Updated: October 5, 2020 10:25:00 am
A team of researchers from the University of Cambridge has found that the ventilation systems used in modern office spaces, designed to keep conditions uniform in all parts of the room, disperse airborne contaminants evenly throughout the space. Their findings were published in the Journal of Fluid Mechanics.
Why are researchers interested in studying the role of airflow in the spread of COVID-19?
Researchers are studying the role of airflow in disease transmission in the case of COVID-19 since it is considered important to estimate the risk of contracting the virus. By now it is well known that poorly ventilated spaces make it easier for virus transmission to take place. It is also believed that virus transmission is easier indoors rather than outdoors. Last week, scientists from the UK began their research to determine how long the coronavirus can survive in tiny aerosol particles.
What does the recent study say about ventilation systems?
The study illustrates that the virus is spreading primarily through large droplets and smaller aerosols, which are expelled when people cough, laugh, talk, breathe or sneeze. The authors note that the data so far indicates that, “indoor transmission of the virus far outstrips outdoor transmission, possibly due to longer exposure times and the decreased turbulence levels (and therefore dispersion) found indoors”.
Professor Paul Linden from Cambridge’s Department of Applied Mathematics and Theoretical Physics (DAMTP), who led the research, said in a press release that inside closed spaces, when people exhale carbon dioxide, the virus is carried along with it and can be spread around the room through ventilation flows.
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Why does poor ventilation increase the risk of disease transmission?
Insufficiently ventilated spaces can lead to higher concentrations of carbon dioxide, thereby increasing the risk of exposure to the virus.
So, what is the role of airflow in disease transmission?
Airflow in a room depends on the placement of vents, doors, windows and the convective flows generated by heat that is emitted by people and the placement of equipment in a building. The authors note that other variables such as people moving or talking, doors opening and closing and changes in outdoor conditions for naturally ventilated buildings also affect the airflow and consequently the risk of transmission.
The study also talks about two methods of ventilation, one being mixing ventilation, which is the most common and involves the placement of vents in a way such that the air in a space is well mixed in order to keep the temperatures uniform throughout the space. But this also means that the contaminants are also spread evenly throughout the room.
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The second method is called displacement ventilation and involves the placement of vents at the bottom and top of the room. This creates a cooler lower zone and a warmer upper zone. This warm air is extracted by the vents placed on the top of the room. Since the exhaled air is also warm, using this method of ventilation means that most of the exhaled air also gets mixed up with the warm air and is hence extracted by the upper vents than breathed in by another person.
So, what does this mean?
The results show that room flows are turbulent and can change dramatically depending on the movement of the occupants, the type of ventilation, the opening and closing of doors and, for naturally ventilated spaces, changes in outdoor conditions. The researchers also found that masks are effective at reducing the spread of exhaled breath, and therefore droplets.
Significantly, they note that while every mask type has a certain amount of leakage through the tops and the sides, it doesn’t matter much because what is important is to slow the momentum of the exhaled contaminants, which essentially reduces the chances of direct exchange of aerosols and droplets.
Breaking the momentum of the exhaled breath means that the air is carried upwards towards the ceiling. They also found that 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.
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