How long could two people survive under water while going up to the surface?...?

They ain't gonna make it. All divers are taught to continuously exhale a small stream of bubbles as we ascend. Failing to do that will cause an embolism and rupture the air sacs in your lungs. There will only be one POSSIBLE survivor in this scenario. Someone has to die and it'll be the person without that lungful.

Additionally they would have been using something other than air at anything past 150 feet. The O2 in the trimix they'd have been on, as they came up, would be hypoxic at and above that depth. The survivor at this point would pass out from lack of O2 during the ascent and certainly at 150 feet or above. He might still live, but I doubt it.

Edit: Nino is assuming that there is a deco obligation. There might not be. There was no time given and deco is a function of depth vs time spent at depth. Nino....get your tables out and relearn them !!! :) Besides, a DCS hit is the least of their worries.

Additionally, if this is a ship, your story can't happen. If there was an airspace they were in, it wouldn't last. Any air trapped in closed compartments aboard a sinking vessel is an area that will implode. Only flooded parts of a ship survive a plunge to depth. Those two would have been crushed to a pulp by steel, before managing any escape. I actively wreck dive , so I've seen what happens to a hull, even at a moderate depth of 150 feet, bulk heads buckle. I'm afraid your story now has NO survivors.

Lol...keep trying, you might get a plausible story yet. :)

WOW, DEEP BREATH.....I have wondered this same exact thing not too long ago. You worded it so good to understand. I think they would have at least 5 more minutes between each other. However, if person A exhaled into person B, Person A wouldn't have as much time before he needs more oxygen while person B has full lungs and will have more minutes to spare on trying to get to the surface. O WAIT, nevermind....you breath out carbon dioxide, so that wouldn't be good at all. Person A who's getting a full exhale from Person B could die very soon.

DAMN, I have taken like 4 deep breaths just thinking about this.

I don't know what else to say. Im gonna see what others had to say.

The air we breathe is composed of 78% Nitrogen,21% Oxigen,1% rare gases.

We do use only the oxigen for our vital functions.

Nitrogen is an inert gas not used from the body.

Rare gases are not considered in this example.

In average you exhale a 19% of oxigen(one of your questions).Therefore,at each breath you are using,in average,only a 2% of it.

Maybe...,knowing the exact depth from where the two ones are surfacing,we can make a theoretical calculation about the O2 consumed,and to give a theoretical answer.

We have to consider,too,that the person that had no volumes of air in depth,while surfacing,the body will release molecoles of gas from the tissues,therefore he will have some gas volume in his lungs.

We consider too that,at big depths,the variations of volume,while surfacing,are small.Increasing more close to surface.

The biggest variations in volume are from 0 to 10 meters(Discending or surfacing).

A diving manual can give to you more details.

So,actually,with the datas that you gave us,I can't give you a good answer,just,this series of information.

For sure,due to the big depths and the surfacing with no deco stops,will bring them to have very serious problems of decompressure sickness,or the death.

Difficult to say.

...A good question,by the way.

Regards.

When person A exhales, wouldn't they be exhaling CARBON DIOXIDE which is NOT what person B would need as they would actually need OXYGEN?

you dont breath out oxygen,which your body needs,you breath out carbon dioxide.try breathing into a paper bag for a little bit,you'll see what i mean.

i don't have an answer, but that is about the best question i've seen on here

Dunno. Are they havin sex on the way up?