Survival and growth of micro-organisms on air filtration media during initial loading

A new type of air filtration medium made from a hygroscopic polymer fibre a nd constructed in three layers was investigated to measure the survival and growth of micro-organisms on this medium in comparison to a widely used fi breglass medium. Both materials were supplied by the manufacturer and teste d "blind". The materials were loaded in an Airotester unit. Micro-organisms were analysed at 2 weekly intervals for 8 weeks by washing filter samples and plating the solution on to agar media and by vital fluorescence microsc opy. Filter samples were also weighed to calculate water content and the pH value of the filter material was measured in the wash out eluate. Vital fl uorescence microscopy revealed fungi were able to grow on fibreglass medium , but not on the multi-layered polymer. The colony forming unit (CFU) count s did not increase at a steady rate. There was a significant increase on bo th materials (P <0.001) during the first 2 weeks which was then followed by a significant decrease in 4 weeks (P <0.001) but the CFU then significantl y increased in 6 weeks (P <0.05) which were the highest CFU counts during t he 2-month trial. There was a significant difference in CFU counts between the filter materials only in week 2 (P less than or equal to0.00 1) and wee k 4 (P = 0.04). Fewer micro-organisms were extracted from the multi-layered polymer than from the fibreglass medium. Fewer fungal species were identif ied on the multi-layered polymer (nine species) than on the fibreglass medi um (13 species). The pH value on the multi-layered polymer was significantl y higher than the fibreglass material but only when clean (P <0.010) and af ter 2 weeks (P <0.001). A significantly higher water content on the fibregl ass medium (P <0.00 1) also indicated a habitat where a wider range of fung al species and bacteria are able to survive. While there was a reduced surv ival and growth of micro-organisms on the multi-layered polymer material in the initial month of service life., this advantage was cancelled by the su pply of nutrients (particulate matter) that were accumulated on the filter materials after 6 weeks. (C) 2001 Published by Elsevier Science Ltd.