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Aerosol containment by airflow in biosafety laboratories
Journal of Biosafety and Biosecurity 1 (2019) 63–67
Contents lists available at ScienceDirect
Journal of Biosafety and Biosecurity journal homepage: www.elsevier.com/locate/jobb
Review Article
Aerosol containment by airflow in biosafety laboratories Xin Feng a,⇑, Yanguo Zhang a, Zhonglin Xu a, Donglin Song b, Guoqing Cao a, Lei Liang a a b
China Academy of Building Research Co., Ltd., 30 North 3rd Ring East Rd., Beijing 100013, China Wuhan Institute of Virology, China Academy of Science, 44 Xiao Hong Shan Middle District, Wu Han 430071, China
a r t i c l e
i n f o
Article history: Received 1 October 2018 Received in revised form 18 December 2018 Accepted 20 December 2018
Keywords: Aerosol Containment Biosafety
a b s t r a c t In this paper, the progress and research related to aerosol containment by airflow in containment environments, e.g., biosafety laboratories, are introduced from a mechanical engineering view. A good airdistribution strategy in the room, a reasonable and stable pressure gradient in the containment area, and a necessary buffer room comprise the integral parts for regulating the air flow and providing the necessary containment. An optimal air-distribution strategy would reduce the residence time of the bioaerosol in the lab room and lower the exposure risk of the work staff. The pressure difference between adjacent rooms provides sufficient isolation protection when the door is closed. Nevertheless, an unfavorable air exchange would occur when the door is open, owing to door movement, passing people, or a tiny temperature difference. A buffer room is therefore necessary to offset the negative impact and maintain the containment effect. Ó 2018 Published by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.
1. Introduction Biosafety laboratories (BSLs) are designed and operated as basic containment and protection measures for the working staff and environment surrounding research facilities dealing with highly infectious biomaterials and toxins. According to modern statistical information, over 80% of experimental activities could generate bio-aerosols. Therefore, aerosol contaminants, rather than gasphase contaminants, are the major exposure risks and control targets in BSLs to lower the occurrence of laboratory-associated infections (LAI) and the contamination of the surrounding environment. Current biosafety containment practices (BMBL1) recommend the use of primary containment equipment, e.g., biosafety cabinets, the proper use of personal protection equipment (PPE), highefficiency particulate air (HEPA) filtration of the exhaust air, and necessary sterilization procedures, as the basic requirements. A good building layout, mechanical system design, optimal airflow pattern in the lab room, reasonable air flow, and stable pressure gradient together comprise the integral physical containment parts in BSLs; they would reduce the infectious-agent residential time within the room, lower the exposure risk for the working staff, and protect the surrounding environment. In this paper, the progress and research related to aerosol containment by airflow, including the air-distribution strategy in the room, pressure-difference functions and malfunctions, and the func⇑ Corresponding author. E-mail address: [email protected] (X. Feng).
tion of buffer rooms are introduced from a mechanical engineering view. We hope it will provide useful and helpful information for the design, operation, maintenance, and updates related to BSLs. 2. Function of air-distribution strategy Because the size of bio-aerosols varies considerably from nano size (virus, DNA/RNA fractions, etc.) to micron size (bacteria) and supermicron size (fungi and pollens), their aerodynamic characteristics are remarkably different. The generally accepted opinion is that the behavior of submicron aerosols is mainly decided by diffusion, while the inertia effect dominates the movement of supermicron aerosols.2 The deposition velocity of particles larger than 10 lm would be around 0.1 m/s in a turbulent flow,3 making it much easier to settle to the ground and other surfaces. Small particles, on the other hand, mainly follow the stream of the air flow, although Brownian diffusion sometimes drives the particles to touch and collect on nearby surfaces during the random motion. Biological experimental activities, e.g., extraction of embryo culture fluid, mixing samples, centrifugation, ultrasonic crushing, or the unexpected breaking of bottles with bacteria liquid, would generate bio-aerosols mainly in the <5 lm range. Thus, it is critical to carefully design the airflow pattern in the containment room to reduce the exposure risk of the working staff to an acceptable level. The basic rule of airflow-pattern design says that the air should flow from the clean area, to the work staff, then to the potentially contaminated area, and then be exhausted. Thus, mechanical engi-
https://doi.org/10.1016/j.jobb.2018.12.009 2588-9338/Ó 2018 Published by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.
