Dust particles and air molecules floating in the air absorb radiation and also reflect it scattering and deflecting it back into space. The higher the elevation the more unfiltered radiation that area will receive. A high elevation simply lies above many of the dust particles and air molecules preventing the radiation's passage. And if an area is high enough, it will also perch above another factor limiting radiation, clouds. It makes sense that clouds play a big role in the amount of UV radiation reaching the ground.
Additional factors limiting UV radiation include haze, smog and other air pollution.
All these factors are taken into account in a scale, called the UV index, measuring the amount of UV radiation reaching the ground and its relative danger to humans. Most can withstand the noon sun for an hour without burning. An average person can stay under the noon sun for 30 minute to an hour without burning. The UV rays may burn the skin in 13-20 minutes. Skin may burn in less than 13 minutes.