Second Edition, June 2012 - Research has shown that hydrophobic membrane filters, with a pore size of 0.22 mm, do not retain micro-organisms under all process conditions. Investigations were conducted in risk assessment of sterilising hydrophobic membrane filters, evaluating the performance of the filters under a range of operating conditions. To validate the bacterial retention ability of sterilising grade hydrophobic membrane filters, a bacterial aerosol challenge test methodology was developed.