Currently, ventilation is the most common strategy being employed to counter the spread of aerosolized COVID-19. The ventilation rate for indoor space is measured in air changes per hour (ACH), defined as the number of times the air in a room is completely changed or renewed with clean air in a single hour. Kohanski et al. (2020) define aerosols as particles having a diameter of 0.1 - 10 µm and droplets as particles having a diameter of 5 - 100 µm. Table 2 shows the suspension time of aerosols and droplets in an indoor environment under various values of ACH. The airborne suspension of particles depends on particle size.

A mechanical ventilation system is typically required to clear the infected space from any virus-laden particles that are still airborne. Such a system with an ACH ≥ 1 can clear the room of suspended aerosols that tend to be airborne for more than 1 h. This level of ACH is not adequate to clear airborne droplets. A mechanical ventilation system with an ACH ≥ 10 can clear the room of suspended aerosols and droplets that tend to be airborne for more than 6 min.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE, 2021) recommends an ACH rate of at least six to abate the aerosolized spread of COVID-19 in school classrooms. Both hospitals and airplanes maintain environments with high air exchange rates, which are ideal for countering the aerosolized spread of the COVID-19 virus. ACH for different environments in industry are shown in Table 3. Commercial airlines typically have a higher total air change rate than hospital operating rooms because of their higher occupancy, which requires higher air change rates.

*Aerosols are defined by Kohanski et al. (2020) as particles having a diameter of 0.1 μm - 10 μm. **Droplets are defined by Kohanski et al. (2020) as particles having a diameter of 5 μm - 100 μm.

*Updated Standard for COVID-19 Protection (ASHRAE, 2021).

As shown in Table 3, the combined outside and filtered ACH of 20 - 30 in an airplane’s cabin consists of 10 - 15 air changes from ventilation, i.e., outside air, and 10 - 15 ACH of filtered air. This means that the air in an airplane cabin is completely refreshed 20 - 30 times an hour or once every 2 - 3 min. The spread of the COVID-19 virus on airplanes should be limited only to those near an infected individual by way of droplet transmission (Morawska & Cao, 2020). Hospitals rely mostly on filtered air (80% of total ventilation) for maintaining a virus-free environment.

As a result of the high ventilation rates in airplanes, in addition to mandatory masking requirements and disinfecting practices, the number of infection cases linked to flights has been low (Bielecki et al., 2020). During the week of January 11-17, 2021, only 0.25% of new COVID-19 cases in Canada were linked to air travel (Government of Canada, 2021a).

The primary virus transmission mode on a flight is via respiratory droplets, which only pose a risk to those seated nearby an infected individual (Khanh et al., 2020). Ensuring adequate and effective ventilation of the COVID-19 virus aerosols in many closed spaces is a challenge. The two main types of ventilation systems are: mixing ventilation systems and displacement ventilation systems. Mixing ventilation systems rely on fresh air being vented into the space to ensure mixing. Opening a window is a type of mixing ventilation. Displacement ventilation typically employs vents at ground level from which fresh air is brought into the space and exhaust vents near the ceiling that extract air out from the space. A research team from the University of Cambridge concluded that mixing ventilation systems could increase viral transmission by dispersing respiratory droplets and aerosols throughout the space (Brierley, 2020). Displacement ventilation systems have the most potential for countering the spread of the virus as they actively extract contaminated air from the space at a faster rate than natural mixing of the indoor air (Brierley, 2020). Furthermore, these researchers stressed that adequate ventilation systems are not in any way a viable substitution for face coverings, which should always be the primary defence against viral spread.

Due to the potential risk associated with airborne respiratory aerosols, the CDC recommends a maximum of only a 30-minute stay in virus-affected, unventilated areas (CDC, 2020b). Windows can be opened either intermittently or continuously as a way of ventilating indoor spaces. However, ventilating a closed space by opening windows in northern cold climates during winter is limited as an option for reducing the risk associated with airborne particulate concentrations and virus-laden aerosols. When access to ventilation is not possible or realistic, residents should opt for a HEPA air purifier (Ontario Labour Relations Board, 2021). Devices utilizing HEPA filtration technology, in theory, can capture 99.97% of particles with a 0.3 µm diameter (Environmental Protection Agency, 2019). Capture efficiencies are at a minimum at this particle size. These efficiencies increase further for particle sizes either smaller or larger than 0.3 µm. Larger particles (>0.3 µm) are primarily captured due to interception with the filter’s fibres, while smaller particles (<0.3 µm) are trapped mainly due to their diffusion after having moved through Brownian motion. In simple terms, Brownian motion can be described as microscopic random fluctuations exhibited by small particles as they interact with each other and the surrounding media in diffusing. Respiratory aerosols that can potentially carry the COVID-19 virus are 5 µm or smaller, meaning that their capture efficiency is at least 99.97% when passing through a HEPA filter. Air purifiers have also been proven to reduce the spread of the common flue and others viruses (Environmental Protection Agency, 2019).

