The short answer is that the radiation risk from the TSA scanners is minimal for a member of the general flying public.
(This is separate from privacy concerns -- or the fact that the bad choice offered to passengers between intrusive x-rays vs. an intrusive physical exam is a problem ultimately caused by our government's inept foreign policy.)
Hence, my personal approach when I fly will be to go through the full-body scanners rather than undergo the aggressive new pat-down searches.
The news media has recently given a lot of attention to the following letter sent several months ago by scientists/physicians at UCSF (Univ. California at San Francisco) to the federal government about the radiation risks:
"Letter of Concern", 4/6/2010
This NPR story from last spring that covers the details more fully:
"Scientists Question Safety Of New Airport Scanners", 5/17/2010
The NPR story also includes a sidebar listing the radiation dose generated by a TSA scanner, and comparing it to the dose one receives merely from being on a transcontinental flight, regular environmental exposure, getting a chest x-ray, etc.
Basically, just getting on a transcontinental flight exposes one to roughly 1,000 times more radiation than undergoing a TSA body scan. (This is because there is less atmospheric protection from natural solar/cosmic radiation at high altitude.)
The FDA has posted its own response to the UCSF letter at:
"Response to University of California - San Francisco Regarding Their Letter of Concern", 10/12/2010
First let me note that I am philosophically opposed to the FDA and other such regulatory bodies, on the grounds that they do not serve proper functions of government. But to the best of my knowledge, the FDA's scientific arguments in that specific response are essentially correct. And the FDA letter also addresses some of the technical issues raised by the UCSF scientists, such as the question of the TSA radiation being deposited mostly in the skin (vs. in the whole body).
Female passengers who are (or may be) pregnant while undergoing a TSA scan may also wonder about radiation effects on a developing fetus.
This web page from Duke University covers this topic nicely:
"Fetal Radiation Dose Estimates"
As a point of clarification, the Duke website uses the older units (rems and millirems) for radiation dose rather than the newer units (Sieverts, milliSv, etc.).
The conversion factor is:
1 Sievert = 100 rem orAs the Duke website notes, if the fetus exposure to less than 1,000 millirem (10 milliSieverts), then there's no known risk to the fetus.
1 milliSievert = 100 millirem
If the fetus exposure is between 1,000 and 10,000 millirem (10-100 milliSieverts), then then the fetus is probably still ok. But, this is the range where bad effects to a fetus start to be observable in some studies, using the most conservative (cautious) statistical criteria.
So if a pregnant passenger wishes to take the most cautious approach and keep her fetal exposure below the 1,000 millirem (10 milliSievert) range, she could still undergo thousands of TSA scans per year. Again, the radiation exposure caused merely by flying would far exceed that caused by the scanner. Furthermore, most of the TSA scanner radiation would be stopped at the skin before it could even reach the fetus, as opposed to the various forms of natural gamma and solar radiation received during the flight which would penetrate deeper into the body.
A pregnant woman might naturally wonder how much radiation she'd be exposed to from the air travel itself?
According to this aviation news website, if she logged 1,000 hours in the air, then she'd be at the 5-10 milliSievert range (depending on the exact altitude/route), which is the level where one might begin to be concerned:
"Radiation Exposure Aloft -- Are You Being Nuked?"
So if she took 10 flights during her pregnancy totaling, say, 40 hours of air time, then that should be no problem. But she were an airline pilot or a frequent business traveler logging 1,000 hours of air time per year, then it might become a genuine issue, using the most conservative estimates for fetal exposure.
This discussion makes two important assumptions, including:
1) The TSA scanners are actually functioning properly and operating within the limits claimed by the government. Of course, if a particular machine malfunctions in a way that it produces too much radiation, then all bets are off.
2) The passenger doesn't have any special medical conditions that make him or her more sensitive to radiation than the general public.
Finally, this discussion applies only to the "backscatter" type of TSA scanner, which uses ionizing x-ray radiation. The other type of whole body TSA scanner uses "millimeter wave" technology, which does not involve ionizing x-ray radiation and does not have the same type of carcinogenic effect. Otherwise, I don't have any specialized knowledge about that particular technology and thus can't comment about any other health effects.
Conclusion: From a radiation safety perspective, it's generally safe to go through the TSA "backscatter" x-ray scanner.