Mechanisms, Techniques and Devices of Airborne Virus Detection: Comparison
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Please note this is a comparison between Version 2 by Lindsay Dong and Version 1 by Rui Chen.

Airborne viruses, such as COVID-19, cause pandemics all over the world. Virus-containing particles produced by infected individuals are suspended in the air for extended periods, actually resulting in viral aerosols and the spread of infectious diseases. Aerosol collection and detection devices are essential for limiting the spread of airborne virus diseases. 

  • :airborne virus
  • particulate matters
  • COVID-19
  • building ventilation
  • aerosol

1. Introduction

The airborne transmission of pathogen is a notable feature of the ongoing global pandemic of coronavirus disease 2019 (COVID-19) [1]. It appears that other significant epidemics and pandemics have not involved airborne transmission. However, it is widely accepted that the airborne transmission mode is an important mode for respiratory viruses and likely drives community spread [2,3][2][3]. Importantly, the mode of exposure of viral aerosols is via inhalation [4]. Viral aerosol contained in human exhaled air ranges from 0.3 to 10 μm. However, some of the viral aerosols will shrink by evaporation after leaving the respiratory tract, and some will form large droplets by combining with other particles. These virus aerosols contribute to pathogen transmission through the air. Airborne transmission includes short-range aerosol transmission (<1 m) and long-range aerosol transmission. Short-range aerosol transmission is characterized by rapid deposition and affects a distance of less than 1 to 2 m, and long-range aerosol transmission is the inhalation of respirable aerosols with particle size less than 5 microns and more than 1 to 2 m from the infected person [5,6,7][5][6][7]. However, the risk of airborne transmission at close-range is generally always higher than that of long-range transmission [8]. These aspects will result in a large number of opportunistic infections, posing a significant challenge to epidemic prevention and control. Aside from COVID-19 in 2019, airborne transmission played a role in the spreads of other pandemics such as SARS pneumonia in 2003 and Influenza A (H1N1) in 2009 [9].
The mechanism by which epidemic diseases are transmitted via aerosol is that viral particles and/or some attached to the environmental particulate matters (PMs) remain active and suspended in the air for long periods until being contacted by humans and deposited on mucous membranes, where they reproduce to form infections [10,11][10][11]. According to studies, the exhalatory actions of virus carriers, such as breathing, talking, singing, shouting, coughing and sneezing, result in the production of a certain amount of viral aerosols [12,13][12][13]. Furthermore, people can shed lots of virus into their exhaled breath and do not need to be coughing to do so. There is great variation in exhaled breath viral shedding and temporal dynamics over the course of infection [14,15][14][15]. Work by Kristen Coleman and others shows that coughing and singing can increase the viral load emitted into aerosols from the respiratory tract [16]. The virus-related PMs have a wide range of particle sizes, from less than 100 nm to more than 1 mm [17]. Large aerosol PMs precipitate faster than small ones, but they are also more likely to impact and stick to other PMs and form droplets due to adsorption [6]. PMs within 10 μm can be suspended in the air for 5 min. Airflow in the room can make the PMs larger than 5 μm move away from the location where it is generated by the airflow. PMs smaller than 10 μm can enter the human chest cavity, and PMs smaller than 5 μm can reach the fine bronchi and alveoli [18]. While factors such as UV light, humidity and temperature inactivate viral PMs, the droplet-forming viral PMs reduce these effects to some extent and are thus more infectious than viral PMs alone [19,20][19][20]. Viral infectivity can remain for many hours [21,22][21][22]. The relationship between the length of period that the person is in the space where the infected person has been and the probability of infection is an important issue. The investigation of this issue has significant implications for the development of prevention, isolation and decontamination policies. However, more accurate qualitative or quantitative analyses rely on high-efficiency and sensitivity viral aerosol particle collection and detection device.
In the natural environment, the concentrations of viral PMs are typically very low [23]. The real-time monitoring and early detection of potential infectious bioaerosls can help meet the significant technical challenges posed by COVID-19. The qualitative or quantitative analysis of aerosol PMs in the environment necessitates the use of bioaerosol samplers capable of isolating and concentrating PMs from air samples, which can then be combined with other biochemical analytical techniques. A good example is the quantitative real-time polymerase chain reaction (qRT-PCR), which is widely used to detect viruses due to its great sensitivity and reliability. Certain comprehensive devices (CDs) are now available to enable real-time determination of parameters such as concentration, dispersion and composition of environment aerosol PMs [24].
The entire procedure for airborne virus detection is divided into three steps. The first step is to separate the PMs from the air and gather them into the solution. This is accomplished by converting the PMs into a hydrosol sample [25,26,27,28][25][26][27][28]. Up until now, there has been a lot of interest in the ability of aerosol to hydrosol (ATH) techniques to help with virus collection and enable downstream detection applications. The second step, called hydrosol to hydrosol (HTH), is to further concentrate and purify the detection target PMs in hydrosol. The third step is to transport the concentrated liquid to a subsequent analytical device, which is a detection technique, such as direct or indirect detection of virus components like nucleic acids or proteins. The development and application of such device is obviously critical to ensuring the biosafety of environmental monitoring and the scientific nature of ventilation in indoor spaces. Combining bioaerosol samplers with other analytical detection techniques to achieve real-time monitoring of airborne viruses is challenging [29].
Theoretical studies can aid in the development of devices that target bioaerosols in a certain particle size range, such as a well-designed high air flow-rate electrostatic sampler (HAFES) that can target the H1N1 (peak diameter: 95 nm), as well as HCoV-229E (109 nm) [24,30][24][30]. However, the viral particles in the real environment are usually not alone in the air but rather attach to other PMs to exist in the air with a wide particle size range. Chains of infection due to sequential presence in the same indoor environment are often frequently hard to track down and establish. Thus, broad-range collection, efficient purification and sensitive analysis of viral PMs in the environment are critical to resolving this issue. Furthermore, combining these three processes to create a CD that collects extremely low concentrations of viral aerosol and transfers them to a detection device with the appropriate limit of detection (LOD) in a small volume of liquid for subsequent detection would reduce the response time for outbreak prevention and control, the range of full nucleic acids and the cost of controlling outbreaks. This will provide significant technical support for the prevention of diseases transmitted by aerosols, such as COVID-19.

