During flu season, sufferers are advised to prevent spreading the virus by covering their mouths when coughing or sneezing and by washing their hands. But these methods may not be enough, according to a new study by Harvard School of Public Health (HSPH) researchers and colleagues.

According to the study, which was published March 7, 2013 in PLOS Pathogens, infected people exhale flu virus particles of varying sizes, and the smallest particles contained the highest concentration of flu virus. These small particles would not be expected to be stopped by traditional flu hygiene methods.

“Our study suggests that flu virus in small particles (less than 0.5 microns) may play a larger role than traditionally thought in the spread of influenza,” said James McDevitt, instructor in HSPH’s Exposure, Epidemiology, and Risk Program. It had previously been assumed that larger particles were associated with flu transmission, he said.

The researchers also found that by wearing masks, study participants infected with the flu had a 25 fold decrease in the amount of large virus particles they exhaled compared with those not wearing a mask. Decreases for small particles were a more modest 2.8 fold reduction.

While the U.S. Centers for Disease Control recommends that doctors and hospitals provide their flu patients with face masks, little is known about the infectiousness of exhaled particles and the effectiveness of masks in preventing their spread.

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