How to Identify an Unknown or Poisonous Gas
Every time me and my colleagues enter this room if the meeting is over ~30 mins we leave with symptoms of fatigue, dizziness and a reduced mental ability, it takes about 1-3 hours to overcome these symptoms.


This sounds very much like Carbon Monoxide(CO) poisoning. It saturates your blood, which slowly suffocates you by preventing your blood from carrying oxygen (it binds better to your hemoglobin). It takes your body some time to recover from it.

You've covered a lot of sources, but does anyone else in the building have something that burns... well, any kind of gas? CO is colorless and odorless, and if someone has, say, an improper venting of their gas heater/stove/grill it could very well be dumping CO exhaust into your area.


  one subterranean level below our meeting room


If this is a garage, it could also be a potential CO source. Many older buildings are not sealed, and if this is an underground garage it could easily concentrate the CO and let it seep into your area.

Another potential source is a natural gas leak. The gas company adds Mercaptan to give it its distinctive smell, but Mercaptan is heavier than air, while natural gas can rise. If a natural gas leak were in the basement, it's possible the natural gas makes it up without the Mercaptan.

Buy a Carbon Monoxide detector

They're not terribly expensive and it can tell you if that's your problem. If you buy a combo smoke detector unit, you can just keep it as an extra smoke detector as well.