32. Sampling Techniques and Devices of Airborne Viruses

3.1. Primary Mechanisms

2.1. Primary Mechanisms

The fundamental mechanism of aerosol particle collection is that the PMs are separated from the air by specific principles (inherent property of the PM or some force). These mechanisms or properties include but are not limited to size filtration, mass differences of components in aerosol, gravity, electrostatic force and centripetal force (Figure 1).
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Figure 1. Schematic illustration of primary mechanisms for collection of airborne virus. (A) Impactor; (B) Cyclone; (C) Impinger; (D) Filtration; (E) Electrostatic precipitator; (F) Amplification. Images were modified from [59][31] with permission.

32.1.1. Mass Differences of Components in Aerosol

Impactor, impinger and cyclone samplers use mass differences to separate PMs from air. The particulate matter’s mass in aerosol varies. When the aerosol flow changes, the larger mass PMs settle on the collection plate due to inertia differences, while the smaller ones and remaining air are discharged from the outlet.

Impactor

As illustrated in Figure 1A, for the Impactor fundamental principle, under the action of a fan or pump, aerosols are sucked or sprayed into a channel with baffles (collection plates); PMs are deposited on the collection plates by inertia; and the remaining aerosols are discharged from the outlet end with uncollected PMs.

yclone

As shown in Figure 1B for Cyclone principle, aerosols are sucked or sprayed into the annular channel under the action of a fan. Centrifugal force deposits PMs on the inner wall of the annular channel, and the remaining aerosol is discharged from the exit end with the uncollected PMs.