More recent scientific findings support the position that air purifiers equipped with HEPA filtration can help reduce the spread of COVID-19 (Christopherson et al., 2020; Zhao et al., 2020; Mousavi et al., 2020; Curtius et al., 2020; Curtius et al., 2021). Mousavi et al. (2020) examined the performance of portable HEPA air purifiers used to isolate air in a hospital patient room from migration into exterior hallways. In their study, the patient’s room was divided into two areas, an isolation room, where aerosol-generating equipment was located, and an anteroom, a dividing room used to separate the isolation room from the rest of the facility. The anteroom, in this case, was set up using a temporary plastic wall. The researchers found that the plastic wall alone, without the use of the air purifier, prevented up to 80% of the surrogate particles from spreading into the anteroom. With the addition of a HEPA air purifier in the isolation room, more than 99% of these particles were contained. The device removed the particles from the air inside the isolation room and discharged filtered air to the outdoors, thus creating negative pressure inside the isolation room and preventing the particles from escaping into adjacent spaces. Curtius et al. (2020, 2021) extensively studied HEPA air purifiers in a classroom setting. They concluded that using air purifiers in classrooms can be a practical strategy for reducing the risk of aerosolized transmission of the COVID-19 virus, especially when no fixed ventilation systems are installed and windows are not accessible (Curtius et al., 2021).

As of September 2021, Canada still considered the use of air purifiers as additional protection against the spread of the COVID-19 virus in situations where natural or mechanical ventilation and physical distancing are not possible (Government of Canada, 2021b). When used, air purifiers with HEPA filters are most effective when located close to the aerosol source (Lindsley et al., 2021). Although these devices capture airborne respiratory aerosols which may contain the COVID-19 virus, their impact on the trajectories of respiratory droplets is limited. Ham (2020) points out that air purifiers could potentially cause respiratory droplets to be suspended in the air longer and travel further. Thus, the air purifier’s position in the room and its exhaust orientation play a crucial role in determining the risk associated with the dispersion of respiratory droplets.

The use of air purifiers to reduce COVID-19 virus transmission in classroom settings has attracted the attention of both governments and researchers. In December 2020, one Quebec public school banned the use of air purifiers brought into classrooms by students (Henriquez, 2020). The Quebec Education Ministry noted that this ban was not official and indicated that maintaining and ensuring effective ventilation practices in the schools is the best strategy going forward (Henriquez, 2020). In early January 2021, the Quebec Minister of Health announced that air purifiers are not to be recommended officially as part of the reopening plan. Instead, opening windows regularly, social distancing, and mask-wearing should remain the primary strategies for stopping the spread of the virus.

According to Lurie and Ross (2021), reporting for CTV news, the decision by the Quebec public school division to ban the use of air purifiers in schools baffled many scientists and experts. In December 2020, the CDC recommended the use of air purifiers to mitigate the spread of the COVID-19 virus. Several accredited studies have already shown their efficacy (Burgmann & Janoske, 2021; Curtius et al., 2021; Lindsley et al., 2021). One study from Germany concluded that HEPA air purifier use in classrooms makes students six times less likely to contract COVID-19 (Curtius et al., 2020).

By December 2020, most school boards in Quebec were still using their windows as the main line of defence against aerosolized spread of the COVID-19 virus. According to Mahoney and Perreaux (2020), some teachers even requested that students be prepared to dress warmly in the classrooms as temperatures were sure to drop with windows left open in the frigid weather. By January of 2021, Quebec requested that all classroom windows be left open for at least 10-15 minutes twice daily, as well as 30 minutes before and after each class (Mahoney & Perreaux, 2020).

