COVID-19: an increased risk of contamination depending on the type of ventilation?
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British researchers warn that a specific type of ventilation system in many modern office buildings can increase the risk of exposure to the coronavirus, especially in the winter. In fact, the droplets and aerosols that a contaminated person exhales can be dispersed within a room if the air does not circulate properly - it all depends on the location of the vents.
The World Health Organization indicates that the virus responsible for Covid-19 is transmitted mainly from person to person through respiratory droplets expelled through the nose or mouth when an infected person coughs, sneezes or speaks. You can get this disease if you breathe in these droplets or touch your mouth, nose, or eyes after touching contaminated objects or surfaces. Given the risk of transmission of Covid-19 by air, it is legitimate to ask the question of the impact of ventilation networks: should they be considered vectors of transmission through droplets and aerosols?
Risk of airborne transmission under certain conditions
Published in the Journal of Fluid Mechanics, the study first recalls that the risk of contamination is greater in an indoor space due to a longer exposure time and a decrease in the dispersal rates of droplets and aerosols that contain virus particles. However, "mixed ventilation" systems would have the particularity of uniformly dispersing pollutants in suspension in space, either droplets or aerosols (by air). These can be produced by breathing and talking, posing a risk of contamination by inhalation if they contain the virus in sufficient quantities.
"It is essential to understand the role of ventilation in estimating the risk of contracting the virus and helping to slow its spread." explains Professor Paul Linden, who led the research. “Although direct control of droplets and aerosols indoors is difficult, we exhale carbon dioxide, which can be easily measured and used as an indicator of infection risk. The small respiratory aerosols that contain the virus are transported together with the carbon dioxide produced by respiration and are carried into a room by ventilation currents. "
Why "mixed" ventilation is not recommended
Therefore, poor ventilation could lead to a high concentration of carbon dioxide, which in turn could increase the risk of exposure to the virus. The researchers were interested in two types of ventilation and firstly mixed or mixed ventilation, when the air vents are placed so that the air and temperature; and therefore the concentrations of pollutants, remain uniform at all times in space. The second mode, displacement ventilation, has air vents located at the bottom and top of a room, creating a cooler lower zone and a warmer upper zone, and the hot air is drawn through the upper part of the room.
It is this type of ventilation that the researchers recommend since a persons breath, and with it what they exhale, is also hot, so most of it accumulates in the upper part of the room. As long as the interface between the zones is high enough, the polluted air is drawn through the ventilation system instead of being inhaled by another person, reducing the risk of exposure. "To model how the coronavirus spreads indoors, it is necessary to know where peoples breath goes when they exhale and how that changes depending on the ventilation. With this data, we can estimate the risk of contracting the virus indoors," says the Professor Linden.
The researchers also found that the masks are effective in reducing the spread of exhaled air. "They allow to slow down the impulse of respiration, slowing down the impulse of any exhaled pollutants reduces the risk of direct exchange of aerosols and droplets while the breath is transported to the ceiling", emphasizes Prof. Linden. They conclude on the importance of wearing masks in a closed environment but also of practicing natural ventilation, that is, keeping the windows open.
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