Effect of Different Mechanical Fans on Virus Particle Transport: A Review

Abstract

In recent years, repeated outbreaks of airborne viruses have normalized human coexistence with these viruses. The complex turbulence and vortices generated by different fan types and operation modes affect virus removal effectiveness. This paper reviews the potential impact and actual effectiveness of different fans in mitigating indoor virus transmission, highlighting their advantages and limitations. Downward rotating ceiling fans can rapidly dilute virus concentration (21–87%) in the breathing zone due to jet cores, with efficiency depending on rotational speed and particle diameter. However, the reprocessing problems of large particles being deposited on surfaces, and small particles settling and rebounding into the air remain unresolved. Upward-rotating ceiling fans do not contribute to indoor virus removal. Exhaust fans generate a negative-pressure environment, which helps expel viruses quickly. But improper vortex zones can increase virus retention time 16–40 times. Air-apply fans effectively dilute and transport viruses only when delivering airflow exceeding 0.5 m/s directly into the breathing zone. Additionally, combined fan strategies remain underexplored, despite potential benefits. This review underscores the need for standardized definitions of particle removal effectiveness and calls for further research on how climatic conditions and thermal comfort influence fan-based interventions.

1. Introduction

To date, various airborne viruses, including those triggering large-scale pandemics, such as the Coronavirus Disease 2019 (COVID-19) and Influenza A, have caused substantial economic and social upheaval globally. Traditional long-term containment and isolation measures are not sustainable under the “new normal” of human–virus coexistence. Existing interventions, including the wearing of masks [1], social distancing [2], and ventilation measures [3], have been demonstrated as effective. Most infections are transmitted indoors [4], and with the relaxation of travel restrictions following pandemic containment, it has become vital to control indoor viral transmission. Indoor airflow dynamics have been found to significantly impact the spread of airborne droplets and viral particles, with both ventilation methods and airflow patterns determining the risk of infection for personnel [5,6,7].

Numerous prevention and control ventilation strategies have been proposed and examined across various settings. Mechanical ventilation (MV) [8], natural ventilation (NV) [9,10], mixed ventilation [11], and fan ventilation [12] have garnered considerable scholarly attention. While MV effectively provides fresh air to rooms, the placement of inlets and outlets influences airflow patterns and virus transmission [13]. Ventilation methods with high-positioned air supply (HPAS) and low-positioned air return (LPAR), such as ceiling air supply, are deemed less effective in virus control. The reason is that the temperature of exhaled particles is usually ~37 °C and the buoyancy drive of the thermal plume induces upward diffusion of warm air carrying virus particles [14]. In contrast, ventilation strategies such as displacement ventilation and underfloor air delivery, as well as low-positioned air supply (LPAS) and high-positioned air return (HPAR), have been recognized for their exceptional virus removal capabilities [15]. However, the associated costs to maintain a low infection risk are considerable [16]. A study by Alexi et al. [17] indicated that air conditioners running on low settings (meaning low air flow) are not effective in removing virus particles; air conditioners on higher settings (meaning high air flow) are effective, but their cost is 2.5–4 times that of a fan.

In contrast, NV offers economic advantages, particularly regarding the virus removal efficacy of the induced airflow patterns through the modification of doors and windows, which has been substantiated by numerous studies [8,18,19]. However, the implementation of NV faces limitations because of factors such as the fixed nature of building structures and the unpredictability of climatic conditions [20]; these factors may impact the anticipated removal efficacy. Additionally, NV strategies are not feasible in cold climates and are susceptible to suboptimal alteration by resident behaviors [21].

As virus transmission control has become the “new normal”, identifying cost-effective and feasible ways to reduce the risk of infection has become paramount [22]. Fans are increasingly recognized as a straightforward and cost-effective solution [12,23,24]. Common fan types include ceiling fans (CFs), exhaust fans (EXFs), air-apply fans (AAFs), and desk fans (DFs). Although their use has declined in developed countries—largely due to advances in HVAC systems—they remain popular in hot climates where the cost of air conditioning is prohibitive [25,26]. EXFs have also drawn attention as energy-saving devices in a growing range of applications [27,28]. While fans may incur additional maintenance costs, such as those associated with airflow disturbances, internal heat generation, and blade soiling, overall expenditures remain relatively low [29]. Previous studies have demonstrated the benefits and viability of fans for enhancing thermal comfort, air quality, and energy savings [30,31]. Many review articles have also made great advancements in these areas [32,33,34].

When the use of fans for the control of viral propagation is the focus of research, it is important not to focus solely on the differences in air changes per hour (ACH) caused by fans. Many studies have identified situations where even a high ACH may result in unsatisfactory virus control [3,35]. Figure 1 shows the methods, the factors influencing, and the effects of four types of fans examined in different buildings. A study by Pandey et al. [36] analyzed the effect of CF speed on indoor airflow and particle distribution in a residential building using computational fluid dynamics (CFDs). CFs provide protection by both altering the transmission pathway and diluting the virus concentration [33]. Moreover, the operational impact of the rotational direction of CFs has been considered in many studies to enhance thermal comfort while meeting virus transmission control criteria [37,38]. Furthermore, Qian et al. [39] removed indoor viruses from a hospital isolation room by creating a negative-pressure isolation zone using an EXF. The effectiveness of such an isolation room depends on the availability of other vents in the room [40]. Ren et al. [20] compared the virus removal effect of EXF and AAF in a school, where the EXF draws virus particles into the outdoors, while the AAF brings fresh air into the room to dilute the virus concentration. Habchi et al. [41] designed a personalized DF ventilation system in an office to provide personal protection by accelerating the exchange of air around individuals.

