Priority of Mixed-Mode Ventilation during Epidemics: A Comprehensive Investigation

Abstract

This chapter provides a detailed analysis of the operation of mixed-mode ventilation during epidemics, concentrating on the pivotal role of indoor air quality (IAQ). It underlines the importance of ventilation in IAQ management, particularly for airborne infection control. However, our principal focus is mixed-mode ventilation, a combined approach of natural and mechanical methods, which we highlight as promising for IAQ management, airborne disease control, and also energy-saving solutions. Our examination includes multiple case studies for each diverse environment, such as educational buildings, hospitals, office buildings, and residential buildings, each evaluated through different methods, including computational fluid dynamics and experimental approaches. Our observations illustrate the significant role of efficient ventilation in improving IAQ, mitigating airborne infection risks, and enhancing occupant comfort, especially during epidemics.

1. Introduction

1.1 Importance of indoor air quality (IAQ)

The importance of indoor environment quality for occupant health cannot be overstated. The quality of indoor air influences the health of those who work, reside, or use enclosed spaces in the same way that outdoor air quality does. Experts emphasize the critical significance of indoor air quality, sanitation, ventilation, and pollution control in the context of the Covid-19 pandemic [1].

The phrase “indoor air quality,” abbreviated as “IAQ,” refers to the state of the air within a building or other enclosed space, taking into account both the thermal comfort levels and the concentrations of various pollutants [2, 3, 4, 5]. ANSI/ASHRAE 62.1-2022 [6] defines acceptable indoor air quality (IAQ) as “air in which there are no known contaminants at harmful concentrations, as determined by cognizant authorities, and with which a substantial majority (80% or more) of the people exposed do not express dissatisfaction.”

Because we spend the majority of our time indoors, IAQ is crucial. Occupants may believe that their enclosed space is exceptionally clean, but do they realize that indoor air contaminants are virtually everywhere? Construction materials, paints, and coatings on walls and ceilings, gas ranges in kitchens, and even personal care products contain contaminants that can be hazardous in an indoor environment. Indoor air can be several times more polluted than outdoor air. Because of this, experts are concerned about IAQ in contemporary times.

The contaminants in the indoor air lead to the degradation of the IAQ. Radon, indoor aerosols, ozone (${\mathrm{O}}_3$), carbon dioxide (${\mathrm{CO}}_2$), carbon monoxide (CO), formaldehyde (HCHO), nitrogen dioxide (${\mathrm{NO}}_2$), sulfur dioxide (${\mathrm{SO}}_2$), total volatile organic compounds (TVOCs), particulate matter (PM2.5 and PM10), and asbestos are all examples of contaminants that may be found in indoor air [2, 3, 7]. Temperature, humidity, air movement, and the effectiveness of ventilation systems are other factors that impact IAQ [3]. Modern buildings are built to be airtight, which eliminates the need for natural ventilation. This is because modern buildings are controlled by heating, ventilation, and air conditioning (HVAC) systems, which recycle a large proportion of the air while replacing minor amounts of it with fresh air in order to maintain a consistent IAQ throughout every season [7, 8]. Imagine a confined space that has a high level of humidity and inadequate ventilation. It should come as no surprise that this space has a poor IAQ. When one person in this enclosed space has influenza, it is quite likely that other persons in the same setting will also get infected with the virus. When someone remains with an individual who suffers from a major infectious condition like severe acute respiratory syndrome (SARS) or COVID-19, the situation is made much worse, and the outcomes may be fatal.

Thus, IAQ is essential. The concentrations of indoor air contaminants are maintained at an acceptable level in an indoor environment with adequate IAQ. The health of the occupants can then be protected. In addition, the indoor temperature, humidity, and ventilation system can all contribute to comfort. Conversely, an environment with inadequate IAQ can result in a variety of problems. In addition to making occupants feel uncomfortable, inadequate IAQ has negative effects on health. One of the most significant building-related health issues is the sick building symptom (SBS), which is caused by inadequate IAQ. SBS side effects include migraines, vertigo, fatigue, shortness of breath, eye irritation, dry cough, and itchiness on the skin [3, 7]. Hence, it is necessary to maintain a good IAQ within a building.

