Scientists built 'cough chamber' to see how far droplets actually travel

A new study out of Western University suggests the two-metre physical distancing rule recommended by the world's doctors to stop the spread of the coronavirus may not be far enough, if someone coughs directly at you without covering their mouth. 

Scientists measured the distance germ-laden droplets released by coughing can travel by building a "cough chamber," a sealed wood and glass box where research subjects could cough without risk of infecting others. 

What researchers at Western University, in collaboration with virologists from Toronto's Sunnybrook Hospital, found is that when someone coughs without covering their mouth, the germ-laden cloud of fine mist they release will not only travel two metres in three seconds, it will continue to travel beyond and even float in the air within that two-metre area for up to three minutes after being released. 

The experiment is the first time scientists have measured what happens to the cloud of fine particles released in a cough beyond a one-metre radius from the mouth and could lead to further understanding of how coughing helps spread airborne germs such as COVID-19. 

Enter the 'cough chamber'

Eric Savory is an engineering professor at Western University who studies fluid mechanics. His team wanted to better understand how illnesses such as SARS or COVID-19 are transmitted through the air when we cough. 

In order to precisely map the cloud of germ-carrying droplets released from the mouth, the scientists had to build a "cough chamber" where subjects could be closely monitored. 

"The idea is it has a little hole in it, and people seat themselves rather like they're going to the optometrist and they cough into the box," Savory said. "

"We have to do this because for bio safety reasons we have to contain what people expel from their bodies." 

Savory said what researchers found is that when a person coughs and doesn't cover their mouth, there's almost no avoiding the cloud of germs they expel if you're standing directly in front of that person, even at two metres away. 

"If a person coughs unobstructedly, then the cough jet that they produce travels quite a long way. Even at a metre away the cough jet is moving about a metre a second, which is quite difficult to avoid."

Anatomy of a 'cough jet'

It's difficult because the study found that the fine mist created by the cough can stay in the air within that two-metre area for up to three minutes after it was released. What's more, the fine droplets continue to travel beyond the area at a speed of about 200 millimetres a second, or about a kilometre an hour. 

"The air is still moving at 200 millimetres a second, which is not insignificant when we're talking about the movement of fine droplets that have been expelled in the fine droplets of a cough," he said. 

Savory said while the study found that there is quite a lot of variability between coughs when it comes to individual people, there's almost no difference in the strength of an individual's cough when they're sick or well. 

While all of the subjects in the experiment had been naturally infected with varying strains of seasonal influenza, Savory said that there would be few differences if the experiment were to be replicated using people infected with COVID-19, or any respiratory illness. 

"I suspect not," he said. "Essentially, the way that a person coughs doesn't seem to vary whether they're sick or healthy."

Savory said while the amount of droplets produced by the cough might vary and how the virus could survive outside a human body would, too, depending on the virus. 

Because the "cough chamber" was only two square metres on the inside, Savory said the research couldn't do computer models that went any farther than that, but he said it's easy to guess what's going to happen to those fine droplets if you get close or walk right through them. 

"They're going to moved by whatever the air is doing. If someone was going to walk by with the airstream, they were going to go with that," he said. 

Is physical distancing distant enough? 

The results of this study might naturally lead one to question whether the two-metre physical distancing rule used by much of the world during the pandemic is distant enough. Savory said it is, most of the time. 

"I think the guidance we're getting is excellent and people should follow it," he said. "Being three metres away is better, four metres away is even better."

"What I think people should understand is that if a droplet comes out of somebody's mouth, it doesn't reach two metres from the mouth and decide, 'Oh, that's it, I'm going back now.'"

"Two metres is a practical distance we can maintain for us to go about our everyday lives and for sure, being two metres away from someone is a lot safer than being half a metre or one metre away," he said. 

Not only do the results suggest we're on the right track with physical distancing rules, it also suggests we're likely onto something when it comes to protecting retail workers with germ shields, or police arresting people for allegedly deliberately coughing on others. 

"I think all of these things are very helpful for sure," Savory said.  

"We know we're not going to eliminate person-to-person transmission, but we've all seen the basic studies. If you can reduce the number of people one person infects, it has a dramatic effect on reducing the number of infections."

"Two metres is a good distance to do that."