When the useable oxygen is gone, they will simply pass out. It's somewhat similar to what happens at altitude when there is loss of cabin pressure and everyone goes to sleep; i.e. the recent jet crash in Virginia where the pilot was seen slumped over in the cabin. That's the only 'mercy' that can come from this scenario. The worse part will have been the realization of their situation while here was still plenty of breathable air still in the crew compartment. That was a mind game for all of those poor souls that is hard for any of us to contemplate.