> McDevitt and his colleagues screened 89 volunteers, 37 of whom tested positive for the flu and were asked to provide exhaled breath samples with and without a surgical mask. Among the volunteers, 43% exhaled large influenza-containing particles while not wearing a mask, compared to 11% while wearing a mask; 92% exhaled small influenza-containing particles while not wearing a mask, compared to 78% while wearing a mask. In addition, they found that the flu virus was 8.8 times more numerous in exhaled small particles than in large particles.

From what I understand that suggests that surgical masks are only slightly effective. But that study only tests surgical masks that don't filter particles less than 0.5 microns, and not N95 masks which can filter out particles as small as 0.3 microns.

http://www.cidrap.umn.edu/news-perspective/2012/04/lab-study...

> A US-Chinese team of researchers created a simulated examination room with mechanized mannequins spaced 6 feet apart to represent coughing and breathing humans. They found that flu viruses floated between the two and were "inhaled" by the breathing mannequin, but that an N95 respirator sealed to the mannequin's face stopped 99.8% of them.

If that's accurate, it's pretty incredible. Out of paranoia I've sometimes worn surgical masks for viral transmission to the office during flu season, but it looks like I should be visiting Home Depot instead of Walgreens.