At the height of the tuberculosis epidemic in the 19th century, doctors began advising people to venture outside to escape the “ bad air ” in their homes. In the 1960s, by contrast, AC became widespread in the U.S., increasing the appeal of staying inside. These days, many of us—the so-called “ indoor generation ”—spend summer mostly inside , relying on AC to beat the heat. This technology saves lives ; our sealed, climate-controlled spaces keep us healthy during dangerous heat waves. AC also