LONDON (TIP): Researchers have found that a human sneeze produces a gas cloud that keeps potentially infectious droplets aloft over much greater distances than previously realized. 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 as previous estimates had assumed. 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.

Researchers used high-speed imaging of coughs and sneezes as well as laboratory simulations with mathematical modelling to produce a new analysis of coughs and sneezes from a fluid-mechanics perspective. “When you cough or sneeze, you see the droplets or feel them if someone sneezes on you,” says John Bush, a professor of applied mathematics at MIT. “But you don’t 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.” The study found that droplets 100 micrometres or millionths of a meter in diameter travel five times farther than previously estimated while droplets 10 micrometres in diameter travel 200 times farther. Droplets less than 50 micrometres in size can frequently remain airborne long enough to reach ceiling ventilation units.

A cough or sneeze is a “multiphase turbulent buoyant cloud,” as the researchers term it in the paper because the cloud mixes with surrounding air before its payload of liquid droplets falls out, evaporates into solid residues or both. “The cloud entrains ambient air into it and continues to grow and mix. But as the cloud grows it slows down and so is less able to suspend the droplets within it. You thus cannot model this as isolated droplets moving ballistically,” the scientists added.