Excerpts from an article in the Travel section of The Telegraph:
“Up front, the pilots will don their own masks and commence a rapid descent to an altitude no higher than 10,000 feet,” he continues. “If the emergency descent feels perilously fast, this isn’t because the plane is crashing: it’s because the crew is doing what’s it’s supposed to do.”..This is important:
According to Airbus, if a plane loses pressure at 40,000 feet, those on board have as little as 18 seconds of “useful consciousness” without supplemental oxygen...
...what you’re supplied isn’t exactly oxygen – nor is it not compressed air in the scuba diving sense. Oxygen tanks are heavy and bulky so aircraft use a more complicated system. The panel above each seat actually contains a cocktail of chemicals that, when burned, release oxygen. They might include barium peroxide, a fine white powder used in fireworks, sodium chlorate, more commonly used as a weedkiller, and potassium chlorate, a staple of school science lab experiments (it reacts violently with sugar).
Tug the mask, like you’re told in the demonstration, and the chemical process starts. Once it starts, it cannot be stopped until everything’s burned up (around 12-15 minutes...).
Do not expect the bag to inflate. Passengers have reportedly suffered hypoxia after believing their mask was broken because the bag wasn’t inflating, prompting them to remove it. Hence the warning given during every safety briefing.I don't know if the standard passenger mask is a "partial rebreather," capturing exhaled air, which even if "once-used" still has usable oxygen available, or a "non-rebreather."
“Oxygen is supplied in a constant flow,” explained a BA spokesman. “The bag does not inflate like a respirator bag used in a medical theatre. How full it gets depends on an individual's rate of breathing. If the rate of breathing is very quick, air is inhaled at a faster rate and so the bag will inflate less. If all the air isn't inhaled, some will remain in the bag, partially inflating it.”
The oxygen generator can also get extremely hot – so don’t touch it – and passengers may even notice a burning smell (so don’t be alarmed).
More information at the FAA.