NRPB published a report describing why us pilots are classed as radiation industry workers and therefore permitted to have a higher annual dose of radiation compared to 99% of the public. Basically, we'd never do more than 2 months a year if kept to the normal limits. Still low quantities but it makes for an interesting read. One day in (IIRC) the early 1970s you could have had an entire years-worth of radiation in one 8 hour sector at FL330+ from northern Europe to North America. Something to do with sunspot activity and the higher doses received in higher latitudes.
Suggest you contact the NRPB (National Radiological something Board) for more details.