[Glass fibre HEPA filters. I. Communication: Microbiological and physico-chemical researchs on used and unused, hydrophobic and hydrophilic filter materials at relative humidities up to 98% in the climatic test chamber (author's transl)].

1. At high humidities up to 98%, an increase in bacteria cannot be detected. This applies to all HEPA filters investigated from the various manufacturers, independent of whether they are hydrophilic or hydrophobic, used or unused. 2. Fungal growth can only be obtained when large amounts of nutrient material (e.g. bacterial cultures) are present. In these cases a streaky growth appears . Under normal conditions of use, however, such intense contamination is not to be expected because of the prefiltration. Penetration only occurs with fungi. 3. Limitation of supply air humidity to 90% or less seems therefore to be unjustified on the basis of these investigations, if prefiltration and a satisfactory intermixing section can be guaranteed technically, and condensation is avoided. 4. HEPA filters of Grade S from various manufacturers and with different behaviour with respect to water do not allow bacterial growth even in the presence of nutrients. Additional measures for the killing of bacteria are therefore not considered necessary.