TSA scanners: are they safe?

  As we have written before here and here and here, a number of scientists and observers have been concerned about the safety of backscatter TSA scanners and the TSA's lack of transparency over the details of that safety.
  Indeed, if you've been through security lately, you've seen people lined up like they're in an old episode of The Twilight Zone, arms up, arms down when they tell you, standing inside the enclosed scanner being sized up and down by an invisible radiation beam. Few people question its safety or efficacy.
  Drudge has linked an article from Propublica concerning a letter 5 scientists have written about these very issues:

For Rez, the real danger occurs if the machine stops in the middle of a scan, allowing the beam to focus on a tiny area for several seconds. Given that the backscatter works with a wheel rotating at a high speed, and that the agency plans to use the scanners continuously 365 days a year, mechanical failures are likely, he said.

  Requested materials from the TSA were heavily redacted and unhelpful.
  Surprisingly. 
  This issue isn't going away. Considering all the conspiracy theories out there right now, including the theory that bin Laden is still alive and that Bush was responsible for 9/11, it's surprising that THIS one hasn't taken off more than it has.