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The SARS outbreak in 2003 has spawned a major controversy concerning protective performance facemasks for healthcare workers. This study reports a study on in-vivo protective performance of surgical masks and N95 respirators.Typical surgical masks and N95 respirators used in Hong Kong hospitals were tested in comparison with those treated with nano-functional materials (called nanomasks) on various physical properties and in-vivo wear filtration efficiency, as well as usability test in hospitals for surgical masks.Tests on physical properties showed that N95 respirators had significantly lower air permeability and water vapor permeability than surgical masks. The in-vivo filtration tests illustrated that N95 respirators filtered out 97% of potassium chloride (KCl) solution, while surgical masks filtered out 95% of KCl solution. Nano-masks show stronger water repellency and antibacterial activities, but no difference in usability, comparing with normal N95 and surgical masks.Surgical masks can provide in-vivo filtration protection of 95% filtration efficiency. N95 respirators provide higher in-vivo filtration efficiency of 97% with significant reduction of air permeability and water vapor permeability. Compared to normal surgical masks/respirators, the nano-masks can provide additional protective functions in stopping capillary diffusion and antibacterial activities. Am. J. Ind. Med. 49:1056-1065, 2006. © 2006 Wiley-Liss, Inc.