Narcosis – Friend, Foe or Both…

Now, I know that this will get controversial, but that’s kinda the point isn’t it?

I’m talking narcosis, or as the textbook puts it Nitrogen Narcosis. Though if we were to be technical or even pedantic, we’d call it Inert Gas Narcosis. This is because it describes the narcotic effects of all gasses, especially the inert ones. Inert means a gas that has no chemical influence on the cellular chemical reactions that keep us alive. This excludes Helium though, which is an inert gas added to the mix to reduce narcosis – and lower the Oxygen level for very deep diving, but that’s for another day.

I’m going to split narcosis down into 2 contributory areas: Scientific / Chemical and Outside factors, which I believe almost have an equal responsibility for what we actually feel in the water. I’ll then talk about the things I have observed over the years.

Scientific / Chemical

As we all learnt in our first dive class, divers are subjected to far more pressure than normal. Over millions of years we have evolved in a pressure of roughly 1 atmosphere, which is approximately 1 bar or 14.7 psi. We also learned that as we descend, we encounter an extra 1 bar of pressure per 10m of water depth (in salt water, fresh is 10.3m). To put this into context, to gain 1 bar of pressure we only have to descend 10m or 33 feet. To lose 1 bar of pressure we must ascend (in air) to something called the Karman line, which is the place where our atmosphere essentially stops and the pressure becomes almost zero. Though this is actually not quite far enough, it’s close enough for our purpose today and it stands at a whopping 100 kilometers, which in old money is 62 miles or 330,000 feet!

I digress (but fascinating no?), back to the diving.

As the pressure increases, the molecules of the gas we breathe are squashed together, meaning that when the gas is delivered to you, it’s at ambient pressure. This is worked out using the simple formula: Depth / 10 + 1. So at 25m, the gas is being delivered to the diver at 3.5 bar. This means that you are getting a much higher concentration of the gas molecules at depth than you would at the surface, the factor is the same number as the pressure, so using the example above, you are breathing 3.5x the molecular quantity or total mass.

Narcosis is caused by the gas being absorbed into the divers tissues and slowing down nerve transmissions. Essentially meaning that the electrical impulses within your brain are delayed to or even missed on the receptors. It’s thought that it happens throughout all your other tissues too, but this is only conjecture and of little importance. With the pressures encountered at normal diving depths, this is only really enough to slow down your thought processes and make you feel a bit “tipsy” but at very high pressures it’s believed to act like a powerful anesthetic and could knock you out.

So when we bring these 2 bits of info together, we have a formula to “explain” narcosis. We take the percentage of the gas we are looking at and multiply it by the depth to reach a “narcotic factor”. For example, if we use air – 21% Oxygen (02) / 79% Nitrogen (N2) – and look at just the 79% N2 at a depth of 30m we end up with .79* x 4 = 3.16 which we’ll call the narcotic factor, though those that have knowledge of nitrox will know that it’s actually partial pressure.

We can also do the same for other depths: 10m (1.58) 20m (2.37) 30m (3.16) 40m (3.95) 50m (4.74) 60m (5.53) & 100m (8.69)

*As we are working with a percentage, we express this number as a decimal, it’s not absolutely necessary but makes for simpler numbers.

From this we can then say that most people will be fine with a factor of 3.5 (34m), and that some people will be fine to up to a factor of 5. This is where the personal preference comes in. Out of interest, the maximum air record is 145m set in 1993 by Bret Gilliam** which would give a “factor” of 12.25!!

** There was apparently a dive to 155m in 1994, but this was very controversial and led to Guinness stopping the SCUBA air depth record.

So if you are happy to dive air to 40m, then you are happy with a narcotic factor of 4 (0.79 x 5). Which is the general gist taught by all the agencies, though they may not explicitly say so as it’s built into the recreational depth limits.

