Abstract: The Alpha Energy Range Discrimination (AERD) method employs first and second proportional counter tubes separated by a foil and an electronic evaluation system, and comprises the following steps:(a) a filter is disposed in the stream of air flow and is thereby covered with dust, no artificial aerosol alpha activity being present;(b) the radiation of natural nuclides is measured by means of the first and second counter tubes in pulses per second, and respective count rates alpha.sub.1 and alpha.sub.2 are determined;(c) the ratio alpha.sub.1 /alpha.sub.2 is plotted graphically against alpha.sub.2 and corresponding constant values a.sub.1 and a.sub.2 are determined for each counter tube in accordance with the least square fit method;(d) the counter tubes are arranged with the second counter tube above the first counter tube, separated by the separating foil, so that the lower counter tube measures the radiation of both natural and artificial nuclides (count rate alpha.sub.