Scientists Say Coughs and Sneezes Stay Airborne for Long Distances

Posted on April 12, 2014

MIT researchers has found that coughs and sneezes have associated gas clouds that keep them airborne for long distances. These clouds, known as multiphase turbulent buoyant clouds, help keep infectious droplets in the air for greater distances than previously realized. This cloud helps droplets travel five to 200 times further than they would if the droplets simply moved as groups of unconnected particles.

John Bush, a professor of applied mathematics at MIT, and co-author of a new paper on the subject, says in a statement, "When you cough or sneeze, you see the droplets, or feel them if someone sneezes on you. 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."

MIT researchers used high-speed imaging to determine that some droplets from coughs and sneezes may carry much farther than previous studies had estimated. Take a look:

The researchers analyzed coughs and sneeze from a fluid-mechanics perspective. Their conclusions upend some prior thinking on traveling mucus droplets. Researchers had previously assumed that larger mucus droplets fly farther than smaller ones. However, researchers found that thanks to the gas cloud smaller droplets can travel larger distances. The researchers found that droplets less than 50 micrometers in size can frequently remain airborne long enough to reach ceiling ventilation units.

Bush says, "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. But by elucidating the dynamics of the gas cloud, we have shown that there's 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. Basically, small drops can be carried a great distance by this gas cloud while the larger drops fall out. So you have a reversal in the dependence of range on size."

The research paper, "Violent expiratory events: on coughing and sneezing," was published in the Journal of Fluid Mechanics.

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