As the COVID-19 pandemic has continued to spread through the US population, the Centers for Disease Control and Prevention (CDC), and the World Health Organization (WHO) have recommended that everyone wash their hands regularly and that they stay home and avoid groups or crowds.
The term “social distancing” (or “physical distancing”) has now become a regular part of the American vocabulary. The WHO defines social distancing as maintaining at least three feet between you and another person, while the CDC recommends six feet of space [1,2].
One scientist, however, is questioning whether or not those guidelines are adequate.
Lydia Bourouiba Says 6 Feet is Not Enough
Lydia Bourouiba is an associate professor at MIT who has been researching the dynamics of exhalations, such as coughs and sneezes, for years. Through her work at the Fluid Dynamics of Disease Transmission Laboratory, she has determined that exhalations cause gaseous clouds that can travel up to 27 feet .
Bourouiba says that current guidelines are based on “large droplets” as the primary method of transmission for the virus and that those droplets are only able to travel a certain distance.
“There’s an urgency in revising the guidelines currently being given by the WHO and the CDC on the needs for protective equipment, particularly for the frontline health care workers,” she said .
In her paper, she explains that large droplets settle faster than they evaporate, while small droplets evaporate faster than they can settle, which is why infection control strategies focus primarily on the transmission of large droplets .
According to Bourouiba, the recommendations by the WHO and the CDC are based on classifications of what constitutes a small droplet and a large droplet, which is why they have determined that COVID-19 is spread through respiratory droplets and not aerosol transmissions (ie- the virus is not airborne). In her opinion, this is not the case .
What Does “Airborne” Mean?
When a virus is airborne, it means that it can be passed from person to person through the air. An aerosol is when particles are held in the air through physical or chemical forces. Fog is an example of an aerosol, in which water particles remain suspended in the air .
These suspended particles can stay floating in the air for several hours, depending on the environmental conditions. In some cases, virus particles can remain in the air, most likely attached to a droplet of mucus or saliva, and remain there for several seconds. When this happens, anyone who walks through that “pathogenic cloud” could contract the virus. The measles is an example of an airborne, or aerosolized, infection .
Is COVID-19 Airborne?
According to the WHO, COVID-19 is not spread through aerosol transmission, but rather by droplets that land nearby, either directly on peoples’ faces, or onto surfaces which could be picked up by peoples’ hands .
The only issue with this is that there is disagreement among the scientific community as to whether or not aerosol transmission matters for the seasonal flu, causing some to believe that it is too early for the WHO to make this claim.
Don Milton, an expert in aerosol transmission at the University of Maryland, believes for this reason that three months is simply not long enough to say for certain that the novel coronavirus cannot be airborne .
The Real Question is: How Far Can the Virus Travel?
Dr. Paul Pottinger, an infectious disease professor at the University of Washington School of Medicine, explains that the more important factor to determine is how far the virus can travel while still remaining a threat.
“The smaller the germ particles, the lower the risk that they might infect somebody who would breathe them in or get them stuck in their nose or their mouth,” he said .
He also says that the “six feet rule” comes from the fact that as of now, experts believe that the biggest threat for transmitting COVID-19 comes from large droplets of saliva. They are large enough that they are still affected by gravity, so they will typically only make it about six feet after leaving someone’s body before falling to the ground .
Pottinger believes that if the coronavirus was effective up to 27 feet, as Bourouiba suggests, this pandemic would be significantly worse than it already is.
“It takes a certain number of individual viruses to actually get a foothold inside the body and cause that infection to get going,” he said .
He said that for this reason, if the virus traveled efficiently by air, it would likely be infecting many more people- nearly everyone would have the virus.
What Does the WHO Say?
The WHO maintains that they are not seeing airborne transmission of COVID-19. Dr. Hanan Balkhy, assistant director-general for antimicrobial resistance at the WHO, explained that the organization would need to see evidence that the virus is spreading via a different route before they change their recommendations.
“If we were to have airborne transmission, we would see cases with no contact before getting ill with that disease…And we’re not seeing that. I think when you look at the sheer number of positive cases, they’re happening with very clear mixing and mingling. They’re very close with each other. … That does not indicate airborne transmission,” she said .
The WHO released a statement in response to Bourouiba’s work, saying that they welcome new studies and information, that they are continuing to monitor emerging evidence, and that they will update their guidelines and recommendations as more information becomes available .
What Does Bourouiba Recommend?
Bourouiba believes, however, that there is not as of yet enough information to know how far the virus can travel, and that policymakers and health officials should be erring on the side of caution when making their recommendations .
Linsey Marr, who studies airborne disease transmission at Virginia Tech, says that you should not be afraid to go outside, as long as you practice proper social distancing.
“People envision these clouds of viruses roaming through the streets coming after them,” she says, “but the risk of [infection] is higher if you’re closer to the source.” 
Bourouiba recommends that you always try to keep as much distance from other people as much as possible, and to avoid shared spaces, like hallways, stairwells, and elevators, as much as possible when other people are using them.
“If you hear neighbors going out, and there are 10 people in the corridor right now, maybe wait and go later,” she said .
She agrees that the biggest risk is still in touching contaminated surfaces, so washing your hands and cleaning frequently-used surfaces is the best way to stay safe.
The COVID-19 pandemic has changed the way we live our lives seemingly overnight, and with new information becoming available constantly, recommendations and policies are changing daily. For this reason, it is of crucial importance that we, as a society, continue to follow the guidelines being given to us by health officials, and do our best to adapt to our new normal.
These are challenging and scary times, but the sooner we commit to social isolation and preventative measures, the sooner we will be able to return to normal life.
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