Here’s a very interesting question. I have a hard time accepting something as fact just because an authority claims it to be so, so I did some research into the 6 foot distance number. What I found was interesting, but also a bit disappointing at the lack of scientific rigor.
The number commonly understood to provide a safe zone for virus transmission used to be 3 feet and was based on work done in the 1930’s at Harvard by a Tuberculosis researcher. He focused on the distance droplets could travel when we breathe, cough, or sneeze. This research was admittedly very limited and apparently wasn’t reproduced. It is still the recommended number by the WHO, and I have seen evidence that this 3 foot number was also put forth as a good precaution during the 1918 Spanish Flu pandemic.
Transmission of the Severe Acute Respiratory Syndrome on Aircraft
The six foot number is new, and the best that I can tell (and Radiolab seems to have arrived at the same conclusion) it came from a pretty limited study conducted by Taiwan’s Center for Disease Control which was published in the New England Journal of Medicine. This study was based on three airplane flights where a known, infectious SARS (another coronavirus) carrier flew on a plane. For the first of these, a flight from Taipei to Hong Kong, the researchers acquired the flight manifest and interviewed people from the flight around ten days afterwards to determine who had caught the disease. The researchers attempted to control for other potential SARS exposures before and after the flight, but it’s unclear how they did this or if it was successful. The results from flight 1 showed that overall 18 travelers became infected with SARS on this three hour flight. 35% (8) of the travelers seated in the same row as the sick passenger or in the 3 rows in front of the infected traveler ultimately caught SARS. 11% (10) of the travelers in other parts of the airplane also developed infections. This flight was the worst case of the three flights that the researchers studied (on the other two flights, from Taipei to Singapore, it appears that no passengers were infected by the sick passenger(s) identified).
A strong confounder in the analysis of the SARS transmission on this flight is that in interviews, the persons who became infected on the first flight pointed to excessive coughing as a likely method of transmission. This coughing may have resulted in the virus being transmitted through the air instead of just through droplets. Even though there wasn’t a strong case that the probability of getting infected was significantly higher within three rows of the sick traveler, it appears that the U.S. CDC made the determination to expand the 36 inch guideline to 6 feet (The distance across 3 rows on a Boeing 737 is 90 inches). This is the belief, at least, because the CDC won’t comment on support for the guideline.
This appears to be the science that dictates a 6 foot radius to protect from COVID-19. It does not appear to be a slam dunk to me, and I can find no studies that compare the efficacy of 3 or 6 feet of distancing with face masks, hand hygiene, or avoiding handshakes. It could well be that any one of those is far more or less effective at preventing virus spread.
There does appear to be evidence, at least, that spending a three hour sitting in or passing through the vicinity of an infected person who is coughing a lot results in a real chance of getting infected. It seems like the wearing of masks to prevent transmission or getting SARS did not start until after these cases, so we don’t know if the situation would have been different had the sick person been wearing a face mask. After this SARS outbreak, the WHO recommended better hand hygiene on airplanes during outbreaks and suggested that it may be appropriate for airlines to place masks on sick travelers.