Laminar Flow Hoods and Biological Safety Cabinets

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Laminar Flow Hoods

Please check out our Laminar Flow Hood section for more information or to find manufacturers that sell these products.

Laminar flow hoods fall in to two basic categories: biological safety cabinets (or biosafety cabinets) and laminar flow clean benches. These categories differ in the level of protection provided to the user, the sample, and the environment. Laminar flow hoods use high-efficiency particulate air (HEPA) filters to sterilize and remove particulates from the air. The right choice of laminar flow hood will depend on the use for the hood, the requirements for sterility, and the potential harm to both the user and the environment.

Laminar flow clean benches: basic specimen protection

A laminar flow clean bench provides a space to work with a product or specimen where it will be protected from contamination by particulates such as microorganisms. This is accomplished by the laminar flow of clean air from a HEPA filter, which is blown across the workspace and out toward the user and the lab. Thus the basic laminar flow clean bench provides no protection for the user from chemically hazardous or infectious materials, including particulate or volatile hazards, and aerosols. Such protection necessitates a biological safety cabinet (discussed below).

Laminar flow clean benches come in two types, vertical and horizontal, and offer different advantages depending on how they will be used. A vertical laminar flow clean bench forces the HEPA-filtered air down across the workspace, and is useful for parts or device assembly, electronics inspection, housing processing equipment, drug preparation, or preparing media, for example. The workspace of a horizontal laminar flow clean bench is bathed in a HEPA-filtered horizontal laminar airflow, and is often used for clinical or pharmaceutical applications, or whenever a sterile, particulate-free environment is needed.

Paradoxically, when an object (such as a container holding your specimen) is placed inside the laminar flow hood, it triggers changes in the airflow that can cause contamination of the laminar flow clean bench workspace. Even the simple acts of inserting and withdrawing one’s hands into the laminar airflow (such as to manipulate the specimen or even just place the specimen on the workspace) disturbs the airflow. Termed obstructions, these events cause turbulence in the airflow, which results in dirty air entering from the lab (known as backwash), and particulate counts to rise sharply. Some laminar flow hoods have special designs that minimize backwash. For example, slots along the edges of a hood’s workspace can act as high velocity air returns, reducing the introduction of dirty air into the laminar flow hood during obstructions.

Class I and II biological safety cabinets: protecting you, too

Biological safety cabinets (biosafety cabinets) must be used when additional protection of the user and the environment is also required. Biological safety cabinets come in several classes (I, II, and III) that offer varying levels of protection from hazardous or contagious materials. A Class I biosafety cabinet protects the user and the environment, but not the product as air is pulled into the cabinet and exhausted (usually out the top) out away from the user. For all classes of biological safety cabinet, the air is then HEPA-filtered before being released as exhaust. Class I biosafety cabinets are not as common in today’s market.

Class II biological safety cabinets fall into four subtypes (A1, A2, B1, B2), all of which protect the product or specimen, the user, and the environment from contamination. This is especially crucial for applications that require sterile or particulate-free conditions, such as cell culture experiments, drug preparations, or in toxicology when working with fumes and gases is required. All types of Class II biological safety cabinet have HEPA filters at their air supply and exhaust points. However, the Class II subtypes differ in the amount of air that is recirculated within the cabinet versus exhausted outward (usually through the top).

For the A1 and A2 subtypes of Class II biological safety cabinets, 70% of the airflow is recirculated, while 30% is exhausted into the lab. The A2 subtype has a higher air velocity than the A1 subtype for greater protection (100FPM intake versus 75FPM intake, respectively). One limitation of Type A biosafety cabinets is that they do not protect the user from vapors and gases. Therefore, they are best used for applications that require aseptic conditions without the need to work with hazardous or volatile chemicals. However, when connected to a building’s exhaust system (through an optional canopy), Class II Type A2 biological safety cabinets can be used for applications that require trace amounts of volatile chemicals and/or radionuclides.

For the B1 and B2 subtypes of Class II biological safety cabinets, either 70% (B1) or 100% (B2) of the airflow is exhausted out of the cabinet. Therefore, Type B biosafety cabinets provide protection for the user when working with vapors and gases where aseptic conditions are required. These cabinets must be connected to the building’s exhaust system. Although larger quantities of volatile chemicals and/or radionuclides can be used in these hoods, limit restrictions may still apply. In addition, flammable or explosive materials should not be used in biological safety cabinets as they are not true chemical fume hoods.

Class III biological safety cabinets for maximum protection

Class III biological safety cabinets, also known as glove boxes or barrier isolators, provide maximum protection for work in biosafety level 4 containment labs. A class III biosafety cabinet is crucial for working with any biosafety level 4 agents, or other dangerous materials, such as aerosols of pathogens or toxins. These biological safety cabinets are hermetically sealed glove boxes with access through a dunk tank or an adjacent, double-door chamber that allows for decontamination. Typically a class III biosafety cabinet has two HEPA filters in the exhaust system for additional environmental protection. Manipulations within the hermetically-sealed chamber of the glove box are performed with built-in arm-length rubber gloves. Class III biosafety cabinets with carbon filtration systems are also available for working with hazardous chemicals.

Please check out our Laminar Flow Hood section for more information or to find manufacturers that sell these products.

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