Hi! I am not sure about your background--which matters for explanation. I cannot speak to 6 feet in particular, but the basic idea starts with the assumption

Hi! I am not sure about your background--which matters for explanation. I cannot speak to 6 feet in particular, but the basic idea starts with the assumption that the primary vector is large respiratory droplets. If that's the case, imagine you are a droplet. You are acted upon by gravity and by air/temperature. As you move along, you will fall. At some distance, most droplets above a certain size will have settled. That's the basic reasoning. You can argue about the particular cutoff, but the bigger question is how relevant aerosols are. But, the problem is that in the early spread of a virus, the growth in infections is very fast. Someone has to guess. If you wait for perfect knowledge, a lot of people will die because you are taking a linear approach to an exponentiatial problem.