Nobody knows for sure but I'm addicted to news and remember this well. A bigshot at CDC was working out the guidelines at home and his 14 yo daughter doing homework they started talking about science. He asked her what might be a safe distance in school...3 feet.....5 feet.....10 feet? And she said 6 feet not too close and not too far. He liked the answer and went with it.
I remember telling this story back then and of course nobody believed me....but then too nobody believed me about lockdowns either.
https://archive.is/Ffkx8
"A FEW DAYS after the April Zoom meeting with the WHO, Marr got an email from another aerosol scientist who had been on the call, an atmospheric chemist at the University of Colorado Boulder named Jose-Luis Jimenez. He’d become fixated on the WHO recommendation that people stay 3 to 6 feet apart from one another. As far as he could tell, that social distancing guideline seemed to be based on a few studies from the 1930s and ’40s. But the authors of those experiments actually argued for the possibility of airborne transmission, which by definition would involve distances over 6 feet. None of it seemed to add up."
"While Jimenez and others sparred on social media, Marr worked behind the scenes to raise awareness of the misunderstandings around aerosols. She started talking to Kimberly Prather, an atmospheric chemist at UC San Diego, who had the ear of prominent public health leaders within the CDC and on the White House Covid Task Force. In July, the two women sent slides to Anthony Fauci, director of the National Institutes of Allergy and Infectious Diseases. One of them showed the trajectory of a 5-micron particle released from the height of the average person’s mouth. It went farther than 6 feet—hundreds of feet farther. A few weeks later, speaking to an audience at Harvard Medical School, Fauci admitted that the 5-micron distinction was wrong—and had been for years. “Bottom line is, there is much more aerosol than we thought,” he said. (Fauci declined to be interviewed for this story.)"