It seems that not all face masks were made equal.

While public health experts have pushed the message that masks are essential in curbing the spread of COVID-19, a new study from Duke University shows that neck gaiters—a face-covering made of thin, stretchy material that shields the neck and lower face—may be inadequate in fighting transmission.

The study measured the effectiveness of various masks and face coverings, finding that some options, like cotton cloth masks, were almost as sufficient as a standard surgical mask—and that fitted N95 masks were the best option. Gaiters ranked as worse than the no-mask control group. Breathable and lightweight, the gaiter that was used was made from a “polyester spandex material,” per researchers.

The study compared 14 masks and coverings with a tool that researchers used to track individual particles released from a person’s mouth when they said the phrase, “Stay healthy, people.” Speakers said the same remark while wearing different masks or no mask at all, with each face-covering tested 10 times.

In a video from the study, Martin Fischer, Ph.D., an associate research professor in the departments of chemistry and physics at Duke, cautioned that neck gaiters are actually “counterproductive.” When a person wore the gaiter, more particles clung to the air after speaking through it than when wearing no mask at all. Fischer explained that the gaiter’s porous fabric seems to break bigger particles into smaller ones, making it more likely for them to stick in the air.

“It’s not the case that any mask is better than nothing,” he said. “There are some masks that actually hurt rather than do good.” The study also discovered that bandanas used as masks, knitted masks, and N95 masks with ventilation valves were inadequate.

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