64
X. Feng et al. / Journal of Biosafety and Biosecurity 1 (2019) 63–67
neers should plan the working area in the room, the major moving route, the location of air inlets, outlets, experimental equipment, and furniture, based on the rule. The actual regulation of the airflow pattern and aerosol transfer is much more complicated, although the rule is quite simple. The air-change rate in the room, the character, number, and location of the air inlets and outlets, the location and size of all impeding furniture, and the air condition, including the velocity, temperature, turbulence intensity, etc., are all important factors that should be taken into consideration. Therefore, since the 1980s, computational fluid dynamics (CFD) simulation has been a powerful design and research tool, together with the appearance and development of computers. In the early stages of CFD, the computational capacity could only provide hundreds of control volumes and could only be used to predict the air flow and aerosol transfer in small areas, such as a clean bench.4 CFD is one of the most important computer-aided investigation tools to benefit from the tremendous development of computer capacity in the past three decades. More recently, a complicated air-supply diffuser and the movement of the operator in the room were simulated, and their performance and impact on indoor airflow patterns were investigated.5,6 In addition to BSLs, CFD tools were used to investigate the spread and prediction of infectious agents in large terrains and contribute to disease control.7,8 Now, CFD is widely used as a popular design tool for optimizing design layouts and developing better air-distribution schemes. 3. Pressure-difference functions and malfunctions 3.1. Pressure-difference function Although the opinion is generally accepted that biomaterials should be handled in a relatively airtight environment to avoid unexpected aerosol propagation, absolute airtightness is not necessary for BSLs. In most cases, the pressure difference between adjacent rooms will lead to a stable and direction-controlled airflow in all gaps and leaks, especially in the gaps of all openable facilities, e.g., doors and transfer boxes. The following equation14 provides a simplified relationship between the pressure difference and the corresponding directed air velocity in a gap. Eq. (3.1), below, is normally used in engineering applications; more complicated calculation methods can be found in other works.9,10
sffiffiffiffiffiffiffiffiffi Q 2 DP V¼ ¼l F q
ð3:1Þ
where V is the air velocity in the gap, m/s; Q is the air-flow rate, m3/s; F is the area of the gap, m2; and l is the flow coefficient, which is normally 0.3–0.5 for small gaps and leakages, and we use 0.4 in this case. DP is the pressure difference, Pa. q is the air density, 1.2 kg/m3 in normal temperature and atmospheric pressure conditions. Thus, for a normal non-air-tight door with average gap dimensions of 6000 mm 5 mm, and an air-tight door with average gap dimensions of 6000 mm 0.11 mm, Table 3.1 summarizes the corresponding leakage air velocity and air-flow rate under different pressure differences. Table 3.2 illustrates the test results of two installed air-tight doors. The function of the pressure difference is to maintain a stable directed air flow into any potential gap or leak in the building boundary of the containment area. The size of the pressure difference is therefore quite critical. On one hand, the generated leakage air velocity in the gap needs to be sufficient to resist negative impacts, e.g., instantaneous air flows caused by the movement of staff and equipment, turbulent air fluctuation, and the natural con-
Table 3.1 Leakage air velocity and air flow rate under different door-pressure differences. Pressure difference Pa
Leakage air velocity m/s
Leakage air flow rate of non-air-tight door m3/s
Leakage air flow rate of air-tight door m3/s
1 2 3 4 5 10 15 20 25 30 35 40 45 50
0.52 0.73 0.89 1.03 1.15 1.63 2.00 2.31 2.58 2.83 3.06 3.27 3.46 3.65
0.015 0.022 0.027 0.031 0.035 0.049 0.060 0.069 0.077 0.085 0.092 0.098 0.104 0.110
0.0003 0.0004 0.0005 0.0006 0.0007 0.0010 0.0012 0.0014 0.0015 0.0017 0.0018 0.0020 0.0021 0.0022
Table 3.2 Leakage air flow rate test results of two installed air-tight doors.
Air-tight door #1 Air-tight door #2
Total gap length (mm)
Test pressure difference (Pa)
Leakage air flow rate (m3/s)
6060 6060
254.5 254.0
0.00178 0.00133
vection driving force caused by a temperature difference between two sides. On the other hand, a considerable pressure difference would not improve the protection level; however, it would lead to unfavorable problems, e.g., increasing the noise and the difficulty of opening the door. As can be concluded from Table 3.1, the leakage air velocity would be 1.63–2.0 m/s when the pressure difference is 10–15 Pa, which is the common practice in biosafety containment requirements. It would be definitely sufficient to overcome all the abovementioned negative factors, if we take biosafety cabinets as a comparison. It has been proved for years that a directed air flow higher than 0.5 m/s is capable of preventing aerosol from escaping through a containment boundary opening.
