Hypercapnia

A buildup of carbon dioxide in the body from skip breathing, overexertion, or dense gas, causing headaches, air hunger, panic, and worse narcosis.

Hypercapnia is an excess of carbon dioxide in the blood. CO2 is the waste gas you exhale, and underwater several things conspire to let it build up: shallow or skip breathing that fails to flush the lungs, hard exertion producing CO2 faster than you can vent it, dense gas at depth that makes every breath more work, excessive dead space in equipment, or a spent scrubber on a rebreather. The symptoms are a throbbing headache, air hunger, confusion, anxiety spiraling toward panic, and in severe cases sudden unconsciousness, a CO2 hit.

CO2 matters beyond its own toxicity. It is a powerful amplifier of other problems: high CO2 markedly worsens nitrogen narcosis, raises the risk of CNS oxygen toxicity, and drives the panic cycle where anxious rapid breathing produces even more retention. The classic scenario is a diver finning hard against current at depth who suddenly feels they cannot get enough air despite a working regulator. The correct response is to stop, hold onto something if possible, and take slow, deep, deliberate breaths until the feeling passes.

Prevention is a calm breathing habit: full, relaxed inhalations and complete exhalations, never skip breathing to stretch gas. Pace yourself, and at depth let the gas density argument win, slow down rather than work harder. Rebreather divers add strict scrubber duration discipline. A pounding headache after a dive is very often a breathing pattern telling on itself, so treat it as feedback to slow down and breathe deeper, not as bad luck.