On deep air dives, thinking slows and simple tasks take more effort, while the diver often feels no concern. This is narcosis: an effect of the breathing gas on the brain, similar to alcohol. It increases with depth and decreases on ascent.
At greater depths the gas also becomes denser. Each breath takes more effort, and the diver can no longer exhale all the carbon dioxide produced. Adding helium to the mix reduces both narcosis and gas density.
Narcosis on air
Nitrogen, and very probably oxygen, dissolve in the fatty membranes of nerve cells and impair their function. Higher pressure dissolves more gas and gives a stronger effect.
An old rule of thumb, Martini’s law, says every 10 m on air has the effect of one martini on an empty stomach. It is an approximation, not a measurement. Narcosis varies between divers and from day to day, and divers are poor at recognising their own.
Equivalent narcotic depth
To compare mixes, divers use the equivalent narcotic depth (END): the depth at which air would be as narcotic as your mix is at the current depth. On air, END equals your depth.
Many technical divers set a limit of about 30 m END. The line on the figure marks that limit. Air reaches it at 30 m.
Helium and END
Helium is not narcotic at these depths. Replacing some nitrogen with helium reduces the narcotic gas at a given depth, so END rises more slowly with depth.
21/35 (21 % oxygen, 35 % helium) reaches the 30 m END limit only below 50 m, and 18/45 only below 60 m. Reducing narcosis is the main reason divers use trimix.
Oxygen in the END calculation
There are 2 ways to calculate END. One counts only nitrogen. The other also counts oxygen, because oxygen is at least as fat-soluble as nitrogen and is probably narcotic.
DiveLogic and every figure on this page count oxygen, which gives the deeper, more conservative END. When you compare an END with another tool, check which convention it uses.
Depth also increases the effort needed to breathe a gas. This effect depends on gas density.
Gas density and breathing
When the pressure doubles, a litre of gas contains twice as many molecules and has twice the mass. This is gas density, measured in grams per litre.
Dense gas flows turbulently through the airways, which increases the work of breathing. Above a certain density, breathing cannot remove carbon dioxide as fast as the body produces it. Air passes 5.2 g/L at about 30 m and 6.2 g/L before 40 m.
Density limits
Anthony and Mitchell reviewed studies of divers breathing dense gas and recommended 2 limits: stay below 5.2 g/L where possible, and do not exceed 6.2 g/L. Above that, CO₂ retention becomes much more likely.
Changing the oxygen fraction has little effect on density. Oxygen is slightly heavier than nitrogen, so EAN32 is slightly denser than air.
Helium and density
Helium has about one seventh the density of nitrogen. Each percent of helium added makes the mix lighter, so density rises more slowly with depth and the 5.2 g/L advisory depth is deeper.
Helium therefore reduces both narcosis and gas density. No other gas in common use does this.
Which limit comes first
Every mix has 3 depth limits: where the ppO₂ reaches its limit, where END passes your limit, and where density passes 5.2 g/L. The shallowest of the 3 sets the mix’s maximum depth.
For air, the narcosis and density limits are both at about 30 m. For the trimixes shown, the density limit is reached first, well before the oxygen limit. A deep mix is chosen for its density at least as much as for its END.
Try it
Set the oxygen and helium in a mix. The lit column runs from the shallowest depth at which the mix can be breathed to the first limit it reaches. The 3 ticks mark the depth of each limit. Adding helium moves the narcosis and density ticks deeper and has little effect on the oxygen tick. Adding oxygen moves the oxygen tick shallower.
- ppO₂ 1.4 bar
- END 30 m
- 5.2 g/L
What to remember
- Narcosis increases with depth and decreases on ascent. Martini’s law is an approximation, and divers cannot reliably judge their own narcosis.
- END is the depth at which air would be as narcotic as your mix. DiveLogic counts oxygen as narcotic, the more cautious convention.
- Gas density increases the work of breathing and allows CO₂ to build up. Stay below 5.2 g/L where possible and do not exceed 6.2 g/L.
- Adding oxygen has little effect on density. Helium is the only common gas that reduces both narcosis and density.
- On deep trimix, density is often the first limit you meet, before narcosis or oxygen.
Mix limits in DiveLogic
The mix limits tool uses the same engine call as the figures above, with your own ppO₂, END and density limits. It shows the depth at which a mix reaches each limit and which limit comes first. Every gas in a plan also shows its density and END.
Sources
- Behnke A. R., Thomson R. M., Motley E. P. (1935). The psychologic effects from breathing air at 4 atmospheres pressure. American Journal of Physiology 112: 554-558.
- Bennett P. B., Rostain J. C. (2003). Inert gas narcosis. In Brubakk A. O., Neuman T. S. (eds), Bennett and Elliott’s Physiology and Medicine of Diving, 5th edition. Saunders, Edinburgh.
- Anthony G., Mitchell S. J. (2016). Respiratory physiology of rebreather diving. In Pollock N. W., Sellers S. H., Godfrey J. M. (eds), Rebreathers and Scientific Diving. Proceedings of the NPS/NOAA/DAN/AAUS 2015 workshop, Durham, NC: 66-79.