carried out on a filter according to the invention identified hereinafter by the symbol f 1 by way of comparison similar tests were also carried out on a filter of the same composition and the same surface weight but modified by addition of an antistatic agent this filter is identified by the symbol f 2 further tests were finally carried out on a third filter of the same surface weight available in a market this known filter is identified hereinafter by the symbol f 3 filter according to the invention in a form of non woven having a surface weight of 135 g m 2 and a weight ratio of fibers 2 to fibers 1 equal to 10 90 the fibers 2 had a 1 5 denier titled filter identical to f1 with in addition an antistatic agent for filter with a surface weight equal to 135 g m 2 available on a market and produced by a triboelectric process as disclosed in u s the results obtained with these different triboelectric filters are as follows in the above table the quality factor is a numerical value which has been calculated with the following equation q ln ue89e 100 efficiency 100 u00d7 lost of pressure the quality factor permits classifying the efficiency of the filters the higher the value of lt lt q gt gt the higher is the quality of the filter as can be seen the filter f 1 according to the invention has proved to be more efficient that the known filter f 3 the poor results obtained with filter f 2 confirm that the electric charges have an important influence of the efficiency of the.