3.2. Consequences of open doors and people walking through The pressure difference’s protection and containment effect is immediately eliminated when the door is open, because the opening in the containment boundary is too big to maintain the necessary directed-air velocity in all gaps. Moreover, as can be seen in Table 3.1, when the door is closed, a small air-flow rate, 0.11 m3/s, would result in a leakage air velocity of 3.65 m/s and maintain a pressure difference of 50 Pa. However, if the door is open, the air velocity of the open door would be as low as 0.055 m/s, which obviously cannot contribute to the containment. Furthermore, the process of opening and closing the door with people walking through would lead to an unavoidable air exchange. The earliest research on the air exchange caused by the movement of doors in a clean controlled environment was conducted in the early 1960s,11 which reported that the air exchange rate was approximately 0.17 m3/s during the door-opening process. More recently,14 reported that the entrained air velocity caused by opening a door could be 0.15–0.30 m/s. Table 3.3 summarizes the visualized air-flow pattern results during the entire procedure when the tested door was opened and closed.13 A similar door opening and closing experiment can also be found in Wiseman’s work.17 All the studies introduced above are about the door-movement impact without any occupant
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X. Feng et al. / Journal of Biosafety and Biosecurity 1 (2019) 63–67 Table 3.3 Visualized air-flow pattern during door movement. Door type
Door opening direction
Pressure difference when the door is closed, Pa
Door movement stage
Observed air invasion
Swinging door
Outwardly
+15
Swinging door
Inwardly
+15
Swinging door
Outwardly
0
Sliding door
Horizontally sliding
+15
Before the door is open As the door opens Door is fully open As the door closes Before the door is open As the door opens Door is fully open As the door closes Before the door is open As the door opens Door is fully open As the door closes All stages
Invisible Quite small Weak Strong Invisible Strong Invisible Invisible Invisible Quite small Weak Strong Nearly invisible
passing through. According to an experimental study, the entrained air resulting from people moving in would be no higher than 0.14 m3/s.12 In other research, measures, including adjusting the supply/ exhaust air volume based on the dynamic monitoring of the door position, etc., were investigated to offset the changing ventilation requirements.18,19 However, for a containment biosafety area with more main lab rooms and corresponding auxiliary rooms, such a control strategy might lead to unfavorable periodic vibrations and unstable pressure gradients in the entire area. 3.3. Temperature impact Theoretically, the air would still be forced to flow from the room with higher pressure to its adjacent lower-pressure containment room, even when the door was fully opened. Unfortunately, a slight temperature difference among rooms served by the same heating, ventilation, and air-conditioning (HVAC) system exists in most cases. The temperature difference would lead to a change in air density. Convection flows and air exchanges would naturally and immediately occur. Air from the warmer side would flow to the colder side through the higher part of the opening, and a reverse flow would occur simultaneously in the lower part, even if the difference was as small as 0.1 °C. Fig. 3.1 illustrates an example of smoke-visualized air-flow pattern test results for a door opening when the temperature difference between two sides was just 0.1 °C. Fig. 3.2 illustrates the calculated increase of the polluted-air leakage rate occurring when a door opens and an occupant walks in as the time difference increases. According to the results, the air leakage rate would increase by nearly 40% when the temperature difference was only 1 °C. In BSL containment areas, the main laboratory room and the auxiliary rooms are served by the same HVAC system; therefore, the supply-air temperature would always be the same. While the thermal load distribution
Fig. 3.1. Results of a visualized air-flow pattern test conducted in Zhenjiang, China in May 2004. (The pressure number was tested when the door was closed.)15
Fig. 3.2. Theoretical calculation results of the pollutant leakage rate.
varies considerably in different function areas, major heat sources, e.g., laboratory equipment, refrigerators, working staff, and even experimental animals, are normally located in the main laboratory room, which inevitably causes a temperature difference among the rooms in the containment area. In a similar case study about an isolation ward,16 the temperature difference between the containment ward and the buffer room varied from 2 °C to 3.8 °C in a day. 4. Containment function of the buffer room Upon being introduced, proper pressure gradients and airtightness in wall panels and openable doors would definitely offer protection and containment in BSLs. Nevertheless, such efforts would immediately fail when the door was opened and a staff member passed through. Therefore, a highly ventilated buffer room would normally be the preferred containment choice. Although 100% containment still cannot be achieved, direct air communication is eliminated, and entrained air from the containment lab room is diluted during the extra buffer time. The exposure risk is consequently reduced. After the disastrous SARS outbreak in China, the quantitative contribution of buffer rooms to the total containment effect was investigated. It was proved that the following equation can be used to calculate the contribution of the buffer room and guide the lab layout design.15
Qk1 Vi ak1 bk ¼ Qk1 i¼1 ðnt=60Þ k2 i¼1 Q i ½e
ð4:1Þ
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X. Feng et al. / Journal of Biosafety and Biosecurity 1 (2019) 63–67
(a)
(b)
(c)
Fig. 4.1. Sampled sedimentation bacteria results. ((a): uncontrolled area outside the buffer room; (b): buffer room; and (c): containment room).
