Pollution can either be dealt with after it has occurred (cleanup) or be prevented from occurring (source reduction). There have been a number of success stories regarding source reduction. Dioxins are a group of chlorinated hydrocarbons that are by-products of the manufacture and use of herbicides such as 2,4,5-T, and the combustion of chlorine-containing hydrocarbons. Dioxins, like DDT, are biomagnified in ecological systems. They cause cancer and are suspected of altering animal sex hormones. Since dioxin-producing herbicides have been banned, dioxin levels in bird's eggs have declined.