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With the current reports of swine flu and the possibility of a pandemic being addressed, the subject of personal protective equipment against the biohazard is also being discussed. In this discussion, I think that the size of the virus should be considered.
"N95" is a NIOSH standard that specifies a mask which will trap about 95 percent of particles with diameters of 0.3 microns or larger. A sneeze or cough creates droplets about 4 microns across (typically from 3 to 10 microns across or about PM2.5 to PM10 in size). A flu virus has a diameter of about 0.08 to 0.1 microns (thus, these are the size of ultrafine particles or UFP), and they cannot be trapped by an N95 mask. As such, N95 respirators only offer protection against the droplets or, more precisely, the flu viruses trapped in the aerosol generated by sneezes and coughs. The flu viruses that not trapped in droplets, however, freely move in the air and can easily pass through the filtered facepiece. Regardless, the N95 respirator is touted because it is relatively cheap and better than nothing.
Here's a link to a good hygiene practice to review: