Arsenic is a potent carcinogen and common contaminant in drinking water. Arsenic causes thousands of cases of cancer each year in the U.S. Click here to read more about arsenic.

Dibromoacetic acid, one of the group of five haloacetic acids regulated by federal standards, is formed when chlorine or other disinfectants are used to treat drinking water. Haloacetic acids and other disinfection byproducts increase the risk of cancer and may cause problems during pregnancy. Click here to read more about disinfection byproducts.

Dichloroacetic acid, one of the group of five haloacetic acids regulated by federal standards, is formed when chlorine or other disinfectants are used to treat drinking water. Haloacetic acids and other disinfection byproducts increase the risk of cancer and may cause problems during pregnancy. Click here to read more about disinfection byproducts.

Haloacetic acids are formed when disinfectants such as chlorine are added to tap water. The group of five haloacetic acids regulated by federal standards includes monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid and dibromoacetic acid.

Nitrate, a fertilizer chemical, frequently contaminates drinking water due to agricultural and urban runoff, and discharges from municipal wastewater treatment plants and septic tanks. Excessive nitrate in water can cause oxygen deprivation in infants and increase the risk of cancer. Click here to read more about nitrate.

Perfluorooctanoic acid (PFOA) is a member of a group of perfluorinated chemicals used in many consumer products. PFOA and other perfluorinated chemicals can cause serious health effects, including cancer, endocrine disruption, accelerated puberty, liver and immune system damage, and thyroid changes. These chemicals are persistent in the environment and they accumulate in people. Click here to read more about perfluorinated chemicals.

Radium is a radioactive element that causes bone cancer and other cancers. It can occur naturally in groundwater, and oil and gas extraction activities such as hydraulic fracturing can elevate concentrations.

Trihalomethanes are cancer-causing contaminants that form during water treatment with chlorine and other disinfectants. The total trihalomethanes group includes four chemicals: chloroform, bromodichloromethane, dibromochloromethane and bromoform.

Toxaphene is a neurotoxic, persistent and bioaccumulative insecticide classified by the EPA as a probable human carcinogen.

Trichloroacetic acid, one of the group of five haloacetic acids regulated by federal standards, is formed when chlorine or other disinfectants are used to treat drinking water. Haloacetic acids and other disinfection byproducts increase the risk of cancer and may cause problems during pregnancy. Click here to read more about disinfection byproducts.

1,1,1-Trichloroethane is a solvent and a degreasing agent used for manufacturing metal products and as ingredient in cleaners and aerosols. Exposure to 1,1,1-trichloroethane can harm the nervous system, liver and circulatory system.

1,1,2-Trichloroethane is a volatile neurotoxic solvent used for manufacturing products such as plastic wrap and carpet backing. It causes liver tumors in animal studies.

1,1-Dichloroethylene is used for plastics production. It damages DNA and causes cancer in laboratory animal studies.

1,2,4-Trichlorobenzene is an industrial solvent that can harm the adrenal glands and may increase the risk of cancer.

1,2-Dibromo-3-chloropropane is a pesticide fumigant banned in the 1970s after scientists discovered it caused sterility in men who worked with it. The chemical causes cancer in laboratory animals and may cause cancer in people.

1,2-Dichloroethane is a volatile carcinogenic chemical used to make plastic products such as polyvinyl chloride.

1,2-Dichloropropane is a volatile carcinogenic solvent discharged from chemical factories and landfills. It also contaminates farmland due to its former use as a soil fumigant.

2(2,4,5-trichlorophenoxy) propionic acid (2,4,5-TP or Silvex) is a cancer-causing herbicide banned in the U.S. in 1985 because of its toxicity. It is still detected as a groundwater contaminant in areas where it was sprayed or stored.

2,4-D (2,4-dichlorophenoxyacetic acid) is a herbicide that can be applied directly to GMO corn and soy plants engineered to withstand this toxic chemical. Americans' exposures to 2,4-D from food and drinking water will likely increase in years to come as farmers plant more of these GMO crops.

Alachlor is a widely used agricultural herbicide. It causes cancer in laboratory animals and may be carcinogenic to people.

Aluminum is a metal released from metal refineries and mining operations. Too much aluminum exposure can impair children's brain development.

Atrazine is a herbicide commonly detected in drinking water that comes from cornfield and other agricultural runoff. It is a hormone disrupter that harms the male and female reproductive systems of people and wildlife.

Barium is a mineral present in rocks, soil and water. High concentrations of barium in drinking water increase the risk of cardiovascular disease and hypertension.

Benzene is a known human carcinogen. It also damages blood cells and the nervous system. Emissions from petroleum processing, hazardous waste landfills and underground storage tanks contaminate drinking water with benzene.

Benzo[a]pyrene is a polycyclic aromatic hydrocarbon, or PAH, released from combustion of fossil fuels and waste incinerators. PAHs increase the risk of cancer; damage the immune, nervous and reproductive systems; and can harm developing fetuses.

