There isn't as much concern about the health implications since the backscatter machines were moved to low volume airports a few years ago (I haven't seen any a

There isn't as much concern about the health implications since the backscatter machines were moved to low volume airports a few years ago (I haven't seen any at major airports anyway), so they're primarily using the microwave machines (non-ionizing radiation) at major airports. Interestingly, it wasn't the risks that led them to opt for microwave scanners - backscatter machines were too slow for the volume of passengers seen at large airports.