However, inappropriate fan operating modes may render virus removal ineffective or even support virus spread. Variations in fan speeds and airflow patterns may also increase both the speed and distance at which viruses can spread indoors, thereby increasing the risk of infection. In addition, fans may be inappropriate or ineffective in certain locations and under certain operating conditions. The advantages, limitations, and influencing factors affecting the removal effectiveness, as well as suitable locations for different types of fans to control virus transmission remain unclear to date. Existing reviews on virus transmission have primarily focused on various ventilation strategies [42], considering factors such as location differences [43,44], human behavior [45], breathing patterns [46], and energy efficiency [22]. However, few of these studies have specifically designed or examined fan ventilation methods. Additionally, while some studies on CFs have reviewed their airflow patterns’ potential to mitigate virus transmission, the primary focus has been on the thermal comfort provided by CFs [32]. Existing studies lack a thorough understanding of the rationale and efficacy behind fan-based ventilation strategies in reducing indoor virus transmission. Moreover, there is an absence of comprehensive investigations into how different types of fans (e.g., CFs, EXFs, and AAFs) individually or collectively influence virus transmission. Therefore, an in-depth investigation of virus removal mechanisms, operational modes, and the effectiveness of various fan types is essential. Such findings can guide more targeted fan-based mitigation strategies and inform evidence-based guidelines for reducing indoor viral transmission.

To outline the role of fans in airborne virus infection control, this review is organized into six sections. Section 2 describes the methodology used in this research. Section 3, Section 4 and Section 5 report the role of CF, EXF, and AAF, respectively. Section 6 describes the roles of other fan types and combination fans. Finally, Section 7 presents the conclusions of this review and proposes future work on virus removal by fans. The results of this review highlight the advantages and shortcomings of multiple types of fans in practical use, especially in dealing with the frequent occurrence of airborne viral diseases in recent years. Thus, it provides a scientific basis for the future improvement of ventilation solutions and a new perspective for the optimization of public health protection measures.

2. Methods

2.1. Literature Collection

This review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The search engines Google Scholar, Web of Science, and Science Direct were used to search for literature related to the use of fans for the control of indoor virus transmission. The keywords “airborne”, “airborne transmission”, “COVID-19”, “SARS-CoV-2”, “ventilation”, “airflow pattern”, and “infection risk” were used as background terms; “ceiling fan”, “external fan”, “exhaust fan”, “mechanical exhaust”, “table fan”, “desktop fan”, “desk fan”, “pedestal fan”, “fan”, “personal ventilation”, “air handing unit”, “supply air”, “air apply fan”, “air conditioner”, “air curtain”, “circulator”, and “fan ventilation” were used as fan-related words to search the literature. Keywords were combined using the Boolean operators “AND” and “OR”.

2.2. Screening

After screening, 51 articles were retained. The PRISMA flow diagram is shown in Figure 2:

• Duplicate publications were excluded.
• Articles not written in English, those for which the full text was inaccessible, or papers available only in unpublished form were excluded.
• Publications unrelated to “fans and virus transmission” were excluded. Specifically, studies focusing on various types of ventilation strategies aimed at mitigating virus transmission were distinguished.
• The focus was on rotating fan ventilation strategies rather than internal fans, such as those found in air conditioners, fan coils, or non-rotating blade-only fans.

The focus of this review was to evaluate the application of fans for controlling virus transmission or reducing the risk of infection. Thermal comfort, air quality, and energy consumption were not the focus of this review.

3. Role of Ceiling Fans in Airborne Viral Infection Control

CFs are commonly used as a ventilation strategy and play an important role in reducing viral infection indoors [47]. Most of the currently applied research methods are measurements [12] or simulations [36].

As shown in Figure 3, the proportion of the measurement methods in the literature was 30%, including methods that record indoor wind speed and pollutant concentration. The advantage of measurement methods is that they provide a direct record of the actual environment with a high level of confidence and incorporate factors that cannot be considered otherwise. However, transient results are not accessible easily through experiments, and most experiments can only read concentrations at specific sampling points over time. Furthermore, the cost and difficulty of experimental measurements ensure that most researchers prefer CFD simulations over measurement methods.

Figure 3 shows that simulation methods represent 70% of the literature, including analyses on velocity distributions, temperature distributions, and concentration distributions. The advantage of CFD is that simulations can be used to visualize the transmission path of virus particles while allowing for considering a wider range of different scenarios.

According to the results of this study, both research methods emphasize the substantial impact of indoor air movement on virus transmission. In addition, all simulations could be validated. There are commonalities in both the analytical methods used and conclusions drawn between simulations and experiments. In addition, the airflow pattern of CF is closely related to the direction of CF rotation (downward or upward), which has been examined before [37,38,48,49]. This section is categorized according to airflow patterns.

3.1. Downward-Rotating Ceiling Fans

3.1.1. Principles and Effects of Downward-Rotating Ceiling Fans

In this type of CF, the blades rotate downward to blow a strong airflow and form a “jet core”, as shown in Figure 4a [32]. Omrani et al. [32] reviewed the energy efficiency, thermal comfort, and air quality affected by CFs, providing a very helpful understanding of the airflow patterns of CFs. Downward-rotating CFs are characterized by high air velocity, downward airflow direction, and clear boundaries [7,50]. After reaching the ground, the airflow gradually spreads out in all directions at a height above ground level of 0.1–0.4 m in general [7,51]. When the airflow reaches a wall, it travels vertically up the wall, eventually entering the area above the CF and rejoining the circulation.

The downward airflow within the “jet core” strongly influences the particle concentration and the proportion of particles floating in the breathing zone. The importance of prioritizing the removal of viral particles from the breathing zone has been emphasized by several studies, while the range setting of the breathing zone is related to the activity of people. For this range, Zhu et al. [52] identified a distance of 5 cm horizontally and 26 cm vertically from the point of inhalation. Li et al. [51] specified the breathing zone for susceptible persons in a Singapore classroom as a sphere with a radius of 5–10 cm, defined as the breathing volume. Ren et al. [20] set the breathing zone within a certain range of height based on the common posture of people in a room. The height of the breathing zone in hospital wards starts at 0.5 m, while the height of the breathing zone in an office is generally 0.8–1.8 m. When the source of viral infection is located directly beneath a CF, airflow forces draw viral particles upwards, thus ensuring their rapid removal from the breathing zone of the person [51]. This is the most effective way to reduce the infection risk. When the location of an infected person is not directly beneath the CF, the concentration of viral particles can no longer rapidly be driven away from the breathing zone; instead, airflow in the room increases to reduce the concentration of viral particles. However, the removal effect of this method is poor and specific viruses are not effectively removed [12].