1.2 Importance of ventilation strategies in improving IAQ and mitigating airborne infections

In 2019, a worldwide outbreak of a hitherto undiscovered virus halted almost all human endeavors. Damage from this pandemic has been catastrophic. The COVID-19 epidemic is, by any measure, one of the most major issues faced by mankind in the twenty-first century [9, 10, 11].

As the World Health Organization (WHO) designated COVID-19 to be a Public Health Emergency of International Concern (PHEIC) on January 30, 2020 [12, 13], very few people possibly could have predicted that it would infect about 691,748,366 individuals and killed 6,901,518 people all over the globe by July 20, 2023. In addition, the world economy was on the edge of entering a downturn on a scale never seen before when the COVID-19 epidemic broke out. COVID-19 is a coronavirus, also referred to as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) [14]. The prevalence of this virus has actually eclipsed the total amount of infections caused by two different epidemics (severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS)) in this century [11, 15]. Patients who are old or who have previous pulmonary diseases have a greater risk of passing away as a result of it [9, 16].

According to the contamination data, the epidemic spread swiftly across the globe. As a result, governments were compelled to implement preventative measures, such as lockdowns, self-isolation, social isolation, the use of facial masks and shields, and the recommendation to cleanse hands as often as feasible [10, 11].

Even though it was once believed that the virus spread by droplets and means of contact and that social distance might assist in disrupting the transmission chain, recent scientific research has essentially proven the possibility that the virus transmits through the air instead and survives for up to 3 hours in the air [9, 11, 17, 18].

Due to the overwhelming majority of infections happening indoors [19], recently published studies have suggested and assessed several strategies to lower or eliminate any virus level, including ventilation systems, high-efficiency filtration, ultraviolet irradiation, air ionization, chemical disinfection, nonthermal plasma, and filter-based air cleaners [20, 21, 22]. Researchers have demonstrated that ventilation strategies are among the most effective techniques for reducing the possibility of viral transmission [10, 11, 20, 23, 24, 25, 26, 27, 28].

In recent times, the Chartered Institution of Building Services Engineers (CIBSE) released guidance on how ventilation can be used to mitigate airborne infections. It has been reported that “there is good evidence that demonstrates room occupants are more at risk of catching an illness in a poorly ventilated room than in a well-ventilated room” [11].

The World Health Organization has stressed ventilation’s importance in improving indoor air quality and health on multiple occasions [10, 20, 29, 30, 31, 32, 33].

Air recirculation by ventilation can be the main route for aerosol transmission, increasing infection risks [10]. The aerosol movement was almost neglected in designing ventilation systems [10]. In addition, an inadequate rate of ventilation and ineffective ventilation technique (mixture of in-room or recycled air, poor mechanical ventilation servicing) have been associated with deteriorated health conditions for high-density building occupants [11].

The probability of infection from airborne disease or mitigation spread through ventilation [34] depends on quanta (viruses released), people exposure time, occupancy, activities, room ventilation flow rate, and room volume.

With the unprecedented COVID-19 outbreak sweeping the globe, many academics and industry professionals are raising the topic of whether present ventilation strategies are outmoded and inadequate for such a contagious disease in today’s environment [11, 34, 35, 36]. This prompted scientists to advocate for a paradigm shift in ventilation. There is an imperative to choose appropriate ventilation strategies and explore the way to structure ventilation systems for distributing healthy air as opposed to facilitating the accumulation of aerosol dispersal in confined environments [10, 37].

This chapter discusses an introduction to ventilation, distinct ventilation strategies such as natural ventilation, mechanical ventilation, mixed mode, or hybrid ventilation, as well as their perks and permits in preventing the spread of viruses. Also covered are the detailed case studies for analyzing mixed-mode ventilation that were published in pertinent articles after the COVID-19 pandemic.