Table 4.1 Test results of the containment performance. Bacteria solution concentration: 8 1010 cfu/ml Nebulized solution: 6.12 ml Nebulizing time: 30 min Pressure gradient: Containment room: 10 Pa, buffer room: 5 Pa, uncontrolled area: 0 Pa Containment room Sampling port No. 1 2 CFU per plate 986 872 Buffer room Sampling port No. 6 7 CFU per plate 186 Uncontrolled area Sampling port No. 11 12 CFU per plate 43 30
where b is defined as the isolation coefficient, representing the target pollutant-concentration ratio of the main lab room and the uncontrolled area; k is the number of rooms in the route from the main lab room to the uncontrolled area; V is the volume of each room in the investigated route, m3; a is the mixing factor; Q is the entrained air volume as the door opens and people pass through, m3; n is the average air exchange rate of all the rooms, h1; t is the time that the people spend in each room in the route, min. Eq. (4.1) calculates how the polluted air in the main containment lab affects the uncontrolled area, in a scenario where one staff member walks continuously from the main lab to the uncontrolled area, following the designed exit route. For a normal laboratory-design layout and room size, the isolation coefficient b would be 42.9–64.4 for a simple layout with one lab room and one buffer room, and 2042–6892 for a layout with one lab room and four auxiliary rooms in the route. Another validation experiment was conducted in a simulated chamber including a containment room with a 27.6 m3 volume and an air-change rate of 12 h1, and a buffer room with a 6.25 m3 volume and an adjustable air-change rate. Settled plates were placed on the ground of the containment room, the buffer room, and outside the door of the buffer room. During the experiment procedure, a B. subtilis var niger solution with a concentration of 8 1010 cfu/ml was nebulized by a collision atomizer for 30 min to generate an airborne bacterial index in the simulated chamber room. Then, an experimental staff member with full PPE protection started from the containment room, opened the door to the buffer room, entered the buffer room and closed the door, opened the outside door of the buffer room, and exited the chamber. Fig. 4.1 illustrates the sampled sedimentation bacteria results. Quantitative test results can be found in Table 4.1. The experimental results show the contribution of the buffer room to the total containment effect during a dynamic condition. The tested isola-
3 823 8 206 13 40
4 688 9 160 14 46
5 720 10 164 15 45
Average 817.8 Average 179.0 Average 40.8
tion coefficient is close to the calculation results based on (4.1), with a slight difference that might be caused by the inevitable temperature difference, difference in people’s effective size, walking speeds, etc.
5. Conclusion In BSLs, aerosol-contaminant containment should be the result of all related parts working well together. From a mechanical engineering view, a good air-distribution strategy in the room, a reasonable and stable pressure gradient in the containment area, and a necessary buffer room comprise the integral parts for regulating the air flow and providing the necessary containment. An optimal air-distribution strategy would reduce the residence time of the bio-aerosol in the room and the exposure risk of the work staff. A pressure difference between adjacent rooms would provide sufficient isolation protection when the door is closed. Nevertheless, an unfavorable air exchange would occur when the door was open, due to door movement, people passing, or tiny temperature differences. A buffer room would therefore be necessary to offset the negative impact and maintain the containment effect.
Conflict of interest statement None. Acknowledgment This research is supported and financially funded by the National Key Research and Development Program of China (2016YFC1202201).
X. Feng et al. / Journal of Biosafety and Biosecurity 1 (2019) 63–67
References 1. World Health Organization, Laboratory biosafety manual (3rd edition), Geneva, 2004. 2. Sheldon K. Friedlander, smoke, dust and haze, fundamentals of aerosol dynamics. second ed. New York: Oxford University Press; 2000. 3. Caporaloni M, Tampieri F, Trombetti F, Vittori O. Transfer of particles in nonisotropic air turbulence. J Atmos Sci. 1975;32:565–568. 4. Kuehn Thomas H. Predicting air flow patterns and particle contamination in clean rooms. J Aerosol Sci. 1988;19:1405–1408. 5. Ma Zonghu, Nan Guoliang, Yang Kun. Experimental investigation and numerical simulation of air distribution in the class III biosafety laboratory. Chin Med Equip J. 2006;27(9):32–37. 6. Yang Suh-Jenq, Fu Wu-Shung. A numerical investigation of effects of a moving operator on airflow patterns in a cleanroom. Build Environ. 2002;37:705–712. 7. Mayer D, Reiczigel J, Rubel F. A Lagrangian particle model to predict the airborne spread of foot-and-mouth disease virus. Atmos Environ. 2008;42:466–479. 8. Pitkin Andrea, Deen John, Dee Scott. Use of a production region model to assess the airborne spread of porcine reproductive and respiratory syndrome virus. Vet Microbiol. 2009;136:1–7. 9. Xu Zhonglin. Fundamentals of air cleaning technology and its application in cleanrooms. New York: Springer press; 2014. 10. Walker Iain S, Wilson David J, Sherman Max H. A comparison of the power law to quadratic formulations for air infiltration calculations. Energy Build. 1998;27:293–299.
