By Adam Townsend
Directives by the CDC and other public health agencies fighting the pandemic still say six feet is the minimum distance to help avoid transmission, but some Florida researchers think that may be too little to avoid infection from people coughing and sneezing nearby. Some simulated cough droplets measured in the study traveled as far as 12 feet -- twice the recommended distance people are told to maintain to protect themselves.
MedicineNet's resident emergency doctor and microbiologist looked at the study and said, based on the results, the six feet recommendation is still a helpful directive, however. The Florida study used synthetic "saliva" and mimicked coughing in a way that most people don't actually cough (straight out without turning the head or covering the mouth), he said.
"The methodology of the study was sound for what it is," said MedicineNet author Charles Patrick Davis, MD, PhD. "It's an innovative way to do research, but it does not translate 100 percent. Based on this data, I think the six-foot recommendation is still a good one."
How Close Is Too Close?
Media discussions among public health experts over the last week on this issue have again illustrated there are no good options to fight the sometimes fatal respiratory disease with a host of frightening and baffling symptoms.
As scientists continue to study the habits of the coronavirus, several papers have outlined previously unknown deadly or virulent aspects of the disease. As researchers scramble to catch up, sometimes they discover the right choices to protect yourself aren't as clear as they seemed in the absence of evidence.
Researchers at Florida Atlantic University used a mannequin and pressurized air to mimic the force of an average cough. They measured the trail of vapor droplets expelled from the synthetic cough using LED and laser lights, according to FAU.
The researchers there showed some forceful coughs can eject vapor droplets – droplets that would contain coronavirus particles in an infected person – up to 12 feet, twice the recommended minimum social distance requirement.
Though masks tested in this experiment did not block all the breath vapor from inhalation by bystanders wearing them, a mask significantly reduced the forward trajectory of the cough vapor when placed on the dummy, the FAU release states.
Interestingly, it was the engineering department at FAU that conducted this study, not the medical school. Described as a "flow visualization" study, the researchers who conducted it come from the university's College of Engineering and Computer Science.
Visualizing fluid mechanics is study leader Manhar Dhanak's bread-and-butter as chair of FAU's Department of Ocean and Mechanical Engineering. He described the pattern of cough droplets using engineering terminology more typically applied to how machines interact with ocean and air currents.
"In the case of light coughing, it can lead to formation of vortex rings in which the particles and droplets ejected in the cough-jet remain confined to evolving toroidal flow structures," said Dhanak. "The rings appear to travel one to nine feet or more before getting diffused and breaking up. We found that wearing a face mask doesn't stop the particles 100 percent, but it does slow down the cough jets."
Can You Get Sick From a Cough 12 Feet Away?
Still, what this means for transmission of coronavirus is unclear. The FAU team used a mixture of water and glycerin warmed to body temperature as a stand-in for saliva, which Dr. Davis said isn't a great substitute.
The study also did not examine the behavior of viruses that could be contained in such particles, or make any assertion as to how likely you are to get infected with COVID-19 from a 12-foot cough.
"Particles and droplets that result from a human cough or sneeze typically range in size from 5 to 500 microns," the FAU release states. "The larger particles fall to the ground within a short distance due to gravity, but smaller particles can be carried longer distances by prevailing breezes of air. The particulates making up the fog from the fog machine Dhanak... used in the emulation of the cough-jet ranged in size from 10 to 20 microns."
Dr. Davis said the actual saliva and sputum droplets expelled by COVID-19 patients is not of uniform consistency like the glycerin solution. Furthermore, researchers could get much more useful data by tweaking the methodology.
"If you could simulate the saliva more accurately and put in a fairly high concentration of non-infectious viruses as stand-ins for the coronavirus, then collect the droplets on petri dishes, you could have a better idea of how infectious the cough is," Dr. Davis said. "I think this data, especially if they take the time to show more how a mask impedes any flow, it’s very useful as a comparison with unmasked coughing."
Dr. Davis also said Dhanak's team, which has already promised further experiments, should try to better mimic how average people cough in public, with a covered mouth or turned head.