When multiple CFs are combined, multiple “jet cores” (Figure 4b) are created that do not interfere with one another [12,53]. The updraft between two neighboring CFs will carry viral particles toward the ceiling, from where they will inevitably disperse throughout the room, which may be influenced by the distance between CFs [26,54].

3.1.2. Factors Affecting Virus Removal by Ceiling Fans

Coupled Effect of Rotational Speed and Particle Diameter

For downward-rotating CFs, the higher the rotation speed, the higher the wind speed in the jet core zone [26]. Many studies have demonstrated that particle size differences are affected differently by the degree of wind speed, but the size range of aerosol particles has not been standardized so far. Many studies have defined aerosols with sizes of only <10 μm or even <3 μm as small particles. The reason for this definition may be that particles smaller than 10 μm in diameter (PM₁₀) can be inhaled into the upper respiratory tract. Particles smaller than 2.5 μm in diameter (PM_2.5) can penetrate deeper into the lungs. The recently emerged SARS-CoV-2 and Influenza A viruses primarily exist as fine particles with diameters of 0.25–2.5 μm [55]. Therefore, Li et al. [12] set the size of all simulated particles to 3 μm, which they considered as representative for all particles in the diameter range of 0.5–10 μm (incorporating airflow, gravity, and other factors). Few particles with diameters smaller than 10–13 μm have been found to remain in the air after 60 s [36]. In addition, Li et al. [51] tested various particle sizes in the ranges of 0.25–0.5 μm, 0.5–1 μm, and 1–2.5 μm; they found that the smaller the particle size, the larger the percentage of particles that remain suspended in the room air, while the larger the particle size, the larger the percentage of settled particles. Activating the CF reduced the total number of particles by 87% in all cases. This result implies that the use of a CF increased the settling of small particles in the room, but the speed of the CF was almost irrelevant to the settling of small particles of various particle sizes.

However, Pandey et al. [36] selected full-size virus particles for a separate analysis. They referred to particles larger than 40 μm as large particles, and to those smaller than 40 μm as small particles. Li et al. [56] showed that the evaporation duration for small particles (4 μm) was only 0.2 s, while that for large particles (50 μm) was 12.5 s. However, during fan rotation, both the evaporation rate and the particle path may change because of the airflow velocity. For small particles, the defined particle size dimensions exceed 10 μm. The results of their study showed that with increasing speed, a greater percentage of large particles (>40 μm) are deposited (implying that more viruses are removed from the breathing zone) [36]. Under the same fan speed, the total particle percentage (including both large and small particles) decreases. Their analysis concludes that the depositing percentage of small particles has a greater influence on the overall simulation results, and that the depositing percentage decreases with increasing CF speed. Li et al. [51] defined 75–400 μm as medium-sized droplets (which are mostly affected by gravity), <75 μm as small particles (which diffuse with the airflow), and >400 μm as large particles (which are mostly affected by inertia). The results of their study indicate that the range of the size of particles within the air of a room without a fan is 1.75–30 μm. Smaller particle sizes (<15 microns) were collected when the fan was turned on. This result, again, shows that with increasing fan speed, small particles in the breathing zone become more difficult to remove than large particles.

Table 1. Impact of ceiling fan speed on particles.

Ref.	Location	Size (m3)	Environmental Temperature (°C)	Rotation Speed (RPM)	Number of CF	Particle Size Range	Method	Conclusions Related to Particle Size	Conclusions Related to the Breathing Zone/Whole Room
[36]	Residential	38	-	160 265 365	1	Large particles: >35 μm Small particles: <35 μm	CFD	Increased rotational speed benefits the settlement of large-particle viruses, but not the settlement of small particle viruses.	In whole rooms, CFs combined with natural ventilation and masks are beneficial for reducing the spread of viruses in indoor spaces.
[12]	Classroom	278	27 ± 0.5	0 100 140	4	5–10 μm	Tracer experiment and CFD	- 1	Increasing RPM 0 to 140 decreases the breathing zone concentration by 21%.
[51]	Classroom	278	26.5	0 100	4	0–30 μm	Tracer experiment and CFD	The fan facilitates the settling of particles that are >15 μm.	Concentrations in the breathing zone are decreased by at least 87%.
[37]	Factory	39,100	Summer: 32, Winter: −17.5	0 16 78	1 (BIG 2)	0.5–10 μm	CFD	-	Rotating downward CFs at high speeds facilitate dilution of virus concentration.

¹—not mentioned. ² BIG: extra-large CF (fan diameter: 6.1 m).

presents a logic of publications based on the effect of CF speed. As the speed of the CF gradually increases, the settling of small particles will play a counterproductive role; because the CF airflow generates higher wind speeds near the floor [48], small particles are prevented from settling and can re-enter the airflow cycle. Additionally, statistical analysis based on floor area revealed that studies in larger spaces typically utilized a narrower range of fan speeds, whereas those in smaller spaces explored a wider spectrum. This variation indicates that effectively optimizing indoor airflow and controlling viral particle transmission requires a comprehensive consideration of room size, fan dimensions, number of fans, and fan speed.

Coupled Effects of Rotational Speed and Particle Concentration

Although many researchers have suggested that CF speed is inversely proportional to the settling percentage of small particles [36], they still suggested that CF speed is beneficial for controlling or reducing the concentration of virus-carrying aerosols [49,51]. On the one hand, the use of a CF creates an airflow effect that influences the direction of small particles, as well as the distance they travel. Li et al. [51] measured cough droplets at the point of release by particle image velocimetry (with initial velocities of up to 2 m/s); with CF, the horizontal cough jet reaches only 0.1 m, with maximum velocities of about 0.5 m/s (the distance between the releaser and the infected person was 1.5 m). Furthermore, because of the downward airflow caused by the CF, the direction of travel of particles is deflected downward, thus reducing the number of particles in the breathing zone of the infected person on the opposite side. On the other hand, because of the increased speed of the CF [48], the increased airflow velocity in the room rapidly dilutes the concentration of particles in the breathing zone. Li et al. [12] found that CFs reduce the concentration of viral particles in the breathing zone of susceptible individuals by 21%. Yang et al. [37] similarly concluded that when the CF was operated at full speed, it effectively decreased the concentration of particles in the breathing zone of workers.

