I once flew with a pilot who had his own Geiger Counter. Setting it on the ground on the centre console at LHR, there was one click every 6 seconds on average. As we climbed to cruise altitude 33,000' heading ESE across Europe, the count went up to 6 counts/second- a 36-fold increase. There was also a marked increase going higher up to 37,000'. On the back of an envelope, a 36 fold increase in ground radiation for about 800 hours a year (less than 1/10 of your life)- you're looking at over 4 times a normal ground dwellers radiation. Enough to create a significant statistical risk factor for airline crews. For those spending a significant time on Polar flying as opposed to tropical flying, you go to higher doses.
Having said that, old pilots do tend to be the fittest people I know, and definitely rather immature (like me)!