In the previous Technically Speaking we introduced the concept of an Airborne Infection risk calculator.
The Fantech COVID-19 Airborne Risk Calculator is a simplified calculator based on the Airborne Infection Risk Calculator Version 3.0 Beta by A. Mikszewski, G. Buonanno, L. Stabile, A. Pacitto and L. Morawska, April 2021. The Fantech tool is limited to simulating simple scenarios where there is only one infectious individual who has a variant of COVID-19. For more advanced scenarios, or for different airborne viruses, the above-mentioned calculator is likely to be more appropriate.
Theory
This tool models a single event (where an event is a gathering of people in an environment for a set period of time). The event could be a 45 minute lesson in a classroom, a 3 hour meeting in a conference room, or 20 minutes waiting at a doctor’s surgery.
To determine airborne transmission risk, a concept called Quanta (or infectious dose) is used. One Quanta is the dose of virus that will infect 63% of susceptible people. The calculator uses the Quanta emission rate (which is the infectious dose exhaled by an infected person) and the removal rate of the virus from the room to determine the amount of virus present in the area.
The removal rate of virus from the area is based on the sum of the air change rate, the viral inactivation rate (the rate at which the virus decays per hour), and the particle deposition rate (the rate at which the virus impacts surfaces).
A susceptible person is a person who is susceptible to catching Covid-19.
The assumption is that the room is well mixed, that the virus levels have reached an equilibrium state, and that the dose of Quanta received by a susceptible person is integrated over the total time.
Since the Quanta Emission Rate (QER) is dependent on an infected person’s breathing rate, the calculator requires information on the activity level of the infectious person. Conversely, the breathing rate of the susceptible people in the room will determine the amount of virus inhaled. Therefore, the more active the infected and susceptible people are, the more likely the virus will be transmitted. It can be observed that mask wearing can reduce a percentage of viruses from being exhaled or inhaled (depending on the effectiveness of the mask and fitment of the mask).
The event time is very important as the longer susceptible people are exposed to a virus, the more likely it is they will receive a large enough dose to become infected.
Lastly, different variants of SARS-CoV-2 display different amounts of transmissibility, so using the relevant virus strain is important and can have a great impact on results. Fantech regularly updates the calculator as transmission data on new variants becomes available.
Results
After entering all the inputs and pressing the calculate button, the individual risk, maximum number of susceptible occupants (before a COVID-19 case will arise), and the probable number of cases is determined.
The Individual Risk field shows the likelihood of a single person being infected, while the Max Occupancy field estimates the maximum number of susceptible people allowed in the room (for the calculated duration) before it is likely that a COVID-19 case will arise.
As an example, if we had a maximum occupancy of 10 people before it was likely that a COVID-19 case would arise, and 11 susceptible people were present in the room, it is likely that a susceptible person would become infected. If only 9 susceptible people were in the room, it is not likely that someone would become infected.
Note - as these numbers are probabilities, even though it may be unlikely that someone becomes infected, there will always be a chance of this occurring.
The goal of the calculator is to provide a means to reduce the risk of transmission to an acceptable level. It is impossible to bring the risk of transmission to zero, even with extreme measures in place.
For more detailed information on the Fantech COVID-19 Airborne Risk Calculator, please consult the help file at www.fantech.com.au/covid-19-risk-calculator
The next edition of Technically Speaking will focus on the concept of Clean Air Delivery rates and what strategies can be used to reduce the amount of airborne virus in an area.
Disclaimer: While the calculator is based on the latest information, several assumptions are made which mean that the results will not be exact. This tool is designed to simulate changes in environment/behaviour and then show whether these changes have a positive or negative effect on the airborne transmission risk. It is not designed to predict an absolute risk of transmission.
