You don't need to have any reason to opt-out from millimeter wave scanners. Just ask for a pat down. I do it all the time when traveling to the US, because I think these machines are useless, expensive and invasive. If enough people did this a simple, legal, act of civil disobedience would force the TSA to rethink the way they operate.

That seems like a weird reason since the radiation counter argument was typically countered by pointing out the radiation from the flight itself likely exceeded the scan by a lot.

This is likely about as safe as wifi. The paper someone linked describes a system using around 24-30 GHz microwaves, one of the company blogs says the power they transmit at those frequencies is even less than a typical router. This isn't unusual. You can see a list of over 800 FCC approved devices in that frequency range here: https://fccid.io/frequency-explorer.php?lower=24000&upper=30... (I also didn't see anything there for Liberty Defense, the company suspects they'll have a product ready by 2020 but we'll see. Radar and imaging applications are tricky, this company could implode beforehand.)

AFAIK, metal detectors (the standing ones) use a simple coil and a receiver on the other side to detect changes in the EM field. The hand-help ones like these ones use either radio waves or microwaves to scan for metal, so non-ionizing radiation. In both cases metal detectors should be safe.

I'd rather be concerned if this is safe for prone for melanoma.