New study reveals how effectively the novel coronavirus spreads through coughs
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.