Impinger

As shown in Figure 1C for the Impinger’s mechanism, aerosol is rushed straight to a liquid surface under the action of a fan; PMs are rushed into and dispersed or dissolved in the liquid by inertia; and the remaining aerosol and uncollected PMs are discharged from the outlet end.

32.1.2. Filtration

As shown in Figure 1D, the aerosol is passed through a channel containing a filter plate or membrane under the action of a fan. The PMs are trapped in the channel by the effects of the screen, and the remaining aerosol is discharged from the outlet end with the uncollected PMs. Filtration generally uses porous media like activated carbon, glass fiber nonwovens and medical stone as filter membranes [64,65,66][32][33][34]. The PMs in the aerosol are adsorbed in the pores inside the media when they pass through.

32.1.3. Electrostatic Precipitator

The electrostatic precipitator mechanism has been widely used in the field of dust removal [67,68][35][36]. Electrostatic precipitators are potential high-flow viral aerosol particulate collection devices because they can use Coulomb forces to all charged PMs in the aerosol to deflect and deposit them on the pole plate. The aerosol enters a collection electric field under the action of a fan, and the PMs are deflected and deposited on the collection plate by the Coulomb force. The remaining aerosol, along with the uncollected PMs, is discharged from the outlet. As shown in Figure 1E for electrostatic precipitator, the Coulombic force applied indiscriminately to all PMs.

32.1.4. Particle Amplifier

High humidity causes each particle to swell with the water which condenses on the particles. Thus, each particle gets larger due to deliquescence/condensation and thus grows and becomes easier to be collected. The PMs are amplified and become easier to deposit before settling into the collection pool. As shown in Figure 1F, the low temperature aerosol enters a warm and extremely humid pipe under the action of a fan. In the pipe, the particles continuously grow to form larger droplets. The amplified droplets are deposited in the collection pool at the back end, and the remaining aerosol along with the uncollected PMs is discharged from the outlet end.

32.1.5. Application Examples

Various devices have been developed based on the fundamental mechanisms stated above. Particle collection using only primary mechanisms has two troubles. First, the whole flow rate is low, and the collection efficiency decreases when the flow rate increases. Second, sampling particle size is limited, and the collection efficiency decreases when the particle size decreases. Even so, these primary mechanisms have been shown to be effective. Enhancement techniques, as well as the combination of several primary mechanisms, may have an extra positive impact on the high-flow efficient particle sampling.

3.2. Enhancement Techniques

2.2. Enhancement Techniques

Researchers have used enhancement methods to increase the collection efficiency and developed comprehensive device to improve the sampler collection efficiency based on the primary methods. The collection of aerosol is divided into three steps as described above: aerosol entry, aerosol collection via a specific principle, and residual aerosol discharge. As an extension, the collection efficiency can be increased from various extra steps of enhancement techniques (Figure 2).
Figure 2. Schematic illustration of enhancement techniques for collection of airborne virus. (A) Precharge; (B) Premixing; (C,D) Liquefied collection plate; (E) Cavities and Ribs.

32.2.1. Aerosol Precharging

In nature, viral particles have relatively weak negative charges [84,85,86][37][38][39]. The charge of the PMs directly determines the magnitude of the Coulomb force on them when using electrostatic precipitation methods to collect PMs. A series of theoretical and experimental studies have been conducted on the particle charging characteristics, and particle charging laws corresponding to various charging devices, particle size and ionic density have been obtained. The precharge on aerosol PMs increases the Coulomb force and shortens the length of the device’s collection section, enabling PMs to be deposited onto the collection plate faster (Figure 2A) [87,88,89,90][40][41][42][43].

32.2.2. Liquefied Collection Plate

The PMs are collected by devices such as impactor, cyclone, precipitation, etc., into a solid collection plate. These methods have a number of limitations:
(1)
PMs have to be eluted into liquid before conducting further monitoring tests;
(2)
There is greater potential for viral infectious decay on dray plates. Additionally, this is also a challenge with most methods;
(3)
The high velocity of wind may cause PMs to re-aerosolize.
Researchers have developed liquefied collection plates to address these issues (Figure 2C,D). In order to improve the collection efficiency, it is possible to collect the viral PMs directly in the liquid and deliver them to the detection module at the back end by turning the collection plate of the above methods into a collection pool.

