Primary Containment for Biohazards: Selection, Installation and Use of Biological Safety Cabinets, 2nd edition. U.S. Department of Health and Human Services Public Health Service, Centers for Disease Control and Prevention and National Institutes of Health, Se

Table 1. Selection of a Safety Cabinet Through Risk Assessment

Biological Risk Assessed Protection Provided BSC Class
Personnel Product Environmental
BSL 1-3 Yes No Yes I
BSL 1-3 Yes Yes Yes II (A1, A2, B1, B2)
BSL 4 Yes Yes Yes III
II – When used in suitroom with suit

Table 2. Comparison of Biosafety Cabinet Characteristics

BSC Class Face Velocity Airflow Pattern Applications
Nonvolatile Toxic Chemicals and Radionuclides Volatile Toxic Chemicals and Radionuclides
I 75 In at front through HEPA to the outside or into the room through HEPA (figure 2) Yes When exhausted outdoors 1,2
II, A1 75 70% recirculated to the cabinet work area through HEPA; 30% balance can be exhausted through HEPA back into the room or to outside through a canopy unit (figure 3) Yes
(minute amounts)
No
II, B1 100 30% recirculated, 70% exhausted. Exhaust cabinet air must pass through a dedicated duct to the outside through a HEPA filter (figures 5A, 5B) Yes Yes (minute amounts) 1,2
II, B2 100 No recirculation; total exhaust to the outside through a HEPA filter (figure 6) Yes Yes (small amounts) 1,2
II, A2 100 Similar to II, A1, but has 100 lfpm intake air velocity and plenums are under negative pressure to room; exhaust air can be ducted to outside through a canopy unit (figure 7) Yes When exhausted outdoors (Formerly “B3”)
(minute amounts) 1,2
III N/A Supply air is HEPA filtered. Exhaust air passes through two HEPA filters in series and is exhausted to the outside via a hard connection (figure 8) Yes Yes (small amounts) 1,2
  1. Installation may require a special duct to the outside, an in-line charcoal filter, and a spark proof (explosion proof) motor and other electrical components in the cabinet. Discharge of a Class I or Class II, Type A2 cabinet into a room should not occur if volatile chemicals are used.
  2. In no instance should the chemical concentration approach the lower explosion limits of the compounds.

Table 3. Field Performance Tests to be Applied to the Three Classes of Biological Safety Cabinets

Test Performed for Biosafety Cabinet
Class I Class II Class III
Primary Containment
Cabinet Integrity N/A A A (A1 Only)
HEPA Filter Leak Req Req Req
Downflow Velocity N/A Req N/A
Face Velocity Req Req N/A
Negative Pressure/Ventilation Rate B N/A Req
Airflow Smoke Patterns Req Req E/F
Alarms and Interlocks C, D C, D Req
Electrical Safety
Electrical Leakage, Etc. E, D E, D E, D
Ground Fault Interrupter D D D
Other
Lighting Intensity E E E
UV Intensity C, E C, E C, E
Noise Level E E E
Vibration E E E
  • Req: Required during certification.
  • A: Required for proper certification if the cabinet is new, has been moved or panels have been removed for maintenance.
  • B: If used with gloves.
  • C: If present.
  • D: Encouraged for electrical safety.
  • E: Optional, at the discretion of the user.
  • F: Used to determine air distribution within cabinet for clean to dirty procedures.
  • N/A: Not applicable.

Table 4. Reference for Applicable Containment Tests

Test Cabinet Type by Class
I II III
HEPA Filter Leak (F.5)¹ (F.5) (F.5)
Airflow Smoke Pattern No smoke shall reflux out of BSC once drawn in. (F.4) N/A
Cabinet Integrity N/A (F.6) [p.138-141]²
Face Velocity Open Front 75-125 lfpm
(F.3.3.3.3)
75 lfpm – type A1;
100 lfpm type A2,
B1& B2:
(F.3)
N/A
Face Velocity Gloves Ports / No Gloves 150 lfpm
(F.3.3.3.3)
N/A N/A
Water Gauge Pressure Glove Ports & Gloves N/A N/A (-0.5 “w.c.”)
[p.145]
Downflow Velocity N/A (F.2) N/A
  1. Parenthetical references are to the NSF/ANSI Standard 49 2004, letters and numerals indicate specific sections and subsections.
  2. Bracketed reference ([ ]) is to the Laboratory Safety Monogragh, Page numbers are indicated.

