Fifty-two of these state-of-the-art machines are already scanning passengers at 23 U.S. airports. By the end of 2011, there will be 1,000 machines and two out of every three passengers will be asked to step into one of the new machines for a six-second head-to-toe scan before boarding.
About half of these machines will be so-called X-ray back-scatter scanners. They use low-energy X-rays to peer beneath passengers' clothing. That has some scientists worried.
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