A single sneeze injects approximately 40,000 infectious aerosols into the room air. When airborne droplets are introduced into a room with a relative humidity below 40%, they rapidly lose up to 90% of their volume. They shrink and manage to float for a long time, covering considerable distances. This increases the chances of reaching another person.

In studies using mannequins simulating people coughing with influenza and people breathing standing two meters away aerosol samples were collected in proximity of the breathing mannequin’s mouth at different moments and their infectiousness was assessed.

Infectivity dropped from 80 to 20% once relative humidity was above 40%RH. This is due to the fact that the influenza virus can no longer remain suspended for a prolonged period of time and to the reduced lifespan of many airborne bacteria and viruses in this relative humidity range.

While higher humidity can reduce the distance that infectious aerosols travel, humidity which is too high is likely to cause increased respiratory infection from mould and bacteria to humans and may even cause damage to buildings themselves.

Most building air conditioning systems are designed and controlled to provide temperature control for human comfort. HVAC systems for many buildings are not always designed to meet specific humidity targets. However, air conditioning systems are used effectively to precisely control humidity in specialist facilities spaces including laboratories and specialist healthcare, museums and art galleries and high technology and pharmaceutical manufacturing. Modifications to existing HVAC systems can be made to improve the ability to regulate the space humidity.