However, Benabed et al. [57] proposed that the airflow generated when people are walking through the room may increase the resuspension of large particles. Small particles that can only be diluted were still suspended in the air 60 s after the fan had been turned on [36]. In their modeling study conducted in a Malaysian mosque, Mat et al. [58] found that the number of infected praying people continued to increase when all fans were activated; the transmission of pathogenic particles could be reduced by 24% when fans were turned off. They concluded that CFs help to spread the virus, as lowering fan speeds or turning off fans entirely to control turbulent air circulation in enclosed prayer spaces apparently minimizes pathogen transmission. Therefore, the use of CF ventilation strategies to control virus transmission in large public places (especially if they are densely crowded) is not a suitable approach.

3.1.3. Effect of Ventilation on the Role of Ceiling Fans

Ventilation helps reduce indoor viral particle concentrations [59,60]. NV and MV, in combination with CF for the removal of viral particles from indoor spaces, were also examined [61,62]. NV and MV provide fresh air to a room, and the effect of inlet air velocity on indoor airflow is almost negligible compared to the influence of CF [12]. However, the role of ventilation in particle removal is undeniable, and an increase in ventilation flushes more particles outdoors, while reducing the risk of infection indoors. In addition, the locations of supply and return vents are important in rooms where CFs are used for virus removal. In this review, based on HPAS with LPAR and LPAS with HPAR, two ventilation modes used in combination with fans were reviewed.

High-Positioned Air Supply and Low-Positioned Air Supply Ventilation

In this subsection, the effectiveness of CF in combination with HPAS and LPAR ventilation systems is reviewed. Pandey et al. [36] showed that HPAS intake velocities of 1, 2, and 3 m/s flushed 1.3%, 6.3%, and 14.4% of the total virus particulate volume from the indoor space to the outdoors, respectively. The probability of infection also decreases exponentially. Li et al. [12] recorded the effects of ACH of 4.5, 5.6, and 7.5 provided by HPSA on indoor particulate matter concentrations. A gradual decrease in the concentration of particulate matter was observed at all sampling points, which further corroborates the effectiveness of ACH in particulate removal. When CFs are combined with HPAS and LPAR, fresh air is rapidly pushed into the room by the CF-induced airflow [12]. This compensates for the weakness of CFs, which can only mix air. However, all of the above studies assumed that the injection source is located directly beneath the CF. If the injection source is not located directly below the CF, viral particles will spread throughout the room. The indoor concentration is much higher than the exit, even if the ACH is as high as 7.47 [12]. The efficiency of ACH for indoor particle removal may also depend on other factors, and therefore, the amount of ventilation in the room with CFs cannot be used as the only reference [12,63]. Other studies have reached similar conclusions, but this feature may be more accentuated under fan conditions.

Low-Positioned Air Supply and High-Positioned Air Return Ventilation

Previous studies relying on MV to remove viral particles from indoor spaces have verified the effectiveness of this removal mode [64,65]. Although the use of inappropriate ventilation strategies may increase the risk of infection [66], displacement ventilation [67,68] and underfloor air delivery [69] are assumed to remove viral particles. Both are based on the principle that the air around the human body is heated, and the resulting thermal plume rises upward because of its higher temperature. Viral particles expelled from the human body into this thermal plume may stay within the confines of the breathing, thereby increasing the risk of infection. The particles are efficiently removed by the induced airflow provided by the upward flow, and are gradually discharged upward and out of the breathing zone. When the air supply is located in the upper part of the room, a higher concentration of viral particles in the breathing zone results, thus increasing the infection risk [11]. Therefore, it is not advisable to install downward blowing CFs for virus control in rooms with displacement ventilation, underfloor air delivery, or any LPAS and HPAR. The collision of the upward airflow with the CF may minimize the dilution effect of the particle concentration in the breathing zone.

3.2. Upward-Rotating Ceiling Fans

3.2.1. Principles and Effects of Upward-Rotating Ceiling Fans

Figure 5 [32] shows a typical airflow pattern of an upward-rotating CF. The CF controls the airflow to gradually spread along the ceiling, the wall, and the floor to the area below the CF [48]. Previous studies have shown that upward-rotating CFs do not have a region of high velocity airflow compared to downward-rotating CFs, a phenomenon that is not conducive to the dilution and deposition of viral particles [37] (as shown in

Table 2. Comparison of ceiling fan rotation directions.

Ref.	Location	Size (m3)	Ceiling Fan Setup	Method	Conclusion of Ceiling Fan Rotating Upwards	Conclusion of Ceiling Fan Rotating Downwards
[37]	Factory	39,100	① UP ② DOWN	CFD	Upward rotation is unfavorable	The best conditions are when operating at the highest rpm
[48]	Laboratory	30	① UP ② DOWN	Tracer experiment	Uniform airflow in the room	High impact on indoor airflow, not uniform
[49]	Hospital	42	① UP ② DOWN	CFD	Upward rotation (RPM 107) in combination with the UR-UVGI system facilitates virus removal	None of the working conditions are favorable
[38]	Hospital	41.6	① UP ② DOWN	CFD	Combined with the UR-UVGI system, the system facilitates virus removal	When the fan (235 rpm) was blowing downward, the residual virus particles increased

). On the one hand, diluting the virus concentration is less effective because the airflow velocity generated within the breathing zone is much lower than that of a downward-rotating CF. On the other hand, because the downward-flowing airflow is reduced, the settling of viral particles on the ground is also reduced. Therefore, the use of upward-rotating CFs to control virus transmission should also be expanded.

