Natural background radiation contains besides the radioactive substances in our environment and in our body also cosmic radiation. Our planet is continually bombarded with high speed particles originating from outer space. Some particles originate from the sun, although most originate from outside our solar system.
We differentiate between primary and secondary cosmic radiation. Primary radiation consists of particles that attack the earth directly from space but do not reach the earth’s surface because of the protective shield created by the atmosphere. The interaction between primary radiation and the atmosphere results in secondary radiation, some of which does reach us. This process also creates other radioactive substances such as tritium and carbon-14.
Cosmic radiation increases as you rise through the atmosphere. The dose incurred approximately doubles with every 1.5 km of elevation. Three to four trans-Atlantic return flights equal the average annual exposure to cosmic radiation in Belgium, i.e. 0,35 millisievert per annum.
More info: In space