2. Ventilation

Ventilation is the act of blending or substituting stale. Contaminated air found inside with the fresh air is drawn in from the outside of the building in order to lower the overall amount of contaminants found within it [38]. Ventilation is a crucial component for achieving high air quality and thermal comfort within a building. The installation of a ventilation system is recommended for a variety of reasons. The prime objective is to maintain the air’s quality by providing an adequate amount of fresh air for metabolic processes and diluting of contaminants. A ventilation system purifies an interior space by removing impurities, undesirable heat, and excess moisture via the process of air extraction. Besides, the purposes of ventilation are (1) protecting the health and comfort of human beings, (2) in the event that it is required, providing adequate air or oxygen for activities such as combustion, (3) eliminating the byproducts of respiration as well as body odor, (4) eliminating potentially hazardous chemicals, and (5) improving air circulation [39, 40, 41].

Three fundamental aspects of building ventilation need to be taken into consideration for this design:

• Rate of ventilation: the quantity of outside air that is brought into the interior space, and the quality of the air that is brought in from the outdoors;

• Airflow direction: refers to the general direction of airflow in a building that ought to flow from clean zones to filthy zones; and

• Air distribution or airflow pattern: outside air ought to be supplied to each section of the space in an effective way, and airborne pollutants should be removed as they circulate around the space.

2.1 Ventilation methods

A building may be ventilated using one of the three methods: natural, mechanical, or hybrid (mixed-mode) [10, 39, 41, 42, 43, 44]. In this chapter, we provide a briefing regarding these approaches and their potential in reducing the dissemination of the virus.

2.1.1 Natural ventilation (NV)

The act of delivering fresh air circulation inside of an enclosure by the utilization of air pressure differentials brought on largely by the effects of wind and buoyancy effect which is again caused by temperature differences in and around the enclosure is referred to as natural ventilation. This method may be defined as a natural way to provide ventilation [10, 45]. The wind that flows along the windward side eliminates the occupants’ body heat through convective and evaporative heat transfer, resulting in chill body. Buoyancy effect is the effect whereby heated and less dense air rises, while cold and high-density air descends [10, 46, 47, 48]. NV is a standard green alternative to mechanical ventilation (MV) systems that contributes to energy saving as it operates without mechanical systems and energy usage by using natural forces [10, 49, 50, 51]. Two categories of NV are controlled, organized NV and nonorganied NV or infiltration [10, 52]. There are three different forms of controlled ventilation: single-sided ventilation, cross-ventilation, and passive stack ventilation [10, 49, 53]. One of the authors [46] in their earlier work has detailed about natural ventilation strategy called wind catcher. Wind catchers are an integral element of the construction process in Middle Eastern countries for many years so that buildings may take advantage of natural ventilation and cooling. The wind catcher allows for naturally occurring ventilation because of both the wind effect and the stack buoyancy effect [46]. Although the WHO strongly recommended NV as a means of reducing the risk of infection [39], it is not commonly used because of some disadvantages, including erratic airflow, a fluctuating ventilation rate, reduced thermal comfort, and it is not adequate for high occupancy levels [54, 55, 56, 57, 58, 59, 60]. As a result of these, the researchers have a hypothesis that mixed mode ventilation (MMV) or a hybrid method made up of natural and mechanical ventilation devices may be considered a more consistent strategy to offer steady airflow, diffuse contaminants, and freshen indoor air while also providing thermal comfort, particularly when paired with controls [10, 61, 62].

2.1.2 Mechanical ventilation (MV)

Mechanical ventilation systems utilize ducts and fans to circulate air. Fans can either be set straight in windows or the walls, or they can be positioned in air ducts, which allows them to either introduce fresh air into an environment or remove stale air from it [39]. Mechanical ventilation methods include distinct benefits and drawbacks. Below are some advantages [39] that may be considered more broadly:

• The integration of this system with the air conditioning system may be achieved with ease.

• The regulation of indoor humidity and temperature is readily achievable.

• The addition of a filtration system to the mechanical ventilation system is a viable option. The removal of dangerous bacteria, particles, gases, smells, and vapors may be achieved.

• Irrespective of the prevailing ambient temperature and wind conditions, continuous access to the necessary flow rate may be achieved.