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11. Prignon Martin, Van Moeseke Geoffrey. Factors influencing airtightness and airtightness predictive models: a literature review. Energy Build. 2017;146:87–97. 12. Wolf HW, Haris MM, Hall LB. Open operating room doors and Staphylococcus aureus. Hospitals. 1961;35:57–64. 13. Xu Zhonglin, Zhang Yizhao, Wang Qingqin, et al. Isolation principle of isolation wards. J HV&AC. 2006;36:1–7. 14. Honda S, Kita Y, Isono K, et al. Dynamic characteristics of the door opening and closing operation and transfer of airborne particles in a cleanroom at solid tablet manufacturing factories. Trans Soc Heat, Air-Conditioning Sanitary Eng Japan. 2004;95:63–71. 15. Xu Zhonglin. Dynamic isolation technologies in negative pressure isolation wards. Singapore: Springer nature; 2017. 16. Feng Xin, Xu Zhonglin, Zhang Yizhao. Influence of different combinations of air changes and cooling load in isolation wards and buffer rooms on temperature difference and isolation coefficient. J HV&AC. 2006;36:7–11. 17. Brian Wiseman. Room pressure for critical environment. ASHRAE J. 2003;45:34–39. 18. Sun Wei. Automatic room pressurization test technique and adaptive flow control strategy in cleanroom and controlled environments. ASHRAE Trans. 2005;111:23–34. 19. Ling Jihong, Xing Jincheng, Zhang Juerong, et al. Control of aerosol containment dispersion by air volume transfer in biosafety level 3 laboratory. J Tianjin Univ. 2009;42:523–527.
Contents lists available at ScienceDirect
Journal of Biosafety and Biosecurity journal homepage: www.elsevier.com/locate/jobb
Review Article
Aerosol containment by airflow in biosafety laboratories Xin Feng a,⇑, Yanguo Zhang a, Zhonglin Xu a, Donglin Song b, Guoqing Cao a, Lei Liang a a b
China Academy of Building Research Co., Ltd., 30 North 3rd Ring East Rd., Beijing 100013, China Wuhan Institute of Virology, China Academy of Science, 44 Xiao Hong Shan Middle District, Wu Han 430071, China
a r t i c l e
i n f o
Article history: Received 1 October 2018 Received in revised form 18 December 2018 Accepted 20 December 2018
Keywords: Aerosol Containment Biosafety
a b s t r a c t In this paper, the progress and research related to aerosol containment by airflow in containment environments, e.g., biosafety laboratories, are introduced from a mechanical engineering view. A good airdistribution strategy in the room, a reasonable and stable pressure gradient in the containment area, and a necessary buffer room comprise the integral parts for regulating the air flow and providing the necessary containment. An optimal air-distribution strategy would reduce the residence time of the bioaerosol in the lab room and lower the exposure risk of the work staff. The pressure difference between adjacent rooms provides sufficient isolation protection when the door is closed. Nevertheless, an unfavorable air exchange would occur when the door is open, owing to door movement, passing people, or a tiny temperature difference. A buffer room is therefore necessary to offset the negative impact and maintain the containment effect. Ó 2018 Published by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.
1. Introduction Biosafety laboratories (BSLs) are designed and operated as basic containment and protection measures for the working staff and environment surrounding research facilities dealing with highly infectious biomaterials and toxins. According to modern statistical information, over 80% of experimental activities could generate bio-aerosols. Therefore, aerosol contaminants, rather than gasphase contaminants, are the major exposure risks and control targets in BSLs to lower the occurrence of laboratory-associated infections (LAI) and the contamination of the surrounding environment. Current biosafety containment practices (BMBL1) recommend the use of primary containment equipment, e.g., biosafety cabinets, the proper use of personal protection equipment (PPE), highefficiency particulate air (HEPA) filtration of the exhaust air, and necessary sterilization procedures, as the basic requirements. A good building layout, mechanical system design, optimal airflow pattern in the lab room, reasonable air flow, and stable pressure gradient together comprise the integral physical containment parts in BSLs; they would reduce the infectious-agent residential time within the room, lower the exposure risk for the working staff, and protect the surrounding environment. In this paper, the progress and research related to aerosol containment by airflow, including the air-distribution strategy in the room, pressure-difference functions and malfunctions, and the func⇑ Corresponding author. E-mail address: [email protected] (X. Feng).
