The radioactive contamination of materials - regardless of whether that is air, water, soil, building materials or food - is usually expressed as activity of a radionuclide per mass (unit Bq/kg) or as activity per volume (unit Bq/l or Bq/m³). In particular cases, the activity can also be related to the surface area, for example in the case of the deposition of radionuclides on the ground after the Chernobyl reactor accident (unit Bq/m² or Bq/km²). In order to be able to make statements about possible hazards to human health, the measured activities (per mass or volume) of the radioactive substances have to be converted into doses (organ dose or effective dose, unit Sv).
