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A biological safety cabinet (BSC)-also known as a biosafety cabinet or microbiological safety cabinet is an enclosed,ventilated laboratory workspace used for the safe handling of materials that are (or may be) contaminated with pathogens requiring regulated biosafety level.Several different types of biological safety cabinets exist,differentiated by the degree of biological containment they provide.
Purpose
The main purpose of the BSC is to protect laboratory workers and the surrounding environment from pathogens.All exhaust air is HEPA filtered as it leaves the biosafety cabinet, removing harmful bacteria and viruses.This is in contrast to laminar flow scrubbers, which blow unfiltered exhaust air towards the user and are not safe for handling pathogens.Most biological safety cabinets are also not safe to use as fume hoods.Likewise, a fume hood cannot provide the environmental protection that HEPA filtration in a BSC does.However, most types of BSCs have a secondary purpose of keeping the material inside (the "product").
The U.S. Centers for Disease Control and Prevention (CDC) divides biological safety cabinets into three categories:These levels and the types of biological safety cabinets within them are distinguished in two ways: the level of protection provided for personnel and the environment, and the level of protection provided for the product
Class I
Class I cabinets provide personnel and environmental protection, but not product protection.In fact, the inward flow of air can lead to sample contamination.Inward airflow is maintained at a minimum velocity of 75 ft/min (0.38 m/s).These biosafety cabinets are often used to enclose specific equipment (such as centrifuges) or procedures (such as aerated cultures) that may generate aerosols.These biological safety cabinets are ducted (connected to the building exhaust system) or non-ducted (recirculated filtered exhaust air back to the laboratory).
Class II
Class II cabinets provide both protections (sample and ambient) as make-up air is also HEPA filtered.There are five types: Type A1 (formerly A),Type A2 (formerly A/B3), Type B1,Type B2, and Type C1.The requirements for each type are defined by NSF International Standard 49,Standard 31, which in 2002 reclassified A/B3 cabinets (the latter type if connected to exhaust ducts) as Type A2,and added Type C1 in the 2016 standard.Approximately 90% of all installed biological safety cabinets are Type A2 cabinets.Principle of Operation Uses a motor-driven blower (fan) mounted in the cabinet to draw directed mass airflow around the user and into the ventilation grille protecting the operator.The air is then drawn under the countertop and returned to the top of the cabinet and through the HEPA filter. A train of HEPA-filtered sterile air is also blown down through the product and process to prevent contamination. Air is also exhausted through a HEPA filter and depending on the type of Class II BSC, the air is either recirculated back into the laboratory or drawn by exhaust fans out of the building through the ductwork.Type A1 cabinets (formerly Type A) have a minimum inflow velocity of 75 ft/min.The downwardly flowing air, which is considered to be contaminated, splits above the work surface (fume splitting of the biosafety cabinet) and mixes with the incoming air.This air is drawn into the rear of the cabinet through ductwork and blown into a positive pressure, contaminated plenum. Here, the air is either recirculated through the HEPA filter, back into the work area, or exhausted from the cabinet (also through the HEPA filter).The size of the HEPA filter and internal damper are used to balance these air volumes.This type is not safe for work with hazardous chemicals even with a "thimble" or canopy vent to avoid disturbing internal airflow.
Type A2 cabinets, formerly designated A/B3, have a minimum inflow velocity of 100 ft/min. A negative plenum surrounds all contaminated positive plenums. In other respects, the specifications are the same as those of the A1 type cabinet.Type B1 and B2 cabinets have a minimum inflow velocity of 100 ft/min and these cabinets must be hard-piped to the exhaust system rather than vented through a conduit connection.Their exhaust systems must also be dedicated (running one BSC per duct, one blower per blower). Compared to Type A1 and A2 cabinets,Type B biological safety cabinets use single-pass airflow (air that is not mixed and recirculated) to contain hazardous chemical vapors. The B1 cabinet splits the airflow so that the air behind the fume diverter is directed to the exhaust system, while the air between the operator and the fume diverter is mixed with the incoming air and recirculated as downdraft. The CDC recommends that hazardous chemical work be performed at the rear of the cabinet as exhaust air is drawn through the rear grill.This is complicated because the smoke split (labeled "rear of the cabinet") is an invisible line that extends the width of the cabinet (approximately 10-14 inches from the front grill) and loads with the internal HEPA filter particles drift.
Type B2 cabinets (also known as fully vented biological safety cabinets) are expensive to run because there is no air recirculation inside.This type is therefore primarily used in applications such as toxicology laboratories, where the ability to safely use hazardous chemicals is important.Furthermore, if the exhaust system of a type B1 or B2 cabinet fails, there is a risk of contaminated air entering the laboratory.To reduce this risk, these types of cabinets typically monitor exhaust flow, shut off supply fans and sound an alarm if exhaust flow is insufficient.
