Dioxins are contaminants released from chemical manufacturing, pulp and paper bleaching, and incineration of products that contain chlorine. 2,3,7,8-Tetrachlorodibenzodioxin (2,3,7,8-TCDD) is the most toxic chemical in the group of dioxins.

Bromate is a carcinogenic disinfection byproduct formed when source waters containing bromide are treated with ozonation or sodium hypochlorite. Studies of laboratory animals show that bromate damages DNA and causes cancer in multiple organs. 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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

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.

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.

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.