According to Fletcher & Olivier (2021), nearly two out of three schools in Quebec currently depend on natural ventilation to prevent COVID-19 virus transmission in schools. Despite Quebec’s official stance on air purifiers in classrooms, many of the largest school divisions in the province invested in HEPA air purifiers for their classrooms. Four months after the initial decision by the Quebec Minister of Health to opt against the use of air purifiers in schools, new data showed a substantial difference in case numbers between Quebec schools that incorporated HEPA air purifiers in their mitigation protocols and those which did not. Out of 677 schools in Quebec, the 61 schools that utilized HEPA air purifiers had, on average, 1.8 COVID-19 cases per school, whereas the 616 schools which did not utilize air purifiers had, on average, 7.3 COVID-19 cases per school (Ross, 2021).

In contrast to Quebec’s recommendations, Public Health in Germany recommended intermittent window opening only during class with no requirement for window ventilation between class periods (Deutsche Welle, 2020, October 10). Curtius et al. (2020) studied the effectiveness of utilizing Germany’s practice of intermittent window opening, as well as the use of HEPA air purifiers during class. These researchers modelled various strategies for reducing the concentration of RNA-containing aerosols in a classroom. The use of four air purifiers with HEPA filtration and a total clean air delivery rate (CADR) of 1026 m³/h was incorporated into their model. The classroom size was 180 m³, and the air purifiers had a CADR rating equivalent to ACH = 5.7. This is slightly below the American Society of Heating and Air-Conditioning Engineers (ASHRAE) recommendation of an ACH of at least 6 for schools (ASHRAE, 2020).

Figure 2 shows the concentration of the RNA-containing aerosol in a German classroom over a series of 60-min classes, each followed by a 30-min break, as modelled by Curtius et al. (2020). One student, who has two successive classes in the same room that day, is assumed to be infected with COVID-19. Natural ventilation in the Curtius et al. (2020) model was assumed to be achieved by opening windows for 3 minutes every 20 minutes, achieving a reduction of either 30% or 99% of the existing virus-containing aerosol during the 3-min period of ventilation. The mitigation potential of windows accessed intermittently depends on

the number of windows, their size, wind conditions, and temperature differentials between the outdoor and indoor spaces. To achieve the 99% reduction in concentration, Curtius et al. (2020) assumed that the classroom must have multiple large windows, each of which is operable, high-wind conditions, and a significant temperature differential between the indoor classroom and the outdoors. To achieve a 30% concentration reduction, Curtius et al. (2020) assumed that the classroom had a single operable window, low wind conditions, and a minimal temperature differential between the indoor classroom and the outdoors.

Figure 2 further shows that a partial level of ventilation (resulting in a 30% reduction in the virus-containing aerosol over a 3-min period, every 20 minutes) is insufficient to mitigate the amount of COVID-19 aerosol dispersed during the previous 20 min that the windows were closed. A step-like escalation in aerosol concentrations results over the 60-min class period and throughout the day. However, this escalation can be avoided entirely if a much higher level of ventilation is applied (e.g., one that would result in a 99% reduction in aerosol concentration over the same 3-min period every 20 min). Such a drastic drop in the concentration requires access to multiple large operable windows and favourable weather conditions, which may not always be possible. This high level of ventilation is used in the current study to gauge the mitigation potential of both Germany’s and Quebec’s intermittent window ventilation strategies.

The Curtius et al. (2020) model was extended in this paper to examine the concentration of the RNA-containing aerosol in Quebec’s school ventilation protocol. Quebec’s school strategy requires that windows and doors be left open for 30-min before and after class. Again, the model assumes that an infectious person is present during each class period, no mechanical ventilation systems are in place, and no HEPA air purifiers are used at all. The results are shown in Figure 3.

The results shown in Figure 3 indicate that, while this ventilation practice is effective in reducing the concentration of the virus-containing aerosol considerably over the 30-min period between classes, the concentration begins to rise again during class as the students are continuously exposed to an infectious individual, and no ventilation mitigation is practised during class.

