Air kerma in air is the sum of kinetic energy of all charged particles liberated per unit mass. A number of publications in the past have expressed measurements in terms of absorbed dose to air. Recent publications and an IAEA Code of Practice point out the experimental difficulty in determining the dose to air, especially in the vicinity of an interface, and that, in reality, what the dosimetry equipment registers is not the energy absorbed from the radiation by the air, but the energy transferred by the radiation to the charged particles resulting from the ionization. For these reasons the IAEA Code of Practice and ICRU Report 74 recommend the use of air kerma rather than absorbed dose to air. The unit is the joule per kilogram (J kg-1) and is given the special name gray (Gy)