tion of buffer rooms are introduced from a mechanical engineering view. We hope it will provide useful and helpful information for the design, operation, maintenance, and updates related to BSLs. 2. Function of air-distribution strategy Because the size of bio-aerosols varies considerably from nano size (virus, DNA/RNA fractions, etc.) to micron size (bacteria) and supermicron size (fungi and pollens), their aerodynamic characteristics are remarkably different. The generally accepted opinion is that the behavior of submicron aerosols is mainly decided by diffusion, while the inertia effect dominates the movement of supermicron aerosols.2 The deposition velocity of particles larger than 10 lm would be around 0.1 m/s in a turbulent flow,3 making it much easier to settle to the ground and other surfaces. Small particles, on the other hand, mainly follow the stream of the air flow, although Brownian diffusion sometimes drives the particles to touch and collect on nearby surfaces during the random motion. Biological experimental activities, e.g., extraction of embryo culture fluid, mixing samples, centrifugation, ultrasonic crushing, or the unexpected breaking of bottles with bacteria liquid, would generate bio-aerosols mainly in the <5 lm range. Thus, it is critical to carefully design the airflow pattern in the containment room to reduce the exposure risk of the working staff to an acceptable level. The basic rule of airflow-pattern design says that the air should flow from the clean area, to the work staff, then to the potentially contaminated area, and then be exhausted. Thus, mechanical engi-
https://doi.org/10.1016/j.jobb.2018.12.009 2588-9338/Ó 2018 Published by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.
64
X. Feng et al. / Journal of Biosafety and Biosecurity 1 (2019) 63–67
neers should plan the working area in the room, the major moving route, the location of air inlets, outlets, experimental equipment, and furniture, based on the rule. The actual regulation of the airflow pattern and aerosol transfer is much more complicated, although the rule is quite simple. The air-change rate in the room, the character, number, and location of the air inlets and outlets, the location and size of all impeding furniture, and the air condition, including the velocity, temperature, turbulence intensity, etc., are all important factors that should be taken into consideration. Therefore, since the 1980s, computational fluid dynamics (CFD) simulation has been a powerful design and research tool, together with the appearance and development of computers. In the early stages of CFD, the computational capacity could only provide hundreds of control volumes and could only be used to predict the air flow and aerosol transfer in small areas, such as a clean bench.4 CFD is one of the most important computer-aided investigation tools to benefit from the tremendous development of computer capacity in the past three decades. More recently, a complicated air-supply diffuser and the movement of the operator in the room were simulated, and their performance and impact on indoor airflow patterns were investigated.5,6 In addition to BSLs, CFD tools were used to investigate the spread and prediction of infectious agents in large terrains and contribute to disease control.7,8 Now, CFD is widely used as a popular design tool for optimizing design layouts and developing better air-distribution schemes. 3. Pressure-difference functions and malfunctions 3.1. Pressure-difference function Although the opinion is generally accepted that biomaterials should be handled in a relatively airtight environment to avoid unexpected aerosol propagation, absolute airtightness is not necessary for BSLs. In most cases, the pressure difference between adjacent rooms will lead to a stable and direction-controlled airflow in all gaps and leaks, especially in the gaps of all openable facilities, e.g., doors and transfer boxes. The following equation14 provides a simplified relationship between the pressure difference and the corresponding directed air velocity in a gap. Eq. (3.1), below, is normally used in engineering applications; more complicated calculation methods can be found in other works.9,10
sffiffiffiffiffiffiffiffiffi Q 2 DP V¼ ¼l F q
ð3:1Þ
where V is the air velocity in the gap, m/s; Q is the air-flow rate, m3/s; F is the area of the gap, m2; and l is the flow coefficient, which is normally 0.3–0.5 for small gaps and leakages, and we use 0.4 in this case. DP is the pressure difference, Pa. q is the air density, 1.2 kg/m3 in normal temperature and atmospheric pressure conditions. Thus, for a normal non-air-tight door with average gap dimensions of 6000 mm 5 mm, and an air-tight door with average gap dimensions of 6000 mm 0.11 mm, Table 3.1 summarizes the corresponding leakage air velocity and air-flow rate under different pressure differences. Table 3.2 illustrates the test results of two installed air-tight doors. The function of the pressure difference is to maintain a stable directed air flow into any potential gap or leak in the building boundary of the containment area. The size of the pressure difference is therefore quite critical. On one hand, the generated leakage air velocity in the gap needs to be sufficient to resist negative impacts, e.g., instantaneous air flows caused by the movement of staff and equipment, turbulent air fluctuation, and the natural con-
Table 3.1 Leakage air velocity and air flow rate under different door-pressure differences. Pressure difference Pa
Leakage air velocity m/s
Leakage air flow rate of non-air-tight door m3/s
Leakage air flow rate of air-tight door m3/s
1 2 3 4 5 10 15 20 25 30 35 40 45 50
0.52 0.73 0.89 1.03 1.15 1.63 2.00 2.31 2.58 2.83 3.06 3.27 3.46 3.65
0.015 0.022 0.027 0.031 0.035 0.049 0.060 0.069 0.077 0.085 0.092 0.098 0.104 0.110
0.0003 0.0004 0.0005 0.0006 0.0007 0.0010 0.0012 0.0014 0.0015 0.0017 0.0018 0.0020 0.0021 0.0022
Table 3.2 Leakage air flow rate test results of two installed air-tight doors.
