Irradiation of members of the public from external radiation produced by nuclear and other establishments is subject to control, and an essential part of control is measurement of the environmental radiation levels in the neighbourhood of these establishments'. The evaluation of the environmental air kerma from X and gamma radiation is difficult. The composition of the background air kerma is complex, with contributions from natural sources such as cosmic radiation and terrestrial radioactivity as well as from manmade radioactivity arising from the operation of nuclear facilities and fall-out from nuclear weapons tests. Furthermore, the natural background air kerma rate varies in time and space.