Check out the math in the article linked below, the quote coming from dosimetry expert Peter Rez.
Protests Mount Over Safety And Privacy Of Airport Scanners : Shots - Health News Blog : NPR:
"Rez agrees the individual risk is still negligible. 'It's a 1-in-20-million chance of dying from radiation for each scan,' he says. 'Your chances of being struck by lightning in the US in any year is 1 in 500,000. But the probability of being blown up in an airplane by a terrorist is around 1 in 30 million. So the risk from the scan is about the same as the thing you're trying to prevent.'"