Strategies to Enhance Contamination Control Performance through Ventilation Improvement in a Biosafety Laboratory Building

Abstract

A biosafety level (BSL) laboratory is a set of biocontainment preventative measures used to prevent and isolate hazardous biological agents or their accidental release in a laboratory. It is vital to provide a negative-pressurized environment for disease infection control. The experimental equipment layout may affect the personnel’s exposure to infection. However, the equipment layout and exhaust air grilles were in a fixed position in this investigated BSL. Due to retrofitting, the layout arrangement of HEPA as supply air is investigated numerically. Computational Fluid Dynamics (CFD) simulation is conducted to analyze and determine a better design for contamination control. This study proposes three ventilation arrangements as an alternative design, including vertical arrangement, horizontal arrangement, and L-shaped ventilation arrangements (instead of the supply being arranged in a single line). In addition, the airflow distribution, concentration decay, air of age, ventilation, and removal effectiveness are all examined in the study. The numerical simulation results were verified by a field measurement test. The results revealed that the L-shaped ventilation arrangement for supply air diffusers would achieve better ventilation and removal efficiency. The local mean age of air was also identified as the most satisfactory ventilation performance measure, as it shows the level of contaminant control. It also indicated that ventilation could be improved by arranging the supply air layout with less expenditure through CFD-aided simulation in identifying strategies for best practices for the design stage to reduce the running cost at full operation.

1. Introduction

A chronic infectious disease with a high fatality rate, tuberculosis (TB) affects people all over the world. Before the COVID-19 outbreak, TB was the most common single-source infection-related cause of death [1]. TB is still recognized as a major public health problem, and global efforts to control TB have expanded significantly in most healthcare facilities and micro-bacteriology laboratories. Kim et al. [2] reported that the risk of personnel contracting TB in the laboratory was twice higher that of non-laboratory workers. Laboratory biosafety is the term used to define the containment concepts, methods, and practices that are utilized to prevent unintentional exposure to infections and toxins or their accidental release, as stated in the World Health Organization’s (WHO) [3] Tuberculosis Laboratory Biosafety Manual. In addition to TB, SARS-CoV-2 also necessitates a biosafety containment level 3 (BSL-3) laboratory to comprehend virus biology and apply new knowledge to the preclinical development of vaccines [4]. The airflow in BSL-3 facilities must be planned to go from “clean” sections into the biocontainment zone. At this level, there is a higher chance of respiratory transmission leading to serious and even fatal infections. Hence, primary and secondary barriers to prevention are given more importance. All laboratory work should be performed inside a biosafety cabinet (BSC) or another enclosed space. Providing a negative-pressurized environment for infectious control in the biosafety level (BSL) laboratory is vital and essential [5]. According to the laboratory implementation method, WHO [6] or US requirements [7], a negative pressure differential of 12.5 Pa (0.05 in w.g.) must be kept between each pressure zone. Greater negative pressure must be created in high-risk rooms in BSL-3 laboratories with several zones. Specific monitoring and control systems, as well as visual readout and alarm devices, must be installed at the entrance to the containment space, in anterooms, or at the entrance to each individual room inside the containment suite to ensure the pressure difference in all containment rooms [8]. For the majority of BSL-3 laboratories, air change rates are typically fixed to a single number between 6 and 12 ACH based on risk assessment and industry standards [9]. A minimum of 6 ACH was suggested for BSL-3 facilities by the laboratory quality stepwise implementation tool. Whether studies are conducted in the lab or not, this minimal airflow must always be maintained [10]. Ventilation rates were established for both odor management in the lab air space as well as for effective protection of airborne contamination [11].

Study on ventilation performance against infection has been investigated. Liu et al. [12] conducted a study of the potential infection risk in a BSL-3 laboratory. The laboratory ventilation mode was established with an airflow that enters the room from ceiling inlets and flows through outlets on the ceiling. Through the use of this ventilation, over 70% of bioaerosol particles were deposited on the walls and other equipment, creating a significant risk of laboratory-acquired infection. As a result, it is crucial to carefully observe the procedures for subsequent laboratory disinfection [13]. Barbarosa et al. [14] investigate the contaminant contention in a BSL with mixed ventilation, which is one of the most common ventilation strategies for infection control environments and most conventional laboratories. The mixed ventilation mode is provided by the supply air from a square ceiling diffuser, and the exhaust grille is located on the ceiling. The findings demonstrated that airflow pattern, turbulence level, and supply air velocity significantly affect BSC’s contaminant contention. The implementation of greater ventilation rates in BSLs should be reevaluated because it increases personnel exposure to potentially hazardous contaminants that leak from the BSC. In several buildings, such as universities or office buildings, higher ventilation rates might be possible to implement. Chen et al. The authors in [15] used a higher ventilation rate to lower the risk of virus contact and take preventive precautions in a university classroom. Furthermore, Szekeres et al. [16] also used a higher ventilation rate in an office building to protect office workers; prevent the spread of pollutants, bacteria, and viruses; and also improve the fresh air rate in the occupied zone. Furthermore, BSL-3 with high potential risk factors for infection requires a reasonable equipment arrangement in order to create a safe and healthy environment. Liu et al. [17] investigated how obstacles and associated experimental equipment affect staff exposure to infection in a biosafety laboratory. Although the results indicate that the equipment layout could decrease contamination, disinfecting is still a key component, particularly for the surfaces of the equipment.

Computational fluid dynamics (CFD) simulation is a widely accepted scientific technique that allows improvement of airflow distribution for contamination control [18]. Some alternative arrangements under a limited budget as well as reduced trial-and-error efforts, could be conducted extensively. Liu et al. [19] used the CFD approach in conjunction with the Wells–Riley model for a BSL-3 laboratory bioaerosol releasing experiment to examine the infection risk of bioaerosols under three common incorrect operations. Wang et al. [20] also used CFD to analyze the distribution of airflow and contamination control using several systems in an operating room, followed by validation using a field measurement test. The air velocity and airflow patterns, such as recirculation, affected the local mean age of air. The concept of age of air, derived from temporal mixing theory, has been widely adopted to evaluate ventilation performance [21]. Federspiel [22] investigated the development of methods for calculating air change effectiveness based on the age of air measurement. The theory and fundamental calculation methods were addressed extensively regarding on-air change effectiveness. In the tracer gas experiment, a concentration decay of CO₂ was used to calculate the ventilation efficiency and air change rate. The results indicate that the ventilation efficiency might be dominantly influenced by the location of the diffuser rather than the air change rate [23].

This study utilizes a BSL-3 laboratory through numerical simulation in order to achieve a better contamination control design. The numerical simulation results were highly verified with the field measurement test results. The proposed supply air layout arrangement was numerically investigated to determine a better design for contamination control. As an alternative design, three supply air arrangements are proposed in this study (vertical, horizontal, and L-type ventilation arrangement). This study adopts the tracer gas method, concentration decay, and age of air to evaluate contamination control performance in this biosafety level laboratory.

2. Biosafety Laboratory

2.1. Layout and Biosafety Laboratory System

The layout of the investigated biosafety level 3 (BSL-3) with a negative-pressurized specifically for the TB laboratory is illustrated in Figure 1. The BSL-3 lab has a length of 8.5 m, a width of 7.0 m, and a height of 2.4 m. It consists of a core BSL-3 laboratory with anterooms, changing rooms, and miscellaneous supporting facilities. In the core lab, there is a main experimental table at the center of the lab with dimensions of 2.6 m × 1.5 m × 0.8 m (length × width × height). Other laboratory equipment, including four incubators, two centrifuges, two refrigerators maintained at 4 °C, and a freezer maintained at −80 °C, were also sketched in the layout. The function of the −80 °C freezer is to preserve the positive strains and provide the use for future pathogen tracing research. At the same time, the infected waste must be sterilized in a double-door sterilizer before being removed and discarded. There are two biosafety cabinets (BSC), each with airflow capacity of 995 m³/h. Some critical areas, including stained region, BSC, and centrifuge, were equipped with exhaust air ducts and grilles. According to ISO 14644-1 [24], the cleanroom classification of the BSL-3 lab is at a cleanliness level of ISO class 8 with an air change rate of 12 ACH. The indoor design conditions are temperature 21 ± 2 °C, relative humidity 60 ± 10%, and pressure difference of −37.5 Pa for the main area within the lab.

