When exhausting a cabinet to atmosphere, three options are available for consideration:

  • Hard ducting with constant room extract
  • Hard ducting with room bypass
  • extract Thimble ducting

Whenever a Class II cabinet is exhausted to atmosphere, the  make–up air to the room to ensure the cabinet operates correctly needs to be borne in mind. This make-up air can be supplied via a simple passive flow system, whereby air is taken from the corridor or surrounding areas and drawn into the laboratory through door grilles, or it can be supplied by an active system whereby conditioned air is fed to the room via a ventilation system external to the room. Whatever system is chosen, the decision should be based on a risk assessment for the work to be carried out. The cost of conditioning the air and then losing it via the exhaust ducting when the extract is on can be considerable if large numbers of cabinets are involved. Passive systems are best for single use hard ducted cabinets (e.g. BSC Class I or BSC2 Type B2) in general laboratories where make-up air can be supplied through door grilles.

Thimble extracts are becoming more popular, as they are relatively simple to install and operate. A thimble will be extracting all the time and ideally will have a 10%-15% higher extract rate than the cabinet extract volume to ensure the room is kept at constant pressure as well and to account for any changes to the cabinet inflow speeds.