We know that wearing a mask can stop the spread of COVID-19 but are all masks created equal – and are all of them actually effective in stopping the spread? From surgical masks and N95 respirators to cotton ones you make at home, or that bandana from the back of your sock drawer – there are so many different types to choose from. Now a new study is shedding some light on which masks are effective at reducing droplet transmission. Big surprise –there was actually one type of mask that increased the spread of droplets.

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The idea for the experiment came about when a professor at Duke’s School of Medicine was helping a local group buy masks for community members in need. The professor wanted to make sure the masks being purchased were actually effective. So, researchers in the Duke physics department set up an experiment using a laser beam and a cell phone to see just how effective the masks were during regular speech.

The study, published Friday, tested face mask efficacy for filtering expelled droplets during speech.

“We use a black box, a laser and a camera,” Martin Fischer, one of the authors of the study, told CNN. “The laser beam is expanded vertically to form a thin sheet of light, which we shine through slits on the left and right of the box.”

The way it works is that a speaker talks into a hole on the front of the box while a cell phone records light that is scattered by the droplets that cut through the laser beams. A computer algorithm counts the droplets.

Fourteen common types of masks were tested, and each mask was tested 10 times. Not surprisingly, the fitted N95 mask was the most effective. Three-layer surgical masks and the cotton masks people have been making at home also performed well.

Gaiter masks, which are neck fleeces that runners wear, turned out to be the least effective form of protection. Wearing a gaiter actually resulted in a higher number of droplets because the material broke down the larger droplets into smaller ones, resulting in more particles carried in the air.

And although that bandana may have come in handy that time you took your kid to a dude ranch, save it for your next cowboy costume. This type of mask may look slightly cooler than a cotton face mask but does little to reduce droplets from spreading in the air when you speak.

Also, keep in mind that the CDC does not recommend N95 respirator masks or surgical masks for the general public, asking that these be saved for medical professionals and frontline workers. Instead, stick to the ones cotton ones you sew at home – adhering to the CDC guidelines – or if you’re as crazed as most parents these days, those three-packs from Old Navy.

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