I once took a small Geiger counter to work with me to do my own experiment. I flew from London to Hong Kong then from Hong Kong to Melbourne and back.
I found a marked increase in radiation above FL250 above which level increases in radiation became roughly linearly proportional to altitude. Radiation was also proportional to latitude being much higher at higher latitudes falling at lower altitudes with a very marked drop when crossing the equator.
My Geiger counter only measured ionising radiation ( Alpha & Beta ) so Gama and X-ray didn’t register at all. I would post the figures here but they were all lost when I upgraded my PDA, sorry. As I recall the radiation levels never quite reached published danger levels but they weren’t far away.
I do have anecdotal evidence however that the levels on aircraft are high enough to cause concern in other industries. I used to work in atomic energy research. A colleague of mine took his family on holiday to Florida. He inadvertently packed his film badge dosimeter in his suitcase. He discovered it while on holiday but thought little of it. Some time after his return to work a routine test of film badges in his section almost caused an emergency shut down of the site. The radiation he had collected on his two sectors was enough to indicate a possible leak in one of the reactors.
Kirk out…