• The only need for its functionality is the provision of electrical power.

• The control of airflow direction is possible.

Nevertheless, mechanical ventilation systems are not without their challenges. Mechanical ventilation systems often encounter operational challenges, leading to disruptions in their intended functionality. These interruptions may arise from several factors, such as equipment malfunction, utility service discontinuity, suboptimal design, inadequate maintenance, or improper management. In the event that the system caters to a vital facility and necessitates uninterrupted operation, it may be essential to implement backup measures for every piece of equipment. However, it is important to acknowledge that such an endeavor might incur significant costs and may not be environmentally sustainable in the long term. The expenses associated with the installation and maintenance of a mechanical ventilation system might be quite expensive. If a mechanical system is unable to be adequately built or maintained as a result of insufficient financial resources, its performance would be adversely affected [39]. Morawska et al. [63] observed the establishment of very large emergency hospital wards, such as those located inside exhibition centers, capable of accommodating hundreds or even thousands of patients. While the mechanical ventilation in these facilities is deemed sufficient for regular exhibition or conference activities, it remains uncertain whether there will be adequate ventilation for patient management and infection control when these facilities are utilized for such purposes, particularly during the COVID-19 pandemic. Studies have shown a correlation between the cleanliness of air filters and HVAC systems and the extent of symptoms tied to sick building syndromes [11]. Moreover, it has been established that inadequate ventilation rates and improper ventilation strategies, such as the mixing of in-room or recirculated air and poor maintenance of mechanical ventilation systems, have been associated with adverse health effects among those inhabiting high occupancy buildings [11]. In their study, Sha et al. [64] conducted an analysis of the rates of mechanical ventilation with the aim of mitigating the risk of COVID-19 transmission. The researchers attempted to establish a correlation between the probability of infection and the requirement for ventilation (the ventilation rate necessary is influenced by five key factors: infection probability, quantum generation rate, social distance with or without mask use, and exposure duration). When the probability of infection decreases to a certain range, there may be a significant rise in the necessary ventilation rate. As an example, while maintaining a social distance of 1.8 meters and being exposed for a duration of 8 hours, the necessary ventilation rate exhibits an increase from 2.6 air changes per hour (ACH) to 5.2 ACH as the infection chance decreases from 2–1%. Nevertheless, when the probability of infection decreases from 1 to 0.1%, there is a corresponding requirement to raise the ventilation rate from 5.2 air changes per hour (ACH) to 52.4 ACH [64]. Based on Chow’s outcomes [65], it can be inferred that in order to effectively manage the spread of COVID-19 throughout various buildings, it is necessary to maintain a minimum air exchange rate of six air changes per hour (ACH). The adoption of this particular value has shown its efficacy; nevertheless, it has also presented several problems. The operational expenses associated with mechanical ventilation systems are significantly elevated. This might perhaps explain why some locations have periods of zero air changes per hour (ACH). The measurement of flow parameters in mechanical air handling systems is a time-consuming process. Determining the ventilation rate of six ACH expeditiously is a challenge. Verifying if the ventilation system is intermittently deactivated for the sake of fuel conservation presents an additional level of complexity. The transmission of viruses may occur effortlessly [65]. A research indicated the use of mechanical ventilation with recirculation as a means to minimize ventilation costs. In this particular case, the ventilation system had a role in enabling the transmission of SARS-CoV-19 to all the individuals residing in a nursing home [66, 67].

The use of mechanical ventilation systems that do not include recirculation mechanisms has been shown to mitigate the potential risk of infection [67, 68, 69, 70]. Mechanical ventilation systems have high efficacy in managing indoor air quality (IAQ); nonetheless, it is essential to acknowledge the concomitant rise in energy consumption associated with their operation. One potentially effective strategy that might make a valuable contribution to this subject matter is the implementation of a hybrid or mixed mode ventilation technique [71, 72], which will be discussed in the next session.

