Radioactivity exists everywhere in nature. We are exposed to intense and continuous natural
radiation coming from the sun, cosmic radiation (from stars other than the sun), telluric radiation
(from the ground), and even to the internal radiation of our own body (about 10 000 Becquerel
for an average individual). The amount of natural radioactivity can vary, from one region to
another. The average background radiation on Earth is by the order of 0.2 |xSv/h, or about 2
mSv/year.