The more we learn about Covid-19, the more the importance of transmission through airborne respiratory droplets, particularly in confined spaces without adequate ventilation, becomes apparent.

Most research has focused on how far droplets from coughs can travel, usually with an accompanying video or image that will make you feel quite ill.

However, less is known about the way the properties of a cough cloud can change as the cloud travels through the air. Researchers from the American Institute of Physics have sought to address that in a new study looking at how cough clouds evolve over a journey, and the effect face masks have on their volume and spread.

"When a person coughs, what is the volume of air that gets contaminated due to the cough ejected out by the person?" the team asks in their paper published in Physics of Fluids. "The answer to this question is not straightforward because the surrounding air gets entrained into the cough-cloud coming out from the person’s mouth and, eventually, becomes its part; therefore, a much larger volume than initially ejected is affected by coughing."

The team hoped that by answering this question, they could discover effective ways to reduce infection indoors, such as by determining the maximum number of people that can safely be housed in an enclosed space, like a hospital ward or cinema. They also wanted to find out the effect of temperature and humidity on the spread of cough clouds, to determine how much ventilation is needed in an office or an aircraft cabin in order to reduce infection.

The team used an analysis based on jet theory to confirm the first 5-8 seconds after a cough are the most important in terms of spread of the disease, after which the cloud begins to disperse.

As the cough travels through the air, however, the cough cloud increases as a cube of the distance traveled before it dissipates. To reduce the size of cough clouds and reduce the chance of spreading infection, however, there's a simple solution.

They found coughing without a mask creates a cloud volume roughly seven times higher than a surgical mask, and 23 times higher than an N95 mask. "The presence of a mask drastically reduces this volume and, consequently, significantly cuts down the risk of the infection to the other persons present in the room," the team wrote. 

"Similarly, actions which drastically cut the distance traveled by the cloud, such as coughing into the elbow and the use of a handkerchief, can reduce the volume of a cough cloud and, therefore, the chances of dispersion of the virus."