Gas Density and CO₂ Buildup

Gas gets heavier with depth

Your regulator delivers gas at whatever pressure surrounds you, so every litre you breathe at depth is packed with more molecules, and weighs more, than a litre at the surface. Gas density, measured in grams per litre, climbs in step with the pressure:

Density at depth (g/L) = surface density × absolute pressure (bar)

Air weighs about 1.29 g/L at the surface. At 30 m (4 bar) that becomes 5.16 g/L. At 40 m (5 bar) it reaches 6.45 g/L. Same air, but each breath is now five times as heavy to move as it was on land.

Density at depth, common gases

*above the 6.2 g/L ceiling · n/a = past the gas's oxygen limit

Nitrox is no help with density. EAN28 is actually a touch heavier than air at the same depth, because the extra oxygen weighs more than the nitrogen it replaces. Only helium, which is far lighter than either, brings the density down.

The 6.2 g/L ceiling

Work by Simon Mitchell and David Doolette showed that the effort of breathing climbs sharply, not gently, as gas gets denser. By about 6.2 g/L your breathing muscles are near their limit under even moderate effort. Past that point CO₂ can build up even when you feel like you're breathing normally, not because you're breathing too slowly, but because the gas itself fights being moved. A working target of 5.2 g/L leaves a sensible margin below that ceiling.

On air, you hit 6.2 g/L at around 40 m. On EAN28 it's about the same depth, which also happens to be that mix's oxygen limit, so on nitrox the gas and the oxygen run out of road together. Trimix is the real fix: swap some nitrogen for light helium and the density drops, which is why a mix like 21/35 stays breathable well past 50 m. In theory this is one of the main reasons technical divers switch to helium beyond 30 to 35 m.

In practice, helium is expensive, and plenty of divers stay on air well past 40 m to keep the fill cheap. Be honest about what that trade is: a budget decision, not a physiological one. The gas down there is still too dense to breathe well, and a CO₂ build-up does not care what you paid for the bottle. If the money rules out helium, the right move is to keep the dive short, the workload low, and the depth conservative, not to pretend the density problem went away.

How CO₂ creeps up

CO₂ is what actually drives you to breathe. Normal arterial CO₂ sits around 35 to 45 mmHg, and your body constantly tweaks how deep and fast you breathe to hold it there.

At depth that control gets harder. Dense gas makes each breath more work, and you can't just breathe faster to keep up: quick, shallow breaths mostly shuffle gas in and out of your airways without it ever reaching the air sacs where exchange happens. Clearing CO₂ takes slow, full breaths, and those are exactly what dense gas makes difficult. Add any real effort and your CO₂ production can outrun your ability to blow it off. Arterial CO₂ climbs, and that's hypercapnia.

What makes it worse

Hard work is the usual trigger, because muscle effort makes CO₂ directly, so finning hard at depth is the fastest road to a problem. Skip-breathing (holding your breath between cycles to stretch your gas) is worse still: it shuts down CO₂ clearance during the hold and saves almost no gas on open circuit. Fast shallow breathing backfires too, moving lots of effort for little real exchange. And on a rebreather, a failing CO₂ scrubber lets CO₂ slip straight back into the loop you're breathing.

CO₂ plus high oxygen: a stacked risk

At depth, especially on nitrox or oxygen-rich gas, your oxygen pressure is already high. CO₂ makes that more dangerous, because it sharply lowers the threshold for a CNS oxygen toxicity seizure. Skip-breathing is the nasty case: CO₂ piles up silently during the hold, and when you finally take a big breath, the combination of high CO₂ and high oxygen can tip you into an underwater convulsion. The technical-diving literature treats this as a likely mechanism behind some otherwise unexplained fatalities.

What hypercapnia feels like

The symptoms creep in and are easy to write off as "just working hard," which is what makes it dangerous.

That headache, a pressure behind the eyes or at the back of the skull that worsens with effort, is often the first warning. The other is breathlessness that's out of all proportion to your gas supply. CO₂ also deepens nitrogen narcosis, and the two feel so alike at depth that a diver caught in both may not be able to tell what's wrong, let alone fix it.

CO₂ and decompression risk

A 2024 review by Daubresse and colleagues found that CO₂ during the bottom phase raises decompression sickness risk two ways: it diffuses easily and can wake up existing gas micronuclei, and it widens blood vessels, pushing more nitrogen into your tissues. The same review described a cluster of eight neurological DCS cases at a French military diving school in 2020, traced to CO₂ build-up from mask-breathing protocols before the dives. Timing matters, though: CO₂ on the bottom is harmful, while CO₂ during decompression may even be mildly protective, although what to do with that finding is still unclear.

Preventing it

Know your mix's density at your planned depth before you splash, and move to trimix beyond 30 to 35 m to keep density below 6.2 g/L with margin to spare. Plan the dive so the hard work doesn't happen at maximum depth: drop in, ascend, and do any real swimming shallower. Never skip-breathe, and breathe slow and full so each breath actually reaches the air sacs rather than just sloshing around your airways.

If symptoms start

1. Stop all effort immediately, including finning.
2. Breathe slow, deep, and full for ten to fifteen cycles.
3. Signal your buddy.
4. If it doesn't ease within a few breaths, begin a controlled ascent.
5. Don't try to push through breathlessness at depth. CO₂ can run to unconsciousness quickly.

Rebreather divers: an extra risk

A closed-circuit diver carries a hazard open-circuit divers don't. If the CO₂ scrubber fails or breaks through, contaminated gas goes straight to your lungs, and because you can't smell CO₂ it can drop you fast. It often shows up as sudden, overwhelming breathlessness, or as the foul taste of a "caustic cocktail" when absorbent gets wet. Careful scrubber packing, pre-dive checks, and respecting the canister's time limit are the defences. It's not a rare problem either: in French Navy data, gas toxicity is behind 68% of rebreather accidents, and 60% of those involve CO₂.

References

• Mitchell SJ, Doolette DJ (2013). Recreational technical diving part 1: an introduction to the activities and the risks. Diving and Hyperbaric Medicine. 43(4):207–214.
• Mitchell SJ, Pollock NW (2023). Gas density: display and alarm recommendation (6 g/L threshold). Rebreather Forum 4 Proceedings.
• Mitchell SJ, Cronjé FJ, Meintjes WAJ, Britz HC (2007). Fatal respiratory failure during a "technical" rebreather dive at extreme pressure. Aviation, Space, and Environmental Medicine.
• Daubresse L, Vallée N, Druelle A, Castagna O, Guieu R, Blatteau J-E (2024). Effects of CO₂ on the occurrence of decompression sickness: a review of the literature. Diving and Hyperbaric Medicine. 54(2):110–119.
• Global Underwater Explorers (2020). Carbon dioxide, narcosis, and diving. GUE technical standards.

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Gas density and CO₂ management sit at the heart of trimix and rebreather training, and we drill them on every deep course I teach. Enquire about training →