32.2.3. Cavities and Ribs on the Inner Wall of the Pipe

Well-designed cavities and ribs on the inner wall of an aerosol channel can improve the collection efficiency of PMs by causing some of them to be deposited in cavities, and such studies are typically carried out using simulation methods. The mechanism of deposition enhancement and deposition efficiency ratio of PMs with different particle sizes, rib spacing (p) and rib height (e) was investigated [92][44]. The presence of ribs on the channel’s inner wall surface improves the particle deposition efficiency significantly, and the entrainment of turbulent eddies caused by ribs is better for the capture of PMs with smaller particle sizes. The smaller particle are refer to dimensionless particle relaxation time <1 [92][44]. The dimensionless particle relaxation time is directly proportional with the square of the particle diameter [92][44]. Particle deposition efficiency improves as rib spacing decreases, especially for large PMs, but rib height variation has no significant effect on deposition efficiency.

32.2.4. Hydrosol to Hydrosol Enrichment

Generally, the concentration of hydrosol samples after collection by aerosol collector is insufficient. Enhancing the concentration of viral aerosol into hydrosol samples with efficient enrichment can enhance the performance of CD and support downstream detection that requires small sample volume. Various HTH enrichment methods have been developed to meet this challenge. Yeh et al. developed a portable microfluidic platform using carbon nanotube arrays with differential filtration porosity for rapid enrichment of viruses [100][45]. Twist Bioscience has developed a variety of SARS-CoV-2 specific platforms that target fragments of viral DNA for enrichment [101][46].

32.2.5. Other Issues

Enhancement from all CD steps is required to improve the performance of airborne virus collection devices. Other methods, in addition to the various enhancement techniques mentioned above, are listed below. As an example, the Ace glass impinger (AGI-30) has a very low collection efficiency (30%) for fine particulate collection, and the glass bead filling has increased the efficiency to 99% [108][47]. The operators’ safety is another critical issue in the development of airborne detection CDs. Some substances are specifically raised here in order to achieve inactivation through premixing in the inlet section (Figure 2B). Tea tree and eucalyptus oils were used as a coating for filter fibers, which effectively improved the particulate collection efficiency while reducing the activity of viral PMs [109,110][48][49].

3.3. Comparisons on Airborne Virus Sampling Devices

2.3. Comparisons on Airborne Virus Sampling Devices

Devices for collecting aerosol PMs using only one collection method already perform well, but they are far from adequate for practical scenarios [117][50]. Real-time surveillance is critical for airborne infectious diseases and epidemics. Further advancements in performance may allow it to be applied to practical epidemic prevention. To achieve increased efficiency, new CDs have been developed that use a combination of multiple primary mechanisms or the method of enhancing techniques.

32.3.1. Combinations of Multiple Primary Mechanisms

Using a cyclone and impactor, Sung et al. designed a CD in which aerosols enter from the outer wall in a cyclonic manner impacting on the collection plate and exiting from the inner channel in the opposite direction. With a flow rate of 1000 L/min, the efficiency of the air sampler is about 50% for a particle size of 1 µm and 78.3% for a particle size of 1.5 µm. For particle size greater than 1.5 µm, collection efficiencies of approximately 100% were observed [31,32][51][52].