Carbofuran is a highly toxic insecticide phased out from use in the U.S. since 2009 due to the risks it posed to children and the environment.

Carbon tetrachloride, a volatile carcinogenic solvent, has been used in industrial chemical production and as a dry cleaning ingredient.

The cancer-causing pesticide chlordane was banned in 1988. Chlordane is persistent and accumulates in the body. It is still found in the environment and in the bodies of adults and newborn babies.

Chromium is a naturally occurring metal, but industrial uses can elevate its levels in water. One form, hexavalent chromium, causes cancer. Total chromium is not a good indicator of the amount of hexavalent chromium in drinking water.

1,2-Dichloroethylene, a neurotoxic solvent, exists in two different forms: trans-1,2-dichloroethylene and cis-1,2-dichloroethylene. A mixture of the two forms is used to manufacture other solvents and chemical products.

Cyanide is a toxic chemical that causes central nervous system and thyroid toxicity. Water contamination is generally the result of metal mining and chemical industry waste, runoff from agriculture and road salts used for melting ice.

The herbicide dalapon has not been used in the U.S. since 1999. In animal studies, exposure to dalapon harms the kidneys.

Di(2-ethylhexyl) adipate is used in PVC plastic, plastic wrap and other consumer products. It is released as a pollutant from industrial sources and sewage treatment plants. In studies of laboratory animals, di(2-ethylhexyl) adipate can harm fetal development.

Di(2-ethylhexyl) phthalate, or DEHP, is a softener added to PVC plastics. Phthalates are hormone disruptors that target the male reproductive system.

Methylene chloride is a common industrial solvent used for paint stripping, vapor degreasing, printing, electronics manufacturing and cleaning. It causes cancer and liver damage in animal studies.

Dinoseb is a herbicide that causes birth defects and reproductive difficulties. It has been banned in the U.S. since 1986.

Diquat is a herbicide used on potato crops and aquatic weeds.

Endothall is a herbicide used to kill aquatic plants and algae. It's also used on potato and cotton crops.

Endrin is a banned organochlorine insecticide that is persistent and bioaccumulative. Endrin damages the nervous, immune and reproductive systems in people. It also harms fish, birds and other wildlife.

Ethylbenzene, a component of petroleum, is a volatile cancer-causing chemical primarily used for production of plastics and rubber. Ethylbenzene is also released from gasoline fuel emissions.

Ethylene dibromide, a likely human carcinogen, was used until 1984 as a gasoline additive and a pesticide. It affects the hormone, reproductive and nervous systems; causes testicular atrophy; and damages the heart, liver and kidneys.

Fluoride occurs naturally in surface and groundwater and is also added to drinking water by many water systems to prevent cavities and support oral health in people.

In 2015, the Department of Health and Human Services updated its recommendation of an optimal level of 0.7 parts per million, or ppm, added to drinking water based on the health benefits of tooth decay prevention. Studies have shown that fluoride added to community water systems at this level reduces dental cavities, especially in children.

And studies have conclusively shown that fluoride in toothpaste and mouthwash can also provide these benefits.

Glyphosate is the weedkiller marketed as Roundup. The state of California classifies glyphosate as a chemical known to cause cancer.

Heptachlor is an insecticide used for termite control until 1988. It is linked to cancer, hormone disruption and neurotoxicity.

Heptachlor epoxide is a breakdown product of heptachlor, a highly toxic and carcinogenic pesticide banned in the U.S. since the 1980s.

Hexachlorobenzene is a cancer-causing, neurotoxic and hormone-disrupting chemical used as a pesticide until 1984. It is still found in landfills and hazardous waste sites, and can be detected in people.

Hexachlorocyclopentadiene was historically used for the manufacture of currently banned pesticides, such as endrin, dieldrin and chlordane. It’s still used for production of flame retardants, resins and plastics.

Lindane is a neurotoxic and carcinogenic insecticide phased out of agricultural use in the U.S. since 2002. It is prescribed to treat lice or scabies, although safer alternatives are available.

Manganese is a naturally occurring element that is common in food and drinking water. Excessive manganese exposures may impair children's attention, memory and intellectual capacity. Click here to read more about manganese.

Methoxychlor is an endocrine-disrupting, neurotoxic insecticide similar to DDT. Banned in the U.S. in 2002, methoxychlor was historically used on farm animals and in animal feed, lumber, gardens and agricultural crops.

Monochlorobenzene is used as a solvent for pesticides, a degreasing agent and an intermediate for manufacturing other chemicals. It causes liver and kidney damage in studies of laboratory animals.

Nitrite, closely related to the fertilizer chemical nitrate, comes from urban runoff and from fertilizer applied to agricultural lands to pollute drinking water. Nitrite is significantly more toxic than nitrate. Excessive nitrite in water can cause oxygen deprivation in infants and increase the risk of cancer.

o-Dichlorobenzene is used as a chemical intermediate for making agricultural herbicides. It is toxic to the liver, kidneys and the nervous system, and causes cancer in animal studies.

