Does your hood really need to be ducted?


Many biological safety cabinets and chemo hoods used in hospitals today are ducted to the outside. This is the only way to protect the user from many non-particulate hazards like chemical fumes.

Venting of the unit’s exhaust to outside the building can make solving airflow problems a lot harder for you. A ducted hood with a problem involves people from different areas of the facility. It’s like a computer problem, with the hardware people blaming the software people, the software people blaming the hardware, and you stuck in the middle.

If your hood won’t come on because of insufficient exhaust or your certifier can’t certify it because the exhaust is too low, the hardest battle can be getting everyone to understand. The certifier says it’s too low, the facilities person says it’s more than enough and you just want to get your hood certified. Controlled Environment Testing Association (CETA) has 2 application guides that will provide helpful hints on understanding and dealing with these and other kinds of problems.

  • CAG-007: Exhaust System Requirements of Class II Biosafety Cabinets
  • CAG-010: Application Guide for Informational Notes to Meet the NSF/ANSI 49:2010a Standard Requirements

LTS can provide a copy of these 2 documents upon request.

There are three types of Biological Safety Cabinets. A Class I is basically a fume hood with a filter. You can’t use large amounts of chemicals in a Class I because they can eat through the filter. Class I cabinets are great for personal protection, but offer no product protection. A Class III cabinet is a ventilated glovebox. Air is filtered in and out and you use gloves mounted in glove ports to work with materials inside the hood.

The five types of Class II cabinets, A1, A2, B1, B2, and C1 are what most of us refer to when we think of a chemo hood or biohood. You reach in through an 8 or 10 high opening below a window. Air is drawn into the hood through that opening like a fume hood. Unlike a Class I cabinet, filtered air is also blown onto the work area from above, so your product is protected from outside contaminates. (Another similar type of equipment is a laminar air flow workstation which blows filtered air over the work space. Since you get product protection but no personnel protection, this isn’t considered a safety cabinet.)

Not all Class II cabinets need to be connected to your building’s exhaust. The Class II types A1 and A2 cabinets do not have to be connected to an exhaust duct to function properly. However, if they are connected to an external exhaust they must be canopy(thimble) connected. The other three types – B1 (sometimes called an NCI hood after the National Cancer Institute), B2 (commonly called a Total Exhaust hood), and the newest C1 all need to be connected to an external exhaust system to function.

If you have the A1 or A2 type, it may be possible to solve your exhaust problem by allowing the unit to vent to the room. If you are dealing with only particulate hazards, the cabinet’s filters should be able to capture the hazard. If those particulates might offgas or if you are using non-particulate hazards like low amounts of ether or other gases, the cabinet’s exhaust may need to be ducted outside. Your facility’s safety officer can help determine this with you.