Various products are advertised and available to provide personal protecton in case of a SARs outbreak.  Based on CDC recommendations, here are the primary considerations (the CDC web site has extensive recommendations):

a) Look for a NIOSH rating of at least 95 ... this will be normally listed as N95.  The 95 represents a 95% efficiency on a rigourous standardized test.  N 99 is a higher rating than N95, and N100 represents the highest rating.  Some masks may have an additional drying material which in some studies has been shown to help prevent virus from contaminating the mask.

b)Properly putting the mask on, and good hygiene is probably more important than the difference in the ratings in most situations.  If worn around a suspect virus, be sure to wash hands before and after removing. Remove using straps. Never dangle mask at your neck. Once used, do not store in a plastic bag (contamination spread).

c) Consider a supply of the cheaper N95's rather than the much more costly N99s or N100's.  If mask is contaminated, be sure to dispose properly.

d)While there are comfort and other differences between brands, any filter or mask with a NIOSH certification will do the job adequately.

e)One option to consider if you anticipate wearing for extended peroids is the exhale value.  This makes the mask easier to tolerate over long peroids (heat, breathing ease).