Oxamyl is a neurotoxic insecticide used on cotton, fruit and vegetable crops. It may harm fetal development.

p-Dichlorobenzene is a carcinogenic insecticide used in mothballs, and added to deodorant blocks made for trash cans and toilets. Long-term exposure can cause liver damage and kidney problems.

Pentachlorophenol is a carcinogenic chemical used as a wood preservative. It harms developing fetuses, and causes immune suppression and hormone disruption.

Perfluorobutane sulfonate (PFBS) is a member of a group of perfluorinated chemicals used in many consumer products. PFOS and other perfluorinated chemicals can cause serious health effects, including cancer, endocrine disruption, accelerated puberty, liver and immune system damage, and thyroid changes. These chemicals are persistent in the environment and they accumulate in people. Click here to read more about perfluorinated chemicals.

Perfluorobutanoic acid (PFBA) is a member of a group of perfluorinated chemicals used in many consumer products. PFOS and other perfluorinated chemicals can cause serious health effects, including cancer, endocrine disruption, accelerated puberty, liver and immune system damage, and thyroid changes. These chemicals are persistent in the environment and they accumulate in people. Click here to read more about perfluorinated chemicals.

Perfluoroheptanoic acid is a member of a group of perfluorinated chemicals used in many consumer products. Perfluorinated chemicals can cause serious health effects, including cancer, endocrine disruption, accelerated puberty, liver and immune system damage, and thyroid changes. These chemicals are persistent in the environment and they accumulate in people. Click here to read more about perfluorinated chemicals.

Perfluorohexanoic acid is a member of a group of perfluorinated chemicals used in many consumer products. Perfluorinated chemicals can cause serious health effects, including cancer, endocrine disruption, accelerated puberty, liver and immune system damage, and thyroid changes. These chemicals are persistent in the environment and they accumulate in people. Click here to read more about perfluorinated chemicals.

Perfluoropentanoic Acid (PFPeA) is a member of a group of perfluorinated chemicals used in many consumer products. PFOS and other perfluorinated chemicals can cause serious health effects, including cancer, endocrine disruption, accelerated puberty, liver and immune system damage, and thyroid changes. These chemicals are persistent in the environment and they accumulate in people. Click here to read more about perfluorinated chemicals.

Picloram is a herbicide that in studies of laboratory animals has been linked with reduced testicular size and damage to the liver and thyroid.

PCBs are synthetic chemicals banned in the U.S. since the 1970s because of their toxicity. PCBs contaminate water by leaching from landfills and hazardous waste clean-up sites. PCBs increase the risk of cancer.

Selenium is an essential element in diets. But too much selenium can decrease thyroid hormone production and cause hair loss, skin lesions and brittle fingernails.

Simazine is a hormone-disrupting herbicide related to atrazine. It affects the male and female reproductive systems. In studies of laboratory animals, simazine increases blood levels of estrogen, decreases prolactin and progesterone, and causes mammary and ovarian tumors.

Styrene is a volatile cancer-causing chemical used for manufacturing Styrofoam (polystyrene) and other plastics. Releases from industrial production sites and hazardous waste landfills cause styrene contamination in drinking water.

Dry cleaning chemical tetrachloroethylene, or perc, can cause cancer. It pollutes soil and groundwater due to emissions from dry cleaning facilities, and automotive, metalworking and other industries.

Toluene is a volatile solvent that harms the nervous and immune systems, damages the liver and may increase the risk of miscarriage and birth defects.

1,2-Dichloroethylene, a neurotoxic solvent, exists in two different forms: trans-1,2-dichloroethylene and cis-1,2-dichloroethylene. A mixture of the two forms is used to manufacture other solvents and chemical products.

Trichloroethylene, an industrial solvent and common groundwater pollutant, damages the immune system, harms the developing fetus and causes cancer.

Uranium is a known human carcinogen. The federal legal limit for uranium is set at 30 micrograms per liter (corresponding to parts per billion), but utilities can also report uranium in picocuries per liter (pCi/L), which is a measure of radioactivity in water. EWG translated all uranium results to pCi/L using a conversion factor developed by the EPA. With this conversion approach, the limit of 30 ppb corresponds to 20 pCi/L. Drinking water with this much uranium would cause more than 4.6 cancer cases in a population of 100,000. California set a public health goal for uranium of 0.43 pCi/L.

Vinyl chloride is a known human carcinogen used for production of polyvinyl chloride (PVC) plastics. Discharges from plastics manufacturing can contaminate drinking water with vinyl chloride.

Xylenes are solvents used in industrial and consumer products, such as cleaning agents and paint thinners. Xylenes cause nervous system damage and may harm developing fetuses.