If you're flying from Amsterdam to the US, you do. They have usually blocked off the metal detectors and claim that you have to use the body scanners (you do have the right to opt out, but it's not stated there).
They're millimetre wave machines, I think, so they likely are safe (certainly not ionising), but there are some theorised mechanisms by which they could impart greater energy on cellular molecules than is otherwise believed to be the case.
What really annoys me is that if these were medical devices, they would require many years of design approval and safety trials, so that their safety wouldn't be assumed but assured (to a high probability) before they could be deployed. Their maintenance regimes would be heavily regulated. However, when it's for security (or rather for lining the pockets of well-connected vendors and contractors), then none of this needs to be done.