3.2.2. Upward-Rotating Fan Combination Applications

Upward-rotating CFs may be used in rooms where upper-room ultraviolet germicidal irradiation (UR-UVGI) is used [38,70] (as shown in Table 2). UR-UVGI uses a radiation source in louvered fixtures so that ultraviolet (UV) radiation is directed to the upper part of the room (to protect occupants from UV radiation) [49]. Ultraviolet germicidal irradiation (UVGI) has been proven effective in eliminating bacteria and viruses, and as an airborne infection control technique [71,72]. Virus inactivation is more effective in the direct vicinity of the UV emission source [73]. Therefore, CF-induced air circulation to the upper part of the room facilitates the contact between air and UV light, and thus gradually inactivates the virus particles in the room [70]. This combination has been validated as effective [74,75]. Pichuroy et al. [49] compared the effectiveness of CFs rotating upward and those rotating downward at three fan speeds on virus removal by UR-UVGI. Medium-speed (107 rpm) upward-rotating CFs were the best combination to improve the efficiency of UVGI. Their finding that there is no linear relationship between CF rotation speed and the effectiveness of UR-UVGI has been shared by Rudnick et al. [76]. They concluded that the addition of a CF improved the effectiveness of UR-UVGI, and that changes in the speed and direction of the fan did not result in statistically significant changes. In addition, upward-rotating CFs combined with LPAS may be advantageous, with the upward airflow increasing the airflow velocity in the area below the fan, thereby reducing the concentration of inhaled virus particles. The removal effect is still poor compared to that of a downward-rotating CF [37].

4. Role of Exhaust Fans in Airborne Viral Infection Control

Many researchers have also used EXFs to mitigate indoor virus transmission [39], which is illustrated in Figure 6. The studies that analyzed airflow velocity and virus particle concentration are almost equally divided among actual measurements and simulations. Among the studies that used actual measurements, 20% focused on velocity, 25% focused on virus concentrations, and 5% focused on indoor–outdoor pressure differences (ΔP). Most of the studies that used simulations focused on airflow velocity (30%). The difference is that actual EXF measurements consider the ΔP between the inside and the outside of a room. This is closely related to the creation of negative pressure isolation rooms by EXF.

4.1. Principles and Effects of Exhaust Fans

Among the many publications on expelling indoor particulate matter through ventilation patterns, researchers have proposed the use of EXFs to increase ventilation, thereby reducing the concentration of pollutants in a room. The application of EXFs can be divided into two main categories: (1) an airflow pattern that is completely dominated by the EXF, with no other air outlets in the room; (2) an airflow pattern in which a certain ventilation pattern exists originally and the EXF assists in the ventilation.

4.1.1. Exhaust-Fan-Dominated Airflow Patterns

In EXF-dominated airflow patterns, the flow field in the room is mainly affected by the location of the EXF. Figure 7a presents a schematic diagram based on the description in the original publication and the EXF airflow patterns found in the literature. There is a main flow field between the air inlet and the EXF, and factors such as the size and height of the main flow field are related to the layout of the room. In addition, vortex areas can emerge in corners and around objects. Additionally, the longitudinal view of Figure 7b demonstrates that vortex zones also exist in the airflow at the top of a room.

Moreover, the effect of virus expulsion in an EXF-dominated airflow pattern is proportional to the number and size of vortex zones [77]: the larger the extent of a vortex zone, the longer the virus particles are retained. Furthermore, depending on the layout of the room, complex indoor situations may lead to the existence of more vortex zones, thus further reducing the probability of virus expulsion. Sinha et al. [78] noted that the time needed for virus particles to be expelled after entering a large vortex zone may be as much as 20 times that of the mainstream field. In their study, the direct injection of virus particles into the largest vortex areas of the room resulted in 93% of the particles being retained in the room. Virus expulsion from the room takes eight times longer compared to injecting particles into mainstream areas.

In addition, the effect of EXF-dominated virus expulsion is negatively correlated with the indoor/outdoor pressure difference; positive indoor pressures may result in certain viral particles failing to be expelled from the EXF or even overflowing from the room [10]. Closed rooms are more conducive to negative indoor pressures, gradually created by EXFs; the lower the indoor pressure, the better the ability of the EXF to control viruses. The ASHRAE standard 170 requires that the pressure difference in hospital isolation rooms should be at least −2.5 Pa to ensure safety from pathogens [79]. The ability of an EXF to create a negative-pressure isolation room is not doubted, it is the speed of the fan that may result in different degrees of negative pressure and additional time cost.

However, all the above airflow patterns are the result of EXFs mounted on the side walls of rooms, and the generality and efficiency of this approach have been recognized by many researchers. Wang et al. [80] examined an EXF mounted on the ceiling of a room and analyzed its airflow pattern. As shown in Figure 7c [80], if the location of the EXF is at the ceiling, the longitudinal airflow field may form upper and lower circulations. The upper air is discharged by the EXF, while the lower air creates a new circulation that is detrimental for virus removal. Therefore, such an installation has not been considered by many studies, and future studies could delve deeper to address this issue.

Figure 7. (a) Floor plan under the exhaust-fan-dominant mode; (b) elevation view under the exhaust-fan-dominant mode; (c) elevation view under the exhaust-fan (located in the ceiling)-dominant mode [80].

Figure 7. (a) Floor plan under the exhaust-fan-dominant mode; (b) elevation view under the exhaust-fan-dominant mode; (c) elevation view under the exhaust-fan (located in the ceiling)-dominant mode [80].

4.1.2. Exhaust-Fan-Assisted Airflow Patterns

Another EXF application is shown in Figure 8a, which presents a schematic diagram of the airflow pattern assisted by an EXF in a room where ventilation (with air intake and exhaust vents) exists naturally. When an EXF is installed for mainstream ventilation, the powerful suction of the EXF pulls the airflow away from the mainstream. However, a portion of the airflow from the mainstream field still remains. Using an EXF in rooms with other vents does not guarantee that the safe zone (neighboring rooms) will not be flooded by virus particles [65]. Khan et al. [81] conducted fan-assisted experiments and also confirmed that it is difficult to achieve a negative-pressure isolation room with opened windows. Saw et al. [82] used two EXFs installed together to remove viruses from a multipotent ward with central air conditioning, with an entrance (outlet) air velocity of 0.05 m/s (exhaust port), and EXFs of 1235 m³·h⁻¹ each; they found that more than 85% of virus particles were expelled from the EXFs. Sinha et al. [78] used a sidewall-assisted EXF in a naturally ventilated toilet. The exhaust velocity of the fan they used was 1.5 m/s and the measured air velocity at the door was 0.57 m/s. In contrast, the optimal virus removal strategy of Wang et al. [80] was to install the EXF 0.3 m from the toilet. All released particles flowed toward the fan. This result illustrates that the close proximity of the EXF installation to the releasing location (despite the presence of a dominant airflow) resulted in a gradual decrease in the virus concentration (see Figure 8b). An interval of 210 s between two toilet uses is recommended.

