they were not safe, and they were only approved because the research that said they were safe was done by the manufacturers themselves. They were taken away once academics and other experts showed that the risks were way too high using them
https://en.wikipedia.org/wiki/Backscatter_X-ray#Health_effec...
Ironically enough, millimeter wave scanning is less well-understood than backscatter x-ray!
Reminds me of the BPA free garbage. Many of the replacements are/were worse than BPA. But no one seemed to care.
BPA has mostly been replaced with BPS ... which is also an estrogen mimic.