"However, researchers at the University of California, San Francisco, have argued that the amount of radiation is higher than claimed by the TSA and body scanner manufacturers because the doses were calculated as if distributed throughout the whole body, but the radiation from backscatter x-ray scanners is focused on just the skin and surrounding tissues"

"Furthermore, other scientists claim the health effects are backscatter are well understood whereas those from millimeter wave scanners are not"

Some of the machines being deployed by the TSA are millimeter wave scanners, BTW.