4.1.3. Ability of Exhaust Fans to Control the Spread of Viruses

Figure 9 represents the ability of EXFs to control virus transmission. Most of the publications affirmed the beneficial role of EXFs [83,84,85,86]. The discharge of virus particles from the main flow zone in the EXF-dominated airflow mode is ideal. In the EXF-assisted airflow mode, the effectiveness of the negative-pressure isolation chamber is reduced, and virus particles may enter through other outlets. However, neither the EXF dominated airflow mode, nor the assisted airflow mode can avoid the existence of the vortex airflow zone. The EXF is incapable of dealing with the vortex zone, and the removal efficiency at the time level can be improved by changing the air velocity.

4.2. Time Until Exhaust Fans Emit Virus Particles

4.2.1. Relationship with the Building Layout

The effectiveness of EXFs in removing viral particles is generally assessed by the time it takes to lower the risk of infection, which is also related to the selection of the building where the EXF is to be installed. Most studies used EXFs in rooms with small indoor areas such as toilets or bathrooms. To create a temporary isolation zone in a home, the EXF was installed in the bathroom rather than the bedroom. When the door between the bathroom and the bedroom remained open, even under negative pressure optimal conditions (differential pressure exceeding −5.0 Pa), the time required to expel the virus was prolonged because of the distance and layout [81]. In contrast, Biswas et al. [87] studied an elevator with an area of 1.44 m² and a height of 2 m, and found that the breathing zone of a typical person ranges between 1.5 and 1.8 m. The top-mounted EXF evacuated all virus particles after 5.5 s of their release. Elevator-mounted top EXFs are advantageous [88], as they set the EXF almost within the breathing zone (the EXF is located at a height of 2 m); therefore, the effect is considerable. Therefore, the distance between the EXF and the particle releaser is an important factor for determining the virus removal efficiency. Wang et al. [80] studied a case of positive room pressure (where the main airflow is not fan-controlled) and demonstrated the advantage of installing the EXF at a short distance. The effectiveness of removing viruses at three times the distance between the EXF and the source of contamination cannot be compensated for by wind speed, even if the wind speed is increased from 1.3 m/s to 3 m/s. In addition, the large size of the EXF was not advantageous when used in close proximity, and smaller-sized EXFs activated faster.

Saw et al. [82] examined the effect of induced airflow on virus transmission in a larger ward with EXF-assisted MV. Two EXFs interfered with the original distribution of particulate matter concentrations indoors (66% of the particulate matter was found at the front end of the ward and 34% at the back end of the ward); the risk of infection increased at the back end of the ward when the EXFs were turned on. The reason is that the particulate matter at the front end is pumped in the direction of the EXF, which may, in a certain sense, extend the transmission distance [78].

4.2.2. Relationship with Speed

The speed of the EXF also affects the time required for the removal of viral particles; the higher the speed, the shorter the time required to discharge particles. Sinha et al. [78] simulated the airflow motion and concentration field of particles discharged outdoors at EXF speeds of 1.5 m/s and 3 m/s, respectively. Their results showed that on the one hand, at an EXF speed of 1.5 m/s, it took approximately 50 s for particles to be fully expelled, whereas increasing the fan speed to 3 m/s reduced the expulsion time to about 25 s. On the other hand, particulate matter in the vortex zone at the same velocity took almost 1000 s to be expelled from inside the room. It still takes 400 s when the rotational speed is increased, which is 16 times longer than the time required in the dominant zone.

4.2.3. Relationship with Environmental Factors

According to the airflow pattern presented in Section 4.1.1, even when emphasizing the influence of other air outlets and vortex zones in the room, fan speed cannot be the only influencing factor when evaluating the removal time and efficiency of viral particles. Climatic and environmental factors also impact the efficiency of the particle discharge time achieved by EXFs, and the main factors considered are the wind direction outside of the room and the temperature and humidity of the indoor environment. The control of airflow and viruses is only favorable because of the negative-indoor-pressure environment created by the EXF [82]. If there are other indoor air outlets (such as open windows) that provide inward airflow, fresh air is beneficial to dilute the indoor particulate concentration, but the negative pressure isolation chamber becomes dysfunctional. Ma et al. [89] stated that a ventilation volume of more than 50 L·person⁻¹·s⁻¹ is detrimental to the removal of particles. Khan et al. [81] showed that when windows were opened, the differential pressure could not satisfy the −2.5 Pa requirement regardless of the increase in the speed of the EXF. Thus, opening the window diminishes the effectiveness of the EXF in controlling airflow, resulting in an increase in the time until discharge.

5. Role of Air-Apply Fans in Airborne Viral Infection Control

5.1. Airflow Patterns and Virus Removal Principles

The AAF draws fresh air from outdoors into the room while affecting the indoor flow field [20,89]. Figure 10a presents a diagram of the airflow pattern for an AAF, where the airflow enters the room through the EXF at high momentum and flows through the room in almost a straight line. The addition of three fans shows that the indoor airflow pattern corresponds to four straight lines with little influence on neighbors (see Figure 10b) [20]. The principle of removing virus particles by AAFs is almost the same as that of CFs; it relies on a strong airflow to dilute and transfer virus particles from the breathing zone. The difference is that CFs can only control and mix air, but AAFs can also introduce a large amount of fresh air.

