For those who wish to read a good brief on this, an article published in a nuclear technology news magazine I once edited,
Nuclear News, provides an excellent overview of the issue of air crew radiation dose:
http://www3.ans.org/pubs/magazines/nn/pdfs/2000-1-3.pdf
Though published in 2000, the article contains a lot of hard data that are still valid.
Incidentally, there is increasing evidence that the current LNT (linear no-threshold--i.e., any dose at any level is bad) theory of dose response and radiation risk to humans vastly overstates radiation risk. It is well known that exposure to low-level dose can actually reduce cancer prevalence below normal rates because it stimulates cell repair mechanisms, a process called "hormesis":
"Hormesis is a biological phenomenon whereby a beneficial effect (improved health, stress tolerance, growth or longevity) results from exposure to low doses of an agent that is otherwise toxic or lethal when given at higher doses."
Sorry for the thread drift, but this subject was brought up. . . .