London, September 30
Ventilation systems in many buildings, which are designed to keep temperatures comfortable, may increase the risk of exposure to the novel coronavirus which causes COVID-19, particularly during the winter season when people prefer to stay indoors, says a new study.
Scientists, including those from the University of Cambridge in the UK, found that widely-used ‘mixing ventilation’ systems, which are designed to keep conditions uniform in all parts of the room, disperse airborne contaminants evenly throughout the space.
According to the study, published in the Journal of Fluid Mechanics, these contaminants may include droplets and aerosols, potentially containing viruses such as the novel coronavirus SARS-CoV-2.
With evidence increasingly indicating that the coronavirus is spread primarily via droplets and aerosols expelled as people cough, sneeze, talk, or breathe, the findings underscore the need for good ventilation and mask-wearing to mitigate SARS-CoV-2 transmission risk.
Based on studies conducted so far, the researchers said indoor transmission is far more common than outdoor transmission — likely due to increased exposure times and decreased dispersion rates for droplets and aerosols.
“As winter approaches in the northern hemisphere and people start spending more time inside, understanding the role of ventilation is critical to estimating the risk of contracting the virus and helping slow its spread,” said study co-author Paul Linden from the University of Cambridge.
Small respiratory aerosols containing the virus are transported along with the carbon dioxide produced by breathing, and are carried around a room by ventilation flows, Linden said.
“Insufficient ventilation can lead to high carbon dioxide concentration, which in turn could increase the risk of exposure to the virus,” he explained.
According to the scientists, airflow within the building works in one of two main modes — mixing ventilation or displacement ventilation.
They said the former mode is the most common, where vents are placed to keep the air in a space well mixed so that temperature and contaminant concentrations are kept uniform throughout the space.
The second mode, displacement ventilation has vents placed at the bottom and the top of a room, creating a cooler lower zone, and a warmer upper zone and warm air is extracted through the top part of the room, the researchers said.
Since exhaled breath is also warm, most of it accumulates in the upper zone, they explained.
If the interface between the zones is high enough, the study said contaminated air can be extracted by the ventilation system rather than breathed in by someone else.
The scientists suggest that when designed properly, displacement ventilation could reduce the risk of mixing and cross-contamination of breath, thereby mitigating the risk of exposure.
“These two concerns are related but different, and there is tension between them, which has been highlighted during the pandemic,” said Rajesh Bhagat, another co-author of the study from the University of Cambridge.
“Maximising ventilation, while at the same time keeping temperatures at a comfortable level without excessive energy consumption is a difficult balance to strike,” Bhagat said.
In order to model how the coronavirus or similar viruses spread indoors, the scientists believe it is important to know where people’s breath goes when they exhale, and how that changes depending on ventilation.
“Using these data, we can estimate the risk of catching the virus while indoors,” Linden said.
In the study, the scientists explored a range of different modes of exhalation — nasal breathing, speaking and laughing, each both with and without a mask — by imaging the heat associated with the exhaled breath.
“When sitting still, humans give off heat, and since hot air rises, when you exhale, the breath rises and accumulates near the ceiling,” said Bhagat.
The researchers said laughing, in particular, creates a large disturbance, suggesting that if an infected person without a mask was laughing indoors, it would greatly increase the risk of transmission.
“One thing we could clearly see is that one of the ways that masks work is by stopping the breath’s momentum,” said Linden.
“While pretty much all masks will have a certain amount of leakage through the top and sides, it doesn’t matter that much, because slowing the momentum of any exhaled contaminants reduces the chance of any direct exchange of aerosols and droplets as the breath remains in the body’s thermal plume and is carried upwards towards the ceiling,” he said.
According to the scientists, a three-layered mask decreases the amount of those contaminants that are recirculated through the room by ventilation.
“Keep windows open and wear a mask appears to be the best advice. Clearly, that’s less of a problem in the summer months, but it’s a cause for concern in the winter months,” Linden said. PTI