Repeated Coughing Seriously Degrades Face Mask Efficiency
University of Nicosia researchers Dbouk and Drikakis return with more important input in reviewing social distancing guidelines. According to their latest paper in Physics of Fluids, small droplets of saliva can emerge from a face mask, with some droplets then traveling as far as 1 metre.
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 paper published this week in Physics of Fluids, by AIP Publishing (On respiratory droplets and face masks), Dr. Talib Dbouk and Professor Dimitris Drikakis, from the University of Nicosia (UNIC) in Cyprus, use precise computer models to map out the expected flow patterns of small droplets released when a mask-wearing person coughs repeatedly.
Previous work from this research group showed droplets of saliva can travel 6 metres in five seconds when an unmasked person coughs, garnering a truly global impact. Their findings have been widely publicized by major news outlets, with a list of press mentions available on the AIP Publishing website. The paper itself is the most downloaded in the history of the journal.
This new work used an extended model to consider the effect of face masks and multiple cycles of coughing. The results show masks can reduce airborne droplet transmission. However, the filtering efficiency of masks is adversely affected by repeated coughing, as might happen when an individual is ill. In such a case, repeated coughs reduce the efficiency, letting many more droplets through.
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 metre, 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.