Toxaphene was once the nation's most heavily used pesticide and accounted for one-fifth of all pesticide use in the United States. It was used mainly on cotton and dozens of food crops and on livestock to kill parasites. It is severely toxic to fish and wildlife and has been implicated in massive kills of fish, ducks, pelicans, and other waterfowl. Because of this, its ability to cause cancerous tumors and genetic abnormalities in animals, and fears that it may be similarly dangerous to humans, all sale and use of toxaphene have been banned except for already existing stocks. However, this chlorinated hydrocarbon pesticide is extremely persistent. Years after application, it has been found in fish, water, wildlife, and the food chain/web, posing a continuing potential threat to the environment and human health.