Actually have seen a Geiger counter/Rem meter sat for measurement purposes just under the wind screen on a B727. The meter showed millirems. Depending on what time of year you fly this meter from sea level to 10,000 would point to around 10 O'clock which would increase to about the 12 O'clock position until you went above 29,000 to 31,000 whereby it increased to approx the 2 to 2;30 position.
I used to fly at 45,000 all the time just because I did not know any better and the jet was much more efficient and used less fuel. The guy who showed me this was a maintenance officer for the SR-71 and was a PHD I think...no matter he was weird but his instruments did not lie. Those guys flew above 85,000 and it is still classified.
Most Airliners cruise at 31,33,35,37,39, and some can make it to 41,000 if they are not heavy.
Usually on a long distance flight we would step climb to our final altitude, IE, maybe to heavy to go much above 31,000 but after taking off, climbing, and cruising for an hour (or so) usually you can make either you intermediate (next step climb like 35,000 and finally 39,000 final cruise.
Why go so high???
Because you won't get there if you don't.
Down low fuel burn might be 9000 pounds per engine per hour at a high power setting. At altitude your cruise fuel consumption might be as low as 2700 pounds per engine per hour at above 93% power...
These are just examples and not aircraft specific.
Many Airline pilots die at 62.5 years which was study done by ALPA several years ago. No reference just a memory but could probably be found. I retired with over 28,000 hours flight time from civilian, military and commercial flying. It was what I always wanted to do so no regrets here. You as a passenger even on a super platinum card will never get that much exposure in several lifetimes..
