A study by Pratik Mehta of University of California, Berkeley and Dr Rebecca Smith-Bindman of University of California, San Francisco has concluded that there is “no significant threat” from backscatter X-ray scanners despite them using ionising radiation.

The study said that the doses are very low – less than one per cent of the additional radiation a person gets from flying in an airplane – that only a handful of cancer cases are likely to result directly from scanner use.

“If individuals feel vulnerable and are worried about the radiation emitted by the scans, they might reconsider flying altogether since most of the small, but real, radiation risk they will receive will come from the flight and not from the exceedingly small exposures from the scans,” wrote Pratik Mehta and Dr Rebecca Smith-Bindman.

The authors wrote that among the estimated one million US frequent fliers who take ten 6-hour plane trips per week for a year, there would be four additional cancers. About 4,00,000 cancers would occur in those fliers anyway.

Mehta and Smith-Bindman said that one would need to undergo 50 airport backscatter scans to equal the exposure from a dental X-ray, 1,000 to equal the exposure from a chest radiograph, 4,000 to equal a mammogram, and 2,00,000 to equal a CT scan.

The authors recommended that the TSA should not use body scanners but approve of independent testing devices. There are 486 of such scanners in use in 78 US airports. By the end of 2011, there will be around 1,000.