Radiation is energy that comes from a source and travels through space or a material medium in the form of waves or particles. It can be produced by unstable atoms that undergo radioactive decay or by machines. Radiation has many beneficial applications, but it can also pose risks to human health and the environment. Different types of radiation require different protective measures, with higher energy "ionizing radiation" requiring more protective measures than lower energy "non-ionizing radiation".

All matter is composed of atoms, which are made up of various parts. Radiation is given off by matter in the form of rays or high-speed particles through a process called radioactive decay. Radioisotopes lose their radioactivity over time, and this gradual loss of radioactivity is measured in half-lives. A half-life of a radioactive material is the time it takes for half of the radioactive atoms in a group of radioactive isotopes to decay.

In the United States, radiation absorbed dose, dose equivalent, and exposure used to be measured and stated in traditional units called rad, rem, or roentgen (R), respectively. EPAs mission in radiation protection is to protect human health and the environment from the ionizing radiation that comes from human use of radioactive elements.