We are constantly bombarded with cosmic radiation from outer space as well as primordial radiation from natural radioisotopes in the earth. We get exter­nal doses from uranium, thorium, and radium in rocks and internal doses from radionuclides in foods. We also get internal doses from breathing in radon that comes from radium in the rocks. With the exception of doses from the food we eat, which depend somewhat on our personal dietary choices, our background levels of radiation depend mainly on where we live, how much time we spend outdoors, and on characteristics of the homes we live in.