Oxygen Masks --- and
Just to clarify - I hope - the different types of oxygen masks that exist - at least in a First Aid/Medical situation {that is excluding such as Scuba equipment, fireman Breathing Apparatus, and such as maybe seen on a flight deck}
Normal air (at seal level) contains 20-21 percent oxygen.
There are basically two types of equipment, for patients who are breathing and those not breathing.
For those breathing, there are generally speaking 3 types:
1) The Nasal Canular (not really a mask, but a tube with two short outlets that insert into the nostrils). Depending on the flow rate (generally 1 to 4 litres a minute) this delivers the patient an oxygen percentage of between 22 and 44 percent.
2) The Face Mask. This fits over the nose and mouth, usually secured by elastic. This mask usually has perforations to the outside air, and again depending on the flow rate selected, delivers between 25 and 45 percent oxygen.
3) The Non Rebreathing mask, Similar to the Face Mask, except that there are one-way valves to the outside air. These generally will have a reservoir - a thin plastic bag - fed with pure oxygen. This mask can deliver upto around 90% oxy.
The big one is the BVM (Bag Valve Mask). For non-breathing (thus unconcious) patients. This mask has a (if used properly) good seal over nose and mouth. Then there is a tough bag that is squeezed by hand to force oxygen into the lungs. Attached to that is usually a reservoir bag too. In this scenario, oxygen flow would be set to max (usually 15 litres/minute)
If you're using a BVM, you're also in a full CPR situation and giving chest compressions too - the recipient isnt flying anything...
The OP and others havn't given a detailed desription of the oxy kit used, but as the pilots continued flying, it COULD NOT have been BVM. As a +bag+ was mentioned, probably a non-rebreathing mask.
STRNGE: It's been mentioned that the first on-board on landing were ambulace/paramedics. Were any noxious fumes/gasses suspeceted, first on would hve been Firemen in Brething Apparatus, with their test equipment.
So, whatever happened, its cler that ground teams had depreciated the seriousness of the issue before landing.