2.1.3 Mixed mode or hybrid ventilation

Mixed-mode or hybrid ventilation refers to the integration of natural ventilation systems with mechanical cooling systems. Hybrid systems endeavor to maximize the utilization of free cooling via the amalgamation of outside and inside air, while using mechanical cooling to maintain thermal comfort when external circumstances are unsuitable for ventilative cooling [10, 61, 62, 71, 72, 73]. There is an increasing inclination toward enhancing research endeavors that investigate the mixed-mode ventilation concept as a means to enhance energy efficiency and improve occupant comfort in buildings [74]. Furthermore, it is well acknowledged that ventilation plays a crucial role in mitigating the transmission of COVID-19, and mixed-mode ventilation is deemed suitable for achieving this objective [75]. The following section provides a comprehensive assessment of published research studies that examine the mixed mode techniques for different buildings and the effects of design on the effective refreshment of indoor spaces, the removal of particles, and the management of indoor airborne transmission across different environments and improves the indoor air quality which aid in the mitigation of airborne viral transmission as detailed in Section 1.2.

3. Case studies

3.1 Case 1: educational classroom buildings

Ren et al. [76] provided recommendations to prevent an infectious disease spread in only naturally ventilated university classrooms. The authors suggest combining natural ventilation with window-integrated fans for hybrid ventilation and optimizing window size. Virtual simulations were done using ANSYS Fluent. The Air Diffusion Performance Index (ADPI) and Wells-Riley infection risk calculation were used to evaluate performance. It was found ADPI may be enhanced by 17% and infection risk reduced by 27%. Infection illness prevention and ventilation efficacy are greatly improved by hybrid ventilation. Figure 1a depicts the study classroom sketch. Rodríguez-Vidal et al. [77] compared natural, mechanical, and hybrid ventilation systems in a university classroom, analyzing their benefits and downsides in the Basque Country, Spain. CO₂ concentrations are compared among scenarios. This research examines IAQ, including CO₂ concentration, thermal comfort, and energy analysis. For this research, 50% room occupancy was considered. The study was done using the Design Builder tool. The hybrid ventilation system efficiently controlled IAQ, creating a healthy and pleasant atmosphere for inhabitants. The hybrid system showed enhanced energy efficiency over mechanical ventilation while preserving comfortable indoor air. It improves indoor air quality and is recommended to decrease viral transmission. Figure 1b depicts the simulated classroom.

Figure 1.

Classroom model used by Ren et al. [76] (a) and Rodríguez-Vidal et al. [77] (b).

Quijada et al. [78] investigated how to use hybrid ventilation for university classrooms in Panama City to improve students’ thermal comfort and indoor air quality. DesignBuilder 6.0 was utilized. A hybrid approach that increases fresh air intake from 5 L/s/person to 10 L/s/person via mechanical ventilation and occupancy reduction is advised. Cheong et al. [79] tested numerous ventilation techniques to reduce instructor-transmitted airborne illnesses in elementary classrooms. Traditional mechanical and hybrid ventilation solutions have been tested. This investigation uses Star CCM + CFD software. The hybrid ventilation approach that was presented showed the potential to effectively eliminating airborne infections.

3.2 Case 2: hospitals

Anuraghava et al. [80] investigated the spread of airborne viruses within a negative pressure room using a mixed-mode ventilation system. Negative-pressure isolation rooms are intended to keep hazardous particles from spreading into the surrounding environment. This is accomplished by creating a negative pressure within the space, so that when the door is opened, the ambient air is sucked in rather than the air already presents inside the room leaving. The computational fluid dynamics (CFD) tool ANSYS FLUENT was used for numerical study. The researchers used discrete-phase modeling to replicate the movement and dispersion of virus droplets while changing the ventilation flow rate parameters. The model consists of a room with two beds. There are also two human bodies on the beds, as well as two rectangular inlets and two circular outlets. Figure 2a displays the model under evaluation from an aerial viewpoint. The K-epsilon turbulence model was applied. The results showed that the mixed-mode ventilation system is more effective in managing virus droplet dispersion within the enclosed environment. It should be noted, however, that this research did not include a comparison with other alternative ventilation strategies. Yu et al. [81] studied the efficiency of ventilation design options in normal hospital wards for virus elimination. CFD tool ANSYS FLUENT was used to simulate airflow field and virus dispersion in a typical six-bed general ward in a Hong Kong hospital. The respiratory viruses included MERS-CoV, SARS-CoV, and H1N1 influenza virus. A 9 h-1 air change rate efficiently reduces respiratory virus particle deposition and floating time while optimizing energy efficiency. Despite not focusing on mixed-mode ventilation, this research on general ventilation techniques for improving IAQ may help hospital management reduce infection risk via improved ventilation design strategies. Figure 2b displays the model used for CFD analyses. Biological particles that may lead to surgical site infections were investigated by Liu et al. [82] in the context of the operating room (OR) air environment. The best ventilation system for maintaining clean air in the OR was discovered to be hybrid ventilation systems with the temperature-controlled airflow (TAF) system included.