2.2. Schematic Diagram of Biosafety Laboratory HVAC System

The schematic diagram of the heating, ventilation, and air conditioning (HVAC) system of the BSL-3 lab is illustrated in Figure 2. Due to biosafety and infectious control concerns, there is no returning air (all fresh air systems). Fresh air supplied from the make-up air-handling unit (MAU) is distributed evenly to the control changing room, anteroom, and BSL-3 lab after high-efficiency particulate air (HEPA) filters. Clean air could be obtained accordingly for all spaces. Exhausted air grilles are located at the corners of the lab for negative-pressurized purposes. The sophisticated venturi air valves with a variable air volume (VAV) control scheme were applied for robust control of stable pressurized conditions for the BSL-3 lab, even the disturbance through the opening and closing of the airtight door and also starting/shutting down of the BSC in the lab. The exhaust air system was installed on the rooftop through an exhaust duct system equipped with a bag-in/bag-out (BI/BO) filter system along with UV sterilization. There is also one backup system for each unit in case of accidental breakdown.

3. Methodology

3.1. Field Measurement Test

Experiments performed in the BSL-3 laboratory are highly dangerous to the human body and belong to the high-level danger level [25]. In order to maintain the necessary environmental protection of the laboratory, effective work after the completion of the laboratory construction is particularly important. This study is based on the regulations of the Taiwan Centers for Disease Control (CDC) [26] to verify the effectiveness of various laboratory air-conditioning system testing items. According to the measurement order, the HEPA leakage test should be conducted at the beginning to make sure the system is in good condition. After that, the testing items can be conducted, including airflow rate, temperature, relative humidity, pressurization, and particle counts. The apparatus for field measurement test items is listed in

Table 1. Apparatus for field measurement.

Parameters	Apparatus Model	Operative Range	Accuracy
HEPA Leak Test	ATI TDA 2H	0.0001%–100%	±0.05%
Temperature Relative Humidity	TSI TA465P	−10~60 °C 5–95% RH	±0.3 °C ±3%RH
Air Velocity Pressure	TSI 8380	0.125–12.5 m/s Diff ± 3735 Pa	±3% ±2%
Particle Counts	AeroTrak TSI 9500	0.3, 0.5, 1, 3, 5, 10 µm	±5%

.

In this study, in order to understand the areas where the laboratory is more likely to cause contaminant retention, the height of the human sitting posture in the main laboratory is 1.2 m, and the breathing height of the human body stands at 1.6 m. It is this height that is used as the measurement [27]. Hence, to match the size of the laboratory and the simulated sampling location, a single height section takes eight points for an average distribution, and each point captures data every minute. The monitoring point location is shown in Figure 3.

3.2. Computational Model

Due to the investigated laboratory being retrofitted from the exiting healthcare building, existing exhaust air grilles and experiment equipment were constrained with few flexibilities. The preliminary study through numerical simulation to evaluate the contamination control for the critical area focus on the alternative arrangement of supply air with HEPA filters. The snapshot of the BSL-3 lab is shown in Figure 4a. The full-scale geometric model for CFD simulation of the investigated lab is shown in Figure 4b. The supply air (SA1, SA2, SA3, SA4, and SA5) is located at the ceiling, as is the exhaust air (EA1, EA2, and EA3). The exhaust air grilles (EAG1, EAG2, and EAG3) are located at the wall corner with a height of 0.2 m above the floor. In addition, leakage is unavoidable in the negative pressure laboratory, so this study also considers the gap of L × W = 1 m × 0.02 m under the door.

Three types of supply air (SA) arrangements were investigated numerically in this BSL-3 lab, including Case (A) vertical arrangement, Case (B) horizontal arrangement, and Case (C) L-shaped arrangement, as shown in Figure 5. Considering the fact that this BSL-3 laboratory is a general floor reconstruction, the location of the air outlet has been fixed, and the design must be feasible in the actual project. The supply air vents are designed to be arranged and combined on the ceiling. Case A ventilation arrangement was carried out to be analyzed to control contamination in one direction. Meanwhile, for Case B, there are some products that need to be controlled on the laboratory table which could potentially be a highly infectious zone. Then, Case C has L-shaped ventilation arrangements rather than the supply being arranged in a single line. This study aims to determine how good and bad the ventilation arrangements for the biosafety level are in preventing contamination in a critical area. It is expected that the best airflow pattern can be obtained under the premise that the actual project can be achieved.

