When atmospheric testing of nuclear devices was resumed in the latter part of 1961 it was found that certain commercial aircraft were being contaminated by airborne radioactive debris to such a degree that decontamination was required, with associated protection of personnel. Some airlines seemed reluctant to publish detailed information on the position. BOAC, however, recognised the phenomenon as a virtually permanent one and in recent months has been setting a lead in developing the process of measurement and control from a somewhat academic exercise to a properly understood engineering routine. When the nature of the hazard is understood—and particularly the all-important distinction between external radiation and internal radiation—it can be seen that, in fact, passengers and crews are not at risk at all, and that handling and maintenance personnel need not be concerned if the contaminated surfaces are washed by an essentially wet process and personal hygiene rules are observed where necessary.