I frequently hear the World Health Organization (WHO) identify diameters of 5-10 μm as the size cutoff between "respiratory dropletes" and "droplet nuclei". For example, in a recent statement they say:
Respiratory infections can be transmitted through droplets of different sizes: when the droplet particles are >5-10 μm in diameter they are referred to as respiratory droplets, and when then are <5μm in diameter, they are referred to as droplet nuclei.
They go on to imply that this distinction is made because:
Droplet transmission occurs when a person is in in close contact (within 1 m) with someone who has respiratory symptoms (e.g., coughing or sneezing) and is therefore at risk of having his/her mucosae (mouth and nose) or conjunctiva (eyes) exposed to potentially infective respiratory droplets... Airborne transmission is different from droplet transmission as it refers to the presence of microbes within droplet nuclei, which are generally considered to be particles <5μm in diameter, can remain in the air for long periods of time and be transmitted to others over distances greater than 1 m.
But the settling velocity of a sphere with the density of water and a diameter in the 5-10 μm range is on the order of several mm/sec at most. This means that a 10 μm sphere, for example, will take over 8 minutes to fall 5 ft (1.5 meters), which is more than ample time to travel over 1 meter horizontally. So why does the WHO link the transition between these two classes of aerosols with such a small diameter?