[sokoloff]

Anywhere that US TSA runs the AIT scanners, you can opt out of them*.

That is domestic US airports plus airports like Toronto and Dublin where you, for practical purposes, clear into the US on foreign soil and land in the US as a domestic flight.

* - I think this only doesn't apply if your boarding pass got tagged with the dreaded "SSSS" enhanced screening tag, but that's a fairly rare corner case for most passengers.

[yencabulator]

My understanding, which may be wrong. It's been a few years since I did this dance.

You can opt out of the millimeter wave radar.

Opting out means you go through a metal detector, a 20-second pat-down and perhaps a hand swab for explosives sniffer.

If you have SSSS on your boarding card, that means the pat-down, hand swab and digging through your carry-on luggage happen whether you opt out of the mmwave or not.

[sokoloff]

It's clear that you can opt out of AIT (mm wave) scanning if you don't have SSSS and uncertain otherwise.

From the TSA website, https://www.tsa.gov/news/press/factsheets/technology , "Most passengers have the opportunity to decline AIT screening in favor of physical screening. However, some passengers will not be able to opt out of AIT screening if their boarding pass indicates that they have been selected for enhanced screening."