Is a 6-Foot Separation Enough to Stop the Spread of Covid-19?

It wasn't quite picked out of thin air, but it's not much better than that.

Keep your distance on Venice Beach
Keep your distance on Venice Beach.

  David McNew/Getty Image

As the kids go back to school and the parents go back to the office, desks have been moved and the floor has been taped so that everyone stays six feet (or two meters) apart. Even deep in the north woods where there there is nobody around for miles, you see these signs. But where did this come from, and does it make any sense?

Sign in the middle of the forest
Sign in the middle of the forest.  Lloyd Alter

Writing in the BMJ, Nicholas R Jones, with Professors Trisha Greenhalgh, Lydia Bourouiba, and colleagues, argue that "rigid safe distancing rules are an oversimplification based on outdated science and experiences of past viruses."

The authors note that there are so many conditions that can affect how far a respiratory droplet can travel, including most notably, the size of the droplet, but also ventilation, patterns of airflow, type of activity, viral load of the emitter, duration of exposure, and susceptibility of an individual to infection. So in fact, depending on circumstances, 6 ft/2m should be considered a minimum, and may not be nearly enough.

The 2-meter rule actually goes back to studies from as early as 1897; the most influential study was from the 1940s, complete with photos of people sneezing, coughing, and talking.

Despite limitations in the accuracy of these early study designs, especially for longer ranges, the observation of large droplets falling close to a host reinforced and further entrenched the assumed scientific basis of the 1-2 m distancing rule. Yet eight of the 10 studies in a recent systematic review showed horizontal projection of respiratory droplets beyond 2 m for particles up to 60 μm. In one study, droplet spread was detected over 6-8 m. 

There is lots of evidence that this has been happening.

Breathing out, singing, coughing, and sneezing generate warm, moist, high momentum gas clouds of exhaled air containing respiratory droplets. This moves the droplets faster than typical background air ventilation flows, keeps them concentrated, and can extend their range up to 7-8 m within a few seconds.

Basically, the six-foot rule applies to large droplets, but small droplets can carry farther if there is air movement to carry them. In our previous discussions about the SARS-CoV-2 virus being airborne, we have shown research indicating that ventilation and fresh air are key to diluting the virus; Jones and his colleagues remind us of clusters in churches, gyms, call centers, all places where people are talking, singing, or heavy breathing in close proximity.

Even though I keep my distance, I might start wearing a mask when I go running, because "the heavy panting from jogging and other sports produces violent exhalations with higher momentum than tidal breathing, closer to coughs in some instances." On the other hand, a new Japanese study reports "an 18.7-fold higher risk of transmission in indoor environments than outdoors." That kind of makes the sign in the middle of a forest kind of pointless.

A Call For a More Nuanced Model

Keep your distance
Risk of SARS-CoV-2 for different conditions. Nicholas R. Jones et al

Jones suggests we should rely less on the rigid six-foot/two-meter rule and instead, look at levels of risk.

In the highest risk situations (indoor environments with poor ventilation, high levels of occupancy, prolonged contact time, and no face coverings, such as a crowded bar or night club) physical distancing beyond 2 m and minimising occupancy time should be considered. Less stringent distancing is likely to be adequate in low risk scenarios. People with symptoms (who should in any case be self-isolating) tend to have high viral load and more frequent violent respiratory exhalations.

As Kate de Selincourt and others point out, classrooms are probably all down there in the lower right-hand corner of high occupancy and poorly ventilated spaces.

Julie Andrews in the Alps
Julie Andrews in the Alps. screen capture 

All of this intuitively makes so much sense; far more important than just staying six feet apart is whether you are are in a well-ventilated space (or outdoors) and whether you are wearing a mask. And no singing, unless you are Julie Andrews in the Alps.