There was another recent union email that actually had some interesting info on that, stuff I've wondered about and conversed with coworkers about many times.

As altitude increases, UV-A and UV-B rays intensify by 15% per 3,000 feet—and the intensification is cumulative. At 36,000 feet, an altitude at which many of our flights operate, the rays are potentially 435% stronger than on the surface. PPG, who makes the windshield on our CRJ aircraft, stated that the windshield blocks over 99% of UV-B rays and approximately 50% of UV-A rays. Although UV-B rays are the primary culprit when you get a sunburn, UV-A rays go further down in the skin—damaging keratinocytes. Exposure to both can lead to skin cancer.