Nitrogen Narcosis: Some Science

Why Nitrogen Narcosis is Difficult to Measure

There are many anecdotes about nitrogen narcosis. It becomes scientifically interesting when individual experiences can be translated into measurable quantities: attention, reaction time, memory, psychomotor performance, or the ability to realistically assess one’s own performance. This is precisely what the studies we examine here aim to do.

Some studies use so-called CFFF tests, i.e., tests for Critical Flicker Fusion Frequency. These investigate whether general arousal levels and attention change under increased ambient pressure. Other studies employ more complex psychomotor tests that assess reaction, information processing, memory, and problem-solving abilities.

Thus, these studies do not measure “nitrogen narcosis” as a feeling, but rather individual functions that are safety-relevant for diving. Can someone remain attentive? Is information processed correctly? How quickly do they react? How well does memory function? And does the affected person even notice when their performance declines?

This page therefore focuses on the most important research approaches. We show what is known about the topic and what open discussions are being held. The next page will then address personal experiences.

Critical Flicker Fusion Frequency

CFFF tests are used relatively often to determine a person’s level of attention.
What do they do in these tests? An LED light flickers at a frequency that continues to increase. At some point, a person only perceives the flickering as constant light, so our ability to distinguish the individual periods of time in which the dot appears and disappears is limited. When exactly this happens can take place at different moments for different people – i.e. at different flicker frequencies. In reality, the dot is never really “continuous”, it just flickers faster and faster. The test is therefore carried out to determine the frequency up to which the test person can still recognize the flickering as such.

In most cases, the transition of perception from “flickering” to “continuous light” occurs at a frequency between 22 and 90 Hz, i.e. at 22-90 pulses per second.

The frequency at the moment at which the test person indicates the perception of the transition is noted: This is the Critical Flicker Fusion Frequency (CFFF), i.e. the frequency at which the flickering of the dot merges into a continuous light for the test subject.

On the one hand, this frequency depends on environmental factors (type of light stimulus, dark adaptation of the eye, an example of this is the situation in the cinema, where even relatively few images per second are perceived as a continuous film). On the other hand, however, it can be a measure of the general level of activation: The longer you perceive the flickering, the more alert and attentive you appear to be. Calibration is therefore carried out first to rule out environmental influences.

In order to measure how attention wanes for different things, it is first determined at what point a person sees the flickering as a continuous light in the “normal state”. The person is then exposed to different things – e.g. higher ambient pressure – and repeats the test. The two values are compared.
If constant light is perceived at a lower frequency, lower brain activity is assumed than if fusion occurs only at higher frequencies.

Several studies have been carried out with CFFF tests. The frequency was evaluated here as a sign of attention. An attempt was therefore made to measure how attention increases or decreases during diving.

Shallow depth narcosis – Paper

Balestra, C et al. “Persistence of critical flicker fusion frequency impairment after a 33 mfw SCUBA dive: evidence of prolonged nitrogen narcosis?” European journal of applied physiology vol. 112,12 (2012)

First of all, there is a study from 2012 on whether narcosis persists on the surface after a 33m dive. We call it a “flat anesthesia study” – it is at the lower end of where anesthesia can be expected.

What did the researchers actually do? They had a group of fit men dive to 33 m for 20 minutes in warm, comfortable conditions, and had them take CFFF tests before, during, and after the dive.
So they measured, during the dive, when the frequency threshold increases and decreases at which participants perceive flicker as continuous light, and from that infer how attentive a person is.
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Result: At the start of the dive, attention increases, but then quickly drops and remains slightly lower than before even after the dive.​
Breathing oxygen after the dive increases attention quickly and significantly​.

You can ask a few questions about the study. First of all, it is quite obvious that you are particularly attentive at the beginning of a reasonably deep dive, but then you relax and your attention wanes. The fact that people are less attentive under water than on land is probably immediately obvious to everyone: we know this, we have already discussed that the situation under water is unfamiliar to many. That you still feel a bit tired afterwards also seems plausible.
But is what was measured here really “nitrogen narcosis”? At such a moderate depth, can it not simply be what happens to people underwater at any depth? Some studies suggest that even at very shallow depths of just 5m, significant cognitive impairment can be detected. Of course, this can also be called narcosis – but whether it is really due to a narcotic effect of a gas cannot be proven.

Environmental conditions – Study

The same authors put the results to the test again a few years later, in 2016, under different conditions. They were in Tenerife under real diving conditions (Atlantic Ocean, water movement, but good visibility and relatively warm), in an indoor dive center with appropriate depth, and for comparison in a pressure chamber.

