I took a few readings with my Geiger counter while flying down to California today. At ground level (in Hillsboro), the level was at ~ 0.11 usv/hr, which is in the typical background range if not a bit low. At 20,000 feet is was 0.64 usv/hr (5.8x) , and at cruise at 42,700 feet it was 3.41 usv/hr (31x). Those are all still relatively low doses, although if you were exposed to 3+ usv/hr for your entire lifetime, that would be a significant increase in risk.