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Bacterial retention properties of heat and moisture exchange filters.

We have examined the properties of six heat and moisture exchange filters (HMEF) to ascertain their resistance to liquid flow and their ability to retain a challenge bacterium, Pseudomonas diminuta, from aqueous and nebulized suspensions. Only one HMEF, the Pall Ultipor was able to withstand a significantly greater pressure of liquid than that found in clinical practice. However, when breached, the HMEF were unable to prevent transmission of micro-organisms from aqueous suspension. Only the Darex Hydrobac filter failed to meet the manufacturer's claim for filter efficiency for nebulized bacteria, mainly because the filter housing failed under test. When the reduction in bacterial cells after passage of the nebulized Pseudomonas diminuta through the HMEFs was analysed statistically, the data showed that the HMEF produced by Pall (Ultipor) and Intersurgical (Filter therm) were superior to those produced by DAR Mediplan (Hygrobac), Intertech (HME 225-2835-800) and Gibeck (Humid-vent).