Air-tight door #1 Air-tight door #2
Total gap length (mm)
Test pressure difference (Pa)
Leakage air flow rate (m3/s)
6060 6060
254.5 254.0
0.00178 0.00133
vection driving force caused by a temperature difference between two sides. On the other hand, a considerable pressure difference would not improve the protection level; however, it would lead to unfavorable problems, e.g., increasing the noise and the difficulty of opening the door. As can be concluded from Table 3.1, the leakage air velocity would be 1.63–2.0 m/s when the pressure difference is 10–15 Pa, which is the common practice in biosafety containment requirements. It would be definitely sufficient to overcome all the abovementioned negative factors, if we take biosafety cabinets as a comparison. It has been proved for years that a directed air flow higher than 0.5 m/s is capable of preventing aerosol from escaping through a containment boundary opening.
3.2. Consequences of open doors and people walking through The pressure difference’s protection and containment effect is immediately eliminated when the door is open, because the opening in the containment boundary is too big to maintain the necessary directed-air velocity in all gaps. Moreover, as can be seen in Table 3.1, when the door is closed, a small air-flow rate, 0.11 m3/s, would result in a leakage air velocity of 3.65 m/s and maintain a pressure difference of 50 Pa. However, if the door is open, the air velocity of the open door would be as low as 0.055 m/s, which obviously cannot contribute to the containment. Furthermore, the process of opening and closing the door with people walking through would lead to an unavoidable air exchange. The earliest research on the air exchange caused by the movement of doors in a clean controlled environment was conducted in the early 1960s,11 which reported that the air exchange rate was approximately 0.17 m3/s during the door-opening process. More recently,14 reported that the entrained air velocity caused by opening a door could be 0.15–0.30 m/s. Table 3.3 summarizes the visualized air-flow pattern results during the entire procedure when the tested door was opened and closed.13 A similar door opening and closing experiment can also be found in Wiseman’s work.17 All the studies introduced above are about the door-movement impact without any occupant
65
X. Feng et al. / Journal of Biosafety and Biosecurity 1 (2019) 63–67 Table 3.3 Visualized air-flow pattern during door movement. Door type
Door opening direction
Pressure difference when the door is closed, Pa
Door movement stage
Observed air invasion
Swinging door
Outwardly
+15
Swinging door
Inwardly
+15
Swinging door
Outwardly
0
Sliding door
Horizontally sliding
+15
Before the door is open As the door opens Door is fully open As the door closes Before the door is open As the door opens Door is fully open As the door closes Before the door is open As the door opens Door is fully open As the door closes All stages
Invisible Quite small Weak Strong Invisible Strong Invisible Invisible Invisible Quite small Weak Strong Nearly invisible
passing through. According to an experimental study, the entrained air resulting from people moving in would be no higher than 0.14 m3/s.12 In other research, measures, including adjusting the supply/ exhaust air volume based on the dynamic monitoring of the door position, etc., were investigated to offset the changing ventilation requirements.18,19 However, for a containment biosafety area with more main lab rooms and corresponding auxiliary rooms, such a control strategy might lead to unfavorable periodic vibrations and unstable pressure gradients in the entire area. 3.3. Temperature impact Theoretically, the air would still be forced to flow from the room with higher pressure to its adjacent lower-pressure containment room, even when the door was fully opened. Unfortunately, a slight temperature difference among rooms served by the same heating, ventilation, and air-conditioning (HVAC) system exists in most cases. The temperature difference would lead to a change in air density. Convection flows and air exchanges would naturally and immediately occur. Air from the warmer side would flow to the colder side through the higher part of the opening, and a reverse flow would occur simultaneously in the lower part, even if the difference was as small as 0.1 °C. Fig. 3.1 illustrates an example of smoke-visualized air-flow pattern test results for a door opening when the temperature difference between two sides was just 0.1 °C. Fig. 3.2 illustrates the calculated increase of the polluted-air leakage rate occurring when a door opens and an occupant walks in as the time difference increases. According to the results, the air leakage rate would increase by nearly 40% when the temperature difference was only 1 °C. In BSL containment areas, the main laboratory room and the auxiliary rooms are served by the same HVAC system; therefore, the supply-air temperature would always be the same. While the thermal load distribution
Fig. 3.1. Results of a visualized air-flow pattern test conducted in Zhenjiang, China in May 2004. (The pressure number was tested when the door was closed.)15
Fig. 3.2. Theoretical calculation results of the pollutant leakage rate.
