Face masks reduce the risk of spreading large droplets linked to Covid by up to 99.9% when speaking or coughing, according to a new study.

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KONRAD K./SIPA

Face masks reduce the risk of spreading large droplets linked to Covid-19 when speaking or coughing by up to 99.9%, according to a new lab experiment performed with mannequins and humans, researchers said on Wednesday .

A woman standing two meters from a coughing man without a mask will be exposed to 10,000 times more such droplets than if he were wearing a mask, they report in the journal Royal Society Open Science.

"Face masks can drastically reduce droplet dispersion"

"There is no doubt that face masks can dramatically reduce the dispersion of potentially virus-laden droplets," says lead author Ignazio Maria Viola, an expert in applied fluid dynamics at the School of Engineering at the University of Edinburgh .

The large postilion-like respiratory droplets - which act like projectiles before being attracted to the ground by gravity - are believed to be the main driver of transmission of SARS-CoV-2, he notes.

The finer droplets, which form aerosols, can remain suspended in the air for longer periods and represent a risk especially indoors in poorly ventilated areas, without drafts, particularly if they are crowded with people who do not wear a mask, or wear it poorly, below the nose.

"We continuously exhale a whole range of droplets, from micro-scale to millimeter scale", and "some droplets fall faster than others depending on temperature, humidity and especially the speed of the current air ”, notes the researcher.

"99.9% reduction"

The study focused on particles larger than 170 microns in diameter - about two to four times the width of a human hair.

Aerosol particles, which tend to follow air currents, are generally described as smaller than 20 or 30 microns.

“In our study, for the largest droplets that we measure, we are talking about a 99.9% reduction,” underlines Ignazio Maria Viola.

The World Health Organization (WHO) recently updated its guidelines for masks to recommend that they be worn indoors in the presence of other people if ventilation is insufficient.

Masks are primarily used to reduce the emission of virus-laden droplets from those who cough, sneeze, sing, speak, or simply breathe, but they can also help prevent droplet inhalation by people who wear them.

Wearing a universal mask would reduce the death toll worldwide by 400,000 by April 1, according to the Institute for Health Metrics and Evaluation (IHME) in Seattle, Washington.

"Sheet masks not only effectively block most large droplets - 20-30 microns or more - but they can also block the exhalation of fine droplets and particles, often referred to as aerosols," according to the US CDC.

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