32.3.2. Applications of Enhancing Techniques

The following describes the CDs using the enhancing techniques in combination with the primary mechanisms. Tan et al. designed a new hemispherical electric field using a Liquefied acquisition plate and a pre-charging, electrostatic precipitator and developed automated bioaerosol collection and output [29]. After the aerosol is charged by the charging electrode, it enters the hemispherical electric field, where the travel path of the PMs is shifted by the Coulomb force and falls into the central charged collection fluid, which is circulated by an external pipe and a peristaltic pump to achieve a continuous cycle and can be integrated with subsequent detection modules. Under the conditions of collection voltage 20 kV, charging voltage 1.5 V and flow rate 1.2 L/min flow rate, the collection efficiency is 70% to 90% for PMs of 0.3–1.2 μm, >90% for PMs of 0.65–0.8 μm and >90% for PMs of 0.8–2.0 μm; under the conditions of 6.2 L flow rate for each particle size segment, the collection efficiency of PM is reduced to 20–60% [29]. This device employs a hemispherical electric field and liquefied acquisition plates to efficiently collect PM into liquid samples.

3.4. Differences between Virus Sampling and Virus Detection

2.4. Differences between Virus Sampling and Virus Detection

It is critical to differentiate between sampling and collection. Because sampling-based devices do not require real-time data, the samples are subsequently re-cultured, and they require less efficiency. In less human-populated environments, such as farms and pastures, sampling equipment can be used for routine monitoring. Collection devices must be more efficient, as all PMs passing through the device must be collected for subsequent testing of certain critical components. Devices for data collection can be used in densely populated environments such as conference rooms, subway stations and large shopping malls.

43. Detection Techniques of Airborne Virus

Detection techniques are important for compounding devices and determine the amount of hydrosol sample per unit time, sensitivity and response time. Virus assays are usually divided into two main categories, namely direct and indirect assays [118][53]. Indirect detection methods involve virus isolation, where viruses are introduced into a suitable host cell line to propagate virus PMs, which are later tested. This method usually requires a longer turnaround time. Direct detection methods include nucleic acid detection and immunoassay. Nucleic acid amplification test (NAAT) refers to the amplification and detection of genes in the virus and is characterized by early diagnosis, high sensitivity and specificity, and the most widely used technique is RT-PCR. Immunoassay uses antibodies as the main means of detecting the virus in the sample, with the greatest advantage of convenience and short detection time. However, immunoassay detection of infection may be of limited use in the early stages due to the time required for the body to develop an immune response to the infection. Table 21 summarized the typical detection techniques. Current conventional diagnostic techniques require expensive device and specialized operators and are inadequate to enable rapid, accurate and on-site diagnosis during pandemics. 
Table 21.
Summary of potential detection techniques for airborne virus.
Detection Techniques (Acronym) Detection Techniques (Full Name) Description Advantages Disadvantages Publications
PCR Polymerase chain reaction Using primers, thermal cycling, thermal cycling needs to go through three temperature changes. High accuracy and specificity, low detection limit, suitable for all kinds of samples. Need a long time and special laboratory environment. [11,41][11][54]
LAMP Loop-mediated isothermal amplification In vitro amplification of nucleic acids at a constant temperature of typically 60–65 °C. Cheap, just a hot plate. High demand for primers, false positive rate may be high. [42,,46][5543,][5644,][57]45[58][59]
CRISPR Clustered regularly interspaced short palindromic repeats Cas enzyme indiscriminately cuts the surrounding single strand after activation. Fast and specific. Relies on the preamplification to detect the targets when concentrations below femtomolar level. [47,48,49,50,51,52][60][61][62][63][64][65]
ELISA Enzyme linked immunosorbent assay An enzyme is used to display and convert readings in a measurable manner based on the interaction between antigen and antibody. The detection takes only ten minutes, no special instrument required. Sensitivity and specificity are limited. Does not apply to all virus types. [41,53,54][54][66][67]
SPR Surface plasmon resonance Light hits a sensor which covered with a metal film. Measures the intensity of the refracted light. The analyte concentrations can be determined and data on biological reaction kinetics can be obtained, with a low LOD character. Sensors are difficult to reuse. [55,56][68][69]
MS Mass spectrometry Mass spectrometry and PCR technology are perfectly combined in nucleic acid mass spectrometry, making it ideal for the investigation of dozens to hundreds of gene loci. It can be used for direct sample detection while the high-throughput features support multi-site multi-targeted detection. Professional needs are comparatively high, standardization plans are few, and automation plans are expensive. [57,58][70][71]

