Halogenated Hydrocarbons and the Environment

A number of halogenated hydrocarbons have been used for a variety of commercial purposes. Some of these substances are detrimental to the environment, however, and their use has been restricted and in some cases, banned. Until the mid-1970s polychlorinated biphenyls (PCBs, 多氯聯苯) were widely used as an insulating fluids in electrical equipment. The structure of a typical PCB is shown below.

Waste fluid and discarded equipment caused large amounts of PCBs to enter the environment and wash into rivers to be deposited on the river bottom. Because PCBs are very stable, they survive a long time in this environment and are ingested by microorganisms They enter the food chain when small fish eat the microorganisms, larger fish eat the small ones, and mammals eat the fish. PCBs are health hazards that can cause mutations in the offspring of affected individuals. The use of PCBs has been banned in many countries, and the disposal of electrical equipment containing PCBs regulated. An additional concern is that PCBs form highly toxic dioxin(二噁夾) when incinerated. Many pesticides are halogenated hydrocarbons, and some, like DDT, were used on a massive scale.

DDT was introduced as a pesticide for mosquitoes to control malaria and was exceedingly successful in this role for about 30 years. Eventually, however, resistance to DDT emerged in the pests it was supposed to control. In the 1960s it became apparent that species of birds such as the peregrine falcon and the bald eagle were threatened because DDT had accumulated in their fatty tissue causing them to lay eggs with thin shells that broke prematurely. The use of DDT then became subject to strict regulation.