Face masks are thought to slow the spread of viruses, including the novel coronavirus that causes COVID-19, but little is known about how well they work.
In a new study, researchers used precise computer models to map out the expected flow patterns of small droplets released when a mask-wearing person coughs repeatedly.
They found that masks can reduce airborne droplet transmission.
However, the filtering efficiency of masks is strongly affected by repeated coughing, as might happen when an individual is ill. Repeated coughs reduce efficiency, letting many more droplets through.
The research was conducted by a team from the University of Nicosia in Cyprus.
Previous work from this research group showed droplets of saliva can travel 18 feet in five seconds when an unmasked person coughs.
This new work used an extended model to consider the effect of face masks and multiple cycles of coughing.
The model was created using complex mathematical equations for turbulence and other flow effects.
A sequence of coughs was modeled by applying several cycles of forward-directed velocity pulses to the initial droplets.
The researchers performed numerical simulations that account for droplet interactions with the porous filter in a surgical mask.
The results are alarming. Even when a mask is worn, some droplets can travel a considerable distance, up to 1 meter, during periods of mild coughing.
Without a mask, droplets travel twice as far, however, so wearing a mask will help. A mask also decreases the number of droplets that leak out the side of the mask but fails to eliminate it entirely.
These calculations also revealed an effect on the droplet size due to turbulent flow encountering the mask, escaping, and entering the environment.
The team says masks decrease the droplet accumulation during repeated cough cycles. However, it remains unclear whether large droplets or small ones are more infectious.
The use of a mask will not provide complete protection. Therefore, social distancing remains essential.
For health care workers, the researchers recommend much more complete personal protective equipment, including helmets with built-in air filters, face shields, disposable gowns, and double sets of gloves.
The authors of the study are Talib Dbouka and Dimitris Drikakisb.
The study is published in Physics of Fluids.
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