Evidently a German whose job was the monitoring of radiation in ground plants had travelled in a flight deck with a geiger meter and it went epileptic. The report was every extensive and quite scary.
It must also have been nonsense. I've travelled quite frequently with a radiation spectrometer (advanced version of a geiger counter) and whilst the radiation is greater than that at ground level it's far, far, far, below the amount deemed to be even slightly hazardous. If you spent every possible second of your life in the air then it might be a problem after a few years, but for crews it's not a problem.
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Why don't we install sensors in every a/c cabin and monitor cabin air at any time? Because we would find something?
There's been a few studies that have done exactly that, to my knowledge none of them found anything. Possibly because there's nothing to find or possibly because they didn't encounter a fume event on the flights that were studied.