Radioactivity has many sources. Some of them are natural, and some man made.
Natural Sources include:
Radioactive nuclides in rock, espeially granite. Granite contains radioactive isotopes of Radon, Thorium and Potassium, among others.
Cosmic rays produce a constant stream of radiation.
The food we eat contains carbon 14, produced by cosmic rays hitting carbon atoms and transforming. Plants absorb the Carbon, and we either eat the plants, or eat the animals that eat the plants.
Man made sources include:
Radiation from X – Ray machines and the isotopes used in medicine.
Fallout from nuclear explosions – both atmospheric and underground - and leaks from power stations, when they explode. Nuclear power stations also produce large amounts of radioactive waste.
The proportions of background radiation due to these different sources are illustrated below.