Figures

  • Figure 1
    Diagram of HEPA filter. These filters are typically constructed of continuous sheets of paper-thin filter medium, pleated to increase surface area, divided by aluminum separators, and affixed to a frame.
  • Figure 1A
    HEPA filters trap particles of 0.3µm with an efficiency of at least 99.97%.
  • Figure 2
    The Class I BSC. A. front opening; B. sash; C. exhaust HEPA filter; D. exhaust plenum. Note: The cabinet needs to be hard connected to the building exhaust system if toxic vapors are to be used. Provides personnel and environmental protection, but not product protection.
  • Figure 3
    The Class II, Type A1 BSC. A. front opening; B. sash; C. exhaust HEPA filter; D. supply HEPA filter; E. common plenum; F. blower.
  • Figure 4
    Canopy (thimble) unit for ducting a Class II, Type A BSC. A. balancing damper; B. flexible connector to exhaust system; C. cabinet exhaust HEPA filter housing; D. canopy unit; E. BSC. Note: There is a 1″ gap between the canopy unit (D) and the exhaust filter housing (C), through which room air is exhausted.
  • Figure 5A
    The Class II, Type B1 BSC (classic design). A. front opening; B. sash; C. exhaust HEPA filter; D. supply HEPA filter; E. negative pressure dedicated exhaust plenum; F. blower; G. additional HEPA filter for supply air. Note: The cabinet exhaust needs to be hard connected to the building exhaust system.
  • Figure 5B
    The Class II, Type B1 BSC (bench top design). A. front opening; B. sash; C. exhaust HEPA filter; D. supply plenum; E. supply HEPA filter; F. blower; G. negative pressure exhaust plenum. Note: The cabinet exhaust needs to be hard connected to the building exhaust system. Connection to the building exhaust system is required.
  • Figure 6
    The Class II, Type B2 BSC. A. front opening; B. sash; C. exhaust HEPA filter; D. supply HEPA filter; E. negative pressure exhaust plenum. Note: The carbon filter in the exhaust system is not shown. The cabinet needs to be hard connected to the building exhaust system.
  • Figure 7
    The tabletop model of a Class II, Type A2 BSC. A. front opening; B. sash; C. exhaust HEPA filter; D. supply HEPA filter; E. positive pressure common plenum; F. negative pressure plenum. The Class II Type A2 BSC is not equivalent to what was formerly called a Class II Type B3 unless it is connected to the laboratory exhaust system. Note: The A2 BSC should be canopy connected to the exhaust system.
  • Figure 8
    The Class III BSC. A. glove ports with O-ring for attaching arm-length gloves to cabinet; B. sash; C. exhaust HEPA filter; D. supply HEPA filter; E. double-ended autoclave or pass-through box. Note: A chemical dunk tank may be installed which would be located beneath the work surface of the BSC with access from above. The cabinet exhaust needs to be hard connected to an exhaust system where the fan is generally separate from the exhaust fans of the facility ventilation system. The exhaust air must be double HEPA-filtered or HEPA-filtered and incinerated.
  • Figure 9A
    The horizontal laminar flow “clean bench”. A. front opening; B. supply grille; C. supply HEPA filter; D. supply plenum; E. blower
  • Figure 9B
    The vertical laminar flow “clean bench”. A. front opening; B. sash; C. supply HEPA filter; D. blower. Note: Some vertical flow clean benches have recirculated air through front and/or rear perforated grilles.
  • Figure 10
    A modified containment cabinet or Class I BSC can be used for labeling infectious microorganisms with I125. A. arm holes; B. Lexan® hinged doors; C. exhaust charcoal filter; D. exhaust HEPA filter; E. filter housing with required connection to building exhaust.
  • Figure 11
    A typical layout for working “clean to dirty” within a Class II BSC. Clean cultures (left) can be inoculated (center); contaminated pipettes can be discarded in the shallow pan and other contaminated materials can be placed in the biohazard bag (right). This arrangement is reversed for left-handed persons.
  • Figure 12
    One method to protect a house vacuum system during aspiration of infectious fluids. The left suction flask (A) is used to collect the contaminated fluids into a suitable decontamination solution; the right flask serves as a fluid overflow collection vessel. An in-line HEPA filter (C) is used to protect the vacuum system (D) from aerosolized microorganisms.
  • Figure 13
    A bag-in-bag-out filter enclosure allows for the removal of the contaminated filter without worker exposure. A. filters; B. bags; C. safety straps; D. cinching straps; E. shock cord located in the mouth of the PVC bag restricts the bag around the second rib of the housing lip.

Acknowledgements

We gratefully acknowledge Baker Company, Filtration Group, Inc, Flanders Filters and Forma Scientific for the use of drawings reproduced herein. The Steering Committee also thanks Dave Stuart, Ph.D., for his technical review and editing assistance and Richard Green for his cover design.