54. Application Scenario-Dependent Devices for Airborne Virus Detection

The combination of an aerosol collector and a detector into a single CD reduces the detection time of airborne viruses and speeds up emergency response. The performances of CDs are crucial for the various application scenarios [9,73,74][9][72][73]. As illustrated in Figure 43, the front-end aerosol collector determines the CD’s flow rate and collection efficiency, while the back-end hydrosol detector determines the CD’s sensitivity and other crucial factors, such as response time.
/media/item_content/202304/64478e7099037ijerph-20-05471-g004.png
Figure 43. Properties and critical performance parameters determined by the aerosol collector and hydrosol detector in airborne virus detection CDs.

5.1. Device Classification

5.1.1. Environmental Monitoring Device

The outdoor environment is multi-factorial and complex, and outdoor aerosol monitoring necessitates collaboration with related fields such as environmental monitoring, air pollution control and weather forecasting, as well as the establishment of large monitoring stations for multi-indicator environmental aerosol monitoring and prediction [169].
Properties and critical performance parameters determined by the aerosol collector and hydrosol detector in airborne virus detection CDs.

4.1. Device Classification

4.1.1. Environmental Monitoring Device

The outdoor environment is multi-factorial and complex, and outdoor aerosol monitoring necessitates collaboration with related fields such as environmental monitoring, air pollution control and weather forecasting, as well as the establishment of large monitoring stations for multi-indicator environmental aerosol monitoring and prediction [74].

High-Flow Environment Aerosol Monitoring Device

The volume of aerosols in the living environment is large for indoor environments that require constant ventilation [171]. For example, an office with a floor area of 20 m
The volume of aerosols in the living environment is large for indoor environments that require constant ventilation [75]. For example, an office with a floor area of 20 m
2
and a floor height of 2.5 m contains nearly 50 m
3 of aerosols. In the environment, viral aerosol PMs have a low concentration and a wide particle size distribution (attached to other PMs) [23,29]. Therefore, aerosol particle collection devices for environmental monitoring are expected to possess the following characteristics in order to meet the requirements:
of aerosols. In the environment, viral aerosol PMs have a low concentration and a wide particle size distribution (attached to other PMs) [23][29]. Therefore, aerosol particle collection devices for environmental monitoring are expected to possess the following characteristics in order to meet the requirements:
(1)
Aerosol collector: high flow rate, high collection efficiency for the full particle size range.
 Aerosol collector: high flow rate, high collection efficiency for the full particle size range.
(2) 
Hydrosol collector: high hydrosol flow rate, high sensitivity, short response time.
(3) 
Many researchers have developed such CDs, and environmental monitoring devices should be able to maintain a high level of particle collection efficiency at high flow rate while also delivering hydrosol samples to the back-end high-sensitivity detection module in real time.

Portable Environment Sensor

For indoor environments with complex ventilation, it is difficult to monitor inlets and outlets uniformly, or its monitoring cannot meet the actual prevention and control requirements. The development of portable environmental sensors for patrolling or the deployment of indoor environmental aerosol detection can help monitor the indoor environment of the virus aerosol. Such portable environmental sensors should possess the following characteristics:
(1) 
Aerosol collector: high flow rate, high collection efficiency for particle size range of human-generated PM.
(2)
 Hydrosol collector: high sensitivity, short response time.

