COVID-19 can spread through respiratory droplets either when a person comes in direct contact with an infected person (when the person coughs or sneezes or talks) or when they touch their face after touching any contaminated object (fomites) like doorknobs, railings and even a thermometer used by an infected person.
Previous studies have suggested that SARS-CoV-2, the causative agent of the novel coronavirus disease, can stay on surfaces for various intervals of time, ranging from a few hours to a week depending on the material. It is also indicated to stay in the air for about three hours so far but there is still much debate around airborne transmission.
Several studies and outbreaks have indicated that the virus can spread more quickly in confined and crowded spaces like workplaces and restaurants as compared to outdoor spaces, mostly due to increased exposure and less dispersion of droplets. In one such study, 10 people from three families were found to be infected when they sat directly in the line of the air conditioner airflow with a presymptomatic person while everyone else at the diner had remained unaffected.
A study published in JAMA Internal Medicine earlier in September also explained how airborne transmission in a closed air-conditioned bus may have led to an outbreak and why masks are important in a closed indoor environment.
Now, a group of researchers at the University of Cambridge claim that certain kinds of ventilation systems can promote the spread of COVID-19 indoors while others may not.
The study is published in the peer-reviewed scientific journal Journal of Fluid Mechanics.
Two types of ventilation system
In the study, the researchers explained that there are two kinds of ventilation systems -- mixing ventilation and displacement ventilation.
Mixing ventilation system, used in most offices, has inlets and outlet ducts placed in a way to help keep the conditions uniform throughout the area. But this is what makes the respiratory droplets, and hence the virus spread throughout too.
On the other hand, in displacement ventilation, the ducts are placed at the top and bottom of a room. This creates a cooler lower zone below while the warmer air moves towards the ceiling and is expelled out through the outlet ducts. Since our exhaled air is warm, it will go to the upper zone and will be expelled. If the interface between the cold and warm zone is high enough, it is less likely to be inhaled in by another person.
"In order to model how the coronavirus or similar viruses spread indoors, you need to know where people's breath goes when they exhale, and how that changes depending on ventilation," said lead researcher of the study, Professor Paul Linden from the Department of Applied Mathematics and Theoretical Physics at Cambridge University, in a news release by the University. "Using these data, we can estimate the risk of catching the virus while indoors," he said.
The authors also suggested that when designed properly, displacement ventilation systems can prevent cross mixing of air and hence virus spread.
More ways to maintain ventilation
The World Health Organisation has suggested some of the following ways to improve ventilation in indoor spaces and prevent COVID-19 spread:
Use natural ventilation instead of mechanical ventilation whenever possible.
If you are using mechanical ventilation, increase the percentage of outdoor air, preferably to 100%.
Install exhaust fans in washrooms and make sure that they are working properly.
Make sure to clean air filters and vents regularly.
If possible increase the total airflow supply in an area.
For more information, read our article on How COVID-19 spreads.
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