Physical Science

How the novel coronavirus spreads through coughs

Researchers study the spread of a cough cloud containing SARS-CoV-2 and used this finding to mathematically formulate the cloud’s spread through moist air in an enclosed area.

February 12, 2021
The Scitech

Infected airborne respiratory droplets play a significant role in the spread of the coronavirus, formally known as SARS-CoV-2. In such a scenario, it is crucial to understand how the virus-carrying fluid particles carried by a person’s cough or sneeze spreads through the ambient air.

Researchers had earlier found that the speed of a cloud of cough containing the airborne virus decreases as it travels away from the mouth. In a recent study, researchers from the Indian Institute of Technology Bombay (IIT Bombay) have used this finding to mathematically formulate the cloud’s spread through moist air in an enclosed area. The study was published in the journal Physics of Fluids.

The researchers found that the virus’ spread is independent of who coughs and how vigorously. The volume of air eventually covered by the cough cloud does not depend on the initial speed with which it is ejected. The mathematical calculation revealed that the volume depends on the distance the cough travels from the mouth and its sidelong spread. “These dependencies arise because the cloud traps air from the surroundings as it evolves,” says Prof Rajneesh Bhardwaj, one of the authors of the study. By analysing the equations of flow for the cough, the researchers found that a large volume of ambient air slowly gets trapped inside the cloud as it spreads out. With time, the droplet concentration inside the cloud thus reduces significantly from its initial concentration. Since the virus requires liquid droplets to survive, the possibility of its spread declines. They also found that the front of the cough cloud covers the first two metres of its total distance from the source within two seconds of being emitted. Hence, the cloud has the maximum probability of spreading the viral liquid immediately after release.

The calculations also enabled the researchers to quantify the effect of masks precisely. Masks reduce the net distance covered by the cloud by blocking it before its spread, earlier experiments have revealed. The researchers now compared the effect of surgical masks and clinical N95 masks on the volume of the cloud. While the cloud remains effective till about 8 seconds before dissipating irrespective of whether the person is masked or not, surgical masks reduce the volume by seven times compared to having no mask. N95 masks perform much better, decreasing it by as much as 23 fold. The researchers also calculated the effect of temperature and humidity of ambient air on the cloud’s spread. They found that the cloud’s temperature and humidity, which depends on the temperature, decrease over the distance of its spread. However, its humidity stays higher than the humidity of the ambient air till the end, as the cloud entraps water vapour from its surroundings. 

(Source: Indian institute of Technology Bombay news release)