Shall we have some numbers?
Exposure increases drastically with altitude.
I don't have any good numbers to hand, but ISTR Concorde crew typically get about 7μSv per hour. 50mSv per year is the "benchmark" upper limit for occupational exposure, so they would have to be in the air 7000 hours per year in order to reach the limit. This would be extremely unlikely!
Exposure (per hour) would be rather lower on conventional airliners.
1. It's very hard to collect lots of accurate data.
2. The 50mSv per year is an upper limit - above this there are known risks, but there may well be (small) risks below this threshold
3. The types of illnesses likely to be caused by repeated x-ray exposure take a long time to appear.
A series of studies have mostly shown slight
increases in malignancies amongst flight crew, but it is extremely difficult to separate xrays from other environmental effects in the cabin, and also from oddities of diet / hormones / body clock disruption.