3.3. Setup and Boundary Condition

A commercial CFD code, Fluent, was conducted to simulate the velocity distribution and concentration contour of the biosafety level. The governing equations solved by Fluent include the three-dimensional time-dependent incompressible Navier–Stokes equation, the time-dependent convection-diffusion equation, and the k-ε turbulence equations. The airflow turbulence simulation uses two simulation methods carried out in this study, transient and steady-state conditions, with the renormalization group (RNG) k-ε as the turbulence model. The inlet boundary condition (velocity inlet) is used for the supply air. The airflow enters the laboratory uniformly from the air supply outlet, and the inlet air velocity is set according to the total air supply volume design value. The airflow rate condition is based on the field measurement data, and the supply air temperature is set to 292.15 K (19 °C). The outlet uses an outlet boundary (outflow), whose mass is conserved and is a free boundary. The air volume infiltrated by the door gap is provided by the volume space outside the laboratory, which is determined by the difference between the total indoor exhaust air volume and the total supply air volume. After solving the velocity distribution, the transient simulations of the concentration contour were conducted accordingly. The concentration decay method based on the mass concentration equation could be derived. The temperature and face velocity of the HEPA filters were measured using a hot-wired anemometer as the boundary conditions and initial conditions for numerical simulation. Particle counts were conducted with a Met-One Model 3313 particle counter, sensitive to particles larger than 0.5 μm.

Table 2. Airflow rates obtained from field measurements for the boundary conditions.

Position	SA 1	SA 2	SA 3	SA 4	SA 5	BSC
Airflow rate (m3/hours)	684	684	684	684	684	−995
Position	EAG 1	EAG 2	EAG 3	EA 1	EA 2	EA 3
Airflow rate (m3/hours)	−400	−400	−400	−350	−350	−350

depicts the airflow rate measured for numerical simulation input of each HEPA, exhaust grille, and BSC. Furthermore, the concentration decay simulation was employed by assuming the initial concentration of CO₂ at 3000 ppm in the lab. The background CO₂ concentration level was assumed to reach 400 ppm, corresponding to the CO₂ concentration of ambient conditions. The parameters are set as shown in

Table 3. Equipment heat load.

Equipment	Heat Load (kW)	Area (m2)
−80 °C freezer	0.414	0.0480
CO2 incubator	0.198	0.0123
Light × 1	0.04	0.1560

. In this study, the simulation was carried out in a transient method. The main laboratory was filled with a high concentration of tracer gas, and the process of indoor concentration decay was simulated in units of 5 s. The total simulation time was 600 s. Fluent uses the finite-volume approach method to establish the differential equation and select the turbulent flow model. The calculation of the entire coupling iteration was solved by the Semi Implicit Method for Linked Equations (SIMPLE) method, and the residual value was set to 10⁻⁶.

3.4. Grid Refinement Test

When using a numerical method to simulate, the choice of grid size and number not only affects the solution convergence time but also has an inseparable relationship with the accuracy of the simulation results [28]. Therefore, a grid test must be performed after the geometric model is established. Through various grid tests, the result data are compared, and then it is known which grid size is the most appropriate. In general, the finer the grid division is, the closer it is possible to the actual value. Correspondingly, increasing the number of grids also means that the computer will spend longer iterative calculation time. Therefore, finding the optimal number of grid points through grid testing and measuring the appropriate calculation time can ensure the stability and correctness of the final solution value. The results can be seen in

Table 4. Grid refinement test.

Grid Number	CO2 Concentration (ppm)	Relative Error (%)
1,284,770	596.8	6.94 4.58 1.42
1,517,922	611.9
1,770,570	632.2
1,960,904	641.3

. The relative error of the three design schemes can be less than 2% when the number of grids is 1,770,570 and the number of grids is 1,960,904. This study uses a grid number of about 1,770,570 as the calculation grid for the three air supply design schemes to compromise the calculation time without affecting the accuracy of the results.

4. Results and Discussion

4.1. Measurement Results

In order to ensure the safety of the future operation of the BSL-3 laboratory after the completion of the construction, this study carried out various necessary measurement operations according to domestic regulations and standards [6]. The actual measured air velocity data of each air outlet is shown in Figure 6a. Except for the high air volume of the air supply outlet SA4, the other air supply outlets are close to the design air volume, while the exhaust grille EAG-2 is slightly lower than the design value, and EAG-3 is higher than the design value. However, the laboratory still complies with the domestic standard that the ventilation rate be greater than 12 times per hour.