CFFF tests were also carried out here. In all three environments, the results were similar to those already shown in the previous study. There is no difference between environments with good visibility and tolerably warm temperatures.

However, no one has ever seriously claimed this difference. What many divers report, however, is that the nitrogen narcosis is stronger (or at least more noticeable!) at night or in cold, dark diving environments. Both have NOT been investigated, so we still don’t know anything for sure.

But: it seems immediately plausible that intoxication is perceived as frightening rather than euphoric when the surroundings are dark and cloudy….

Breathing gas comparison – Study

Rocco et al. (2019) then went to a genuinely interesting depth: air, trimix, and heliox compared (hereafter: “breathing gas comparison study”)​.
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Here, real dives to 50 m were conducted, with a bottom time of 20 minutes. Here too, the participants took CFFF tests at the beginning, at the end and at 5m before the ascent. The people involved were all young, healthy men. In order not to distort the statistics, the few registered women were not allowed to take part. Here you can see a problem that unfortunately affects many diving studies: the test group is usually very small and doesn’t reflect the diversity of real-world divers.
Result: At the start of the dive, attention increases, then slowly decreases, but remains elevated.​ The results are better the lower the nitrogen fraction in the breathing gas.
That is the exact opposite of the previous findings. While it was previously always assumed that attention was reduced and that this could be proven by CFFF tests, our divers are consistently more attentive at 50m with air than our less experienced test subjects at 30m.

MATB-II and others: grasping psychomotoric skills?

So far we have seen a few interesting studies, but they all have one problem in common: only the CFFF is measured, the interpretation as “attention” is just that, an interpretation. And whether this is really the only relevant sign of a nitrogen narcosis is open to debate.

What do we want to know when we are traveling really deep? We want to carry out our dive safely – this can be done relatively mechanically, it can be trained – and we want to react appropriately, flexibly and quickly in the event of problems. This becomes difficult in a state of intoxication – just as you can probably still turn the ignition key and put the car into gear after two pints of beer, but may not be able to brake quickly enough in traffic if an unexpected obstacle appears, you may not be able to get to grips with unexpected problems under water if you are intoxicated. And that can be dangerous….

Studies with more extensive tests are therefore extremely informative. Psychomotor skills come into view in different ways: What does depth do to memory? Is it still possible to react? How well can information be processed? And can you get problems under control?
Some studies use NASA’s MATB-II tests, which are designed for relatively simple devices. The skills tested here are those expected of air traffic controllers, for example, but as the tasks are relatively unspecific and simple, the results can provide good indications of general skills.

Psychomotor skills study

A study by Freiberger et al. is somewhat more complicated than the simple CFFF tests. 2016: Influence of N2, O2 and CO2 on psychomotor performance (hereinafter: “Psychomotor study”) This is freely available – if you would like to read it, you will find it linked above. It’s really worth it!

The conditions here are different to those for studies in water: in the Navy study, the test subjects sit with their heads out of the water in a pool in a pressure chamber, where they are exposed to the pressure that is to be measured. Here, pN2 up to 5.6 bar, pO2 up to 1.22 bar and an increased pCO2 at rest and in motion were used to adequately cover the spectrum of gas mixtures with regard to air/nitrox. In this position, they then performed MATB-II tests.
How they respond, how quickly they respond, how they process information, how well each part of their memory works—this was measured in detail.

What the study revealed:

• Attention and reaction speed suffer from high pN2
• Memory also deteriorates under the influence of too much N2
• The reaction time for things to do with memory is also limited by O2 and CO2
• Planning and problem-solving skills are significantly limited at higher pN2 and pCO2
• Motor skills are not affected by the pN2
• Motor skills can be trained: divers and people who regularly play video games perform better

Perhaps the most interesting result of this study was not so much how the candidates actually performed under certain gas pressures, but how they themselves assessed the results.

Is oxygen narcotic?

One question that is often discussed is whether the oxygen content in the gas should be counted as narcotic or not. Do we have to assume that oxygen makes the narcosis worse or less bad? Is oxygen narcotic or not?

According to the theory of fat solubility, oxygen – an extremely fat-soluble gas – should be narcotic. At the same time, however, oxygen is metabolized, which means that it may not accumulate at the synapses in sufficient quantities for a narcosis.

Unfortunately, it is impossible to directly measure the narcotic effect of oxygen under high pressure, as toxicity sets a limit long before a possible narcotic effect. However, indications can be gleaned from some studies on nitrox.

