Problem was, for years the borderline between "airborne" and "droplet" transmission of particles was anything smaller than 5 microns will tend to be airborne (distances much longer than 6 feet) -- i.e. transmitted though aerosols rather than cough/sneeze droplets. That borderline should have been set at a particle size of 100 microns, but an incorrect 1955 paper with the 5 micron size got adopted as the standard.

Precautions for airborne isolation vs. droplet isolation (at least in the hospital) are rather different in their priority setup.