Market forces and other concerns continued to drive innovations. Led by Thomas Midgley, chemical engineers at Frigidaire solved the dangerous problem of toxic, flammable refrigerants—which had been known to leak, with fatal consequences—by synthesizing the world's first chlorofluorocarbon, to which they gave the trademarked name Freon. It was the perfect refrigerant, so safe that at a demonstration before the American Chemical Society in 1930 Midgley inhaled a lungful of the stuff and then used it to blow out a candle. In the late 1980s, however, chlorofluorocarbons were found to be contributing to the destruction of Earth's protective ozone layer. Production of these chemicals was phased out and the search for a replacement began.

At about the same time Frigidaire was introducing Freon it also turned its attention to the other side of the mechanical refrigeration business: air conditioning. Comfort cooling for the home had been hampered by the fact that air conditioners tended to be bulky affairs that had been designed specifically for large-scale applications such as factories, theaters, and the like. In 1928 Carrier introduced the "Weathermaker," the first practical home air conditioner, but because the company's main business was still commercial, it was slow to turn to the smaller-scale designs that residential applications required. Frigidaire, on the other hand, was ready to apply the same expertise in engineering and manufacturing that had allowed it to mass produce—literally by the millions—the low-cost, small-sized refrigerators that were already a fixture in most American homes. In 1929 the company introduced the first commercially successful "room cooler," and a familiar list of challengers—Kelvinator, GE, and this time Carrier—quickly took up the gauntlet. Window units came first, then central whole-house systems. Without leaving home, Americans could now escape everything from the worst humid summers of the Northeast and Midwest to the year-round thermometer-busting highs of the South and desert Southwest.

At about the same time that both refrigeration and air conditioning were becoming significantly more commonplace, both also went mobile. In 1939 Packard introduced the first automobile air conditioner, a rather awkward affair with no independent shut-off mechanism. To turn it off, the driver had to stop the car and the engine and then open the hood and disconnect a belt connected to the air conditioning compressor. Mechanical engineers weren't long in introducing needed improvements, ultimately making air conditioning on wheels so de rigueur that even convertibles had it.

But as wonderful as cool air for summer drives was, it didn't have anywhere near the impact of the contribution of Frederick McKinley Jones, an inventor who was eventually granted more than 40 patents in the field of refrigeration and more than 60 overall. On July 12, 1940, Jones—a mechanic by training but largely self-taught—was issued a patent for a roof-mounted cooling device that would refrigerate the inside of a truck. Jones's device was soon adapted for use on trains and ships. Hand in hand with Clarence Birdseye's invention of flash freezing, Jones's refrigeration system made readily available—no matter what the season—all manner of fresh and frozen foods from every corner of the nation and, indeed, the world.

Small but incrementally significant improvements continued as the century unfolded, making refrigeration and air conditioning systems steadily more efficient and more affordable—and increasingly widespread. The range of applications has grown as well, with mechanical refrigeration playing a role in everything from medical research and computer manufacturing to space travel. Without, for example, the controlled, air-conditioned environment in spacecraft and spacesuits, humans would never have made it into space—or walked on the Moon—-even with all the other engineering hurdles overcome. But most of us don't have to go quite so far to appreciate the benefits of keeping cool. They're right there for us, each time we open the refrigerator door and reach for something cold to drink.