Atmosphere blocks not only Ultraviolet rays. It completely shields us from alpha, beta, and gamma radiation.
Note that on the ground (zero altitude), Geiger counter registered zero Gamma radiation. But as the balloon took off, Gamma radiation increased dramatically, peaking at around 60,000 feet.
- Flying an airplane exposes passangers to ~3 uSv/hr of ionizing radiation.
- Background radiation near Chernobyl nuclear plant reads ~6 uSv/hr today