22 April 2020
Just how far respiratory pathogens, such as viruses, can travel through the air and how they do it is one of the keys to controlling the spread of respiratory diseases such as COVID-19.
Aside from being spread via contact with contaminated surfaces, respiratory viruses such as SARS-CoV-2 are spread when an infected person coughs, sneezes, talks or sings and expels droplets containing virus into the air.
Historically, these droplets have been thought of as being either large, heavier droplets that settle close to the infected person or smaller droplets that travel further and may evaporate faster than they settle, creating respiratory aerosols.
Large droplets are heavier and tend to fall to the ground and nearby surfaces, or land on a person in close proximity to the infected person expelling them. Most respiratory infections can be spread by this short-range, large-droplet transmission.
Smaller respiratory droplets travel further and may evaporate, leaving behind aerosols of dried residues such as virus particles that can stay suspended in the air for hours. An aerosol by definition is a suspension of fine solid particles in a gas, such as air.
Management of respiratory infections has largely been based on classifying infections as having either droplet spread or airborne transmission via aerosols. But increasingly, scientists are questioning this dichotomous model classifying respiratory infections into droplet or aerosol infections.
Recent research has shown that human sneezes, coughs and exhalations are primarily made up of a turbulent gas cloud (see photo and video) that carries droplets of varying sizes enveloped in a moist warm cloud which allows the droplets to avoid evaporation for longer than if the droplets were isolated. The cloud has a forward momentum which propels the droplets onwards, settling out or evaporating according to their size, the speed of the cloud and the ambient conditions. 
How far respiratory viruses can travel in such a cloud and how much viral load needs to be delivered by droplets or aerosols to infect someone are key questions to answer.
On the question of the horizontal distance travelled by respiratory droplets, Australian scientists from the University of New South Wales and US scientists from Massachusetts Institute of Technology (MIT) have published a systematic review of 10 studies.
They reported that 8 of the studies found the distance travelled by droplets from various types of exhalations was more than 2 metres from the person emitting them. And they could travel up to 8 metres from the person.
As well as impacting advice to protect healthcare professionals, this has wider implications for social distancing.
Social distancing guidelines are based on the belief that 1-2 metres is a safe distance to avoid being sprayed by large droplets, and that these droplets would be the main emissions containing the virus and able to cause disease, according to Professor Raina MacIntyre, Head of the Biosecurity Program, The Kirby Institute, Sydney, and one of the study’s authors.
“But the body of evidence shows that droplets can be expelled further than 2 metres. Smaller particles that can’t be seen or felt may remain suspended in a ‘cloud’ and then carried for hours in ambient air, so they can end up travelling much longer distances,” she said.