Lost in Saigon, On any flight where depressurisation occurs an emergency descent is likely to get the aircraft down to an altitude where the passengers will be able to breathe with the aid of the masks. On large aircraft there are bound to be a few passengers who have taken an hypoxia course at some stage in their life, even if it was more than 40 years ago.
I give my wife an additional safety briefing over and above the one given bt the CC. I tell her that if the masks fall, get it on asap, breathe in normally, then immediately exhale fully. Breathe in normally, but forcibly exhale.
It should only take about 10 - 15 breaths before the aircraft is down to an altitude where normal breathing is possible.