Cosmic Rays collisions
A primary cosmic particle which enters the Earth's atmosphere gives rise to a shower of secondary particles. Collisions with nuclei of oxygen or nitrogen create some radioactive nuclei such as radiocarbon or tritium. Some of these secondary particles will reach the ground and hundreds of them pass through our bodies every second. This cosmic bombardment makes up a part of the natural radioactivity we are constantly exposed to.
INFN/IN2P3