How high does light pollution reach into the sky?
Light pollution occurs because light from the ground refects off atoms in the atmosphere. So you can reduce light pollution either by getting away from  light, or getting above the atmosphere. 50% of the atmosphere lies below 5500m,  if you can get 5500 m high, you half light pollution.  Getting high also improves clarity, and reduces the disturbance caused by atmospheric turbulence, which is why most major telescopes are situated on mountain tops.