UV has already been well covered above. Exposure to ionizing radiation is a function of:
1) Altitude (because the atmosphere attenuates the secondary radiations which cause this exposure, the primary radiation having had nuclear interactions with the upper atmosphere well above aviation altitudes)
2) Magnetic Latitude (because as you near the magnetic poles the magnetic field lines are increasingly parallel to the trajectories of the energetic particles and impede their progress less)
3) Solar cycle (because, perhaps a little counter intuitively, the primary source of this harmful radiation is galactic cosmic rays, and magnetic fields created by increased solar plasma help deflect these from Earth)
3a) ...the caveat!.. however certain solar events that can send streams of energetic particles towards Earth can cause localized increases.
Now the interesting trivia part....there is a region in the Southern hemisphere that behaves differently *BUT* it only affects altitudes 200km+ (so satellites, not aircraft)
https://en.wikipedia.org/wiki/South_Atlantic_Anomaly