Chlorofluorocarbons
CHLOROFLUOROCARBONS
Chlorofluorocarbons (CFCs) are a class of chemicals that contain only atoms of carbon, chlorine, and fluorine. As a group, they are unreactive, stable, and poorly soluble in water. Commercially, the most important CFCs were derivatives of methane and ethane. These included trichlorofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) and 1,2-dichloro-1,1,2,2-tetrafluoroethane (CFC-114). CFCs were first introduced in the 1930s as safe replacements for refrigerants such as sulfur dioxide, ammonia, chloroform, and carbon tetrachloride. During World War II they were used to produce aerosols of insecticides. During the next fifty years the applications expanded to include foam blowing, precision cleaning, air conditioning, refrigeration, and propellants for medicinal, cosmetic, food, and general-purpose aerosols. These uses eventually resulted in large emissions of CFCs into the atmosphere. Because of their low chemical reactivity, CFCs typically have long atmospheric residence times, and as a consequence are distributed globally.
In 1974, M. Molina and F. Rowland hypothesized that when CFCs reached the stratosphere they would break down to release chlorine atoms. The chlorine atoms would then react with stratospheric ozone, breaking it down into oxygen. Since stratospheric ozone absorbs much of the sun's ultraviolet radiation, decreased stratospherel ozone levels could lead to increased ground-level ultraviolet radiation. This could adversely affect crop growth, and also lead to increases in cataracts and nonmelanoma skin cancer. Following reports of a marked drop in "column ozone" over Antarctica (the "ozone hole") during the Antarctic winter of 1986, most of the nations of the world drafted and signed an agreement calling for the phaseout of CFCs. This agreement is known as the Montreal Protocol. Included were all CFCs and bromochlorofluorocarbons (halons), which are used in fire suppression systems.
The banning of CFCs has lead to research to identify other chemicals that can be used in the same applications but without the same environmental concerns. Two classes of chemicals that have been identified are the hydrochlorofluorocarbons (HCFCs) and the hydrofluorocarbons (HFCs). The presence of hydrogen in the molecule promotes attack by hydroxyl radicals in the atmosphere leading to more rapid breakdown and shorter atmospheric lifetimes. While HFCs do not contain chlorine and therefore can not contribute to ozone depletion, HCFCs do contain chlorine and can contribute to ozone depletion. However, due to the presence of hydrogen, their atmospheric lifetimes are much shorter than the CFCs and the corresponding ozone depletion values are smaller, typically by a factor of between 10 and 100. In subsequent amendments to the Montreal Protocol, the HCFCs have been classified as transitional substances and they are also scheduled for a phase-out, but at much later dates.
One of the reasons the CFCs have been used so extensively and in such a wide variety of applications is their low level of toxicity. The acute, median lethal concentration for a four-hour exposure to many of these materials is greater than 50,000 parts per million (ppm) (5% in the air). In longer term exposure studies, rarely are effects seen below 20,000 ppm (2% in the air). The one exception to this is the potential of all of these compounds, as well as hydrochlorocarbons and hydrocarbons, to sensitize the heart to the action of adrenaline. In the 1960s, it was first reported that teenagers were abusively inhaling CFCs to get a preanesthectic "high." However, in some cases, the individual would get excited, run around and then die, with no apparent cause of death. Subsequent research demonstrated that this effect could be reproduced in laboratory animals which are now used to test possible CFC replacements.
GEORGE M. RUSCH
(SEE ALSO: Ambient Air Quality [Air Pollution]; Atmosphere; Hazardous Air Pollutants; Melanoma; Skin Cancer; Ultraviolet Radiation)
