Fig. 2. Scanning electron microscopy (20× magnification) of the clean filtering membrane of the three breathing filters tested: DAR Barrierbac S® (Mallinckrodt DAR, Mirandola, Italy) (  A ), Baxter Airlife® (Baxter Canada, Mississauga, Ontario, Canada) (  B ), and Pall BB25A® (Pall Canada Ltd., Mississauga, Ontario, Canada) (  C ). The filtering membrane of the Pall BB25A® is much more compact and its pore size are much smaller. 