Radiation at 50,000 feet

"When cosmic rays crash into Earth's atmosphere, they produce a spray of secondary particles. With increasing depth in the atmosphere, the primary cosmic radiation component decreases, whereas the secondary radiation component increases. This complex situation results in a maximum of the dose rate at an altitude of ~20 km, the so-called 'Pfotzer maximum,' named after physicist George Pfotzer who discovered the peak using balloons and Geiger tubes in the 1930s." - (spaceweather.com)   The secondary, ionizing radiation increases to a maximum elevation between 40,000 to 72,000 feet.  

Commercial passenger planes get up to this altitude.  Airline crews and people who use planes a lot for business purposes accumulate a radiation dose that can incapacitate them.  Sometimes radio waves will shake some of the radiation down, making the dosage even higher. Sometimes there is a coronal mass ejection (from the sun) that hits our ionosphere, exaggerating this problem.

In late 2015 two planes had incapacitated flight crews a few hours apart from each other.  Both of them made emergency landings safely.  If you are inexperienced, here's how to fly a commercial plane.

For more esoteric information, see "Ionizing Radiation in Earth's atmosphere and in Space Near Earth" by the FAA