Wouldn't you need something like 10% or less in the air? I have no idea what systems airliners use, just thought that it was the halon that did the suffocating, I guess it's the resulting CO2.
You are right about the percentage. Computer rooms, etc. usually get a 5 or 6% concentration by volume and it is allowed to "soak" for 15 minutes.
Aircraft engines never really build a concentration as the Halon is discharged into the airflow at the front of the engine. I've not had much to do with civilian aircraft and learguy is probably 100% right on about those. I have dealt with C-141, C-130, and C-5 systems and there are two bottles on each of those engines. At least there was in the 70s.
Even Dupont, the inventor of Halon, doesn't know, exactly, how Halon does what it does. It is a knockoff of Freon, dichlorodifloromethane, and can be used as a refrigerant. I know that you can't even get a lighter to spark in a Halon environment. Also, at temperatures above 750 degrees Faranheit Halon turns into Phosgene, the "mustard gas" used by the Germans in WWI. In that way I guess it can be toxic, but in normal fire situations it is totally safe and doesn't create any CO2 and is not corrosive. It's a darn sight better than dry chemical for car engine fires.