One example of this is an experiment in a pressure chamber in which 10 test subjects breathed either air or EAN40 in a pressure chamber. During a simulated dive to 30m, a CFFF test was carried out beforehand, on arrival at depth, after 20 minutes (directly before the ascent), on reaching the surface and 30 minutes after the dive. The result: In both groups, alertness increases at the beginning of the dive, with nitrox significantly more than with air. Towards the end, and also afterwards on the surface, it then decreases significantly – with Nitrox, however, significantly less than with air, and after diving you immediately reach a normal level again.

If one wants to understand this decreasing alertness as a sign of narcosis, one can read this study as an indication that nitrox is less narcotic than air – and consequently that oxygen is probably not narcotic, or at least not as strong as nitrogen.

There are also recent studies of brain waves in subjects who have breathed air at normal pressure and pure oxygen at normal and elevated pressure, which indicate that at elevated pressure, breathing pure oxygen causes different changes in brain activity than those known to occur when breathing air and may be associated with the narcotic effect.

However, the question is really much less relevant in practice than one would expect from the heated debates: one may believe that one feels less narcosis at 40m with an EAN28 than with air. With the 7% difference, it might be difficult to prove more than a placebo effect – but since there is no harm in using an EAN28, anyone who likes it can simply do so. On the other hand, when it comes to determining the EAD in a trimix, the difference between assuming O2 is or is not narcotic is so small that the whole discussion is rather academic.

Can you train for nitrogen narcosis?

Many divers report that they are experienced enough to no longer have a narcosis – they are convinced that it can be trained away. But is that really the case?

In fact, there is at least some evidence that experience helps in different ways to cope with being narked. In the psychomotor skills study, those with more diving experience performed better – an indication that dealing with high pressures can perhaps actually be trained. In the breathing gas comparison study, the results of the test subjects, all experienced divers, were also better than in the shallow water narcosis study, in which less experienced divers took part.

In addition, studies on brain chemistry have shown changes after repeated exposure to high pN2. However, it is not clear whether this is actually a sign of habituation and what effects it has on the skills relevant underwater.

What you can train is how to deal with intoxication. You can get used to not being quite so clear-headed. And above all, you can train your own diving skills so that they work without thinking – as we have also seen in the psychomotor skills study, mechanical skills are largely retained.

Is nitrogen narcosis stronger in dark and cold waters?

Divers in particular, who also spend a lot of time in deep lakes, often describe how the narcosis is something completely different in the cold and dark than in warm, light-flooded waters. What’s the story?

First of all, no differences can be proven under different diving conditions – at least if you believe the “environmental conditions study”. But: In this study, no really dark and cold conditions were tested.

Now there are two things that can actually have an influence here.

Firstly, you usually dive in cold water in a dry suit, and often with a twin tank. So you have to move more equipment than in a thin neoprene – and that is a little more strenuous. However, exertion in combination with the increasingly dense breathing gas at depth can cause the CO2 level to rise – and this in turn feels quite oppressive: CO2 is highly narcotic, and it also triggers a reaction that is definitely suspected of turning a “mild, euphoric” deep dive into an unpleasant, bad one. The air hunger associated with an increased CO2 level, the feeling of breathlessness, can intensify worrying feelings. So it really can have a physiological cause – which doesn’t just play a role in the cold, but also in other exertions, e.g. when you have to fight against a slight current.

On the other hand, you can well imagine that darkness in combination with depth is simply somewhat unsettling. The same narcosis can therefore simply feel different. But that doesn’t necessarily make it stronger, you just perceive it differently.

Can you do anything about nitrogen narcosis?

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If you can’t actually get used to the narcosis and it is very dependent on your daily form, is there anything you can do to minimize the risk?

Strictly speaking, there is only one thing to do: keep the pN2 below the threshold that becomes problematic. And there are two ways to do this: you can replace some of the nitrogen with helium, or you can stay shallower.

Of course, there can be no hard limit here; as we have seen, the effect of narcosis sets in gradually and becomes stronger and stronger with higher pN2. Perhaps a useful guideline is the following: If you really want to be careful – up to 30m the narcosis is not particularly limiting for most people in most situations. In the range between 30m and 40m, most people feel a noticeable effect, and it is already worth thinking carefully about the possible consequences and, if it becomes uncomfortable, to get shallower immediately or to think about alternative breathing gases. From 40m the effects increase significantly. Dives to these depths are possible with air and are also made. But they absolutely require an informed decision, caution, and the knowledge that if in doubt, choosing a different breathing gas will be the safer option. And above all, you shouldn’t try to turn the maximum depth that can be dived with air into a test of courage or a baptism of fire.