5.2. Virus Removal Effect

The height of the AAF directly affects the airflow pattern, virus removal effectiveness, and occupant comfort. In their study of a classroom, Ren et al. [20] tested three heights of AAFs. Figure 10b shows the AAF installed at a height of 1.2 m. The height of the “jet core” varied with the height of fan placement. Virus particles could only be effectively removed when the fan was turned on and installed at a height of 1.2 m. However, the air velocity exceeded 0.5 m/s, which made some of the students uncomfortable (the ASHAER standard states that the acceptable air velocity for occupants is ≤0.35 m/s). In addition, Ren et al. [20] mentioned that while wind speeds of 0–0.5 m/s in the space between “jet cores” were within the acceptable range for occupants, they were not effective for the removal of virus particles. In non-emergency situations, the use of AAFs to remove virus particles sacrifices much personal thermal comfort.

AAFs have an unexpected effect in certain special places. Clegg et al. [90] monitored the results of nitrogen dioxide before and after the installation of bidirectional fans on station platforms; they found that the overall particulate matter concentration decreased significantly, which can help to prevent the spread of infectious diseases. In a study of a nasopharyngeal swab sampling chamber for airport personnel, Ma et al. [89] compared the effect of AAF modes of operation on exposure risk. The results showed that AAFs were the most effective in reducing the exposure risk. EXFs cannot achieve these results, even with NV and adequate ventilation. They also noted that because healthcare workers were all wearing protective clothing, they preferred the strong airflow provided by AAFs in terms of thermal comfort.

6. Role of Other Fans in Airborne Viral Infection Control

6.1. Desk and Pedestal Fans

In addition to CFs and EXFs, other fan-assisted virus removal measures have also been studied. These measures include the personalized use of DFs or PFs for targeted virus isolation. DFs are used as an adjunct to infection control ventilation as a form of personalized ventilation [91]. For this application, DFs are typically installed in close proximity to personnel and produce a jet that transports air to the breathing zone, thus creating a canopy that acts as a barrier against viruses (see Figure 11 [41]). This air transport provides relief because of the rise of the body heat plume as well as more efficient air transportation. The result is a clogging effect that creates a canopy of air, which provides a barrier that isolates surrounding viruses [92]. It has been noted that the efficiency of personalized ventilation sites increases from 10% to 22% when DF-assisted equipment is installed, which represents a more than twofold increase [91]. The removal efficiency is optimal in fan-assisted personalized ventilation systems with DFs of appropriate air speed (10 L/s is recommended). Excessive fan flow may break the barrier protection and increase the particulate matter concentration in the breathing zone [41]. Such an approach can reduce the overall MV configuration requirements in large spaces, thus reducing costs and ensuring the safety of occupants. In addition, when the distance between two desks is doubled, the ability of DF in removing viruses decreases. However, the removal effect was still significantly better compared to traditional MV ventilation models. Low-cost operating air conditioner systems (breezes) are not as effective as high-speed rotating DFs. Low-cost air conditioners not only pose a higher transmission risk, but also require 2.5–4 times more energy than fans. Therefore, it is recommended that inefficient air conditioning configurations should be avoided.

PFs play a role in mixing the air in a room, and the fastest wind speeds can be measured in the direction of the fan’s direct face. Wang et al. [93] analyzed the role of PFs on virus transmission in terms of charge effects. They concluded that in the case of PFs alone, virus particles are removed through particle adsorption (the effect of charge on aerosol airborne propagation is shown in Figure 11b). PFs placed in a room to mix the indoor air increase the probabilities of collision between the particles themselves as well as between particles and surfaces. This increased probability results in a higher negative charge generation within a short period of time. In addition, the increased turbulence makes the particles more inclined to adsorption on surfaces rather than being removed by the original ventilation system. This increases the potential risk of re-suspension in the air.

6.2. Multiple Types of Fan Cross Actions

The abilities of fans have been well recognized; yet strategies for fan use should be adjusted in moderation according to the actual situation. The importance of the fan rotation speed has been highlighted in studies of all types of fans. In fact, rotational speed is not the only factor that affects viruses, and it can only affect efficiency but not effectiveness. Therefore, the combined use of multiple fan types has received increased attention in recent studies. EXFs can control airflow in almost all ventilation methods, but airflow from CFs may decrease the effectiveness of negative pressure isolation rooms, as part of the airflow may leak into other rooms through gaps in internal doors [10]. Nevertheless, Sinha et al. [78] suggested that the use of CFs to induce airflow interferences in the vortex zone of a room can increase the removal effectiveness of EXFs. This level of airflow interference can also be achieved with PFs.

7. Future Work

In the past, the use of a variety of fan ventilation strategies to remove—or assist in removing—viral particles from indoor environments have seen substantial progress, offering valuable insights into fan configurations, airflow patterns, and overall infection control. However, the diversity in fan designs, operational parameters, and performance evaluation methods has resulted in fragmented findings, making it challenging to draw direct comparisons between different approaches. The following discussion synthesizes existing knowledge in this area, highlights unresolved challenges, and proposes directions for future work aimed at establishing more consistent and comprehensive evaluation criteria.

One core issue is the “jet core” generated by downward rotating CFs, which reduces suspended viral particles in the breathing zone but may be more suitable for spaces with relatively short durations of viral exposure. In long-term occupancy settings such as workplaces and residential areas, residual viral reprocessing requires careful attention. On the one hand, small particles that settle on floors or walls due to fan-induced airflow may re-enter the air if the fan’s speed or angle causes them to rebound, potentially creating a new cycle of airborne transmission. Future studies could address this by conducting controlled experiments under diverse environmental conditions, monitoring real-time particle behavior, and employing CFD to determine optimal fan speeds that minimize re-entrainment. On the other hand, the risk of large particles settling on surfaces and remaining infectious for certain durations has been overlooked; these attached viruses may later be transferred through contact. Further research should therefore quantify particle deposition rates on surfaces in real-world or laboratory conditions and consider fan-induced airflow in the distribution of deposited particles. Optimized strategies that jointly address large particle deposition and small particle rebound through fan design and operation would offer clearer guidance for infection prevention. In contrast, an upward-rotating CF alone is generally ineffective for reducing airborne viral levels, but its combination with UR-UVGI shows significant potential for viral inactivation. Future investigations should explore the synergistic use of upward-rotating CFs alongside UR-UVGI, particularly in settings such as hospital wards. Efforts to refine these integrated systems will ensure that both airflow patterns and ultraviolet disinfection are harmonized, ultimately improving overall viral removal efficacy.

