Dodge that cough cloud entering your nose!

Travelling inside a crowed Metro or a jam-packed public transport bus? Make sure that you raise your elbow to cover up that turbulent buoyant cloud your neighbourhood commuter is about to expel.

Coughs and sneezes stay airborne for long distances in gas clouds that keep their potentially infectious droplets aloft over much greater distances than previously realised, the researchers said.

"When you cough or sneeze, you see the droplets, or feel them if someone sneezes on you. But you do not see the cloud, the invisible gas phase. The influence of this gas cloud is to extend the range of the individual droplets, particularly the small ones," explained John Bush, a professor of applied mathematics at Massachusetts Institute of Technology (MIT).

The smaller droplets that emerge in a cough or sneeze may travel five to 200 times further than they would if those droplets simply moved as groups of unconnected particles.

The tendency of these droplets to stay airborne, resuspended by gas clouds, means that ventilation systems may be more prone to transmitting potentially infectious particles than had been suspected.

"You can have ventilation contamination in a much more direct way than we would have expected originally," added Lydia Bourouiba, an assistant professor in MIT.

The researchers used high-speed imaging of coughs and sneezes, as well as laboratory simulations and mathematical modeling, to produce a new analysis of coughs and sneezes from a fluid-mechanics perspective.

"If you ignored the presence of the gas cloud, your first guess would be that larger drops go farther than the smaller ones, and travel at most a couple of meters," Bush noted.

"But by elucidating the dynamics of the gas cloud, we have shown that there is a circulation within the cloud - the smaller drops can be swept around and resuspended by the eddies within a cloud and so settle more slowly," researchers emphasised.

Basically, small drops can be carried a great distance by this gas cloud while the larger drops fall out, said the paper published in the Journal of Fluid Mechanics.