Coughs and sneezes can spread germs further than we thought

Harold Mandel's picture
A businessman coughing
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Concerns about catching a cold or flu from someone coughing or sneezing are well justified, particularly in view of recent research showing violent respiratory events can spread respiratory diseases further than we previously thought. The public health consequences of people coughing and sneezing demand that greater efforts be taken to protect people from infected droplets spread in such manners. The new findings that these droplets may can spread a lot further than previously realized means more aggressive initiatives are needed to cope with this problem.

Respiratory diseases are spread between infectious and susceptible individuals primarily from violent respiratory events such as coughs and sneezes, reports the Journal of Fluid Mechanics. To better understand this problem researchers have studied the fluid dynamics of such violent expiratory events. Direct observation of sneezing and coughing events has revealed that such flows are multiphase turbulent buoyant clouds made up of suspended droplets of various sizes. The researchers developed discrete and continuous models of droplet fallout from the clouds with a goal of predicting the range of pathogens.

Coughs and sneezes float farther than you think

Researchers have discovered how coughs and sneezes float farther than you think, reports MIT on April 8, 2014. In a novel study MIT researchers have showed that coughs and sneezes have associated gas clouds which keep their potentially infectious droplets floating over much greater distances than were previously realized. John Bush, a professor of applied mathematics at MIT, and co-author of a new paper on the subject, has said, “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.”

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Ventilation systems may be more likely to transmit potentially infectious particles

It was observed in this study that the smaller droplets which develop in a cough or sneeze may travel five to 200 times further than they would if those droplets were not part of a cloud and simply moved as groups of unconnected particles. The fact that these droplets may remain airborn due to being resuspended by gas clouds, means that ventilation systems may be more likely to transmit potentially infectious particles than has been previously suspected. It is pointed out that this understanding should prompt architects and engineers to re-examine the design of workplaces and hospitals and circulation on airplanes in order to lower the chances that airborne pathogens will be transmitted among people.

A cough or sneeze resembles a puff emerging from a smokestack

In their new analysis of coughs and sneezes from a fluid-mechanics perspective the researchers used high-speed imaging of coughs and sneezes along with laboratory simulations and mathematical modeling. What they came up with is the interesting observation that the cough or sneeze "resembles, say, a puff emerging from a smokestack." Bush says, “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." Therefore the smaller drops can be carried a longer distance while the larger drops fall out. Researchers had assumed previously that larger mucus droplets fly farther than smaller ones due to having more momentum. It is now understood that droplets which are less than 50 micrometers in size can often remain airborne long enough to reach ceiling ventilation units.

This research raises new considerations for taking preventive measures to prevent colds and flus and other respiratory illnesses. It remains wise as usual to strive to maintain a strong immune system by eating well and getting adequate exercise and sleep. This new information showing that infectious droplets may spread a lot further than previously thought should alert public health officials to advise both people who are suffering from respiratory illnesses and susceptible individuals to take more precautions than ever to try to prevent the spread of the infectious droplets. In the meantime, the suggestion that architects and engineers get to work on this problem is also a good idea.

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