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The Class I BSC
The Class I BSC is essentially a biological version of the traditional laboratory fume hood (chemical hood), and provides personnel and environmental protection, but no product protection.
It is similar in air movement to a chemical fume hood, but has a HEPA filter in the exhaust system to protect the environment. In the Class I BSC, unfiltered room air is drawn across the work surface. Personnel protection is provided by this inward airflow as long as a minimum velocity of 75 linear feet per minute (lfpm) is maintained5 through the front opening. Because of the product protection provided by the Class II BSCs, general usage of the Class I BSC has declined. However, in many cases Class I BSCs are used specifically to enclose equipment (e.g., centrifuges, harvesting equipment or small fermenters), or procedures (e.g. cage dumping, aerating cultures or homogenizing tissues) with a potential to generate aerosols that may flow back into the room.
The Class I BSC can be hard-ducted to the building exhaust system, thimble-connected, or recirculated back into the room depending on use. If it is hard-ducted, the building exhaust fan provides the static pressure necessary to draw room air into the cabinet. Cabinet air is drawn through a HEPA filter as it enters the exhaust plenum. Sometimes a second HEPA filter is installed in the building exhaust system.
A steel panel with 8" arm holes to allow access to the work surface can be added to the Class I cabinet. The restricted opening results in increased inward air velocity, thereby increasing worker protection. For added safety, arm-length gloves can be attached to the panel. Makeup air is then drawn through an auxiliary air supply opening (which may contain a filter) and/or around a loose-fitting front panel. To permit access to the cabinet interior with the panel installed, a double-door air lock is attached on either side of the cabinet. Consideration must be given to the chemicals used in a BSC with HEPA filters as some chemicals can destroy the filter medium, housings and/or gaskets causing the loss of containment.
A. front opening
B. sash
C. exhaust HEPA
D. exhaust plenum