The temperature and humidity measurements were conducted to confirm whether the main laboratory’s indoor environment can maintain the staff’s comfort. Figure 6b is the monitoring data of the main laboratory location for a week. The red line represents the temperature, and the blue line is the humidity measurement result. The dotted line is the allowable range of the regulations. The temperature is stipulated to be within 21 °C ± 2 °C, and the humidity is stipulated to be controlled by 60% ± 10%. When the temperature and humidity exceed this range, relying on the monitoring system to transmit signals to the air-conditioning system is necessary. Adjusted to make it return to the standard range, the peak in the curve is the operation of the host to maintain the set temperature and humidity.

Since the BSL-3 laboratory does not have strict requirements for cleanliness, from the measurement results of cleanliness in Figure 6c, it can be seen that the average number of fine dust particles with a particle size of 0.5μm is about 18,143. Its 95% UCL is 20,797, so the cleanliness level of this laboratory is Class 100,000 at the height of 1.2 m, and the number of dust particles with a particle size of 0.5 mm is less than 20,000. In contrast, Positions 1, 2, and 5 are areas with a higher number of dust particles. The area is relatively clean in Positions 6, 7, and 8. We also measured the cleanliness at the height of the human breathing zone at 1.6 m, which is the evaluation height of concentration removal used in the simulation stage. The measurement results are consistent with the simulation in the design stage. It is indeed difficult to remove concentration at Positions 1 and 5. The high number of fine dust particles measured at Position 6 may be because the actual measurement position is quite close to Position 5, where it is difficult to remove concentration. The number of dust particles at the height of 1.6 m is significantly higher than the measured value at 1.2 m. From the previous simulation results, it can be inferred that the air supply configuration of the top supply air and the side exhaust easily lead the air close to the ground to the exhaust port. At the same time, the airflow in the upper area may be affected by the eddy current formed next to the air supply port, making it difficult to remove fine dust.

The pressurization was recorded for 24 h for the entrance dressing buffer room, the entrance front room, and the main laboratory for one week for analysis, as shown in Figure 6d. The curve in the figure is the pressure difference between the public walkway outside the laboratory and each space. The system records one data per minute. The dotted line represents the pressure standard in order to understand the pressure difference in the laboratory for around 24 h. It can be seen from the figure that the pressure will change greatly during working hours and with large fluctuations. The points are roughly distributed at three time points: work, noon, and leave work.

4.2. Validation Results

The ultrasonic anemometer is used to measure the turbulence intensity based on the measurement location with the same cleanliness. The measurement and simulation results of turbulent intensity are plotted as shown in Figure 7a. It can be found that the actual measurement and simulation have the same trend, representing the scene. The airflow state roughly matches the simulation. The difference between the turbulence intensity measured at the height of 1.2 m and the height of 1.6 m is not large, and the airflow in the potentially dangerous areas from Position 1 to Position 5 is quite stable. The turbulence intensity is less than 15%. Positions 6, 7, and 8 near the supply air area have higher turbulence intensity, which means that the airflow near the supply air area is more unstable than in other areas. The vortex forms on both sides of the air supply grilles, increasing the airflow instability in that area. In addition, the validation of temperature also plotted in the Figure 7b. The results between measurement and simulation are quite close.

4.3. Pressure Difference

This laboratory includes two buffers as an anteroom. The pressure is maintained at the standard below −37.5 Pa for the biosafety lab and −25 Pa for the anteroom. In addition, the main laboratory pressure difference is calculated according to the standard definition. When the pressure is lower than −37.5 Pa, it is qualified. The actual measurement results show that the pressure of the anteroom is barely in line with the adjacent area. The difference is at least 12.5. The pressure specifications of the main laboratory and the rest of the buffer zone are within the safe range. Figure 8 illustrates the pressure difference between the biosafety lab and the anteroom. It shows that the pressure difference result has complied with the standard of the adjacent area and could be maintained at around 17.77 Pa.

