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Biological-safety Cabinets: Choosing the right class

Bio-safety Cabinets are divided into three broad classes – called I, II and III – based, among other things, on the level of protection they provide. This article explores features and functionalities that are unique to each of the classes, highlighting the key differences between them.

 Class I

 The first class of BSCs offers the most basic protection. While these cabinets ensure that laboratory personnel and the surrounding environment are protected from exposure to potentially hazardous substances, they do not protect the actual product being manipulated inside.

 The air going through the cabinet’s exhaust passes through HEPA (High Efficiency Particulate Arrestance) filters, and may be discharged outside the building or recirculated, depending on the type of work that is being performed. However, the air flowing into the cabinet does not necessarily pass through any such filter. Because this air may be unclean, Class I cabinets cannot be used for materials susceptible to contamination.

 Class I cabinets are mostly used to enclose equipment, such as mixers and blenders, and to do work that may generate aerosols, like tissue homogenisation and sonication. They can safely handle micro-organisms at Biosafety Levels 1- 3 (low- to moderate-risk micro-organisms). The cabinets usually have an open front, use negative pressure, and have a minimum face velocity of 0.38 metres per second (m/sec).

 Class II

 As indicated by the number, Class II cabinets offer mid-range protection. Like Class I cabinets, they can safely handle micro-organisms at Biosafety Levels 1- 3. However, a key difference is that air flowing inside and outside the cabinet passes through HEPA filters, and the air inside is kept moving in a downward, vertical, unidirectional manner.

 Class II cabinets make up the majority of BSCs. They are used widely in clinical, hospital, research and pharmaceutical laboratories. Many Class I cabinets are now being replaced by Class II models as they have greater functionality.

 Class II cabinets are further subdivided into four types - A1, A2, B1 and B2 - based on their construction, air flow velocities and patterns, and exhaust systems.

A1: Of the four types of Class II cabinet, this one offers the lowest level of protection. It has an inward airflow velocity of 0.38m/sec, may have positive pressure on the ducts and plena, and recirculates 70% of the air. This type of cabinet may be exhausted inside or outside the laboratory.

A2: An estimated 95% of all BSCs are A2 cabinets (previously known as B3 cabinets). They have an inward airflow velocity of 0.51m/sec, use negative pressure on the plena, recirculate 70% of the air, and are exhausted outside.

B1: These cabinets also have an inward airflow velocity of 0.51m/sec and may use negative pressure. However, for greater protection, they must be hard-ducted to an outside exhaust, and they only recirculate 30-40% of the air inside the cabinet.

B2: These cabinets provide the most protection out of the Class II BSCs. They are exhausted outside, and no air is recirculated at all. They have an inward airflow velocity of 0.51m/sec and use negative pressure on the ducts and plena. They ensure both biological and chemical containment.

Volatile chemicals can only be used in A2 and B1 cabinets in minute quantities. They can be used in B2 cabinets in slightly larger quantities. They cannot be used in A1 and Class I cabinets at all without structural modifications.

 Class III

 These cabinets provide maximum protection for personnel, the environment and the product. They are completely enclosed and gas-tight, and can handle micro-organisms at Biosafety Level 4 (high-risk micro-organisms, such as emerging diseases). All work is done using arm-length rubber gloves that are attached to the cabinet, and or half suits.

 The supply air is HEPA-filtered and exhaust air passes through either two HEPA filters or one and an incineration, before being discharged outside. The cabinets use negative pressure. They are connected to a double-doored autoclave and or chemical dunk-tank for the disinfection of all materials. Several Class III cabinets are often set up together as a contained system.

 The class of BSC needed in a given laboratory will be determined by the type of work being done and the level of protection required.

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