Biological safety cabinets (BSCs) are a form of engineering control that is used throughout the microbiology laboratory. These hoods are a type of containment barrier that protects the laboratory worker from the aerosolized transmission of organisms. Any procedure that involves the risk of generating aerosols should be handled in a BSC. Samples sent to microbiology laboratory should also be handled within a BSC. There are three types of BSC: Class I, Class II, and Class III.
- In Class I biological safety cabinet, it uses an exhaust fan to move air inward through the open front. The air is circulated within the safety hood, passing through a high-efficiency particulate air (HEPA) filter before reaching the environment outside the hood. This type of safety cabinets will provide personnel and environmental protection, but not product protection.
- In Class II biological safety cabinet, the air is pulled inward and downward by a blower and passed up through the airflow plenum where it passes through a HEPA filter before reaching the work surface. A percentage of the remaining air is HEPA filtered before reaching the environment. This type of safety cabinets will provide personnel, environment and product protection. The main point to be noted is that HEPA filters are effective for trapping particulates and infectious agents, these filters will not capture volatile chemicals or gases.
- In Class III biological safety cabinet, there is totally enclosed ventilated cabinet of gas-tight construction. It is generally used for handling highly infectious microorganisms or materials and provides the highest level of personal protection. The closed front contains attached gloves for manipulation on the work surface. When in use, the Class III cabinet is maintained through negative air pressure. Supply air is drawn into the cabinet through HEPA filters. The cabinet exhaust air is filtered by two HEPA filters, installed in series, before discharge outside of the facility. The exhaust fan for the Class III cabinet is generally separate from the exhaust fans of the facility’s ventilation system.
The most common type of BSC used in a microbiology laboratory is the class II. There are four types of Class II biological cabinets:
- Class II, type A: This type of cabinet does not need to be vented, which makes it suitable for use in laboratory rooms which cannot be ducted. This cabinet is acceptable for use of low to moderate risk agents in the absence of volatile toxic chemicals and volatile radioisotopes.
- Class II, type B1: This type of cabinet must be vented, with 30% of the air exhausted from the cabinet while 70% is recirculated back into the room. This cabinet may be used with etiologic agents treated with minute quantities of toxic chemicals and trace amounts of radioisotopes required as an adjunct to microbiological studies if work is done in the directly exhausted portion of the cabinet, or if the chemicals will not interfere with the work when recirculated in the downflow air.
- Class II, type B2: This type of cabinet must be totally exhausted, with100% of the air exhausted through a dedicated duct. This cabinet may be used with etiologic agents treated with toxic chemicals or radioisotopes required as an adjunct to microbiological studies.
- Class II, type B3: this must be vented. 70% of the air is exhausted from the cabinet while 30% is recirculated. This cabinet may be used with etiologic agents treated with minute quantities of toxic chemicals and trace quantities of radioisotopes that will not interfere with work if recirculated in the downflow air.
Points to follow for proper use of Biological Safety Cabinet
- Be aware of the functioning and limitations of the unit. You and your specimen are being protected solely by an air curtain barrier, and anything that disrupts this curtain threatens your safety. Ensure that the cabinet has been inspected within the last year and that the air pressure readings across the HEPA filter are within specifications.
- Plan your work, anticipating the order of events.
- Turn on the incandescent lights and the blower fan. Be sure the UV light is off. Wait 15 to 30 minutes to ensure satisfactory establishment of the air curtain.
- Wash your hands, and then put on gown and gloves. Wear mask or other personal protective equipment as appropriate.
- Decontaminate the interior work surfaces by cleansing thoroughly with 70% ethanol.
- Organize all necessary work materials and place them in the cabinet. Do not place any extra items in the cabinet. Ensure that both the front intake grill and the rear-wall or floor exhaust grills are unobstructed.
- Segregate clean and contaminated items and place them to minimize subsequent movement with the BSC. The discarding jar should be near the rear of the cabinet but not obstructing the exhaust grill.
- Do not use open flames in the unit. Instead, use disposable supplies or a microburner/ incinerator.
- All arm movements into and within the cabinet should be slow and deliberate so as to minimize disruption of the air curtain.
- Allow adequate time at the conclusion of movement for the air curtain to reestablish itself.
- After the work is done, remove all nonpermanent items from the BSC and allow the cabinet fans to continue running for at least 30 minutes to ensure thorough filtering of the inside air (assuming the fans are not left on permanently). Turn on the UV lights to disinfect the interior of the cabinet.
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