Researchers created a test to determine which masks are the least effective

This is how masks protect us from the coronavirus 3:43

(CNN) - Schools reopen, amusement parks welcome visitors, and cookouts are the new way to eat out. But despite signs that life is returning to normal, the coronavirus pandemic has gone nowhere.

That's why a group of researchers from Duke University created a simple technique to test the effectiveness of various types of masks that have become a critical component in stopping the spread of the virus.

The search began when a professor at Duke School of Medicine was helping a local group buy masks in bulk to distribute to community members in need. The professor wanted to make sure that the group bought masks that were really effective.

In the study published Friday, researchers from Duke's physics department demonstrated the use of a simple method that uses a laser beam and a cell phone to assess the efficiency of masks by studying the transmission of respiratory droplets during regular speech.

A visual aid showing how the laser beam and light sheet work to capture respiratory droplets.

"We used a black box, a laser and a camera," Martin Fischer, one of the study's authors, told CNN. "The laser beam expands vertically to form a thin sheet of light, which we illuminate through the slots on the left and right of the box."

At the front of the box there is a hole where you can speak. A cell phone camera is placed on the back of the case to record the light that is scattered in all directions by respiratory droplets that cut the laser beam when they speak.

Encourage the use of effective masks

Public health experts have spent months emphasizing that masks are one of the most effective tools to help fight the pandemic, and many US states have introduced some type of mask requirement.

But when testing their effectiveness, the researchers found that some masks are literally useless.

The researchers tested 14 commonly available masks, including a professionally fitted N95 mask, generally reserved for healthcare workers. First, the test was conducted with a person speaking without wearing a mask. Then they did it again while a speaker was wearing a mask. Each mask was tested 10 times.

The most effective mask was the fitted N95. The three-layer surgical masks and cotton masks, which many people have been making at home, also worked well.

The 14 masks used in the test.

Neck fleeces, also called neck masks and often used by runners, were the least effective. In fact, wearing such a mask resulted in a higher number of respiratory droplets because the material seemed to break down the larger droplets into smaller particles that are more easily carried through the air.

Scarves and knitted masks also performed poorly and did not offer much protection.

"We were very surprised to find that the number of particles measured with fleece actually exceeded the number of particles measured without wearing a mask," said Fischer. "We want to emphasize that we really encourage people to wear masks, but we want them to wear masks that really work."

While the test setup is fairly simple - all that is required is a box, a laser under $ 200, a lens, and a cell phone camera - Fischer does not recommend that people install them at home. .

Unless a person is familiar with laser safety or has experience in optics, mishandling powerful lasers can cause permanent eye damage. However, the researchers hope that companies, museums and community outreach centers will set up the test to show people which masks are the most effective.

"This is a very powerful visual tool for raising awareness that very simple masks, like these homemade cotton masks, do very well to stop most of these respiratory droplets," Fischer said. "Companies and manufacturers can set this up and test their mask designs before producing them, which would also be very helpful."

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