After airline passengers walked through the metal detectors at Albuquerque airport last fall, some volunteered to be scrutinized one more time by a device that "sniffed" them to detect chemical explosives. One by one, they stepped into a portal about the size of two shower stalls, where nozzles puffed jets of air from their shoulders to their knees. A few seconds later, a computer screen handed down the verdict: "clean" or "dirty."

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