Hypercapnia is the condition in which there is too much carbon dioxide in the blood. Hypercapnia is often accompanied by hypoxia, the condition in which there is too little oxygen in the blood. There is a scene from the movie Das Boot during which the submarine must stay submerged too long, causing the submariners to suffer from carbon dioxide poisoning, or hypercapnia. There is a similar scene in Arthur C. Clarke’s book, A Fall of Moondust, which I recently read, during which the passengers of a tourist suffer from hypercapnia when their vessel is submerged for many hours in a sea of moon dust and the carbon dioxide conditioning unit begins to fail.

I learned the word hypercapnia from a long and fascinating post, “Human (amphibious model): living in and on the water,” by Greg Downey on the PLoS Blog Network website. There is so much information packed into Downey’s post (which he states in a comment is the draft of a chapter for a book he is writing) that I recommend you read the post in its entirety.

When you hold your breath too long, your body forces you to breathe. I’ve always wondered if this reflex is triggered by a lack of oxygen. Scientists have learned instead that the breathing reflex is triggered by hypercapnia, and the reflex occurs long before there is real danger of hypoxia. That is, it is impossible for most ordinary persons to hold their breath until passing out from lack of oxygen.

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