Some mathematical expressions are given for the calculation of the activity concentration (13N,15O) in the surroundings of medical electron accelerators with a maximum photon energy up to 25 MeV. The author differentiates between prolonged irradiations typical of physical operation or maintenance work and shorter irradiations separated into fractions which are typical of the treatment of patients. In most cases the calculated concentrations are considerably lower than the limit values valid e.g. for persons of category B professionally exposed to radiation, so that the margin of error assumed for this calculation method is of no significance. A measure of the activity concentration which is always time- and cost-consuming would be unnecessary in that case.