Natural radioactivity

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.