If you will accept that one atmosphere of pressure is 15 psi (for easy figuring), then the partial pressure of oxygen within one atmosphere of pressure is 3 psi (air is 20% oxygen and 80% nitrogen). So we normally breath oxygen at 3 psi when we’re sitting on a beach (sea level).

33 feet of sea water is the equivalent of the pressure exerted by the earth’s entire atmosphere. At the surface, you are exposed to one atmosphere of air (15 psi) The deeper you go, the more pressure you are exposed to. So on scuba at 33 feet in the ocean, you would breath air at 30 psi (two atmospheres… one of air and one of water). At 66 feet deep in the ocean on scuba, you’d breath in air at 45 psi (two atmospheres of water and one of air) and so on.

Mixture of gases such as air are made up of the partial pressures of the gases present in the mixture. So oxygen, making up 20% of air, also accounts for 20% of the total pressure. So 20% of 15 psi (1 atmosphere) is 3 psi; 20 % of 30psi (2 atmospheres) is 6 psi, and so on.

We are accustomed to breathing oxygen at 3psi; at some point less than that we will pass out from a lack of oxygen. Note that everything I’ve mentioned up to this point was on scuba, where one had almost infinite amounts of air available. But suppose you were a breath holding diver:

You’d gulp down some air at the surface (3 psi of oxygen), dive down to 33 feet to grab some pearls (6psi of oxygen), and then immediately rise back to the surface. The closer you get to the surface the less the total pressures; also the less the partial pressure of oxygen. But you started at 3, went to 6, and resurfaced back to 3 psi. No worries, right?

But you’re a typical pearl diver… you don’t just dive and immediately ascend. You’re going to stay on the bottom until you just can’t stand it, and then surface. Hyperventilating will allow you to stay down longer, so you huff and puff until you get dizzy in preparation for the dive.

Here’s the fly in the ointment. Your physiological need to breath isn’t determined by the lack of oxygen; it’s controlled by the buildup of carbon dioxide (your exhaust gas). So you huff and puff on the surface, then make your dive. The pickings are good so you’re on the bottom longer than you’d normally stay. You finally get the urge to breath later in the game than normal, but you’ve finally gotten it and have headed to the surface. Remember you’ve spent more time than normal on the bottom. You’re returning to the surface with less than the normal amount of oxygen. As you ascend, the total pressure decreases. As the total pressure decreases, the partial pressure decreases.

And at some point shortly before you reach the surface, there isn’t enough oxygen left to support consciousness, and you pass out, followed by drowning. The closer you get to the surface, the more rapidly the partial pressure drops, and the more at risk you are. But what choice do you have? If you stay, you drown. If you surface, you’ll probably drown as well.

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