If you had asked most people what the significance of “6ft”was just a couple of months ago, many of them would have thought about a reasonably tall person or the height of their backyard fences. In more recent months, “6ft” (just under 2 meters) has become the distance that people are trying to separate themselves from others in an attempt to reduce the transmission of SARS-CoV2, the new coronavirus that has caused a worldwide pandemic, infecting millions and causing over 160,000 deaths so far.

But is the recommended 6ft of physical distancing really enough? The answer, as with most topics regarding COVID-19 and the coronavirus, is not entirely clear.

“I think the exact distance to maintain is less important, it would be more important to avoid prolonged close contact with other people in crowded areas,” said Ben Cowling, Professor in Infectious Disease Epidemiology at the University of Hong Kong. “If people are coughing or ill they should stay at home as much as they can,” Cowling added.

People who are obviously sick and coughing are likely to spread droplets containing coronavirus particles further than people who are simply breathing or talking. In most places, the current guidelines for people who are obviously sick is to not to go out at all.

In support of this, a recent study from researchers at Western University in Canada looked at how people with influenza might transmit the virus to others. The study showed that at least 10% of droplets produced by a cough traveled further than 6 ft.

“We suspect that there really isn’t going to be any difference in the way people cough depending on the virus people have,” said Eric Savory, Professor of Fluid Mechanics in the Faculty of Engineering at Western University and lead author of the research. “Our study showed that the type and strength of coughs are the same with healthy and sick people, but we don’t yet know whether different droplets are produced with different viruses,” Savory added.

The research has been accepted for publication after scientific peer review, but has not been published yet and the study was done on influenza, not the SARS-CoV2 virus which causes COVID-19. But are the findings regarding some droplets travelling further than 6ft likely to be similar with people who have COVID-19?

“All human expiratory activities, including breathing, speaking, coughing, etc, result in exhalation of small droplets,” said Professor Lidia Morawska, Director of the International Laboratory for Air Quality and Health at the Queensland University of Technology in Australia. “When coughing, speaking or singing, large droplets are also generated. This is no different between healthy people, people infected with influenza for example, or SARS-CoV2,” added Morawska.

The researchers in the Canadian study used a “cough chamber,” a sealed plastic box to track the number, size and distance traveled of droplets released by coughs in people with influenza and healthy controls. They found that 10% of the droplets which came out of someone’s mouth could be detected 2 meters away, 4 to 5 seconds after the initial cough.

“These are smaller droplets, they can stay in the air much longer before falling to the ground,” said Savory.

The “cough chamber” used by Western University researchers can show how coughs comprise of droplets of different sizes, some able to travel further than others. In the video above, smaller droplets (dark and light blue) travel further than larger droplets, with some traveling further than 6ft. Credit: Western University.

“It is very likely that COVID-19 laden droplets will travel further than 6 feet. Of course, the ones that are large and heavy will fall close to where they were expelled, but the smaller will be able to travel much further than this and stay suspended in the air for sufficiently long periods of time to increase the risk of transmission,” said Morawska, who has previously done similar research to track droplets using a tunnel-based system.

So what if someone coughs and several seconds or even minutes later, another person walks into that same space even while maintaining adequate physical distancing?

“The fine droplets will likely remain suspended in the air for quite a long time – they are moved around by whatever the air is doing, they can be influenced by airflow, such as wind, but they can stay in the air for a very long time, like fine pollen,” said Savory.

Despite these results, Savory still believes that recommending 6 ft physical distancing is a practical recommendation at the moment.

“Being 2 meters away is better than being 1 meter or half a meter away, but if you can be at a greater distance, do it,” said Savory.

He and colleagues plan to do similar experiments simulating coughs and sneezes with particles containing SARS-CoV2 coronavirus, albeit they cannot do this with actual patients as they did with influenza for numerous safety and ethical reasons.

“We are just about to start work with a specialized bio-containment facility. A chamber where we can safely release droplets containing the virus that causes COVID-19. It isn’t human but it will be the virus and will be simulated coughs and sneezes,” said Savory.

A key question in determining whether an individual is likely to transmit the virus to another person even if they are practicing adequate physical distancing seems to be the size of droplets they produce when coughing, speaking or even breathing. Larger droplets are likely to quickly fall to the ground in calm, still conditions, but smaller ones are likely to persist in the air for longer.

“I think the jury is still out on the sizes of COVID-laden droplets and whether they tend to be larger or smaller or both,” said Julian Tang, virologist and honorary associate professor in the Department of Respiratory Sciences at the University of Leicester in the U.K. “We have seen that COVID-19 patients are not highly contagious, but that does not directly inform the size of droplets involved in transmission,” Tang added.

There is growing evidence that many people who test positive for the virus are asymptomatic and may even have high viral loads, similar to those who have symptomatic COVID-19. And healthy people cough and sneeze too, especially with allergy season fast-approaching in many parts of the world.

“The main risks of airborne transmission are not from sneezing and coughing (which we don’t do so often) but simply breathing and talking to each other within one meter and sharing the same air. If I can smell the garlic, curry or alcohol you had for lunch, I am almost certainly inhaling any viruses that you are also exhaling – and vice-versa,” said Tang.

Connected to this, the million dollar question at the moment seems to be “to mask, or not to mask?” Whether or not to do this is, again, an example of an incredibly complex and seemingly controversial topic, confounded by the fact that proper medical masks, such as surgical masks, are currently in short supply in most of the world. Many people agree that healthcare and front line workers should be first in line for these scarce supplies, but if people who are coughing absolutely have to leave their homes, should they try to wear a mask of some type?

“If people are coughing or ill they should stay at home as much as they can. If they need to go out, wearing a mask should be a good way to capture virus-laden droplets and reduce the amount of virus getting into the environment,” said Cowling.

“Surgical masks are very good at containing the infection – particularly larger droplets – if worn by an infected patient/person. In hospitals with tuberculosis patients, for example, such patients are routinely masked when being taken to X-ray. However, the mask reduces the risk – but it does not make it completely zero,” said Tang, who has published work looking at whether masks are useful for infection control in people who are coughing.

So, based on everything that is currently known about how the coronavirus is transmitted and spreads, is 6 ft between people truly enough distance to significantly reduce the risk of transmission?

“This is the right precaution against infection by close contact, which is by large droplets. If you are sharing a room with an infected person, and the ventilation is not effective and adequate, which means that the concentration of the virus in the air of the room can build up, the distance of 6 ft is not sufficient to protect against infections,” said Morawska, who recently published a paper suggesting that airborne transmission of the virus could be a major risk, particularly in indoor environments.

For now, it would seem that 6 ft physical distancing is a practical suggestion that will likely protect people from contracting or transmitting the coronavirus. It is however likely that further separation, if practical, would be better.

“A 2 meter separation is safer than 1 meter, for sure – but is it absolutely safe? No, we cannot say that,” said Tang.