Not only is it possible to study mask effectiveness against viruses, it has actually been done scientifically several times. The conclusions of multiple real scientifically valid studies has been that masks (even N95 masks) are NOT effective at preventing or even significantly diminishing influenza or similar virus infections. This has been discussed at length in several Free Republic threads over the past 15 months.
I am not aware of ANY scientifically valid study that demonstrates any more than minimal virus protection.
Many commercially available N95 masks even include a disclaimer in their packaging that it does NOT protect against virus infection.
Could you point out one of those studies?