Researchers say that social distancing guidelines should differ depending on where you are and what you (and those around you) are doing.
Review shows eight of ten studies find that respiratory droplets can travel beyond six feet. One concludes that a sneeze can send aerosols as far as 26-feet inside a room.
CAMBRIDGE, Mass. — Six feet seems to be the new standard of measurement in our world. Whether we’re sitting, walking, or running, we’re all urged to practice social distancing. There’s even a country song sporting the phrase in its title. Some research claims that six feet simply isn’t far enough away. So just how accurate is this coronavirus outbreak rule of thumb?
In a new report in The BJM, scientists from the Massachusetts Institute of Technology and the University of Oxford argue that the six-foot rule is part of oversimplified, overly rigid, and outdated science.
The history of droplet emission science
Researchers say modern social distancing guidelines are based on scientific studies from over 100 years ago. The six-foot rule was originally proposed in 1897 by Carl Flügge, a German bacteriologist who studied the movement patterns of fluid droplets expelled from the mouth when speaking, coughing, or sneezing. In his research, he found that visible fluid droplets containing pathogens travel about six feet.
In the 1940’s, a study of a bacterium known as haemolytic streptococci confirmed Flügge’s original findings. That study shows that most bacteria-containing droplets stay close to their source. It also finds that a minority (10%) of people produce droplets that travel as far as 9.5 feet away.
Despite the general and persisting belief that six feet is a safe distance in most circumstances, recent studies continue to prove this belief wrong. In a recent review of 10 studies on droplet emissions, eight of the 10 studies find that respiratory droplets can travel farther than six feet.
One of those studies even reveals small droplets from a sneeze indoors can travel between 20 to 26 feet!
Large droplets and small droplets both carry pathogens
The original six foot rule is based on old scientific methods, which only allows scientists to visualize large respiratory droplets. Modern technology gives scientists the ability to visualize tiny droplets (aerosols) that can’t be seen with the naked eye.
While large droplets do tend to fall within six feet of their source, smaller droplets can travel much farther and can hang around in the air for longer periods of time. Moreover, small droplets can travel different distances depending on ambient airflow, like a person talking.
Given that small droplets can carry pathogens, it is important to consider their movement dynamics when developing social distance guidelines. This is particularly true for SARS-CoV-2 (the virus responsible for COVID-19), which can attach to air particles and remain in the air for up to 16 hours, researchers say.
Safe social distancing depends on what you do and where you are
The MIT and Oxford scientists write that it is also important to consider exactly what activities people are engaging in when social distancing. Some activities cause droplets to travel farther and stay in the air longer than other activities. Singing, talking loudly, and breathing heavily all increase the travel distance of emissions.
Activities that cause people to breathe heavily are of particular concern. That’s because they can cause high momentum droplet expulsions that are similar to coughs. These activities include things like running and other sports. On the bright side, respiratory droplets become more diluted in outdoor environments, making outdoor settings much safer than indoor settings for sports and other athletic activities.
The scientists conclude by urging for more nuanced social distancing guidelines. Specifically, they argue that distancing guidelines should be based on graded levels of risk.
While six-foot distancing is likely adequate in low-risk settings, greater distancing measures should be considered in high risk settings. Those include indoor areas with poor ventilation, situations in which large groups of people are together for long periods of time, and in the absence of face covering.