Effect of altitude on background radiation

There are several sources of background radiation. One source is the presence of radioisotopes in the soil, typically uranium and thorium (and their decay products) was well as potassium and other trace nuclides. The other is secondary radiation from cosmic rays.

The graph below shows the change in background radiation while aboard an airplane flight, vs the ground level radiation. The increase is due to additional radiation from cosmic rays, normally blocked on the ground by the atmosphere.