Dr. Cillian De Gascun:

That is what the WHO is quoted as stating, which is a nice communicable message for members of the public. It is important to realise that this comes down in essence to droplets, which may or may not contain virus particles. If people cough, sneeze, laugh or even speak or breathe, we know that the large respiratory particles, or droplets, coming out of their mouths drop very quickly - it is gravity - typically within 1 m to 2 m. We also know, however, that there are smaller particles that take a longer time to drop. They can stay in the air for a number of hours, and that is really the concern. They can also be moved around by wind currents. We know, therefore, when it comes to 1 m versus 2 m versus 3 m, the farther away one is, the better. However, we obviously need to get the balance right in the context of what risk we are willing to accept and what we can tolerate as a society. When we looked at this from a starting point of 1 m versus 2 m, we found there is not a huge evidence base that compares 1 m versus 2 m and tells us which is better.