It's much easier to understand the potential dangers of x-ray radiation backscatter scanners, but it has been difficult to know if the millimeter wave scanners pose any health dangers. There is some talk about breaking DNA, but I have not been able to find any sources that confirm the energy at which this happens and compare it to the energy of the scanners.
Now there's a report of a woman with a pacemaker dying minutes after passing through one of these scanners. It wasn't at an airport (it was at the Gaza-Egypt crossing). Maybe it was a rare occurrence, but that is precisely the point of the public health experts who point out that using the scanners as primary screening means that very rare incidences will occur because of the large volume of people screened.