Bob Gardner wrote:

Based on my experience in a USAF altitude chamber, after the chamber has
been pumped up (actually down...it is a vacuum) to 25000 feet and the
pressure suddenly dumped as would be the case in a decompression, whatever
air you have in your lungs comes out forcefully...no way do you have any
control over it. Gotta get that mask on and get it on in a hurry, because
time is fleetingly short.

In US regulations, the magic altitude is flight level 350 (more or less
35000 feet, depending on ambient atmospheric pressure). The Cypriot plane
was reportedly at FL 340, if we can believe the media. Under US rules, above
FL 350 both pilots must be at the controls and each must have a
quick-donning mask at hand. If one pilot has to leave the cockpit for any
reason, the other has to put on and use oxygen until he returns.

It's FL 250 (121.333 I believe). Above 250 one crewmember must be on O2 when
the other leaves the flight deck.

As to decompression of an aircraft the size of the 737, there is rapid
decompression and explosive decompression. It the packs both trip off or a
window blows out it results in rapid decompression, which is quite different
than an altitude chamber experience. The cabin ascends at perhaps 10,000 feet
per minute. But, if there is some type of structural failure resulting in a
whole larger than a window, then the cabin can explosively decompress, similar
to your experience in the altitude chamber.