ due to plumbing, service lines, and localized conditions. Private well owners have no regulatory safety net at all. Testing your own water is the only way to know with certainty what you are drinking.
Test Immediately If:
- You have infants, pregnant women, or immunocompromised individuals in the household
- You live in a home built before 1986 and have never tested for lead
- Your water has changed color, taste, or odor
- There has been a construction, mining, or agricultural development nearby
- Your CCR shows an MCL violation or lead action level exceedance
- You are on a private well and have never tested, or it has been more than one year
- There has been flooding, a water main break, or a boil-water advisory in your area
Recommended Testing Schedule
| Water Source | Annual Test | Every 2-3 Years | Special Circumstances |
|---|---|---|---|
| City Water | Lead, copper (at the tap), pH, TDS | Comprehensive panel including VOCs, pesticides, PFAS if not in CCR | After plumbing work, if CCR shows violations, if home was built before 1950 (lead service lines) |
| Private Well | Total coliform, E. coli, nitrate, pH, TDS | Arsenic, lead, radon, radium, uranium, VOCs, pesticides, manganese, iron | New well, flooding, nearby construction, gastrointestinal illness in household, changes in taste/odor |
For guidance on selecting a certified laboratory or at-home test kit, see our detailed guide on how to test your water quality.
7. Which Filter Removes What: Quick Reference
Not all filters are created equal. The following table matches common filter types to the contaminant categories they can address, based on independent NSF/ANSI certifications. A filter without certification claims is not verified.
| Filter Type | Certification Standard | Removes | Does NOT Remove | Best For |
|---|---|---|---|---|
| Pitcher Filter (activated carbon) |
NSF/ANSI 42, 53 | Chlorine taste/odor, some VOCs, lead (if NSF 53 certified), particulates, some DBPs | Nitrate, fluoride, arsenic, bacteria, PFAS (most), radionuclides, TDS | Budget chlorine/lead reduction; aesthetic improvement |
| Faucet-Mount Filter | NSF/ANSI 42, 53 | Chlorine, lead (if certified), cysts, some VOCs, particulates | Nitrate, fluoride, arsenic, PFAS (most), bacteria, TDS, radionuclides | Convenient point-of-use chlorine and lead reduction |
| Countertop Carbon Block | NSF/ANSI 42, 53 | Chlorine, lead, cysts, VOCs, THMs, HAAs (if certified), mercury, asbestos, some pesticides | Nitrate, fluoride, arsenic (V), PFAS (most), bacteria, TDS | Broad health-contaminant reduction without plumbing modification |
| Under-Sink Carbon | NSF/ANSI 42, 53 | Same as countertop, typically with higher capacity and flow rate | Nitrate, fluoride, arsenic (V), PFAS (most), bacteria, TDS | Higher-volume point-of-use filtration; certified lead/VOC reduction |
| Reverse Osmosis (RO) | NSF/ANSI 58 | Lead, arsenic, nitrate, fluoride, TDS, chromium, selenium, uranium, radium, PFAS (if P473 certified), cysts, most VOCs, DBPs | Bacteria (most RO membranes alone are not certified for bacteria; need UV or NSF 58 microbiological claim), dissolved gases (some chloramine may pass) | Comprehensive contaminant removal; well water; high-TDS water |
| RO + UV | NSF/ANSI 58 + Class A UV | Everything standard RO removes, plus bacteria and viruses | Some dissolved gases; very small organics below membrane cutoff | Maximum protection; well water with pathogen risk; immunocompromised households |
| Distillation | NSF/ANSI 62 | Nearly everything non-volatile: lead, arsenic, nitrate, fluoride, TDS, chromium, bacteria, cysts, most minerals | VOCs that boil near water temperature (e.g., some VOCs may carry over); requires carbon post-filter for VOCs and chlorine | Maximum contaminant removal for specific concerns; small batch use |
| Whole-House Carbon | NSF/ANSI 42 | Chlorine, chloramine (if KDF or catalytic carbon), taste/odor, some VOCs, particulates | Lead (at the tap), nitrate, fluoride, arsenic, PFAS (most), bacteria, TDS, radionuclides | Chlorine/chloramine removal for bathing and cooking; appliance protection |
| UV Purifier | NSF/ANSI 55 (Class A) | Bacteria, viruses, cysts (Class A), algae | Lead, nitrate, fluoride, arsenic, VOCs, PFAS, TDS, metals, DBPs | Pathogen inactivation for well water; supplement to filtration |
| Water Softener (Ion Exchange) |
NSF/ANSI 44 | Calcium, magnesium (hardness), barium, radium (some resins) | Lead (at the tap), nitrate (standard softener; anion exchange removes nitrate), PFAS, bacteria, TDS (overall), VOCs | Hard water treatment; some radium reduction; appliance protection |
Critical point: Always verify NSF/ANSI certification for the specific contaminants you are targeting. Manufacturers frequently make broad marketing claims ("removes 99% of contaminants") that are not backed by independent testing. Look for the NSF mark on the product packaging, or verify on NSF's online certified products database (nsf.org). The specific Standard matters: NSF/ANSI 42 addresses aesthetic claims (taste, odor, chlorine), while NSF/ANSI 53 addresses health-effect claims (lead, cysts, VOCs, asbestos).
