This scanner is not an x-ray scanner like the ones deployed to scan air travelers. X-rays are ionizing radiation, the kind which can cause cancer. The machine in the picture uses millimeter waves which are not ionizing, and produces a much fuzzier image.

Although the TSA claims the x-ray dose given to air travelers is safe, there are reasons for concern. For one, they refuse to disclose the x-ray flux. Also, the comparisons the TSA makes with other radiation sources are flawed because they compare to the absorbed x-ray dose and average it over the entire volume of a person. Anyone in radiation safety understands the important number is the dose absorbed in specific portions of the body. In this case, the x-ray radiation is absorbed in the skin (minus the very small scattered fraction), not your entire body volume, resulting in a much higher local dose than you would expect if you modeled the exposure as uniform through the volume.

Since the x-ray source strength is quite low (assuming the machine isn't multifunctioning), the health risks to occasional travelers are likely to be low. Maybe frequent travelers or airline employees should have greater concern, or those at risk of cataracts.

See:

http://www.npr.org/assets/news/2010/05/17/concern.pdf