4.1.2. Individual Detection Device

5.1.2. Individual Detection Device

An individual detection device is a device that collects air directly from a person’s exhalation [76]. The amount of air exhaled by a person in a single breath is about 10 mL/kg, and with an average weight of 70 kg, the amount of aerosol exhaled by a person in a single breath is about 70 mL [77]. PMs contained in human exhaled aerosols range from 0.3 to 10 μm, which requires individual detection devices to have ultra-high sensitivity with very small sample sizes [78]. This requirement brings an extremely high challenge to both the front-end collection technology and the back-end detection technology. In addition, people directly use the device and exhale; without settling, the exhaled PM particle size segment is more stable [79][80]. Therefore, the individual CD should possess the following characteristics:
An individual detection device is a device that collects air directly from a person’s exhalation [172]. The amount of air exhaled by a person in a single breath is about 10 mL/kg, and with an average weight of 70 kg, the amount of aerosol exhaled by a person in a single breath is about 70 mL [173]. PMs contained in human exhaled aerosols range from 0.3 to 10 μm, which requires individual detection devices to have ultra-high sensitivity with very small sample sizes [174]. This requirement brings an extremely high challenge to both the front-end collection technology and the back-end detection technology. In addition, people directly use the device and exhale; without settling, the exhaled PM particle size segment is more stable [175,176]. Therefore, the individual CD should possess the following characteristics:
(1) 
Aerosol collector: low flow rate, high collection efficiency for particle size range of human-generated PM.
(2)
Hydrosol collector: ultra-high sensitivity, short response time.
 Hydrosol collector: ultra-high sensitivity, short response time.

5.2. Building Ventilation and Airborne Virus Detection

4.2. Building Ventilation and Airborne Virus Detection

Airborne virus collection and detection for various indoor environments are achievable using a combination of the various devices mentioned above. Common indoor environments ventilation is categorized into three types: mechanical (full ventilation), hybrid ventilation (partial ventilation) and natural (natural ventilation) [177]. Full ventilation means that air exchange through ventilation equipment, no air leakage or air leakage can be ignored. Partial ventilation means, on the basis of ventilation equipment, are also accompanied by windows, doors and other air leaks [178]. Natural ventilation refers to ventilation only through windows, doors and other vents.
Airborne virus collection and detection for various indoor environments are achievable using a combination of the various devices mentioned above. Common indoor environments ventilation is categorized into three types: mechanical (full ventilation), hybrid ventilation (partial ventilation) and natural (natural ventilation) [81]. Full ventilation means that air exchange through ventilation equipment, no air leakage or air leakage can be ignored. Partial ventilation means, on the basis of ventilation equipment, are also accompanied by windows, doors and other air leaks [82]. Natural ventilation refers to ventilation only through windows, doors and other vents.

5.3. Forward to an Intelligent Detection Strategy of Airborne Virus

4.3. Forward to an Intelligent Detection Strategy of Airborne Virus

Numerous studies show that improving the quality of the indoor environment can reduce viral transmission [184,185,186,187][83][84][85][86]. To improve indoor air quality, the airborne virus sampling devices can be combined with air purification/filtration techniques and other processes. Airborne virus sampling devices, particularly those with high throughput, enable virus detection to be combined with indoor space ventilation in future building design. Indoor aerosol environmental monitoring requires three types of devices: conventional monitoring devices with certain collection efficiency at high flow rates, high efficiency collection devices with a portable premise at high flow rates and individual monitoring devices with nearly 100% collection efficiency at ultra-low flow rates.
(1) 
Individual detection devices with ultra-high sensitivity and low flow rates are used to ensure that no viral aerosol PMs are produced by people entering a critical site.
(2)
 A high-flow environment aerosol monitoring device with an ultra-high flow rate is used to collect and monitor viral PMs in the full ventilation environment’s inlet and outlet, as well as the partial ventilation environment’s inlet and return pipe.
(3)
 A portable environmental sensor is used as a supplement because the air leakage from partial and natural ventilation is insufficient for regular monitoring.
(4)
 The collection device at the front end determines CD collection efficiency, but the detection module at the back end determines the specific length of response time, and the detection limit is influenced by both the collection efficiency and the detection method.
(5)
 The enhancement technique improves indoor air quality and contributes to the prevention and control of airborne diseases in indoor environments by lowering PM concentrations and virus activity in inlets and return pipes.
 

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