8. Frequently Asked Questions
Is tap water safe to drink in the United States?
For the vast majority of Americans, yes. The U.S. has one of the safest public drinking water supplies in the world, thanks to the Safe Drinking Water Act and the EPA's regulatory framework. However, "safe" does not mean "pure." The EPA regulates over 90 contaminants, but thousands of chemicals are used in commerce that have never been assessed for drinking water impacts. Local violations occur, aging infrastructure introduces lead, and emerging contaminants like PFAS are only now being addressed. If you are in a sensitive group (pregnant, immunocompromised, have infants), or if your CCR shows detections approaching MCLs, point-of-use filtration is a reasonable additional precaution.
What is the most dangerous contaminant in drinking water?
There is no single answer, as danger depends on concentration, exposure duration, and individual vulnerability. That said, lead is the most dangerous contaminant for the largest number of U.S. households because it has no safe level of exposure (MCLG = 0), it affects children's brain development permanently, and it enters water inside the home through corroded plumbing, meaning water that meets federal standards at the plant can still be contaminated at the tap. For infants, nitrate is uniquely dangerous because it can cause methemoglobinemia (blue baby syndrome) at levels near the MCL. PFAS is the most significant emerging threat due to its persistence, bioaccumulation, and links to multiple cancers and immune effects.
Do I need a water filter if my water meets all EPA standards?
Not strictly, but there are valid reasons to filter anyway. EPA standards balance health protection against economic and technical feasibility, meaning MCLs are often set well above the level where health effects begin to appear. The MCLG for many contaminants is zero, while the enforceable MCL is higher. Additionally, EPA standards are designed to protect the "average" healthy adult; they may not provide adequate protection for infants, pregnant women, the elderly, or immunocompromised individuals. If your CCR shows any contaminants detected above 50% of their MCL, or if you have specific health concerns, a certified filter provides an additional margin of safety.
Which contaminants are most commonly found above safe levels?
Based on EPA violation data and the EWG Tap Water Database, the most frequently detected contaminants at concerning levels are:
- Total Trihalomethanes (TTHMs): Found in virtually all chlorinated surface water supplies; exceedances are common in systems with high organic matter.
- Haloacetic Acids (HAA5): Second most common DBP exceedance, often co-occurring with TTHMs.
- Lead: Action level exceedances have affected thousands of systems, most notably in Flint, Michigan, but also in smaller communities nationwide.
- Arsenic: Exceedances are concentrated in the Southwest (Arizona, New Mexico, California), parts of New England, and the Upper Midwest.
- Nitrate: Common in agricultural regions (Iowa, Nebraska, California Central Valley, Kansas) where fertilizer use is heavy.
- PFAS: Widespread detections in all 50 states, with exceedances of the new 4 ppt MCL expected to affect hundreds of systems.
Can boiling water remove contaminants?
Boiling is effective for killing bacteria, viruses, and protozoa, making it the recommended response to boil-water advisories. However, boiling concentrates non-volatile contaminants like lead, nitrate, arsenic, and radionuclides because water evaporates while the contaminant remains. Boiling does not remove chemicals, metals, PFAS, or radionuclides. For chemical contaminants, reverse osmosis, activated carbon, or distillation are the appropriate treatments.
Does bottled water have fewer contaminants than tap water?
Bottled water is regulated by the FDA, not the EPA, and FDA standards are generally aligned with EPA MCLs. Bottled water is not necessarily safer than tap water. Independent testing has found contaminants including arsenic, PFAS, microplastics, and bacteria in bottled water brands. Some bottled water is simply filtered tap water. From a cost and environmental perspective, a certified point-of-use filter is typically the better choice for consistent contaminant reduction.