Figure 2.

Model overview of Anuraghava et al. [80] (a) and Yu et al. [81] (b).

3.3 Case 3: office buildings

Srivastava et al. [83] examined the effects of adding an air disinfection system to an office building’s mixed-mode ventilation system. Increased air changes per hour (ACH) in confined spaces may reduce infection risk. However, this strategy significantly raises HVAC system running costs. The same goal may be achieved using region-specific air disinfection technologies. The research employed RM3 UV-C air disinfection. A CFD numerical simulation was performed on a real-life office building with offices and workstations. The model is in Figure 3a. This research used ANSYS FLUENT software and the RNG $k-\epsilon$ turbulence model for CFD simulation. The Eulerian approach was chosen over the Lagrangian method for viral concentration modeling for more accurate prediction. Four cases were studied. The numerical results suggest that a hybrid system with 100% outdoor air intake and UV-C technology in HVAC ducts is ideal. This arrangement disinfects HVAC air using RM3 UV-C devices. In this research, infection risk dropped from 27% (10% outside air) to 3.1% utilizing 100% outdoor air. If infection must be reduced below 2%, it is recommended to utilize UV-C devices. Cai et al. [84] examined mixing ventilation options for upgrading an office meeting room to reduce airborne infectious virus and particle concentrations. Infection risk has been demonstrated using the Well-Riley model. The base scenario was analyzed by measuring CO₂ concentrations and particulates in the meeting room with one, two, or no individuals. Phoenics CFD tool was used to analyze improvement models. With a low-pressure filter with 99.9% efficiency and mixing ventilation, CFD models suggest an infection risk $<1\%$. Figure 3b shows the virtual model. Duan et al. [85] modeled mixed-mode ventilation to analyze the energy savings of upgrading the ventilation strategy of an office in Beijing. DesignBuilder 7.0 and CFD were used for the research. Hamdy and Mauro [86] presented three hybrid ventilation control technologies for an open-plan office building in Glasgow, Scotland. The three solutions were compared to a mechanical ventilation system using IDA ICE software. For indoor comfort, performance assessment includes interior temperature and predicted the percentage of dissatisfied (PPD). Assessment of indoor air quality (IAQ) includes CO₂ monitoring. Though these last two works do not involve the assessment of infectious risk, they provide good insights into optimizing the mixed mode ventilation to improve the IAQ.

Figure 3.

Illustration of models used by Srivastava et al. [83] (a) and Cai et al. [84] (b).

