Infected airborne respiratory droplets play a significant role in the spread of the coronavirus, formally known as SARS-CoV-2. The widespread use of respiratory masks has curbed the spread of the virus and brought down the number of people affected by the disease. But, as the pandemic rages on, general fatigue in following the behavioural restrictions has set among the public. Festive gatherings continue to be celebrated, and governments across the world have had a difficult time convincing people to adhere to the safety guidelines. 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. This quantitative estimate sheds a clear light on why masks have been so effective in curbing the spread.
“In case a person is not using a mask while coughing, it is possible to reduce the spread by simply blocking the mouth with a palm or an elbow,” says Prof Amit Agrawal, the other author of the study.
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.
“Only during the course of the pandemic have people realised the importance of studying coughing and sneezing in the context of disease transmission,” says Prof Agrawal. While the data related to coughing has been generated only recently, another team is already conducting experiments on sneezing, and the researchers will use these results to consider the effects of sneezing. “Such a study will help us determine the maximum number of people that can be safely accommodated in a hospital ward,” adds Prof Bhardwaj.
Moreover, the airflow in the surroundings may change how a cough or sneeze evolves, for example, if there is a strong wind in the room. Although conducting detailed experiments in such flow conditions is not easy, work is in full swing. Once the results start coming in, the researchers will modify their findings considering additional practical situations.
“This will enable us to study the minimum rate at which air in a room, elevator, cinema hall, car, aircraft cabin, or restaurant needs to be circulated to maintain freshness and reduce the chances of infection,” signs off Prof Agrawal.
This article has been run past the researchers, whose work is covered, to ensure accuracy.