Oxygen
There have been several mentions of lack of oxygen being responsible for the thick black smoke which appears in the videos.
The relative mixture of atmospheric gases is the same at 30,000 feet as it is at sea level, only the pressure is lower.
Cabin pressure in a high flying aircraft is maintained by outside air being compressed and vented into the cabin. If there were less oxygen at altitude, then this technique wouldn't work.
In event of depressurisation occuring, supplementary oxygen masks are deployed for passengers because the human body can't extract sufficient oxygen from low pressure air, not because the air itself has any less oxygen.
Sufficiently fit and acclimatised individuals are capable of surviving at equivalent altitudes for several hours without supplementary oxygen as proved by unassisted ascents of Mount Everest.