Comment Geiger data (Score 1) 275
I've been playing with Geiger counters (and scintillation tubes) for a while. On one trip I took a small basic Geiger tube on an international flight. At cruise altitude the radiation was about 15x the level on the ground and when plotted with the altitude it correlated almost perfectly - down to the small 2000 foot altitude changes.
By the way, I haven't measured it, but standing at a store with a bunch of smoke detectors stacked up should provide a significant exposure too.