They say they are safe, but ever since I was full body scanned the first time (when they first were introduced) I've developed some funny skin marks:
First is a mark on the pad my pinky finger... it appeared and has been growing. it's now the size of a .25 carat diamond, and all my doc says is "let's keep monitoring it"... LOL. Until when, it covers my finger tip? It's not a mole, it's under the skin, and I swear it throbs each time I've been scanned since.
Second mark: on my forehead near the hairline. same deal... started off as a dark mark the size of a pin-pick and has grown so that it can be easily seen.
As a woman, I don't like the pat downs... and often when offered this as an alternative, there is an underlying "We're going to do a VERY thorough search if you don't acquiesce and get scanned"... so in the past 2 years I was scanned 10 times at least. (Yikes. I had not counted before...)
I know this is stupid and have started opting out more. I no longer have to travel for business as frequently and choose to take the train or other land transport when convenient... (and sometimes when not. But a 28 hour train ride over a weekend beats a 4 hr wait at the airport, scanning, and horrible flight in my books!)
I'm rambling to say "better safe than sorry. Opt out. I wish I had never subjected myself to the scanners."