[Comparative studies of gelatin and cellulose ester membrane filters for their suitability in determining the microorganism count in the air].

Filtration is one of the classical methods for the determination of the microbial content of the air or the number of microbe-bearing particles in the ambient air. Hereby, the opinion prevails that gelatin foam filters yield better results than filters made of cellulose esters. Since gelatin filters are themselves by no means free from disadvantages, several tests were carried out under the conditions encounted in practice in two packaging rooms used for pharmaceutical preparations. The objective was to ascertain whether the customary cellulose ester filters really are less suitable for determination of the microbial content of the air than the filters made of soluble gelatin. Based on the results of various preliminary experiments, gamma-irradiated filters of cellulose nitrate, with a pore size of 0,45 micron, were selected from a range of tested membrane filters (Tables 1,2,4-6 and Fig. 1) and compared directly with the gelatin filters. The statistical evaluation of these tests showed that, under the chosen experimental conditions, both filters yielded the same count of organisms (Tables 3, 9 and 10, Figs. 2 and 3). Moreover, by means of further tests, it could be proved that neither cellulose nitrate filters with a pore size of 0,8 micron, which had undergone gamma-radiations, nor filters made of the mixed esters of cellulose, which had been treated with ethylene oxide or autoclaved, differed significantly from the selected type (Tables 5-8). This study therefore clearly demonstrates that - in addition to gelatin filters - a whole range of membrane filters made of cellulose esters yield equally good results and may be used for supervision of the microbial content of the air in closed rooms.