Wearing the same face mask more than once might be more dangerous than not wearing one at all, researchers say.
A new study has looked into the effectiveness of the now-common three-layer surgical masks.
What would have been an unusual site a year ago is now all-to-familiar for Brits who are required to wear coverings when out and about.
Boffins have found that when the masks are worn for the first time they filter out three-quarters of particles that linger in the air.
Those particles are responsible for transmitting infections.
But if the same mask is worn for a second or third time, they only filter out a quarter of the tiny droplets.
Masks become more deformed with each wear, The Sun reported.
The research was conducted by experts at the University of Massachusetts Lowell, and California Baptist University.
Co-author Dr Jinxiang Xi, an associate professor of biomedical engineering at UMass Lowell, said: "It is natural to think that wearing a mask, no matter new or old, should always be better than nothing.
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"Our results show that this belief is only true for particles larger than five micrometres, but not for fine particles smaller than 2.5 micrometres."
Researchers said that wearing a mask with less than 30% filtration efficiency could be worse than not wearing one at all.
For the study, professors developed a computer model of a patient wearing a surgical mask with pleats and tracked particles through the material.
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They also studied the aerosols after they went through the layers, looking at where they landed in the nose or pharynx.
Their modelling showed that when people wear masks, it changes the airflow around their face.
The team found that air does not enter through the nose and mouth at specific points but through the entire mask surface at low speeds.
Researchers found that a new mask being worn for the first time can filter 65% of particles, but a used one only keeps out 25%.
According to the study, this is because of how the pleats in the mask affect airflow patterns, and when worn more than once it can change shape and diminish efficiency.
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