When it comes to homemade face masks, two or three layers of fabric is best, researchers say.
That's what you need to keep droplets from your nose and mouth from spreading the virus, the Australian scientists found.
Several kinds of material have been suggested for making masks, but there's little or no evidence of how effective they are, the team noted.
For the study, the researchers compared single- and double-layer cotton face masks (with a thread count of 170 per inch) with a 3-ply surgical face mask.
The single-layer mask was made from a folded cotton T-shirt and the double-layer mask was made using the sewing method recommended by the U.S. Centers for Disease Control and Prevention.
The findings were published online July 23 in the journal Thorax.
A video recording showed that the 3-ply surgical mask was the most effective, but even a single-layer mask reduced the spread of droplets.
And the double-layer mask was better than a single-layer mask, the researchers said in a journal news release.
Effectiveness may also depend on other factors, including the material used, design and fit, and how often the mask is washed, according to C. Raina MacIntyre, of the Biosecurity Research Program at the Kirby Institute at University of New South Wales, and colleagues.
"Guidelines on home-made cloth masks should stipulate multiple layers," the researchers concluded. "There is a need for more evidence to inform safer cloth mask design, and countries should ensure adequate manufacturing or procurement of surgical masks."
For more on making your own face mask, head to the U.S. Centers for Disease Control and Prevention.
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