4.4. Concentration and Airflow Distribution

To evaluate the ventilation performance and investigate the effect of different supply air arrangements in BSL-3, three cases were conducted to compare the concentration distribution and velocity vectors. The total airflow rate was set to be the same for three cases. The height of z = 1.6 m was chosen to represent the typical standing breathing level for personnel. Some critical monitoring points (as shown in Figure 3) at the height of 1.6 m located in front of BSC (Point 1 and Point 2) and in front of centrifuges were chosen (Point 3 and Point 4) due to the potentially high risk for contamination. These points were chosen to survey the concentration decay rate at the critical area of the lab. Simulation results were performed by the concentration decay method with transient simulation, which was conducted by assuming the initial contamination concentration at 3000 ppm and then diluting to the background concentration level at 400 ppm.

Figure 9 shows the state of the concentration field at t = 180 s. Figure 9a, under the air supply configuration of Case A, shows that there is still a concentration close to 2000 ppm in areas P1 and P5 that are difficult to remove. The Y section shows that most high-concentration pollutants are accumulated around the main experimental table. In contrast, the X section shows that the potentially dangerous areas of Positions P2 to P4 have better concentration removal status than Positions P5 to P7. It can be seen from the velocity field section of Figure 10a, Y = 3.75 m, that the main experimental table blocks the airflow from the supply air area at the beginning. Then, the generation of local vortices makes it difficult for the air supply configuration of Case A to make the airflow evenly. When it flows to the P1 area at the Y = 3.75 m section, it can be observed that the airflow in the P5 to P7 areas is mainly close to the low area. So, the concentration in the upper area is not easily diluted. The P2 to P4 areas with better removal effects are close to the lower area.

The concentration field of Case B is shown in Figure 9b. Contrary to Case A, the concentration removal effect of Case B at Positions P5 to P7 is better. Still, the concentration in the areas with relatively poor removal effect is only 1000 ppm. The high concentration part is not as high as that of Case A, which means that the airflow of the air supply configuration of Case B is more evenly distributed. In Figure 10b, the airflow trend from P2 to P4 in the Z section is similar to Case A. It can be seen from the Y = 2 m section that no matter what the air supply mode is, the wind speed of the door opening due to the high negative pressure is quite high, and this high-speed airflow will obviously guide the airflow direction of the entire laboratory. Case B could distribute the airflow in the whole laboratory because it can be observed through the cross section of X = 3.9 m, and the arrangement direction of the air supply port of Case B can pass through the main experimental table smoothly. It is observed that since the air supply vents are located closer to the areas of Position P1 and Position P5, the concentration can be removed before they accumulate.

From the cross section of the concentration field Z = 1.6 m in Figure 9c, the arrangement of the air supply ports of Case C combines the directions of Case A and Case B, and more air supply ports are arranged in the longer direction of the laboratory. So, the pollutants in the areas P4, P6, P7, and P8 can be quickly eliminated, and it can be seen from the cross section of Y = 3.75 m in Figure 10c that the airflow in the area P8 is also affected by the main experimental table. However, since Case C and Case B have a similar arrangement of air supply ports, the airflow can still flow smoothly to the potentially dangerous area at Positions P6 to P7. The pollutant concentrations in the P1, P2, and P5 areas are relatively high. It can be seen from the velocity field that there are some eddy currents in the areas on both sides of the air supply port, which may be the reason for the poor pollutant removal efficiency.

4.5. Concentration Contamination Decay

Figure 11 depicts the transient simulation of concentration decay rate at specified monitoring points at the lab with three cases of supply air HEPA arrangement alternatives. Case A with vertical arrangement presents an acceptable performance for concentration decay for Points 2, 3, and 4, but slow decay for Point 1 takes about 400 s to dilute the concentration to 400 ppm. Case B with horizontal arrangement represents a similar trend for all monitoring points, including Point 4, which reveals better ventilation performance than Case A. It indicated that the concentration decreased faster for Case B with a horizontal arrangement. The concentration decay curve for Case (C) with L-shaped arrangement demonstrated less time (about 100–200 s) needed for all monitoring points to reach 400 ppm concentration level than in previous cases, representing better ventilation performance. These simulation results also revealed that the ventilation performance could be improved just by relocating the supply air HEPA without any extra cost.

