Abstract: The permissable level of radioactivity due to airborne actinides is extremely low: the satisfactory discrimination of actinide emissions from other sources is, consequently, a problem. This specification discloses the combination of (a) selecting only the larger airborne particles, (b) measuring the activity of emissions at the energy characteristic of actinide decay of these particles and (c) decay analysis of the measurements to reject emissions which are characteristic of high energy processes but which have lost sufficient energy to fall within the measurement band.