For EXFs, the challenge posed by vortex zones can only be mitigated by increasing rotation speed, thereby enhancing airflow velocity and removal efficiency. Future studies should examine how room design might influence the position and magnitude of vortex zones or how other personalized ventilation methods might be introduced to disrupt these zones and expedite the removal process. CFD-based visualization and quantitative analysis could further clarify the correlations between vortex formation, airflow rate, and the time required to reduce indoor virus concentrations. Although studies on AAFs remain relatively scarce, existing findings generally indicate that AAFs can be highly effective in diluting viral concentrations and reducing infections. However, the intense airflow sensation they produce may cause occupant discomfort. Future work should therefore seek optimal trade-offs between ventilation efficacy and occupant comfort, potentially through adjustable airflow settings or personalized controls. DFs and PFs often serve as complementary strategies in tandem with other ventilation methods, providing localized assistance. However, the literature contains minimal exploration of how multiple fan types might be used together in a coordinated manner. Investigating the synergistic mechanisms of combining different fan types—harnessing the strengths of each—could yield more efficient virus removal, lower infection risks, and maximize the overall benefits of fan usage.

Beyond fan-specific considerations, research on virus removal currently lacks standardized definitions of effectiveness and unified evaluation criteria. Consequently, results tend to be applicable only under specific circumstances, preventing fair comparisons of fan types and operating strategies. Existing studies also underscore two primary research directions in fan effectiveness: occupant-centered control (CF, DF, AAF) and whole-room rapid reduction (EXF, AAF). The varying airflow patterns in different fan designs lead to distinct outcomes for localized and global viral reduction. Establishing comprehensive, standardized metrics that encompass both localized breathing-zone protection and broader indoor air quality is crucial. These metrics must be sufficiently adaptable to account for diverse fan configurations and operating modes while ensuring consistency and reproducibility in cross-study evaluations.

Another important area of inquiry involves the influence of environmental conditions—such as temperature, humidity, and wind speed—on evaporation rates and viral particle behavior. Since most existing work on 0.5–10 μm particles overlooks evaporation, larger particles (10–40 μm) may remain suspended if fan-induced airflow accelerates evaporation. Conversely, in cold and humid environments, particle settling tends to be faster, whereas dry, hot environments enhance evaporation, leaving more viral particles airborne. Ceiling-mounted EXFs typically perform better in dry, hot conditions due to lower settling rates, but future research must more thoroughly address how various fan designs alter environmental parameters, fan speed–evaporation interactions, and different climatic conditions (e.g., hot–humid and dry–cold). Optimizing fan design according to these climatic factors promises to substantially improve virus control substantially.

Non-environmental factors also demand attention. Most existing studies neglect occupant thermal comfort, which determines the viability of different fan types, especially in regions with extreme climates. Furthermore, the interplay of seasonal noise, psychological responses, and energy use can shape the practicality of fan ventilation strategies. Occupant posture and height likewise exert considerable influence on ventilation efficacy; children and adults, as well as seated and standing occupants, each occupy distinct breathing zones. Future research should incorporate varied occupant models and usage scenarios, followed by comprehensive spatial and infection-risk analyses to refine ventilation strategies for diverse occupant configurations.

In conclusion, addressing these multifaceted research gaps will foster a more systematic understanding of fan ventilation strategies and enable the development of scientific guidelines for real-world applications. Future investigations could further explore energy-saving control strategies—such as intelligent wind speed regulation and adaptive ventilation modes—to maintain high virus removal efficacy while minimizing energy consumption. By integrating these insights, fan-based interventions can be optimized to protect public health more effectively and support safer indoor environments.

8. Conclusions

In this paper, all the literature on the use of “fan” to reduce the risk of viral infection is reviewed. The conclusions are as follows:

Regarding CFs, the “jet core” created by downward-rotating CFs is the main reason for the reduction in virus particles suspended in the breathing zone. However, in the future, the reprocessing of settled and diluted particles should become a research focus. Moreover, the spatial risks associated with differences in height and posture of personnel should also be considered. In addition, whether “large particles” over 10 μm (less than 50 μm or 40 μm) can also leave viruses in the air after being instantly vaporized by the addition of CFs requires scientific examination. In contrast, the upward-rotating CF indoor airflow patterns are detrimental to the deposition and dilution of viruses. This could be solved by addition of an auxiliary UR-UVGI for direct virus inactivation.

Furthermore, EXF has been recognized for its effectiveness in controlling viral gases, especially in negative pressure isolation rooms (e.g., hospitals). The size of the room and the location of the EXF were the main influences on virus expulsion. Research on EXFs has focused on the time required to remove viral particles from a room, while the effect of viral particle size and the expansion of the mainstream field approach should be addressed in the future. In addition, while many studies have shown that AAFs provide excellent performance, the strong sensation of blowing air they generate may discomfort personnel. Acoustics are another such discomfort that needs to be addressed when using AAFs in office and learning environments.

Studies on DFs and PFs have generally used them in combination with other modes of ventilation that may act as personalized ventilation, thus forming a protective shield or barrier over the breathing zone. However, evidence to demonstrate the stability of their effects is insufficient. Future studies should consider the effects of a combination of DFs and PFs in different scenarios to explore the optimal fan ventilation strategy.

Future research could explore the synergistic use of multiple fan types to overcome the limitations of individual fan types and improve virus removal efficiency. Additionally, establishing standardized definitions and unified evaluation criteria for virus removal effectiveness is essential for comprehensively assessing various fan-based strategies and guiding their optimal selection. Meanwhile, fan-ventilation-related studies should further consider regional characteristics due to climatic conditions and environmental parameters, and consider indoor occupant thermal comfort levels.