How Long Can I Survive if I Move to the Bottom of the Ocean?

While most people assume that suffocating in an enclosed space means running out of oxygen, in fact it’s often the buildup of carbon dioxide that gets you first.

Oregon Coast Aquarium. (Victor Phan)
By Marty Smith

I’m thinking of doing a boneheaded challenge of trying to survive underwater in an oxygen-pressurized half dome for 50 hours. Is this possible if my dome is 500 cubic feet? Also, would I need an exhaust system to keep the air fresh? —Nick G.

You’re my kind of guy, Nick: an idiot. Still, I’ll bet you’re smart enough to tell the difference between my column and actual medical advice—and frankly, anybody who isn’t probably died two years ago from injecting bleach because they heard it cures COVID. Nevertheless, a disclaimer: I’m not a physician. You’re not Aquaman. Don’t try this at home.

Anyway, a 500-cubic-foot dome is about 6 feet high and 12 feet across, and has an air capacity of about 3,740 gallons. (Or 14,200 liters, but American units seem better suited to measuring stupid ideas.) Of that air, 20%—748 gallons—is sweet, sweet oxygen.

That doesn’t sound like much, but it’s enough—the human body consumes just 100 gallons of oxygen in 24 hours. You’ll finish your 50-hour ordeal with 540 gallons to spare, at an oxygen level of 14.4%. That’s not great, but it’s manageable; for comparison, the oxygen concentration at the summit of Mount Hood is 13.6%.

However, before you start David Blaine-ing yourself into watery immortality, there’s another problem: While most people assume that suffocating in an enclosed space means running out of oxygen, in fact it’s often the buildup of carbon dioxide that gets you first. CO2 is toxic at relatively low concentrations—and unfortunately, you exhale 5.5 gallons of it every hour.

Your air will begin with the normal concentration of CO2, about 0.05%. After 24 hours, however, that figure will reach 3.6%—more than 70 times what you’re used to, and enough to cause headache, dizziness and painful shortness of breath.

By this time you will almost certainly have rung the “get me the fuck out of here” bell, but if you somehow keep going past 5% you can add heart palpitations, tremors and unconsciousness to the menu.

It’s true that you can avoid these unpleasant effects with a simple CO2 scrubber. Be careful, though: Since it’s mostly CO2, rather than lack of oxygen, that triggers that “need more air” reflex, if you do miscalculate and run out of oxygen, you may nod off to the soothing hum of your scrubber and never wake up. (Though I suppose there are worse things.)

Questions? Send them to dr.know@wweek.com.

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