Droplets from coughs and sneezes travel farther than you think

It is common knowledge that when we cough or sneeze, we should cover our mouth and nose with a tissue to prevent germs from becoming airborne. Now, new research from the Massachusetts Institute of Technology suggests this instruction is more important than ever; they found that droplets from coughs or sneezes can travel up to 200 times farther than previously thought.

According to the research team, including John Bush, professor of applied mathematics at MIT, the droplets that are produced when we cough or sneeze are accompanied by "gas clouds" that enable the droplets to travel greater distances.

The study has recently been published in the Journal of Fluid Mechanics.

To reach their findings, the researchers used a combination of high-speed imaging of coughs and sneezes, laboratory simulations and mathematical modeling. This allowed them to analyze the fluid mechanisms behind coughs and sneezes.

The team found that, contrary to previous beliefs, each droplet from a cough or sneeze is connected through interaction with a gas cloud.

Past research has suggested that larger drops of mucus travel farther than smaller drops because they have more strength behind them. However, the investigators of this most recent study found that when droplets merge with the gas cloud, their trajectory is altered.

"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," says Bush.

"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 MIT video below shows the gas cloud carrying droplets as a person sneezes:

Droplets travel much farther than previous estimates

The investigators found that, compared with previous assumptions, droplets from coughs and sneezes travel much farther.

Droplets that are 100 micrometers in diameter were found to travel five times farther than past estimates, while droplets 10 micrometers in diameter were found to travel 200 times farther. In addition, the team found that droplets less that 50 micrometers in size are often able to stay airborne long enough to enter ceiling ventilation units.

According to the research team, a cough or sneeze is a "multiphase turbulent buoyant cloud."

Study co-author Lydia Bourouiba, assistant professor in the Department of Civil and Environmental Engineering at MIT, explains that the gas cloud encourages surrounding air into it, which causes the cloud to grow and mingle with the droplets.

"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," she adds.

Now that the researchers have uncovered more information as to what happens when we cough and sneeze, their next step is to find out what happens to pathogens in the droplets carried by the gas cloud.

The team plans to focus on "fluid breakup" - a process responsible for the formation of pathogen-containing droplets that allow infection transmission.

"We're trying to rationalize the droplet size distribution resulting from the fluid breakup in the respiratory tract and exit of the mouth," explains Bourouiba. "That requires zooming in close to see precisely how these droplets are formed and ejected."

But until the underlying mechanisms of fluid breakup are uncovered, one thing is clear: keep a tissue handy at all times.