The radiation from airport full body scanners is negligible and poses no real threat to health, say US experts.
Many US airports and some UK airports now have the security devices to help combat the threat of terror attacks.
The scans use X-ray beams, similar to those used in hospitals, and some have raised concerns about their safety.
But California scientists writing in Archives of Internal Medicine calculate they contribute under 1% of radiation people are exposed to during a flight.
Most of this comes from the cosmic rays at high altitude.
Mr Patrick Mehta and Dr Rebecca Smith-Bindman, experts in public health and radiology at the University of California, say even the most frequent flyers who clock up 60 hours a week in the air will face only a minute increase in cancer risk.
For example, the scans might cause four extra cancers among a million of these frequent flyers, they say.
In comparison, 600 cancers could occur in these travellers from the radiation received during the flight itself and 400,000 cancers would be expected to occur throughout their lifetime anyway, regardless of their travel exposure.
And the threat to children is also low, they say.
They estimate that for every 2m five-year-old girls who travel one round-trip a week, one additional breast cancer would occur from the scans.
However, 250,000 breast cancers will occur in this group over their lifetimes owing to the 1% lifetime incidence of breast cancer.
Paul Hadfield, a spokesman for Manchester Airport, said the findings should help reassure air passengers.
"We now have full body scanners at all three of our terminals for the 18.5m passengers that we have every year.
"The upside of the scanners is that they cut queuing - checks now take 30 seconds rather than over two minutes per person - and it means passengers do not have to be touched to be checked as part of the patting down process.
"Obviously if people hear the word X-ray they think radiation and they can start to worry, but the expert opinion we rely on - the Radiation Protection Board and the Health Protection Agency - confirms that the scans are safe.
"And acceptability levels among our passengers are extremely high. Since February 2010 when the scans became compulsory we have had only nine refusals. And only one of those was health-related."