Have you ever used Scotch or packing tape? Incadescent or flourescent lighting? If so, then you've been exposed to Terahertz waves in quantities far in excess of these scanners.
Incandescent or florescent lighting: You have some cites? Doesn’t make sense that it would be a higher dose — a super wattage incandescent bulb maybe. But in proximity to areas of the body normally covered by clothing?
Air is opaque to terahertz, and we are talking dosage at exposed surface. While terahertz might be higher at the filament, there’s always plenty of air between skin and source.
I remember you are the one hung up on “ionizing” effects — I mean you tried already to gloss over bio-hazard by saying terahertz is non-ionizing. So what? Like microwaves are also non-ionizing yet on the weak polar bonds between water molecules they’ll cook you to death by resonance.
I’ll bet there are a lot more bio-molecules that absorb resonant energy from tera hertz radiation than yet we yet know, and that some of those will have some very strange and mutagenic effect.