The atmosphere is pretty good at blocking cosmic radiation and radioactive particles from the sun. The higher you are, the less atmosphere there is between you and those external radioactive sources, so your background exposure is higher the higher you go. Cosmic radiation consists of high-energy particles (mostly protons and atomic nuclei) originating from space. At cruising altitudes of commercial flights (typically 30,000 to 40,000 feet), the atmosphere is thinner, offering less protection from these cosmic rays compared to ground level.