be applied to office buildings and finally to residences as well. Air conditioning played a key role in the years following World War II in promoting urban growth in the region known as the Sun Belt, as well as in tropical areas around the globe. From Florida to Texas to southern California, the massive influx of new residents depended in no small measure on the ability of buildings to protect their occupants from summer heat. Air conditioning became a fact of life in such places, so much so that it is hard to imagine urban life in the Sun Belt without it. Its significance can be captured by two phenomena of striking environmental significance: the shift in the seasonal consumption of electricity from peak load during the winter months (when energy consumption for lighting and space heating had always traditionally been at its highest) to peak load during the summer; and the steeply upward slope in the production and consumption of chlorinated fluorocarbons. The upward trend in CFC production was also aided by the development of still other uses for CFCs: as nontoxic propellants in aerosol sprays and later, in the 1960s and 1970s, as solvents in the manufacture of integrated circuits.

CFCs are very stable gases: that is in fact one of the properties that made them seem so benign when measured by their toxicity and immediate environmental effects. But the very stability that made CFCs so attractive for so many applications proved finally to be their greatest hazard. Once released into the environment—and the proliferation of refrigerators, freezers, and air conditioners meant that Freon escaped at an ever increasing rate—CFCs began to permeate the atmosphere, eventually reaching its upper regions. There they encountered the ozone layer, the thin belt of unstable tripartite oxygen molecules that filters out much of the sun's ultraviolet radiation and protects living organisms on the surface of the planet from the effects of that radiation. In the presence of sunlight, CFC molecules became chemical agents capable of destroying many times their number of ozone molecules. This effect was first hypothesized in 1974 by the chemists Mario Molina and Sherwood Rowland of the University of California at Irvine, writing in the wake of the controversy over supersonic transport aircraft and with recent knowledge, developed through new detection technology, that CFCs were present in the atmosphere (Molina and Rowland, 1974). Their hypothesis was controversial but convincing enough to produce action by the United States and eight other countries to ban the use of CFCs in aerosol sprays in the late 1970s (unquestionably the most marginal of their uses). Significantly, the suggestion that CFCs might possibly be damaging

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