An anonymous reader sends this quote from a story at NPR about the accelerated deployment of new scanning machines at airports: "Fifty-two of these state-of-the-art machines are already scanning passengers at 23 US 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. ... The San Francisco group thinks both the machine's manufacturer, Rapiscan, and government officials have miscalculated the dose that the X-ray scanners deliver to the skin — where nearly all the radiation is concentrated. The stated dose — about .02 microsieverts, a medical unit of radiation — is averaged over the whole body, members of the UCSF group said in interviews. But they maintain that if the dose is calculated as what gets deposited in the skin, the number would be higher, though how much higher is unclear."