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In 1946, ventilation standards for U.S. buildings were reduced by two-thirds — from an exchange rate of 30 cubic feet of air per minute per person, to a rate of 10 cubic feet of air. The old ventilation standard — enforced by law in 22 states — was swept aside by a new architectural concept: the mechanically ventilated building. Wartime advances in heating and air-conditioning had convinced designers that sealed, climate controlled buildings were not only possible — but desirable. Still, you could open a window if you needed to. With the first world energy crisis in 1973-and-4, however, the drive for “hermetically sealed” buildings began in earnest.