Masks block 99.9% of large droplets linked to COVID: study

Large respiratory droplets are believed to be primarily responsible for the transmission of SARS-CoV-2.  Banksy Illustration

Large respiratory droplets are believed to be primarily responsible for the transmission of SARS-CoV-2. Banksy Illustration

Face masks reduce the risk of spreading large drops linked to COVID when speaking or coughing by up to 99.9 percent, according to a laboratory experiment with mechanical mannequins and humans, the researchers said on Wednesday.

A woman who is two meters from a man coughing without a mask will be exposed to 10,000 times more of these drops than if he were wearing one, even if he is only 50 centimeters away, reported in the newspaper Royal Society Open Science.

“There is no longer any doubt that masks can drastically reduce the dispersion of droplets potentially loaded with viruses,” senior author Ignazio Maria Viola, a specialist in applied fluid dynamics at the University of Edinburgh School of Engineering, told AFP.

Large respiratory droplets – which act as projectiles before being pulled toward the ground by gravity – are believed to be primarily responsible for the transmission of SARS-CoV-2, he noted.

Smaller ones, sometimes called aerosol droplets, can remain suspended in the air for longer periods.

“We continuously exhale a wide variety of drops, from the micro scale to the millimeter scale,” said Maria Viola by phone.

“Some of the drops will fall faster than others” depending on temperature, humidity and mainly the speed of the air, he said.

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

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

Droplets of intermediate size can behave anyway, the study concluded.

Using universal mask

“If you wear a mask, you are mitigating virus transmission by an order of magnitude – 10 times less,” said Maria Viola.

“In our study, for the largest drops we measured, we are talking about 99.9 percent less.”

According to the Institute for Health Metrics and Evaluation (IHME) in Seattle, Washington, 55,000 lives could be saved in the United States in the next four months if a universal mask policy is adopted.

The Institute’s modelers – who accurately predicted in mid-July that US COVID-19 deaths would reach 224,000 on November 1 – project 561,000 deaths in current trends as of April 1, 2021.

The use of universal masks would reduce the global death toll by 400,000 in the same period, from 2.9 million to 2.5 million, they estimate. To date, the virus has claimed about 1.7 million lives.

Earlier this month, the World Health Organization (WHO) updated its COVID-19 guideline on masks to recommend that they be used indoors in the presence of others if ventilation is inadequate.

The guidelines apply especially in areas of known or suspected community transmission.

The masks serve mainly to reduce the emission of virus-droplets loaded by people when they cough, sneeze, sing, speak or simply breathe, but they can also help prevent the inhalation of droplets by the person wearing them.

“Fabric masks not only effectively block most large drops – 20-30 microns and larger – but they can also block the exhalation of fine particles and droplets, also often called aerosols,” according to the CDC.

Masks are not enough to prevent the spread of COVID-19 without distance: study

More information:
Lucia Bandiera et al. Facial coverings and droplet dispersion of the respiratory tract, Royal Society Open Science (2020). DOI: 10.1098 / rsos.201663

© 2020 AFP

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