UPDATE: many readers have pointed out the data from the cited simulation is a pre-print and has not been peer-reviewed yet. Many have cited an article in Vice condemning the media attention garnered by these researchers. I want to be clear the purpose of this post is to help reduce transmission risk by raising awareness that the “6 foot rule” is a recommendation made with many assumptions. The provided information can help us reduce transmission by recognizing heightened distance precautions near fast-moving individuals. As a medical community we are continually gathering new data to adapt to the evolving COVID-19 crisis. This post is not a recommendation to avoid exercise outdoors. It is absolutely not meant to instigate anger towards individuals who are exercising outdoors. We must all be aligned in our efforts to physically distance, and should recognize that 6 feet is likely insufficient in many real-life, outdoor scenarios. So is social distancing effective? It appears so! Let’s do it as effectively as we can without stoking undue anxiety.
Social distancing is effective at making a dent in our worldwide effort to “flatten the curve” of coronavirus spread! Not without reason: social distancing certainly makes “common sense”, but how do we decide what distance is effective? To be as safe as possible against COVID-19 when outside, let’s look at the practical research on this topic.
Bottom line: 6 feet is not always enough to stay safe!
Where does the 6 foot rule for social distancing come from?
For over half a century researchers have studied how infectious disease spreads. Let’s recall that the spread of infectious disease, like COVID-19, typically falls under three categories:
- Contact: virus lives on surfaces, and we touch those surfaces and then touch the mucous membranes of our face
- Droplet: virus lives in water droplets that are expelled from our nose and mouth when we cough or sneeze, and those droplets come in contact with the susceptible parts of our bodies to infect us
- Airborne: very small suspensions of virus persist in the air for extended periods of time when we breath, cough, or sneeze, and come in contact with the susceptible parts of our bodies to infect us
We believe that contact and droplet routes are the most common methods of coronavirus spread in our communities. Airborne spread is also possible, but we believe to account for a smaller portion of transmission (see my previous post).
How far SARS-CoV-2 droplets spread when standing still
We are interested in how coronavirus spreads via respiratory droplets, as that is where the rationale for social distancing comes from. These water droplets refer to sizes greater than at least 5 micrometers that fall relatively quickly to the ground from gravity (recall this is in contrast to smaller particles that can remain suspended in air for much longer and travel farther). There are various definitions for droplet size (many larger than 5 micrometers), but those specifics are not part of our pragmatic approach here.
When these droplets are expelled from our nose and mouth during a cough or sneeze they typically fall to the ground relatively quickly, but they don’t drop dead like rocks. The vector they take as they settle to the ground depends on numerous environmental factors, consider:
- Direction and strength of local airflows
- Temperature and relative humidity (how quickly does the droplet evaporate as it falls?)
- Exhaled air velocity
- Viscosity of the fluid
- Flow path (through the nose, the mouth or both)
A study that nicely summarized these effects concluded that large droplets leaving our faces at slow speeds (when exhaled) are likely to evaporate before falling 6 feet away. Those same large droplets leaving our faces at higher speeds (such as sneezing) may travel more than 20 feet. Coughing distributes droplets at a distance between breathing and coughing.
It is very important to note that there is tremendous variability in these numbers, so none of them are hard and fast absolute rules. Furthermore, if a symptomatic person is wearing a mask, those numbers should drop lower.
So coronavirus respiratory droplets can indeed spread farther than 6 feet
We can draw two simple conclusions so far:
- 6 feet may not be an adequate distance if somebody is actively sneezing (droplets might spread farther!)
- Sick people, please wear a mask when around healthy people!
But is social distancing effective if we are moving? The effect of the slip stream
There were many assumptions that went into the 6 foot guideline. All of those assumptions and guidelines go out the door once we start to move or there is wind and directional changes of airflow. A group of researchers in the Netherlands and Belgium tried to characterize the radius needed for effective social distancing if people are moving. They did not include the effects of external wind, but they specifically studied the effect of slipstream (the movement of air behind a moving object).
While there are many assumptions in their calculations, as well, the take-home is that when people are moving, there is greater spread of droplets in the air around them. This means that for safety, the 6 foot rule must be expanded to an even larger radius. By way of summary, their findings showed:
- Walking 2.5 mph has potential droplet spread up to 16 feet
- Running 9 mph has a potential droplet spread up to 32 feet
importantly, a key assumption is that there is no headwind, tailwind, or crosswind. As before, all of these distances depend on the droplet size emitted from peoples’ noses and mouths, and there is significant variability in that droplet size, and therefore the distances that those droplets can travel.
The researchers produced several very helpful images to better appreciate what the respiratory droplet spread looks like visually:
How do we effectively socially distance if walking or running during COVID-19?
These unsurprising results have several practical action items for us to protect ourselves while walking or running nearby others:
- Coughing, and particularly sneezing, can send virus droplets more than 10 feet away
- Running fast (eg 9 mph) can spread droplets up to 32 feet behind you (infectious potential is unclear, but likely not zero)
- Walking (eg 2.5 mph) can spread droplets up to 16 feet behind you (infectious potential is unclear, but also likely not zero)
- The faster you are moving, the farther you should distance from others
- When walking or running with others, avoid staying in their slipstream (run staggered or side-by-side)
- If minding your own business and somebody else walks or runs past you, move out of their way and stay out of their way as long as possible because of the trail of droplets that will be left behind
- If you are walking or running, avoid passing directly in front of others so they don’t fall into your slip stream
I hope you can all use this information to enjoy your physical activity outdoors while minimizing your risk of coronavirus exposure. Physical activity is so important to support our immune systems and protect against COVID-19, so let’s use this knowledge to safely exercise outdoors.
Please share with your family and friends – particularly anyone who leaves their home, and especially any runners out there!
Chartier, Y., and C. L. Pessoa-Silva. Natural ventilation for infection control in health-care settings. World Health Organization, 2009.
Xie, X., et al. “How far droplets can move in indoor environments–revisiting the Wells evaporation-falling curve.” Indoor air 17.3 (2007): 211-225.
Blocken, et al. Towards aerodynamically equivalent COVID19 1.5 m social distancing for walking and running