The plan for broad use of X-ray body scanners to detect bombs or weapons under airline passengers' clothes has rekindled a debate about the safety of delivering small doses of radiation to millions of people -- a process some experts say is certain to result in a few additional cancer deaths. The scanning machines, called 'backscatter scanners,' deliver a dose of ionizing radiation equivalent to 1 percent or less of the radiation in a dental X-ray. The amount is so small that the risk to an individual is negligible, according to radiation experts. But collectively, the radiation doses from the scanners incrementally increase the risk of fatal cancers among the thousands or millions of travelers who will be exposed, some radiation experts believe. Full-body scanners that are already in place in some airports around the country and abroad use a different type of imaging technology, called millimeter wave, that uses less powerful, nonionizing radiation that does not pose the same risk. But those machines also produce images that are less clear. And in the wake of the attempted bombing of an airplane traveling to Detroit from Amsterdam on Christmas Day, the United States is turning to backscatter scanners for routine security checks. Congress has appropriated funds for 450 of the scanners to be placed in American airports. In a 2002 report on the safety of backscatter scanners, the National Council on Radiation Protection and Measurements, which is highly influential in setting regulatory standards, said it 'cannot exclude the possibility of a fatal cancer attributable to radiation in a very large population of people exposed to very low doses of radiation.' One author of that report, David J. Brenner, a professor of radiation biophysics at Columbia and director of the university's Center for Radiological Research, said that risk might be increased as the transportation agency moves from using the scanning machines as a second-round check after metal detectors and hand searches to using them as a first-line screening system. 'When we were looking at these a few years back, it was always going to be as secondary screening tool,' he said. 'In that scenario, I don't think there's too much concern.' But, he said, if millions or tens of millions of passengers a year were scanned with the backscatter X-ray, he said, the risk would be higher. 