Is your COVID-19 mask working? If it’s a bandanna, maybe not much.

The shopper darts into a store, quickly tying on a bandanna or pulling up a shirt collar at the last minute. Enough to satisfy the overworked clerk who is tasked with enforcing COVID-19 mask requirements? Probably.

But not so effective at preventing the spread of coronavirus, a new study suggests.

The authors of the study did not use actual viruses or real people, but simulated coughing with a mannequin and a pump that sprayed a mixture of gelatin and water. The results, from engineers at Florida Atlantic University, were consistent with previous research, not to mention common sense: Not all masks are created equal.

Masks continue to be a flashpoint in the COVID-19 wars, with some opponents claiming they cause breathing problems (they don’t) and others viewing them as an infringement on personal freedoms. (Hello, seat belts?)

When it comes to the key question of whether they reduce the spread of disease, the evidence is clear that they can, said Abrar Chughtai, an epidemiologist at the University of New South Wales in Sydney, Australia, who was not involved with the new study.

As public-health officials have been advising for weeks, a homemade mask may not prevent the wearer from becoming infected, but it reduces the risk that he or she infects others, especially in conjunction with hand-washing, said Chughtai, who co-authored a review of 19 mask studies. That fact is especially relevant for the coronavirus, as infected people can spread disease before developing symptoms.

“We don’t know who is healthy and who is sick,” he said. “That’s why it is recommended that everyone should use masks.”

In the new study, published in the journal Physics of Fluids, researchers measured how far simulated saliva droplets traveled through various types of coverings. The average distances varied:

Manhar R. Dhanak, one of the study authors, said the researchers used the artificial mannequin-and-pump setup to generate the same amount of droplets over and over.

“It gives you a way to repeat the experiment,” said Dhanak, a professor in Florida Atlantic’s engineering school.

The fog of droplets was illuminated with green laser light.

While the simulation was not much like the real thing, it was a reasonable approach for comparing the permeability of various materials, said Chughtai.

The findings were consistent with what others have found, the epidemiologist said. Generally, two layers are more protective than one. Fabric with a high thread count is better than loosely woven material. And a snug fit is key.

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“It should fit around your face, so it is not leaking out,” he said.

Further research will involve analyzing the effectiveness of masks in containing droplets of various sizes, Dhanak said.

In the meantime, he and Chughtai agreed with the advice from health agencies worldwide: Mask up.