Measurements of the germ emissions in various activated sludge units with different ventilations systems have shown that the lowest density occurs with small bubble ventilation; with medium bubble and brush ventilation rather high values were measured; the spray devices used for foam elimination had a particularly unfavorable influence. In great germ densities above the pools (50-100 000 germs/m3) - depending on the weather - between 500 and 1500 germs/m3 were measured at a distance of 50 and 100 meters. At distances of 200 and 400 m the number of germs was about the same as normally measured in the outside air (100-500 germs/m3). In a closed pool altogether considerably higher values were measured: In the immediate neighbourhood of the activated sludge unit they amounted to abt. 10 000-25 000 germs/m3; in the hall 3000-4000 germs/m3 were measured, whereby the part of coliform bacteria amounted to 1-2% and that of enterococci to 2-4%. It has to be taken into account that besides the measured intestinal flora further pathogenous agents are present in the aerosols. In closed units, therefore, a greater risk of aerogenous infections exists, whereas in open-air pools this danger can be judged as very small.