Radiation dosimetry in the field of health physics is the measurement, calculation and assessment of the ionizing radiation dose absorbed by an object, usually the human body. This applies both to ingested or inhaled radioactive substances and to irradiation due to an external source of radiation. The worldwide average background effective dose for a human being is about 3.5 mSv per year, mostly from cosmic radiation and natural isotopes in the Earth.
The largest single source of radiation exposure for the public is naturally occurring radon gas.