The monitoring results of pollutant removal are shown in Figure 11. First, we compare the pollutant removal status of the four monitoring points in the potential risk areas P1 to P4. In Figure 11a Case A, the removal rate of pollutants at Position P1 is quite slow and begins to decay downwards after about t = 50 s. It does not approach the air supply outlet concentration value of 400 ppm until after t = 500 s, while the situation at Position P2 is relatively better. The pollutant removal curve began to decline at about t = 20 s, the decline rate gradually eased at t = 120 s, and the concentration gradually stabilized after t = 500 s. Before the incubator, Position P3 has a good concentration removal rate, and the pollutant concentration begins to decrease rapidly at about t = 5 s, while the concentration removal rate at Position P4 is the fastest. Almost as soon as the air conditioner starts to operate, the pollutant concentration curve decays immediately.

Figure 11b shows Case B arranged in a vertical arrangement with the side with more exhaust ports. The monitoring results of this air supply configuration at four potential risk locations show that the overall trend is relatively concentrated, and there is no Case A. The phenomenon that the concentration removal rate of the configuration is too different occurs, which means that the airflow distribution in the potentially dangerous area is relatively uniform. In addition, in Case C, with an L-shaped arrangement, as shown in Figure 11c, the pollutant removal rate at Position P4 is not as fast as in Case A, which immediately decays at the beginning of the air conditioner operation. However, its pollutant removal rate is still quite fast, while the downward trend of P1, P2, P3, and Case B is similar. Compared with Case A, Case B and Case C show a more concentrated downward trend in the potentially dangerous area, indicating the arrangement of the air supply outlets in the L-shaped case. The L-shaped arrangement may produce a more uniform airflow field.

From Positions P5 to P8 outside the potentially dangerous area, it can be observed that different air supply configurations have different results from P5 to P8. Position P5 is described in the previous concentration and velocity fields because of the main experiment of the table’s block and the eddy current formed by the air supply port. In addition to the fact that Case B can have better concentration removal ability, the performance in Case A and Case C is not ideal, and Position P6 and Position P7 are provided in Case A. At the height of Z = 1.6 m, the wind speed is low, and there are eddy currents, so it is not easy to eliminate concentration compared with the other two air supply configurations. Position P8 can obtain a good concentration removal effect in Case A and Case C. Still, the removal efficiency in Case B is slightly poor because Position P8 is close to the supply air area, which provides downdraft at the same time. It is easy to form eddy currents around it.

4.6. Mean Age of Air

The local mean age of air was calculated using the theoretical method to assess and quantify the ventilation performance of the operation room. The concept of age of air based on numerical concentration level has been incorporated. Calculating the mean age of air gives the time elapsed, and it can be used to characterize the airflow pattern and ventilation performance. Transient simulation for concentration field at an arbitrary point is vital for calculating the age of air. Furthermore, the calculation of concentration decay after 600 s, room local mean age of air, is shown for all monitoring points of three cases in Figure 12.

In order to clearly present the simulation results, we used the local average air age to assist in the evaluation and further quantified the concentration removal effect of the three types of air supply. P4, P5, and P8 are quite close, but the air ages of Case A at Positions P1, P5, P6, and P7 have the longest local air ages among the three cases. Case B and Case A also have four locations where the local air age is longer. The difference is that among the four locations, P2, P3, and P4 are not significantly different from the other two cases, and the younger ones can be obtained at locations P5 to P7. As for Case C, except for Position P5, the local average air age of the other positions can be controlled within 2 min, and each position performs well in the three cases.

Considering the possibility of accidental occurrence of the practical actions performed by the BSC and the surrounding area of the incubator, we believe that in the selection of the air supply type, Positions P1, P2, P3, and P4 must have a higher ability to remove contamination than the rest. It is better that the concentration removal efficiency of the location area should not be too bad. The performance of Case C in the potentially dangerous area is better than the others. The air supply arrangements in P1 and the difference between P2, P3, P4, and Case A is not large, and the local air age of Case C in other areas is not the longest. In our view, the L-shaped air supply arrangement of the Case C ceiling is the best choice, which can take into account the even distribution of the airflow in the potentially dangerous area and the whole laboratory room at the same time. We suggest this type of air supply port be the actual air supply port design in this laboratory for construction. We will also conduct on-the-spot measurements and verification after the construction of this laboratory is completed.

