Suffolk County Water Authority, Area 12

Possible carcinogen that attacks the nasal cavity, kidney and liver. A clear liquid that dissolves in water and is used primarily as a manufacturing solvent, although it can also be found in personal care items such as soap. Of 1,062 tests nationwide that exceeded the level, 221, or 20 percent, were on Long Island. Prolonged exposure above the level could lead to one additional cancer case per 1 million population.

Used as an ingredient in paint, varnish remover, solvents and degreasing agents. Prolonged exposure above the level could lead to one additional cancer case per 1 million population.

Can cause anemia and nervous system problems in children. Used as a disinfectant, it prevents the body from absorbing iodine, important to the normal functioning of the thyroid gland. Prolonged exposure above the level could lead to adverse health effects.

Used in chrome-plating, dyes, pigments, tanning leather and preserving wood, if inhaled it can cause lung cancer, stomach tumors and skin ulcers. The government is reviewing its toxicity in drinking water and has set no threshold for dangerous exposure. It was detected in 1,247 of 1,417 tests on Long Island.

Used as a refrigerant, as a low-temperature solvent, and in fluorocarbon resins, especially tetrafluoroethylene polymers. It is chlorofluorocarbon and occurs as a gas.