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Before 1930, refrigerators were not only bulky and expensive but extremely dangerous. Chemicals used as refrigerants—ammonia, methyl chloride, and sulfur dioxide—were not only toxic but highly combustible. In 1929 a leak in a methyl chloride refrigeration system caused an explosion that killed more than 100 people in a Cleveland hospital. It was no wonder that consumers preferred their old iceboxes. That would all change in 1930 with the invention of Freon by a General Motors researcher.

At the end of World War I, GM had bought the Frigidaire Corporation, but it had not proved a lucrative investment. It lost $2.5 million in 1921, a year when only 5,000 refrigerators were built in the United States, of which just 365 were Frigidaires. As the decade wore on, Frigidaire became modestly successful, but Charles Kettering, GM’s research chief, knew that finding an improved refrigerant would determine Frigidaire’s future success. A nontoxic and nonflammable refrigerant might also produce dividends in the nascent field of air conditioning. At the time, artificial cooling was reserved for the occupants of a few large public buildings and a handful of wealthy homeowners.

Kettering, famous for creating the automobile self-starter, the high-compression engine, and the modern diesel locomotive, had spent the past decade working with mechanical engineer Thomas Midgley Jr. at Dayton Research Laboratories, a subsidiary of GM, on trying to improve the notoriously inefficient early gasoline engine. Encouraged by Kettering, Midgley had developed tetraethyl lead gasoline (TEL), which silenced engine “putt-putt” and “ping” sounds, eliminated sputtering, and helped prevent the overheating that often occurred when a motorist accelerated or climbed a hill. Their collaboration proved a big success, although tainted by claims that leaded gasoline was poisonous.

Kettering tasked Midgley with developing a new refrigerant, although the Cornell-trained engineer had taken only two formal chemical courses in his life. Yet Kettering knew that Midgley had a quality that he called “intelligent ignorance”: an ability to think, coupled with an absence of preconception, that is the hallmark of truly innovative inventors.

“It took a whole Saturday afternoon to sell Midge on the idea that this was quite an important project,” recalled Kettering. Midgley dropped his work on synthetic rubber tires and turned to refrigerants. In the fall of 1928, at Ohio State University’s laboratories in Columbus, he started with the periodic table. A quick survey of known refrigerants revealed that all were compounds of hydrogen, carbon, nitrogen, oxygen, sulfur, and the halogens. Within this set of compounds, flammability decreased from left to right on the table, while toxicity decreased from bottom to top. The two trends pointed to fluorine as a promising candidate. These results surprised Midgley, because many of fluorine’s compounds are poisonous. Although sodium chloride is innocuous table salt, its cousins sodium fluoride and hydrofluoric acid are used as an insecticide and to dissolve glass, respectively. Yet the carbon-fluorine bond appeared to be particularly stable.

Midgley and GM chemist Albert Henne set about synthesizing compounds of carbon, fluorine, and other halogens or hydrogen. They tested each compound for boiling point, flammability, toxicity, and other physical properties. This research uncovered that dichlorodifluoromethane (CC12F2), later dubbed Freon 12 or simply Freon, had a boiling point in the middle of the desired range and was nontoxic and nonflammable. Its success led to a whole class of compounds known as chlorofluorocarbons (CFCs).

Midgley dramatically demonstrated Freon’s characteristics at the 1930 meeting of the American Chemical Society. He carried a small vial of Freon and a candle to the stage. After lighting the candle, he poured the Freon into a bowl and heated it to a boil with a burner. He breathed in a lungful of the vapors and then gently exhaled toward a candle flame and extinguished it. No member of the audience could now argue that Freon was poisonous or flammable.

Immediately after the demonstration, GM and DuPont appointed Midgley vice president of Kinetic Chemicals Inc., their joint venture to market CFCs. Within five years, Frigidaire and other manufacturers of refrigerators that used Kinetic Chemicals’ Freon had blown the competition out of the market, selling more than 8 million new refrigerators nationally. Air conditioner sales also skyrocketed within the next half century. By 1978 17.6 million American homeowners had central air conditioning, and 25.1 million owned room units, nearly all of which used CFCs.

Freon had other uses, unintended by Kettering and Midgley. After World War II, CFC propellants were widely used in aerosols, ranging from paints to deodorants—one of innumerable civilian applications of war technology. This application released into the atmosphere far greater quantities of CFCs than refrigeration and air conditioning, which use closed systems that recycled fixed amounts.

The consequences of this profligacy would not become known until decades after Midgley’s death (in 1944), when two University of California scientists published a paper entitled “Stratospheric Sink for Chlorofluoromethanes: Chlorine Atom-catalyzed Destruction of Ozone” in the June 28, 1974, issue of Nature. Mario J. Molina and F. S. Rowland reported that although CFC atoms are extremely stable, high levels of ultraviolet radiation in the upper stratosphere cause them to decompose, releasing chlorine that diminished the protective ozone layer by converting ozone (O3) to oxygen (O2). The ozone layer has been instrumental to the development of life on Earth by blocking most of the sun’s high-frequency ultraviolet light.