This model was further expanded in this paper to include the effect of HEPA air purifiers in order to characterize their impact on reducing the concentration of the virus-containing aerosol, both alone and in combination with different intermittent window ventilation strategies. The results are shown in Figure 4 and Figure 5. These results indicate that introducing a HEPA air purifier, in addition to ventilation, consistently offers an additional layer of protection against virus-laden aerosol concentrations. Moreover, their presence ensures that any escalation in COVID-19 virus-laden aerosol concentration between classes can be effectively avoided. For both models, an infectious person is assumed to be present during each class period, no mechanical ventilation systems are installed, and several HEPA air purifiers are run continuously throughout the day. A combined air exchange rate (ACH) of the air purifier(s) = 5.7 was assumed, which is the same as that used by Curtius et al. (2020).

In contrast to intermittent ventilation strategies, continuous ventilation, such as that provided by mechanical HVAC systems or by leaving windows/doors open at all times, ensures a continual and sharp reduction in indoor virus-laden aerosol concentrations, both before and during the class period.

Like HVAC systems, HEPA-equipped air purifiers provide continuous mitigation and are relatively less expensive than HVAC systems. As pointed out by Curtius et al. (2020), utilizing multiple HEPA air purifiers simultaneously in an indoor space offers cumulative performance, as measured by the total CADR or ACH. Notably, Quebec Public Health still does not recommend using portable air purifiers as a primary means for mitigating aerosolized COVID-19 virus. Quebec has since updated its window access strategy to request that windows in school classrooms be kept open at all times, if possible (Government of Quebec, 2021).

In addition to the concentration of the RNA-containing aerosol, Curtius et al. (2020) examined the dose of RNA copies inhaled by a susceptible person over a one-hour period for various scenarios of ventilation (see Table 4). Table 4 shows that a combination of good ventilation and the use of HEPA air purifiers provides occupants with the most protection against aerosolized COVID-19 virus. In rooms with limited ventilation (poor or some), HEPA air purifiers significantly improve protection.

Ranking mitigation protocols by the viral dose absorbed over an hour-long period in a classroom setting can be used to evaluate further virus transmission associated with human behaviour in comparably sized and occupied indoor spaces. This evaluation is discussed in Section 3.3.

Figure 4 and Figure 5 show that HEPA air purifiers, used in combination with good ventilation practices, offer a high level of mitigation against aerosolized spread. However, opening a window could potentially elevate the overall transmission risk, in a similar fashion described by Ham (2020), if a wind draft causes respiratory droplets, which generally fall abruptly to the ground, to remain airborne for extended periods of time. The precise nature of the overall risk associated with indoor aerosolized and droplet transmission of the COVID-19 virus when HEPA air purifiers and windows are utilized has yet to be comprehensively examined. However, it is clear that without either ventilation or HEPA air purification, respiratory aerosols remain airborne indoors for much longer periods than the maximum time recommended by the CDC (30 minutes) for anyone to remain in a non-ventilated space.

Note: Ventilation is achieved with intermittent window access in a classroom setting. *Poor Ventilation: 0% airborne viral concentration reduction over a 3-min period (window not opened). **Some Intermittent Ventilation: 30% airborne viral concentration reduction over a 3-min period every 20 min. ***Good Intermittent Ventilation: 99% airborne viral concentration reduction over a 3-min period, every 20 min.

Even if masks are worn indoors, an inherent risk is associated with aerosolized spread without proper ventilation or air purification. Out of the 26 largest school boards across Canada contacted by the Globe and Mail in December 2020, only eight had opted to install HEPA air purification units in classrooms (Mahoney & Perreaux, 2020). As of July 2021, schools in Winnipeg, Saskatoon, and Calgary currently have no plans for introducing similar systems in their schools.

Scientific evidence shows that air purifiers utilizing HEPA filters, together with good ventilation, effectively reduce aerosolized particles in a classroom setting (Curtius et al., 2020; Han et al., 2020; Bullister, 2021). Combined with mask use, these strategies counter the spread of COVID-19 even more effectively than social distancing alone (Foster & Kinzel, 2021).

