Around two thirds of existing radioactivity comes from natural sources:
- cosmic radiation a;
- radiation from the earth and buildings (construction materials) b;
- radiation from the human body c;
- soil gas radon d;
- medical applications e: some radionuclides can be used for tests (for example X-rays) and medical treatments (such as radiotherapy);
- industrial products f : radionuclides produced in nuclear installations and mining.