Where does the radiation in human body come from ?

Natural radionuclides such as radon, carbon-14, tritium, postassium-40, radium-226, polonium-210 and lead-210, are present in air, food and drinking water. These radionuclides enter the human body through inhalation and ingestion, and give off radiation when they decay. The radionuclides are absorbed during digestion, but at the same time their amounts are reduced due to decay and excretion. When the intake and excretion of radionuclides reach a balance, a stable level of radioactivity will be maintained in our body. Please also refer to question (9).