The British Committee on Radiation Units and Measurements (BCRU) Guide to the Measurement of Environmental Gamma-ray Dose Rate (Spiers et al, 1981) recommended the use of the quantity “absorbed dose to air free-in-air (i.e. scatter-free except for air scatter) and that the measuring device has a wall thick enough to give electron equilibrium. Air kerma, which uses the same unit, gray, as absorbed dose, can be taken to have the same numerical value as absorbed dose to air under conditions of electron equilibrium”. Conversion factors from exposure, air kerma and absorbed dose to air effective dose equivalent were included as Chapter 7 in this BCRU guide. The authors of the Guide noted that the International Commission on Radiation Units and Measurements (ICRU) had set up a working party to put forward proposals on the most appropriate quantity to use for measuring ambient radiation for routine protection purposes and suggested that their recommendations could influence the choice of quantities used in future. The International Commission on Radiation Units and Measurements has now published its recommendations (ICRU, 1985) and has proposed various quantities of which ambient dose equivalent is recommended for the measurement of penetrating environmental γ radiation. The National Radiological Protection Board (NRPB, 1986) and the BCRU (BCRU, 1986) have endorsed the ICRU recommendations (ICRU, 1985) for use in assessing the exposure of people.