Chemical carcinogens: How dangerous are low doses?

The question of what happens after exposure to very low doses of carcinogens is examined. The threshold hypothesis assumes that there is a no-effect dose of carcinogen below which induction of cancer cannot occur or occurs with extremely low probability. The single-event or one-hit hypothesis assumes that cancer is an expression of a permanent, replicable change in cellular genetics resulting from the interaction of one molecule of carcinogen with a critical receptor in one cell. Evidence frequently cited to support the hypotheses is discussed