Studies have shown that a low humidity, less than 40%RH, causes infectious droplets people release whilst talking, breathing and coughing, to remain airborne for longer. Expelled droplets shrink through evaporation in dry atmospheres, causing them to be smaller and therefore float for longer.
Some viruses, such as influenza, have been shown to survive for longer whilst airborne in atmospheres of less than 40%RH. At a humidity of 40-60%RH, studies have shown that airborne viruses remain infectious for the shortest possible duration.
The human body's first line of defence against airborne infection is the mucociliary clearance process, which occurs in the nose and throat. Our mucous membranes capture and remove airborne pollutants that we inhale, preventing them from causing an infection. In dry air, below 40%RH, these mucous membranes dry out and become much less effective at preventing infection.
By maintaining 40-60%RH in occupied areas, viruses remain airborne for a shorter duration, they remain infectious for the shortest possible time and our body's respiratory immune system is most efficient at fighting them off.