And if the air feels a little fresher at Kohl's, that's because newer buildings feature fresh-air ventilation and no smoking within 25 feet of the entrance.

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Current HVAC systems get rid of stale, germ-ridden indoor air by cooling or heating fresh outdoor air and blowing it through ventilation ducts, a process that gobbles electricity.

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Though it was intended as a temporary shelter, its amenities were impressive: some 600 above-ground doors from which someone can enter the underground city from, 15, 000 ventilation ducts to provide fresh air, as well as multiple wineries, cellars and a complex network of passages, tunnels, and corridors.

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Ventilation can involve either 100 percent fresh air in which outside air enters and leaves the cabin in a single pass or a system in which various fractions of air are recirculated from the aircraft cabin and mixed with fresh air.

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