Dean Armstrong is taking his USB Geiger-counter on civilian aircraft and measuring the radiation levels that he can detect at 12,000 feet and higher.
Southwest Airlines flight to LAX. I turned on the detector at roughly 12,000ft, and turned it off at the official 10,000ft announcement. According to the pilot cruising altitude was at 39,000ft. The big drop at 3/4 of the way across the graph was a temporary disconnection.Link (via Make Blog)