So far, we have just looked at air, so what about other gasses. Well, if we exclude Oxygen for a minute we can say that all gasses give a narcotic value in relation to their atomic weight or mass (there are other factors, but mass is the main one). So for example, if we ignore everything but the narcotic value, then we can say that the most narcotic is Radon with a mass of 122, and the least is Hydrogen with a mass of 1. So from this, as Nitrogen has a mass of 14, we can say that Radon is 8.7x more narcotic and Hydrogen is 14x less. Whilst Radon is definitely not used in diving, Hydrogen can and has been in the past, though it’s felt to be a bit unstable with it’s willingness to blow up – see Hindenburg! A gas that is commonly used is Helium with a mass of 4, so that does work out to be the best choice as it’s 3.5x less narcotic than Nitrogen.

With Oxygen, although it is actually more narcotic than Nitrogen with a mass of 15, it’s thought not to have much affect due to the different storage, transportation and use in the body over the inert gasses.

 Outside Factors

I believe that when it comes to narcosis, the mathematical ” narcosis factor” plays only some part to it. In researching this, I came across a Wikipedia article that stated “no diver will every build up tolerance to narcosis”                                     I think that this is total rubbish, as when I was scalloping full time and diving 50m every day, I felt great doing it. Now that i’m not doing it so much, the narcosis is definitely more evident.

When I think back to my deepest local dives, some of the best ones have been 58m on air. When I say best, i’m scoring by: clarity, fluidity, ease of sending the shot up, tying knots, wreck navigation, finding the shot, focus on plan & time, gas management, problem solving and everything else about the dive.

At the same time my worst ever dive for narcosis was at 45m on trimix. It should have felt like being at about 28m, but I was off my tits! I put this down to several factors in descending importance:

Rushing to get in the water. We were on a boat with about 8 other tech divers and I was team of 3 others. So after running about helping 8 others to get in the water, I was left trying to manage all my own stuff alone. At the time it was twin 10’s and 2 deco gasses. Also, the boatman had told my team to get in the water and i could see them struggling in screaming tide at the surface so i was trying to get in asap.

Speed of descent. I think that this plays a massive role in onset of narcosis, and although it’s related to the “narcosis factor” it’s not often talked about.

Light & Visability There is no doubt in my mind that narcosis effects are stronger when in low light and visibility conditions. It’s one of the reasons many warm-water divers get a shock when they dive locally.

C02 & Workrate One factor that is coming into common knowledge and teaching is that excessive amounts of C02 will worsen the narcosis. It’s thought that C02 is a catalyst for many of the diving nasties, though not much is known about the exact mechanics of it as so little research is being done. C02 levels are lifted due to either excessive workrate and poor breathing through technique or poorly maintained equipment – and rebreathers, but that’s beyond the scope of this.


With this section i’m hoping that some other people will write and add their personal experiences as they may be different to mine. Below is  written as my own feelings and what I have witnessed in others.

Personally, i’m quite happy diving to normoxic depth limits on air. In other words 57m ish. Having done these sort of depths literally hundreds of times, i’m saying this with the experience to back it up. Though we don’t really get the opportunity locally to go deeper than that easily, I think that it’s the limit for me. That said, on one dive in Jordan, I switched from Trimix to air on ascent at 66m and that was fine too.

For me narcosis used to be fun and enjoyable, i’d find myself in a heightened mood in the deep as i went about my business. These days a lot of the time i don’t really feel it,  on the worst days it just tends to add a slight fuzziness that I need to concentrate a bit harder to overcome. I honestly cannot say that it ever feels anywhere near dangerous to me on a good day – more on this later.

Anyone that has been to 30m+ will have experienced some level of narcosis, many though will say that they haven’t. They did, they just didn’t know about it because for most people it’s no big deal. It’s often that you’ll hear people draw parallels between narcosis and alcohol, which is a good way to explain it. In small quantities, it makes most people happy and allows them to get more enjoyment out of whatever they may be doing. Of course, in large quantities it can lead you to do stupid things and lose your mind. There has to be a personal limit.

One thing that i have heard a few times over the years as i travel, is the resort instructor telling students to not go below 30m or they’ll “get narked”                  I can never sit back and listen to this so will argue the point. It’s not that i want everyone to dive deep (for one they’d be in my way), but at the same time I don’t like the idea that people are scared of an imaginary line and that if they ever venture to 30.1m they’ll get knocked out as if they have just been caught by a McGregor left hook.

