The physics and biology of the TSA's backscatter security scanners

From ars technica:

The recent deployment of backscatter scanning devices meant for airline passengers has caused controversies focused on both the privacy issues of the scans and the safety of the devices themselves (not to mention the unpleasant alternative of an aggressive frisking). The discussion of safety issues has been clouded by two competing narratives. On one side, there's radiation exposure that's comically low compared to what comes from simply boarding the aircraft and being lifted above a lot of the Earth's atmosphere. On the other, there are arguments that the sort of exposure generated by backscatter devices is somehow different.