I work as a safety engineer/design engineer - while I don't particularly get involved with LEV systems/ductwork - it does seem rather unwise to tie in the general room ventilation with the LEV ductwork. It would seem to be obvious that an LEV/fume cupboard system is being installed to control the risk of emissions from the pharma materials entering the general room atmosphere and hence being inhaled by people in the room. An obvious problem/failure mode would seem to be back flow of extracted air from the fume cupboard/cabinets back along the general ventilation ductwork. How would this be prevented? Fitting dampers/NRV valves in the system. Also, general holes in the ductwork - through age/corrosion etc - probably not highly possible, but nevertheless a credible failure mode. This would also possibly allow air leakage into the general work area. Design reconsideration required, I think. A simple HAZOP/HAZID study should flag up these issues.

@Ian - Thanks for your response Ian, those are my primary concerns and it's a common sense call really. I have some others issues related to proposed positioning of the cabinets; single door access to some rooms; T-connectors on the ductwork and positioning of the exhaust stacks. All these other concerns are well documented in the guidance notes but nothing I've seen covers my question about the ductwork.

It's a fine balance to get local exhaust ventilation working with general room extraction as the possiblity of blowback is quite high if not balanced correctly. I know that when this is put in it will work fine, but as soon as you start to add items, another fume cabinet or a dust extractor, then the balance will change and you could get blowback. Then, as Ian says, you start to add dampers and fans and other interim measures to combat the issue and to me, this has never been satisfactory. We had a system that added in a number of different elements into one system and we experienced blowback of the welding fumes. These were all industrial exhaust systems, not general ventilation. Eventually we had to dismantle the system and put independent welding fume exhausts in to avoid this issue. It would be easier to redesign as two systems at the outset as I am sure it will save money in the longer term.