Researchers often use the simple mathematical Wells-Riley model to simulate the spread of the COVID-19 virus within an indoor space to estimate infection probability (Foster & Kinzel, 2021; Noakes & Sleigh, 2008). This model, however, is limited in that it does not account for complex stochastic particle behaviour arising within the indoor space. Instead, it assumes complete indoor air mixing, implying uniformity of infectious aerosols (Noakes & Sleigh, 2008; Foster & Kinzel, 2021). As a result, Wells-Riley models do not provide insight on potential localized risk zones that may arise within the indoor space itself (Noakes & Sleigh, 2009). Similarly, the model developed by Curtius et al. (2020) assumes perfect mixing of the indoor air at all times and, therefore, does not help describe aerosolized risk variance within the classroom space itself. The risk of infection in a real-world environment will not always be uniform within a given space. Depending on where occupants reside within the space, infection transmission routes can vary significantly (Foster & Kinzel, 2021). However, these models allow researchers to easily rank mitigation strategies employed in an indoor space (Foster & Kinzel, 2021).

A number of researchers have used computational fluid dynamics (CFD) approaches to obtain a clearer understanding of how COVID-19 aerosols spread locally inside a classroom setting. Burgmann & Janoske (2021), for example, used the Numerical CFD code OpenFOAM^® to examine the effectiveness of a HEPA air purifier in reducing the concentration of aerosols in a classroom. Abuhegazy et al. (2020) used computational fluid-particle dynamics simulations to study the spread of aerosol from an infected individual in a classroom environment and the effectiveness of glass partitions and window openings as a way of reducing aerosol transmission. Narayanan and Yang (2021) used the Eulerian-Lagrangian approach to examine the effectiveness of portable air purifiers to reduce the concentration of aerosolized COVID-19 virus in a music classroom. Pavilonis et al. (2021) examined the risk of SARS-CoV-2 transmission among students and teachers in 19 schools in New York using a modified Wells-Riley equation under both steady-state and varying exposure conditions. The researchers found that the mean probability of transmission was generally low, which they attributed to the fact that the study was conducted between December 2017 and September 2018, before the surge of SARS-CoV-2 in New York City. Foster & Kinzel (2021) used mathematical (Wells–Riley) and numerical CFD methods to examine the probability of infection in a typical classroom scenario with masked individuals. They found that both methods predicted a 40% - 50% lower risk of infection when ventilation, in combination with filtration, was used.

The research addressing the spread of aerosols inside a classroom setting is predominantly theoretical (Mirzaie et al., 2021). However, some experimental evidence is available. Burgmann & Janoske (2021), for example, used artificially generated aerosol to examine the effectiveness of a HEPA air purifier in reducing the concentration of aerosols in classrooms. The reduction in concentration ranged between 70% and 90% depending on the location when an ACH of 6 was applied. Curtius et al. (2020) conducted both theoretical and experimental examination of the efficiency and practicability of operating four air purifiers equipped with HEPA filters in a high school classroom during regular classes. They observed that, with windows and doors closed, the aerosol concentration in the classroom was reduced by more than 90% within less than 30 min when running the purifiers (ACH = 5.7).

The computational fluid mechanics approach in modelling droplet motion and distribution is well-documented in the fluid mechanics literature (Dolatabadi et al., 2021; Hoseinzade et al., 2021; Yazdani & Lakzian, 2020; Foroozeh et al., 2020; Hosseini & Lakzian, 2020). Several researchers have also used a CFD approach to examine the distribution of airborne COVID-19 virus droplets, under various environmental conditions, in school as well as in other environments. For example, Mirzaie et al. (2021) used a 3D turbulence model and a Langrangian particle trajectory analysis method to track the motion and dispersion of cough droplets by an infected individual in front of a classroom. One of the parameters tested was the use of transparent barriers placed in front of the seats to reduce the concentration of droplets. The researchers found this intervention to be “somewhat effective.” D’Alessandro et al. (2021) and Motamedi Zoka et al. (2021) used the Eulerian-Lagrangian CFD approach to simulate the distribution of infectious exhaled droplets in various environments.

The airborne dispersion of particles due to various indoor human activities (i.e., coughing, singing) was the subject of study by Mariam et al. (2021) and Rosti et al. (2020), who used CFD to study the safe distance for the transmission of the COVID-19 virus. Mariam et al. (2021) used the Reynolds-Averaged Navier-Stokes (RANS)-CFD approach, while Rosti et al. (2020) used a Lagrangian model. Mesgarpour et al. (2021) used a combined CFD and artificial intelligence (AI) approach to study the spread of droplets produced by a sneezing passenger in a bus. The results showed that the concentration of droplets could decline to 87% in the first 3 m.