[aperiodic]

The health risks can't just be quantified by the overall radiation dose. It depends on how that dose is deposited in the subject. The letter raises the concern that the since almost all the radiation is absorbed by the skin, the dose to your first centimeter or two of epidermal tissue may be fairly high. If the calculations that concluded the devices were safe assumed that the radiation was evenly distributed throughout the body, then the safety of the devices could be suspect.

[pygy_]

Thanks for the details.

I could not find the energy of the X-Rays used for these scans, but they would likely be low-energy (high energy rays don't scatter much), hence absorbed mostly superficially, as you said.