Man that's annoying. I don't feel like these scanners have been impartially vetted by the medical community and while they are potentially safe, it's going to be a bummer to finally be forced through one.
And previously pregnant women, those with compromised immune systems, and parents of young children could opt out. Not so much now.
I will say that the current Millimetre Wave machines are likely safer than the now defunct Backscatter X-ray machines. At least we're dealing with non-ironising radiation with a lower amount of raw energy.
It's only got about 120 degrees of rotation, so it's tricky trying to tune it to emit a particular color.
I had it turned to ELF for a while, but then the Navy called and said my flashlight was disrupting their submarine communications.
Earlier this year, Terminal 5 at Heathrow wouldn't let me opt out of the scanner. Kinda felt like a personal violation, but the duty guards just said I was uneducated for having any concerns outside of their training.
Whenever I travel to a country that uses body scanners, I carry a printed copy of the airport's FAQ page (if available) and the government FAQ page or law that says that passengers are allowed to opt out. Then I know that I can show this to the people operating the checkpoints or to their supervisors if necessary. So far it was only necessary once (in the Netherlands, where body scanners are used routinely and some checkpoint staff aren't trained well about the opt-out option). But I still have a printout of the UK one and remember very clearly when the policy changed (prior to the change, I took the train through the Chunnel to Brussels instead of flying out of Heathrow).
Maybe I can set up a web page that has links to the policy and law documents so that people can find them more easily. There are already some pages that try to track which airports have body scanners, but I don't think I've seen one that tracks "what to show checkpoint staff to convince them that their country allows opt-outs".