The other story that gets told often is that a diver suffering from narcosis will try and give their regulator 2nd stage to a fish. Now, i’ve not witnessed this myself, but I have heard it from people i trust. I think that if this does indeed happen, I don’t think that the diver thinks the fish needs the reg, I think that the diver does it for a joke, and as they are “narked” it tends to go wrong, or at least be taken the wrong way.

One person that I used to dive a lot with used to get an experience that he called “nitrogen psychosis” Essentially, when at depths over 40m he used to think that everything was out to get him and was going to go wrong – air was going to run out, reg was going to stop working, shark was going to eat him, he was going to get the bends, that sort of thing. Obviously, he stopped diving deep once he decided that it wasn’t a one-off as that’s not a good state to be in underwater!

Another that I used to dive with a lot, got such a high from the narcosis, that he would be in an amazingly good mood for the whole dive and even afterwards – he would often be seen with a big grin on his safety stop singing along to a tune in his head started on the bottom!

Where narcosis does get dangerous, is when it distracts from the important things like the monitoring of gas and deco obligations. Which is why it’s a good idea to push yourself under the supervision of an instructor or buddy that is far more experienced than you in diving those depths and more likely to be actually paying attention to you, rather than being too busy with their own stuff. It’s silly to think that an instructor can dive in Havelet a couple of times a month and then be on point in the deep! One time I was with a student on a 40m dive. We came across a lost parlour pot and I had a quick look in through the mesh (nothing inside) and then “stepped” back to allow the student to do the same. He then spent the next 5 mins looking into the pot from every possible angle multiple times, as if there was something really interesting in there. I was convinced that he was narked and to prove my point, I allowed it as long as I could and then called for him to follow me. By this time we were at the end of our bottom time and were ready to ascend. I don’t know how long he would have stayed looking without the intervention, but from a decompression and gas perspective it could have led to an issue if it had been another 5 mins. Once back on the boat, I asked him about it and he was adamant that he’d had 1 quick look and then carried on!

As I think about all the divers that I know, there is quite a varied “tolerance” to narcosis, I know some people that don’t like to dive below 40 without Trimix, and that’s perfectly fine. It’s all about personal choice, provided that they remember that Helium only reduces narcosis, it doesn’t effect or enhance any other skill. As Mark Ellyatt quotes on his website “Helium provides only sobriety, not experience or buoyancy skills!”

So… Please absorb your inert gasses responsibly…

3 Replies to “Narcosis – Friend, Foe or Both…”

  1. Good post Matt, enjoying these you should write for a dive mag! I’d agree with everything you said especially your ‘outside factors’ section, I think these are the biggest contributors. Alex Guilbert thought this might be due to an interaction with adrenalin which is likely to increase with any of the ‘outside factors’ you mentioned which seems a plausible explanation to increased narcosis. Personally 45m is my limit on air in Guernsey waters and then after that it has to be trimix for me. In cold dark local water I find things start to to get ‘fuzzy’ at 40m and then I really loose concentration on anything below 45m, this has actually led to me wandering off following something insignificant or not paying proper attention to my gauges. My first trimix dive a year ago locally was on the Bizon and it was like a ‘revelation’. In clear warm waters i’ve happily gone well beyond 45m and felt fine, so the outside factors are an issue for me, especially low viz and cold and I’ve also noticed from rapid descent flatty dives on the south coast that the narcosis seems to come in a bigger hit. Now I do much slower descents and it seems to reduce narcosis significantly.

  2. Another good post – I can feel narcosis at 25m at the start of the season in a bit of tide.

    For myself I don’t fancy 60m+ on air the more Helium the better for me.

    The quote I liked was ‘If he is that dumb on the surface it ain’t going to get better with depth regardless of helium content’.

    1. This is why i like to write this blog stuff. To show that all these things are personal, and that there should be no peer pressure or stigma in the choices that people make – providing that they get the job done.

Leave a Reply

Your email address will not be published. Required fields are marked *