While life has returned to a sense of normality for most people after the various COVID-19 lockdowns, the reality is that COVID-19 is still with us. As time passes we are seeing new COVID-19 variants and we are learning more about how the virus is transmitted and how to limit transmission between people.
In the early days of the pandemic it was thought that SARS-CoV-2 was only transmitted via large heavy droplets that travel a short distance and quickly fall to the ground. Social distancing and disinfection of surfaces were the recommended preventative options to combat transmission.
However, it is now known that a person exhales a range of particle sizes in every breath and that most of the SARS-CoV-2 particles exhaled are very light and small. The smallest particles can remain airborne for a number of hours or even days in some circumstances. While the virus cannot survive indefinitely outside of the body, it can survive for a period of time and is therefore an infection risk. (Mask wearing was used as a method of controlling droplet and airborne transmission of the virus).
While the requirements for mask wearing are relaxed in many areas, we can still utilise ventilation to reduce the risk of spreading SARS-CoV-2 by airborne transmission
Ventilation
Ventilation works on the principle of dilution to reduce airborne transmission of SAR-CoV-2. If fresh, virus-free air is replacing contaminated air, then the amount of SARS-CoV-2 present in the environment is diluted in proportion to the ventilation rate. If there is less SARS-CoV-2 in the area, then there is less chance of an infection occurring
Ventilation (or the replacement of indoor air with fresh air) can either be natural (by opening windows and doors) or mechanical (by using a fan to control the airflow).
Natural ventilation can be effective when the conditions are suitable, such as when there is a wind outside, and when the outside temperatures allow for the opening of windows and doors. However natural ventilation may not give reliable, consistent ventilation and could introduce unwanted noise, dust and changes of temperature into an environment.
Mechanical ventilation gives controlled ventilation that ensures the environment can be continuously ventilated at a known rate. The potential downsides of natural ventilation can be solved using mechanical ventilation, but this is obviously a more costly solution than opening a window!
Methods of removal of SARS-CoV-2
There are a number of methods that can be used to reduce the concentration of SARS-CoV-2 in an indoor environment. These include:
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Dilution – replacing contaminated air with fresh air by ventilation and therefore reducing the virus concentration in the environment.
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Filtration – using filters to remove the virus from a recirculating duct system. [Note: high efficiency filters (with high pressure drops) may be required to capture the very small particle sizes of viruses.]
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Deactivation – using technologies such as UVGI or plasma to modify the genetic structure of a virus, thus stopping it from replicating.
All three of these methods result in a reduction of infectious SARS-CoV-2 from the environment and can work individually or together. This concept is called Clean Airflow.
Calculation tool
Fantech has recently released an online calculator to determine the risk of airborne transmission of SARS-CoV-2.
The tool considers the room size, clean airflow rate, occupancy, exposure time, the variant of SARS-CoV-2 (e.g., Omicron BA.2), exhalation and inhalation rates of occupants, and mask wearing strategies. Based on these parameters we can calculate the number of occupants that are likely to become infected due to airborne transmission of SARS-CoV-2.
The next edition of Technically Speaking will go into more detail of how to use the calculator and the concept of Clean Airflow rates.
