Biosafety Cabinets & Clean Benches

When you walk into a research laboratory, there is a piece of equipment that is often referred to by many different names:  cell culture hood, tissue culture hood, laminar flow hood, PCR hood, clean bench, or biosafety cabinet.   An important thing to note, however, is that not all of these “hoods” are created equally; in fact, they have very different protective capabilities.   The common thread is that the equipment provides laminar air flow for a “clean” work area, but it is important to know that not all equipment provides additional personnel or environmental protection.

Below is a general guidance on the differences between two common pieces of lab equipment at UNH that both provide laminar flow of air; biological safety cabinets (BSC) and the laminar flow “clean bench”.  Both of these pieces have high efficiency particulate air (HEPA) filters, which control airborne particulate materials by removing the most penetrating particle size of 0.3 ?m with an efficiency of at least 99.97%.  The direction of the airflow out of the cabinet is very different, however, and therefore each piece of equipment is appropriate for specific types of work.  

Laminar Flow Clean Bench

The laminar flow “clean bench” discharges HEPA-filtered air from the back or top of the cabinet across the work surface and toward the user. These devices only provide product protection. They can be used for certain clean activities, such as the dust-free assembly of sterile equipment or electronic devices. Clean benches should never be used when handling cell culture materials, drug formulations, potentially infectious materials, or any other potentially hazardous materials. The worker will be exposed to the materials being manipulated on the clean bench potentially resulting in hypersensitivity, toxicity or infection depending on the materials being handled. Horizontal airflow “clean benches” must never be used as a substitute for a biological safety cabinet.

The horizontal laminar flow “clean bench” (A) front opening; (B) supply grille; (C) supply HEPA filter; (D) supply plenum; (E) blower.

The vertical laminar flow “clean bench” (A) front opening; (B) sash; (C) supply HEPA filter; (D) blower.

Types of work that can be done in a laminar flow “clean bench”:

• Work with non-hazardous materials where clean, particle-free air quality is required.
• Media plate preparation
• Plant tissue culture
• RNA/DNA
• Assembly of sterile equipment
• Assembly of electronic devices 

Hazardous chemicals, potential biohazards, radionuclides and any material that is a potential allergen cannot be used in this type of equipment.  

A clean bench must never be substituted for a biological safety cabinet.

Class II Biological Safety Cabinet

The Class II biological safety cabinet (BSC) comes in a variety of types.  All Class II BSC types provide personnel, environmental and product protection.  Airflow is drawn into the front grille of the cabinet, providing personnel protection.   The downward flow of HEPA-filtered air provides product protection by minimizing the chance of cross-contamination across the work surface of the cabinet.   Exhaust air is passed through a certified HEPA filter and is particulate-free, which provides environmental protection.  Depending on the type of Class II cabinet, exhaust air may be re-circulated to the laboratory (Types A1 and A2) or discharged from the building via a canopy or “thimble” connected to the building exhaust.   Exhaust air from Types B1 and B2 BSCs must be discharged directly to the outdoors via a hard connection.

HEPA filters are effective at trapping particulates and thus infectious agents but do not capture volatile chemicals or gases.  Only Type A2-exhausted or Types B1and B2 BSCs exhausting to the outside should be used when working with volatile, toxic chemicals, and amounts must be limited.

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.

Types of work that can be done in a biological safety cabinet:

• Work involving microorganisms assigned to biosafety levels 1, 2, and 3.
• Mammalian tissue culture
• Work with mammalian blood and tissues
• Manipulations of infectious or potentially infectious materials that may cause aerosols
• Formulation of drug compounds

Limited amounts of volatile chemicals in Types B1 and B2 ONLY

Biological safety cabinets must be used with good practices in order to provide protection to the worker.  Practices such as the following are recommended for optimum protection:

1. If the cabinet has been shut down, the blowers should be operated at least four minutes before beginning work to allow the cabinet to “purge.” This purge will remove any suspended particulates in the cabinet.
2. The work surface, the interior walls, interior surface of the window, and materials placed in the cabinet should be wiped with 70% ethanol, a 1:100 dilution of household bleach, or other appropriate disinfectant.  When bleach is used, a second wiping with sterile water is needed to remove the residual chlorine, which may eventually corrode stainless steel surfaces.
3. Place materials in the BSC before beginning work. This minimizes the number of air curtain disruptions.
4. Only the materials and equipment required for the immediate work should be placed in the BSC.
5. Arrange the workflow to be from “clean to dirty”.
6. Make sure that the sash is returned to its original position before work is initiated.
7. Move arms in and out slowly so as not to disrupt the air curtain and compromise the containment barrier.
8. Manipulation of materials should be delayed for approximately one minute after placing the hands/arms inside.
9. All operations should be performed on the work surface at least four inches in from the front grille.
10. Collect pipettes inside the cabinet to avoid frequent inward/outward movements.  Decontaminate all potentially contaminated materials inside the cabinet before removing them.
11. Limit other activities in the room (movements near the cabinet, walking traffic, open/closing room doors) as this may also disrupt the cabinet air barrier.

Proper maintenance of cabinets used for work at all biosafety levels cannot be over emphasized.   An active cabinet is a primary containment device.   A BSC must be routinely inspected and tested by trained personnel, following strict protocols, to verify that it is working properly.  Certification is performed annually, and whenever a cabinet is moved.

For additional information, contact the UNH Biological Safety Officer at 862-0197.

This information has been excerpted from the U.S. Department of Health and Human Services, Centers for Disease Control and Prevention and National Institutes of Health publication Biosafety in Microbiological and Biomedical Laboratories (BMBL), Fifth Edition (2009).  For more specific information, refer to Appendix A in the BMBL.