There are 3 classes of Biosafety cabinets (BSC's), cleverly called class 1, class 2, and class 3, so you think understanding them would be straightforward. It’s not because class 2 BSC’s are further divided into types. Class 2, Type A1, Type A2 (formerly type B3), Type B1, and Type B2.
The Class 1 BSC is designed to protect personnel and the environment, but not the product. It draws air from the room into the cabinet similar to a fume hood, but adds a HEPA filter on the exhaust to filter the air prior to being exhausted outside. You can see this does not protect the product because the air going across the product originates in the laboratory and is drawn into the hood across whatever is in front of the hood, generally the operator.
All Class 2 cabinets are designed for work involving microorganisms. They provide the personnel and environmental protections of the Class 1 but add HEPA filtered air across the product to protect it as well.
Class 2 Type A1 BSC’s draw air through the front grill, filter it through a HEPA filter, and then provide 75 linear feet per minute (lfpm) unidirectional, (virtually) particle free air to the product before re-circulating 70% of the air and exhausting 30%, either back into the room or to the outside, after passing it through a second HEPA filter.
Class 2 Type A2 BSC’s are similar to Type A1’s except they provide 100 lfpm of air and the plenums are under negative pressure to keep any of the air in the hood from leaking back into the lab.
Class 2 Type A1 and Class 2 Type A2 are designed for routine microbiological work where personnel, product and the environment all require some level of protection. In order to provide that protection, the airflow through the BSC is maintained at a constant flow rate regardless of any fluctuations in the building exhaust or supply pressures. This constant airflow is ensured by ducting the BSC through a canopy hood with a thimble connection. The air gap at the thimble connection allows filtered air from the BSC to escape back into the room or plenum to maintain the air balance in case air flow from the exhaust fan or the building pressure changes. Class 2 BSC’s are never suitable for use with solvents, because the air gap at the thimble connection has the potential to allow solvents to escape back into the laboratory and endanger the facility or its occupants.
An example of a thimble connection is shown here.
This arrangement also allows numerous type A1 or A2 BSC’s to be serviced with the same exhaust duct. As long as everything is designed, installed and balanced correctly, each BSC will be able to maintain the proper airflow regardless of the status of the other connected BSC’s. If the thimble connection were not installed and each had a hard duct, the danger of reversing the airflow in any one of the hoods would increase.
The thimble connection is not a panacea however. There should still be airflow monitors, interlocks and alarms installed to ensure that the operator is aware of any airflow issues and can take the appropriate steps to correct the situation.
Class 2 Type B1 BSC’s differ from Type A1, and Type A2 in that they are installed with a dedicated exhaust fan and duct and thus a canopy or thimble connection is not needed, and no air is returned to the room. They also provide 100 lfpm of air across the product. They re-circulate 30% of the air, and exhaust 70%.
Class 2 Type B2 BSC’s are similar to Class 2 Type B1 except no air is re-circulated. It is 100% exhausted through a HEPA filter. Because of this, they can be used with small amounts of solvents.
Lastly, Class 3 BSC’s are fully enclosed glove boxes and are used for the most virulent products. They filter the incoming air through a HEPA filter and send the exhaust air through 2 HEPA filters in series through a dedicated, hard connected duct.
For more information, you can look in the Biosafety in Microbiological and Biomedical Laboratories also known as the BMBL.
BSC’s are not the simplest things to understand if you don’t work with them a lot. If you have more questions contact us!