Halogenated hydrocarbons are derivatives of hydrocarbons (that is, organic compounds that only contain carbon and hydrogen atoms) which include some halogen atoms within their chemical structure. The most commonly encountered halogens in halogenated hydrocarbons are fluorine and chlorine, but sometimes bromine or iodine occur, or combinations of any of these.
Some halogenated hydrocarbons occur naturally, being synthesized by halogenation reactions occurring during combustion of biomass containing the constituent atoms (that is, carbon, hydrogen, and halogens). For example, these syntheses occur commonly but at low rates during forest fires. However, most species of halogenated hydrocarbons are synthetic, and are manufactured by humans as industrially useful materials, or are incidentally produced as a by-product during industrial chemical reactions, or during the incineration of municipal waste.
Chlorinated hydrocarbons are a well known group, with a wide variety of uses. A number of these chemicals have been used as insecticides, including DDT, DDD, lindane, chlordane, aldrin, and dieldrin. Others have been used as herbicides, especially 2,4-D and 2,4,5-T. Polychlorinated biphenyls or PCBs have been widely used as dielectric fluids in electrical transformers and for other purposes. Dioxins, including the deadly TCDD, are trace contaminants synthesized during the manufacture of other chlorinated hydrocarbons and in spontaneous chlorination reactions in incinerators and pulp mills. Chlorinated hydrocarbons are associated with some well known environmental problems, because most of these chemicals are persistent in the environment, and they accumulate in organisms, sometimes causing toxicity.
Chlorofluorocarbons or freons are another group of halogenated hydrocarbons that have been used extensively in refrigeration, air conditioning, and for cleaning electrons. After their use these chemicals are often discharged to the atmosphere, where they are very persistent, and appear to be involved in ozone-destroying reactions occurring in the stratosphere. This is an important environmental problem, because ozone is critical in screening life on Earth's surface from the deleterious effects of exposure to solar ultraviolet radiation, which can cause skin cancers, cataracts, and other problems. In recognition of the environmental problems associated with these chemicals, the manufacturing and use of chlorofluorocarbons are rapidly being curtailed through international agreements.