varies considerably in different function areas, major heat sources, e.g., laboratory equipment, refrigerators, working staff, and even experimental animals, are normally located in the main laboratory room, which inevitably causes a temperature difference among the rooms in the containment area. In a similar case study about an isolation ward,16 the temperature difference between the containment ward and the buffer room varied from 2 °C to 3.8 °C in a day. 4. Containment function of the buffer room Upon being introduced, proper pressure gradients and airtightness in wall panels and openable doors would definitely offer protection and containment in BSLs. Nevertheless, such efforts would immediately fail when the door was opened and a staff member passed through. Therefore, a highly ventilated buffer room would normally be the preferred containment choice. Although 100% containment still cannot be achieved, direct air communication is eliminated, and entrained air from the containment lab room is diluted during the extra buffer time. The exposure risk is consequently reduced. After the disastrous SARS outbreak in China, the quantitative contribution of buffer rooms to the total containment effect was investigated. It was proved that the following equation can be used to calculate the contribution of the buffer room and guide the lab layout design.15
Qk1 Vi ak1 bk ¼ Qk1 i¼1 ðnt=60Þ k2 i¼1 Q i ½e
ð4:1Þ
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(a)
(b)
(c)
Fig. 4.1. Sampled sedimentation bacteria results. ((a): uncontrolled area outside the buffer room; (b): buffer room; and (c): containment room).
Table 4.1 Test results of the containment performance. Bacteria solution concentration: 8 1010 cfu/ml Nebulized solution: 6.12 ml Nebulizing time: 30 min Pressure gradient: Containment room: 10 Pa, buffer room: 5 Pa, uncontrolled area: 0 Pa Containment room Sampling port No. 1 2 CFU per plate 986 872 Buffer room Sampling port No. 6 7 CFU per plate 186 Uncontrolled area Sampling port No. 11 12 CFU per plate 43 30
where b is defined as the isolation coefficient, representing the target pollutant-concentration ratio of the main lab room and the uncontrolled area; k is the number of rooms in the route from the main lab room to the uncontrolled area; V is the volume of each room in the investigated route, m3; a is the mixing factor; Q is the entrained air volume as the door opens and people pass through, m3; n is the average air exchange rate of all the rooms, h1; t is the time that the people spend in each room in the route, min. Eq. (4.1) calculates how the polluted air in the main containment lab affects the uncontrolled area, in a scenario where one staff member walks continuously from the main lab to the uncontrolled area, following the designed exit route. For a normal laboratory-design layout and room size, the isolation coefficient b would be 42.9–64.4 for a simple layout with one lab room and one buffer room, and 2042–6892 for a layout with one lab room and four auxiliary rooms in the route. Another validation experiment was conducted in a simulated chamber including a containment room with a 27.6 m3 volume and an air-change rate of 12 h1, and a buffer room with a 6.25 m3 volume and an adjustable air-change rate. Settled plates were placed on the ground of the containment room, the buffer room, and outside the door of the buffer room. During the experiment procedure, a B. subtilis var niger solution with a concentration of 8 1010 cfu/ml was nebulized by a collision atomizer for 30 min to generate an airborne bacterial index in the simulated chamber room. Then, an experimental staff member with full PPE protection started from the containment room, opened the door to the buffer room, entered the buffer room and closed the door, opened the outside door of the buffer room, and exited the chamber. Fig. 4.1 illustrates the sampled sedimentation bacteria results. Quantitative test results can be found in Table 4.1. The experimental results show the contribution of the buffer room to the total containment effect during a dynamic condition. The tested isola-
3 823 8 206 13 40
4 688 9 160 14 46
5 720 10 164 15 45
Average 817.8 Average 179.0 Average 40.8
tion coefficient is close to the calculation results based on (4.1), with a slight difference that might be caused by the inevitable temperature difference, difference in people’s effective size, walking speeds, etc.
5. Conclusion In BSLs, aerosol-contaminant containment should be the result of all related parts working well together. From a mechanical engineering view, a good air-distribution strategy in the room, a reasonable and stable pressure gradient in the containment area, and a necessary buffer room comprise the integral parts for regulating the air flow and providing the necessary containment. An optimal air-distribution strategy would reduce the residence time of the bio-aerosol in the room and the exposure risk of the work staff. A pressure difference between adjacent rooms would provide sufficient isolation protection when the door is closed. Nevertheless, an unfavorable air exchange would occur when the door was open, due to door movement, people passing, or tiny temperature differences. A buffer room would therefore be necessary to offset the negative impact and maintain the containment effect.
Conflict of interest statement None. Acknowledgment This research is supported and financially funded by the National Key Research and Development Program of China (2016YFC1202201).
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