Study: The best and worst face masks for preventing COVID-19 spread

The study at Duke University says that N-95s, surgical masks, and homemade cloth masks are good, bandanas bad.

A study at Duke University has determined which face masks are the most of effective at preventing the spread of droplets emitted by people when talking, a crucial aspect of limiting exposure to COVID-19.

The results of the low-cost study into 14 different styles of face mask were shared in Science Advances, and shows a significant disparity between the best performing and worst.

Unsurprisingly, the best at preventing droplets are the N-95 masks (without an exhalation valve) that are used heavily in medical settings, which the study found blocked more than 99.9% of droplets emitted by a person wearing the mask.

Also scoring highly with medical-grade 3-layer surgical masks, which only emit slightly more droplets than an N95 mask, followed by face masks made with polypropylene.

Cotton masks also fared pretty well, though there was variation based on the type of mask being worn. The best-performing homemade cotton mask, pictured below, is a "2-layer cotton, pleated style mask," which over the course of 10 trials produced a result of blocking around 90% of droplet emissions.

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The worst performing was neck fleeces, which was actually worse for emitting droplets than wearing no mask at all, finding they emit 10% droplets more than talking normally without a mask.

This is because neck fleeces "seemed to disperse the largest droplets into a multitude of smaller droplets," which remain airborne longer than larger droplets.

"Considering that smaller particles are airborne longer than large droplets (larger droplets sink faster), the use of such a mask might be counterproductive," the study found.

Bandanas also performed poorly, blocking less than 50% of droplets on average, while knitted masks were the third worst performing with around 65% of droplets blocked.

Here's a look at the masks tested, their description, and the results. 

The masks were all trialed using a simple, inexpensive test involving an optical laser and a light sheet. You can read the study results here.

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