Face masks slow the spread of viruses, including the novel coronavirus that causes Covid-19, but their filtering efficiency is adversely affected by repeated coughing, say researchers.
"Masks decrease droplet accumulation during repeated cough cycles. However, it remains unclear whether large droplets or small ones are more infectious," said study researcher Talib Dbouk from the University of Nicosia in Cyprus.
For the findings, published in the journal Physics of Fluids, the research team used 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 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 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.
Repeated coughs reduce 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 modelled 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.
According to the team, 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, 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 droplet sizes change and fluctuate continuously during cough cycles as a result of several interactions with the mask and face," said study author Dimitris Drikakis.
"The use of a mask will not provide complete protection. Therefore, social distancing remains essential."
( With inputs from IANS )