Covid-19: N95 mask most effective in stopping cough spread

A 54-year-old Cardiothoracic surgeon from Sri Jayadeva Institute of Cardiovascular Sciences and Research, Dr Prasanna Simha Mohan Rao and his 24-year-old son Padmanabha Prasanna Simha, an aerospace engineer at Vikram Sarabhai Space Centre, ISRO, have visualised and researched the cough flow fields under various common mouth covering scenarios.

They found that N95 masks are the most effective at reducing the horizontal spread of cough. This was published in the journal Physics of Fluids on August 24.

It has also elicited the interest of The Optical Society of America who has reached out to the duo after the publication.

Jayadeva's Director Dr CN Manjunath, a member of Karnataka's Technical Advisory Committee (TAC) plans to have the study findings on the meeting agenda of TAC so that a public advisory is sent out for naysayers who are spreading misinformation about the usage of masks.

Dr Rao told DH that he was peeved with the videos on social media against the usage of masks and that is when he and his son used schlieren imaging, that shows the density of the cough droplets, its motion, velocity and spread.

Credit: Dr Prasanna Simha Mohan Rao

"Schlieren imaging measures temperature and density gradients. When you cough, there is warm air, which is less dense than the ambient air. So we could see how it travels. We positioned our subjects farther away from each other. When we cough, there is a contamination bubble around us where we can infect each other. With a three-ply surgical mask, the bubble is 0.5 to 1.5 metres," Dr Rao explained.

"We are not suggesting everyone buy N95 masks. Wearing any mask, including cloth masks, is better than wearing none. An N95 mask is most effective because it limits the spread of the salivary droplets expelled while coughing between 0.1 and 0.25 metres. With a disposable mask on, the droplets travel up to 2.5 metres, and when the mouth is not covered, the droplets travel up to three metres," Dr Rao said.

"There is also a reduction of velocity in the droplets by up to ten times when one is wearing an N95 mask," he added. The father-son duo started the study before the first lockdown in March in February when the first cases were being reported in the country. "We used five subjects including my son and his friends. In all, four men and one woman," said Dr Rao.

He went on to explain that Indians tend to use their arms or elbows to cover their mouths while coughing which was proved to be ineffective during their study.