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.

Bromodichloromethane, one of the total trihalomethanes (TTHMs), is formed when chlorine or other disinfectants are used to treat drinking water. Bromodichloromethane and other disinfection byproducts increase the risk of cancer and may cause problems during pregnancy. Click here to read more about disinfection byproducts.

Chloroform, one of the total trihalomethanes (TTHMs), is formed when chlorine or other disinfectants are used to treat drinking water. Chloroform and other disinfection byproducts increase the risk of cancer and may cause problems during pregnancy.

Chromium (hexavalent) is a carcinogen that commonly contaminates American drinking water. Chromium (hexavalent) in drinking water may be due to industrial pollution or natural occurrences in mineral deposits and groundwater. Read more about chromium (hexavalent).

Dibromochloromethane, one of the total trihalomethanes (TTHMs), is formed when chlorine or other disinfectants are used to treat drinking water. Dibromochloromethane 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.

Haloacetic acids are formed when disinfectants such as chlorine are added to tap water. The group of nine haloacetic acids includes monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid and dibromoacetic acid, which are regulated as a group by the federal government (HAA5); and bromochloroacetic acid, bromodichloroacetic acid, chlorodibromoacetic acid, and tribromoacetic 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.

Nitrate and nitrite enter water from fertilizer runoff, septic tanks and urban runoff. These contaminants can cause oxygen deprivation for infants and increase the risk of cancer. Nitrite is significantly more toxic than nitrate. Click here to read more about nitrate.

Perfluorohexane sulfonate 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.

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.

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.

1,4-Dioxane is a solvent classified by the EPA as a likely human carcinogen. It contaminates groundwater in many states due to industrial wastewater discharges, plastic manufacturing runoff and landfill runoff.

1-butanol is an industrial petrochemical used in the production of butyl acetate. Exposure to 1-butanol can harm the nervous system, liver and kidneys.

2-propen-1-ol is used in the production of glycerol and other chemicals. Long-term exposure to 2-propen-1-ol may harm the liver and kidneys.

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.

Chlorate forms in drinking water as a byproduct of disinfection. Chlorate impairs thyroid function, making chlorate exposure most harmful during pregnancy and childhood.

Chlorodifluoromethane (Freon 22) is a refrigerant, solvent and aerosol propellant banned in 2000 under the Montreal Protocol because of its ozone-depleting properties. Freons can cause heart, nervous system and liver damage.

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.

Cobalt is a metal found throughout the environment; people may be exposed to cobalt in air, food and drinking water.

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.

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

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.

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.

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.

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.

Germanium is a naturally occuring element used as a component in semiconductors and fiber optics. Excessive exposures to germanium may harm the kidneys.

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

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.

Molybdenum is a metal that occurs naturally in soil, minerals and water. People who ingest large amounts can have increased levels of uric acid and gout-like symptoms.

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.

Strontium is a metal that accumulates in the bones. Radioactive strontium-90 can cause bone cancer and leukemia, and any form of strontium at high doses can harm bone health.

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.

Vanadium is a metal used in steels and other alloys. People are commonly exposed to vanadium in water and food. Excessive exposure can be toxic during pregnancy and childhood.

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.