Five heat and moisture exchangers were investigated to compare their efficiency of humidification, their ability to filter bacterial spores and their various physical properties. The results are presented and the various mechanisms of heat and moisture exchange are reviewed. The Pall Ultipor BB50, because of its hydrophobic properties, has a slightly different action from heat and moisture exchangers already in use, The place of the Pall Ultipor BB50 in clinical practice is discussed.