3.4 Case 4: residential buildings

The study done by Al-Hilfi et al. [87] focused on the improvement of indoor air quality (IAQ) via the management of an environmental-controlled fan. The research was carried out in Malaysia. In tropical nations such as Malaysia, it is insufficient for air quality improvement to rely only on natural ventilation. However, the implementation of home isolation measures during the lockdown period has resulted in a surge in the use of air conditioning systems. This has therefore heightened individuals’ susceptibility to respiratory ailments and compromised their immune system, rendering them more susceptible to the COVID-19 pandemic. The concept of designing automated fan systems that monitor indoor air quality (IAQ) in order to enhance air velocity and provide sufficient ventilation has been motivated by several environmental elements, including humidity levels, airflow velocity, concentrations of CO₂ and CO, and temperature levels. The study included the collection of data from the master bedroom of a residential unit in Malaysia over a period of about 2 weeks. Various sensors were used to measure the characteristics indicated above. The analysis of the thermal comfort state was conducted using the predicted mean vote (PMV) approach. The benchmark outlines the ten primary circumstances including all potential indoor air conditions and their associated fan speeds required in accordance with the ASHRAE 55-2020 and EN-16798 standards. While ensuring the desired level of IAQ by implementing an environmental fan system, it is also found that a significant decrease of 31.4% in energy consumption of air conditioning units is accomplished. This research primarily focuses on the optimization of hybrid ventilation systems via the use of sensors and controllers, with a specific emphasis on ensuring the maintenance of sufficient indoor air quality (IAQ) and also energy savings. Tognon et al. [88] used a co-simulation methodology to examine two distinct case studies, and one of the studies is residential building. The objective was to assess various control techniques for hybrid ventilation systems, focusing on their impact on the mitigation of risks and also energy savings. The simulations were conducted with the TRNSYS and CONTAM software, with the subsequent use of the Wells-Riley model to predict the risk of airborne infection. The study determined that when properly managed using an appropriate management method, the hybrid ventilation system shows potential in effectively sustaining indoor settings that promote health, while also lowering energy usage.

4. Conclusions

The paramount significance of appropriate ventilation strategies during periods of epidemic crises is indisputable. In this book chapter, we have undertaken an exhaustive examination of multiple dimensions pertaining to the operation of mixed-mode ventilation strategies in such challenging contexts. Our central goal is to explore the full spectrum of ventilation techniques that can be deployed to mitigate the transmission of airborne diseases.

A core theme of our discussion centers on the critical role that indoor air quality (IAQ) assumes in safeguarding health and well-being, more specifically within the framework of airborne infections during epidemics. The subject of IAQ is complex and multifaceted, involving a myriad of components, each contributing to the overall air quality experienced by individuals inside buildings.

We shed light on the importance of diverse ventilation strategies as effective tools to manage IAQ. Our examination extends to an in-depth survey of ventilation standards, both those particular to Singapore and those recognized internationally. Through this survey, we underscore the imperative of adhering to these benchmarks in striving for and sustaining optimal IAQ.

In this chapter, various ventilation strategies, such as natural, mechanical, and hybrid ventilation strategies are discussed. However, the primary emphasis of this chapter lies in the domain of mixed-mode or hybrid ventilation. As an innovative approach that amalgamates the merits of natural and mechanical ventilation techniques, mixed-mode ventilation emerges as a plausible and potentially energy-efficient solution to manage IAQ efficaciously. An extensive dissection of this technique is provided, delineating its principles, potential advantages, and the contexts in which it can be aptly applied.

To supplement our analysis, we delve into a range of case studies that portray the practical applications of the aforementioned ventilation strategies. These case studies elucidate the implementation and optimization of these techniques in real-world settings, further emphasizing the effectiveness of mixed-mode ventilation across different environments. Our case studies include various building types, such as educational buildings, hospitals, office buildings, and residential buildings. Each case study employs a unique analytical approach, ranging from computational fluid dynamics (CFD) to empirical experimentation. This diverse set of methodologies provides a holistic picture of the challenges and opportunities involved in improving IAQ across different environments.

In conclusion, robust ventilation strategies, and in particular mixed-mode ventilation, are instrumental in sustaining IAQ, mitigating the risk of airborne infections, and augmenting occupant comfort, particularly during epidemic crises. However, this chapter is a stepping stone in this ever-evolving field. Further research is warranted to foster the development of robust, adaptive strategies for the effective implementation of mixed-mode ventilation. Future investigations should aim to enhance energy efficiency, comfort, and safety across a diverse portfolio of building types and environmental conditions. In another chapter Machine Learning Techniques in Indoor Environmental Quality Assessment, we have discussed recent advancements in ML-based techniques for Indoor Environmental Quality Assessment.