When we travel by airplane at high altitude, we are generally exposed to higher cosmic radiation, compared to the amount we receive on the ground.  Between 6km and 12km altitude where the commercial airplanes usually fly, we receive an elevated level of cosmic radiation, mainly from galactic and solar origins.

Sources of galactic cosmic radiation originate from the deep space.  Galactic cosmic radiation consists of high-energy electromagnetic component (gamma radiation) and nucleonic component.  Radiation enters our atmosphere and interacts with the air molecules, producing cascades of secondary galactic cosmic radiation (Figure 1).  They attenuate when travelling through the atmosphere where their energies are absorbed by the surrounding air molecules.  The primary and secondary galactic cosmic radiations share the largest portion of the radiation dose we receive during air travel at high altitude.