Face masks recommended to help prevent flu transmission

March 13, 2013 — 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.

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.

M. Patricia Fabian, a research fellow in Exposure, Epidemiology, and Risk also contributed to the study. Lead author of the study was Donald Milton of the University of Maryland Institute for Applied Environmental Health.

In a previous paper published online January 2, 2013, in Aerosol Science and Technology, McDevitt, Fabian, and colleagues described the air sampler that they designed and built for testing exhalations from influenza patients. Known as the G-II, the device allows researchers to test subjects while they perform various respiratory functions such as talking and coughing, with and without a mask.

More research is needed on how the flu is spread and how to prevent transmission, McDevitt said. “However, the first line of defense against getting the flu is getting the flu vaccine.”

— Amy Roeder

Photo: iStockphoto.com/Jimmy Kim