Wells curve

[3] In undisturbed moisture-saturated air, all respiratory droplets fall due to gravity until they reach the ground or another horizontal surface.

Sufficiently large droplets still reach the ground or another surface, where they continue to dry, leaving potentially infectious residues called fomites.

However, the high surface area to volume ratios of small droplets cause them to evaporate so rapidly that they dry out before they reach the ground.

[1] Wells summarized this relationship graphically, with droplet size on the X-axis and time to evaporate or fall to the ground on the Y-axis.

Diseases such as measles, whose causative viruses remain highly infectious in droplet nuclei, can be spread without personal contact, across a room or through ventilation systems and are said to have airborne transmission.

The internal turbulence of such clouds may also delay the fall of large droplets, increasing the chance that they will evaporate before reaching the ground.

[10][11] A face shield protects the wearer against impacts by large droplets that may be expelled horizontally by an infected person's cough or sneeze or during medical treatments.

[13][14] N-95 respirator masks are designed to filter out even small dry particles, but they must be individually fitted and checked to prevent leakage of air around the sides.