4.7. Ventilation and Removal Efficiency

For the biosafety lab in particular, efficient airflow distribution is necessary for eliminating contaminants. Utilizing ventilation and removal efficiency as an indicator to evaluate the ventilation performance between the concentration of contaminants in the spaces and the concentration of contaminants in the exhaust air, we measure how well the ventilation system replaces the stale air in a space with fresh air. For evaluation, the ventilation efficiency index should be employed. Equation (1) expresses ventilation efficiency as:

$$\mathsf{\epsilon }{ }_{\mathrm{ventilation}}=\frac{{\mathrm{C}}_{ \mathrm{exhaust}}-{ \mathrm{C} }_{\mathrm{supply}}}{\left({\mathrm{C} }_{average}\right)-{ \mathrm{C}}_{ \mathrm{supply}}} \times 100\%$$

(1)

Contaminant removal effectiveness is the ratio between contaminant concentration in the exhaust air and the concentration at a point in the occupied space. It is a measure of how quickly an airborne contaminant is removed from the room. The removal efficiency Equation (2) is as follows:

$$\mathsf{\epsilon }{ }_{\mathrm{removal}}=\frac{{\mathrm{T}}_{\mathrm{particle} \left(0\right)}-{ \mathrm{T}}_{ \mathrm{particle} \left(\mathrm{t}\right)}}{{\mathrm{T}}_{ \mathrm{particle} \left(0\right)}}$$

(2)

where T _{particle (0)} represents all the particles that were expelled at the beginning (0 s). According to the time rate, T _{particle (t)} represents the total number of residual particles in the space, and t represents time in seconds (s).

Table 5. Ventilation and removal efficiency in different ventilation arrangement.

Case Study	Supply Air Arrangement	C exhaust (ppm)	C supply (ppm)	C average (ppm)	Ventilation Efficiency (%)	T particle (0) (ppm)	T particle (t = 600 s) (ppm)	Removal Efficiency (%)
A	Vertical	562.1	400	598.5	81.7	3000	568.5	81.1
B	Horizontal	519.5	400	541.3	84.6	3000	512.3	82.9
C	L-shaped	474.2	400	482.8	89.6	3000	445.4	85.2

shows the results of ventilation and removal efficiency of the biosafety level. Three cases are carried out with a numerical simulation. Case A with vertical arrangement of supply air presents the highest concentration profile compared with other cases, followed by Case B. With horizontal arrangement of supply air, the concentration results are lower than Case A. However, Case C with the L-shaped arrangement of supply air presents the lowest concentration profile. The concentration in the biosafety lab can be removed efficiently in all cases. However, the L-shaped design could present the highest ventilation and removal efficiency. With good ventilation strategies, contamination removal can be more effective. Case C presents the most satisfactory ventilation performance and better contamination control. It also indicated that the ventilation performance could be improved by arranging the supply air HEPA layout.

5. Conclusions

Nowadays, there few official guidelines about the layout arrangement for airflow distribution systems in a negative-pressured environment. In this study, a numerical simulation of a full-scale BSL-3 laboratory has been carried out to determine a better layout ventilation for contamination control. CFD simulation with three supply air arrangement alternatives has been conducted to investigate the airflow distribution, concentration decay, age of air, and ventilation and removal efficiency in the BSL-3 laboratory. Air distribution arrangements could be extensively assessed by airflow simulation and field measurement to achieve contamination control. The tracer gas method for the concentration decay and the local age of air was used to quantify it. The results show that the supply air configuration on the side is arranged horizontally, which is easily hindered by the central laboratory table. The L-shaped air supply configuration can achieve better concentration removal efficiency in potentially dangerous areas and other indoor areas. With the L-shaped ventilation arrangement, the ventilation and removal efficiency could be around 89.6% and 85.2%, respectively. Regions also achieve better contamination removal capability. Ventilation performance could be achieved effortlessly with less expenditure through the proper arrangement of supply air HEPA. It is also expected that CFD-aided simulation could identify strategies for best practices and achieve contamination control. Other than that, it also could provide some alternative arrangements under a limited budget and reduced trial-and-error efforts.

Since maintaining the designated pressure difference is critical for BSLs as in this study, the authors have conducted the research with a focus on pressurization, which is critical in preventing contamination at the biosafety level, and this could be the subject of future research. It is possible to examine and determine the effects of a BSC defect, pressurization loss, a varied ventilation rate, and contamination leakage by comparing the best-case and worst-case scenarios.