How do I know which filter to buy?
Start with your Consumer Confidence Report or a water quality test to identify which contaminants are present in your water. Then match those contaminants to a filter with the appropriate NSF/ANSI certification:
- Chlorine taste/odor only: NSF/ANSI 42 certified pitcher, faucet, or under-sink filter
- Lead: NSF/ANSI 53 certified for lead reduction, or NSF/ANSI 58 RO system
- PFAS: NSF/ANSI 53 with P473 certification, or NSF/ANSI 58 RO system
- Nitrate or Fluoride: NSF/ANSI 58 RO system, or activated alumina for fluoride specifically
- Pathogens (well water): NSF/ANSI 53 for cysts + NSF/ANSI 55 Class A UV for bacteria/viruses
- Comprehensive protection: NSF/ANSI 58 RO system with post-carbon filter
For detailed recommendations, see our Water Contaminants & Treatment Guide.
What is the difference between NSF/ANSI 42, 53, and 58?
- NSF/ANSI 42 (Aesthetic Effects): Tests for chlorine taste and odor reduction, particulates, and turbidity. Does not address health-contaminant removal.
- NSF/ANSI 53 (Health Effects): Tests for reduction of contaminants with known health effects, including lead, cysts, VOCs, mercury, asbestos, some pesticides, and select pharmaceuticals.
- NSF/ANSI 58 (Reverse Osmosis): Tests RO systems for TDS reduction, plus specific claims for arsenic, nitrate, fluoride, chromium, radium, and other inorganics. The P473 addendum tests for PFOA/PFOS.
- NSF/ANSI 401 (Emerging Compounds): Tests for reduction of trace pharmaceuticals and chemicals not yet regulated by the EPA.
Are there contaminants in tap water that the EPA does not regulate at all?
Yes. The EPA has established MCLs for approximately 90 contaminants, but over 80,000 chemicals are registered for use in the United States, and most have never been evaluated for drinking water safety. The EPA's Unregulated Contaminant Monitoring Rule (UCMR) requires water systems to test for a rotating list of 30 unregulated contaminants every five years. Data from UCMR monitoring informs decisions about future regulations. PFAS was monitored under UCMR 3 (2013-2015) before receiving federal MCLs in 2024. Currently unregulated contaminants of concern include many pharmaceuticals, microplastics, perchlorate, 1,4-dioxane, and additional PFAS compounds beyond the six now regulated.
Related Guides from Filter Tested
Water Contaminants & Treatment Guide
Deep dive into water treatment technologies, how they work, and which contaminants each is certified to remove.
PFAS Water Filter Guide
Everything you need to know about "forever chemicals," the new EPA regulations, and certified PFAS filtration options.
Lead Water Filter Guide
Understand lead sources, health risks, and find NSF-certified filters proven to remove lead from drinking water.
Fluoride Water Filter Guide
Learn about fluoride sources, the MCL controversy, and filtration methods that actually remove fluoride.
How to Test Water Quality
Step-by-step guide to testing your water at home or through a certified laboratory.
Sources & References
- U.S. Environmental Protection Agency, "National Primary Drinking Water Regulations," 40 CFR Part 141. epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations
- U.S. EPA, "PFAS National Primary Drinking Water Regulation," Final Rule, April 10, 2024. epa.gov/sdwa/and-polyfluoroalkyl-substances-pfas
- U.S. EPA, "Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules." epa.gov/dwreginfo/stage-1-and-stage-2-disinfectants-and-disinfection-byproducts-rules
- U.S. EPA, "Radionuclides Rule." epa.gov/dwreginfo/radionuclides-rule
- Environmental Working Group, Tap Water Database, Disinfection Byproducts Review, 2024. ewg.org/tapwater
- National Toxicology Program, "Report on Carcinogens: Haloacetic Acids," 2018.
- International Agency for Research on Cancer (IARC) Monographs on TCE, Chromium-6, and Glyphosate.
- World Health Organization, "Microplastics in Drinking-water," 2019.
- NSF International Standards: NSF/ANSI 42, 53, 55, 58, 62, and P473 certification protocols.
- Agency for Toxic Substances and Disease Registry (ATSDR), Toxicological Profiles for Lead, Arsenic, PFAS, Nitrate.