Airports in Britain, the Netherlands and Canada have said they plan to use full-body scanners to foil future terror attempts like the Christmas Day attempt to blow up a Detroit-bound flight.
The United States has tested 40 whole-body scanners as part of a pilot program started after the September 11 attacks, and this past October ordered 150 more.
There are two types of machines -- millimeter wavelength imaging and backscatter X-ray scanners. Both are used to see under clothes and identify unusual objects.
Only one -- backscatter X-ray machines -- expose individuals to ionizing radiation such as that used in common medical X-rays.
But the radiation levels are well below the threshold that could be considered a risk to an individual's health, said Dr. James Thrall of the American College of Radiology and chief of radiology at Massachusetts General Hospital in Boston.
"All of the concerns that we have about the medical use of X-rays really don't apply to these devices," Thrall said in a telephone interview.
"The exposure is extremely low and the energy of the X-rays is also very, very low," he said.
"When X-rays are used for medical imaging purposes, they have to be energetic enough to get through the human body. The X-rays used in the backscatter machines in airports have such low energy that they literally bounce off the skin. That is what backscatter implies," Thrall said.
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