Spring is upon us in the Northern Hemisphere, and for many people the season comes with an onslaught of sniffles and sneezes brought on by allergies. Researchers from the Massachusetts Institute of Technology are advising potential allergy sufferers to be sure to cover their mouths well when sneezing, as new research indicates unbarricaded sneezes (and coughs, too) travel much farther than previously believed.
It turns out that an uncovered cough or sneeze doesn't just send unsavory droplets of spit flying out of your mouth, but that they are propelled by a multiphase turbulent buoyant cloud of gas.
"When you cough or sneeze, you see the droplets, or feel them if someone sneezes on you," said research co-author 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."
Bush and his collaborators found that tiny droplets emitted by a cough or sneeze and that are bound in this gas cloud can travel five to 200 time farther than if they were somehow unconnected. Previous research on this topic, the MIT researchers said, incorrectly assumed the droplets were unconnected.
"You can have ventilation contamination in a much more direct way than we would have expected originally," said Lydia Bourouiba, an assistant professor in MIT's Department of Civil and Environmental Engineering, and another co-author of the study.
Prior research that assumed sneeze and cough droplets were unconnected also presumed that the largest of the droplets traveled the greatest distance. Their momentum, after all, is defined by mass times velocity, which would suggest that larger particles travel farthest.
But after using high-speed imaging techniques, laboratory simulations and mathematical modeling, the MIT researchers offer a point to the contrary.
"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 said in an MIT statement. "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 researchers determined that droplets 100 micrometers in diameter traveled five times farther than previous estimates, and droplets just 10 micrometers in diameter can travel 200 times farther.
The revelation sheds important light on hygiene standards and stopping the spread of diseases. The researchers report that droplets less than 50 micrometers in size are able to remain airborne long enough to reach typical ceiling ventilation ducts.
The researchers published their study in the Journal of Fluid Mechanics.
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