In 1972, the life of atmospheric scientist F. Sherwood Rowland took a critical turn when he heard a lecture describing Lovelock's work. Like other researchers at the time, Rowland had no inkling that CFCs could harm the environment, but the injection into the atmosphere of large quantities of previously unknown compounds piqued his interest. What would be the ultimate fate of these compounds? Rowland, joined by Mario Molina, a colleague at the University of California, Irvine, decided to find out.

The scientists showed that CFCs remained undisturbed in the lower atmosphere for decades. Invulnerable to visible sunlight, nearly insoluble in water, and resistant to oxidation, CFCs display an impressive durability in the atmosphere's lower depths. But at altitudes above 18 miles, with 99 percent of all air molecules lying beneath them, CFCs show their vulnerability. At this height, the harsh, high-energy ultraviolet radiation from the sun impinges directly on the CFC molecules, breaking them apart into chlorine atoms and residual fragments.

If Rowland and Molina had ended their CFC study with these findings, no one other than atmospheric scientists would ever have heard about it. However, scientific completeness required that the researchers explore not only the fate of the CFCs, but also of the highly reactive atomic and molecular fragments generated by the ultraviolet radiation.

In examining these fragments, Rowland and Molina were aided by prior basic research on chemical kinetics--the study of how quickly molecules react with one another and how such reactions take place. Scientists had demonstrated that a simple laboratory experiment will show how rapidly a particular reaction takes place, even if the reaction involves the interaction of a chlorine atom with methane at an altitude of 18 miles and a temperature of -60 degrees Fahrenheit.

Rowland and Molina did not have to carry out even a single laboratory experiment on the reaction rates of chlorine atoms. They had only to look up the rates already measured by other scientists. Basic research into chemical kinetics had reduced a decade's worth of work to two or three days.

After reviewing the pertinent reactions, the two researchers determined that most of the chlorine atoms combine with ozone, the form of oxygen that protects Earth from ultraviolet radiation. When chlorine and ozone react, they form the free radical chlorine oxide, which in turn becomes part of a chain reaction. As a result of that chain reaction, a single chlorine atom can remove as many as 100,000 molecules of ozone.

Unknown to Rowland and Molina, the same chlorine atom chain reaction had been discovered a few months earlier by Richard Stolarski and Ralph Cicerone. In 1974, Rowland and Molina made a disturbing prediction: If industry continued to release a million tons of CFCs into the atmosphere each year, atmospheric ozone would eventually drop by 7 to 13 percent.

To make matters worse, other scientists had demonstrated that an entirely different group of compounds could further reduce ozone levels. Paul Crutzen first showed in 1970 that nitrogen oxides react catalytically with ozone, playing an important role in the natural ozone balance. Soil-borne microorganisms produce nitrogen oxides as a decay product, and Crutzen's work spotlighted how microbe-rich agricultural fertilizers might lead to reduced ozone levels. His research and that of Harold Johnston also focused attention on the effect of nitrogen oxides spewed by high-altitude aircraft. These emissions may also reduce ozone levels in the stratosphere.

Earlier studies, which had investigated whether exhaust emissions from the supersonic transport and other high-speed aircraft could damage the environment, had already begun to document the effects of ozone loss. Compiled because of the perceived threat from these aircraft, the data were brought to bear on the very real threat from CFCs and nitrogen oxides.

With less ozone in the atmosphere, more ultraviolet radiation reaches Earth. Scientists estimated that increased exposure would lead to a higher incidence of skin cancer, cataracts, and damage to the immune system and to slowed plant growth. Because some CFCs persist in the atmosphere for more than 100 years, these effects would last throughout the twenty-first century.

Concluding that such long-term hazards were unacceptable, Rowland and Molina called for a ban on further release of CFCs. Alerted to this clear and present danger, the United States, Canada, Norway, and Sweden in the late 1970s banned the use of CFCs in aerosol sprays.