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Nitrogen Narcosis

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New Member
Aug 17, 2004
What is the relationship between the depth at which a freediver experiences nitrogen narcosis and the depth at which a scuba diver experiences nitrogen narcosis? I ask this question with an ideal mathematical relationship in mind, even though I understand that the issue is one of personal physiology.
For me there are four situations as far as N2 narcosis (BTW, in freedivers, CO2 narcosis and N2 narcosis are never seperable):

1. Line diving in warm clear water
2. Line diving in cold dark water
3. Recreational diving in warm clear water
4. Recreational diving in dark cold water

The max depth I have gone line diving in warm clear water without narcosis is 67m. At 75m I get narcosis even in warm clear water.

For recreational diving in warm clear water, I have never had narcosis.

For line diving in cold dark water, I generally get narcosis on dives of 57m or more, but it depends how cold I am, and how scared I am. It also depends on how slow/fast the dive is. I did a hang at 52m and got narked.

For recreational diving in cold dark water, I usually notice narcosis on the descent, starting at 35-40m, and it becomes severe after 45m. Keep in mind that on recreational dives, the descent takes far longer than on a line dive. A 50m-53m recreational dive would last 2'10" - 2'25", whereas a line dive of the same duration would be 70m-75m or more.

The less I hyperventilate (i.e. the more CO2 I have), the earlier and worse the narcosis is. The colder and darker it is, the worse the narcosis. The more apprehension/fear I have, the faster and worse I get narked.

On line dives, most divers only get narked on the ascent, because that is when the CO2 is generated. In cold dark water, as I sink without effort on a line dive, narcosis can hit me on the descent, starting at about 68m. Then, as I turn and start the ascent, the narcosis becomes more severe. The narcosis continues to get worse until about 45m on the ascent, then it starts to get weaker.

The shallowest dive of any type with any sort of weird narcosis effect was a 33m dive in a lake. I got to the surface (no narcosis on the bottom), and then started laughing hysterically for 60 seconds once I surfaced. There was no reason to laugh. Soon I realized, 'why on earth am I laughing?'

Eric Fattah
BC, Canada
i think you become less susceptible to the effects of narcosis if you dive deep on a regular basis. i've heard this is true for deep air scuba diving too. during the summer months i was diving deep 3 times a week and never noitced any narcosis during my dives. my mind always felt quite clear.

i have felt quite strong narcosis during freedives in the past so i know the feelings. i can remember experiencing narcosis on dives around 50m when i wasn't diving to depth very often. the shallowest (and strongest) i've ever felt it was during a 40m variable weight dive.
I guess that I had estimated that ~60m was probably where it happened most usually for most free divers. My brother SCUBA's and he gets narked before he passes 30m. I had worked out in my head that a free diver has 10/([Depth in meters]+10) the volume of a SCUBA diver at the same depth, which means there is that much more nitrogen in the SCUBA divers lungs. So I had reasoned out that there was an inverse square relationship and if a free diver gets narcosis at 7 atm then the same scuba diver should get narcosis at sqrt(7) atm ~= 2.6 atm. But after reading some people talk about narcosis experienced during SCUBA diving, I am not so sure anymore. Is my math all wrong?
BTW, I dive in cold water, as in cold like the seventh layer of hell, not just a little chilly. More of the mind numbing, need to warm my bones up sort of cold. Quarry diving sucks. I need a vacation.

I don't think your concept is correct.

Even though a freediver has less total nitrogen molecules in his lungs, the nitrogen partial pressure he experiences is the same at a given depth. If he remains for 20 minutes (holding his breath), at 30m, then he would absorb almost all the nitrogen in his lungs, at which time his nitrogen partial pressure would start to decrease.

For example,
Air = 80% N2, 20% O2
Freediver descends to 30m
Freediver is at 4atm
Freediver has 4atm *0.8 = 3.2atm nitrogen in his lungs at time T=0

Scuba diver descends to 30m
Scuba diver has 4atm *0.8 = 3.2atm nitrogen in his lungs at time T=0

However, now, as they both remain at 30m, they both absorb nitrogen. The scuba diver has a limitless supply of nitrogen as he breathes, so he is exposed to 4atm*0.8 = 3.2atm nitrogen forever.

The freediver gradually absorbs the nitrogen out of his lungs. After perhaps a couple of minutes at 30m, the freediver may no longer have 80% N2 in his lungs, because he is absorbing the N2. Now, he may have 65% N2, giving him (65/80)*3.2atm of N2 pressure in his lungs.

N2 narcosis is a function of the N2 in the blood plasma. It takes a non-zero amount of time for the blood plasma to reach equilibrium with the lung N2 pressure.

So, upon arriving at 30m, the freediver is OUT OF EQUILIBRIUM, the blood still has very low N2, but as he waits, his blood N2 rises, reaching an equilibrium after perhaps one or two minutes.

So, what does it all mean?
It means that a freediver, if he hangs at his target depth for one or two minutes, will get almost as narked as a scuba diver at the same depth..... almost (not the same), because the freediver has absorbed a small fraction of his N2, decreasing his N2 pressure slightly.

Eric Fattah
BC, Canada
Being aware of narcosis and being affected by the narcotic effects of N2 under preasure are not the same thing. My memory for detail is weak, but I'm pretty sure that for scuba divers there is considerable documentation that mental sharpness begins to decrease well before the diver is aware of it.

I would be interested in knowing how fast narcosis takes effect on mid depth free dives, 30-40 meters, 1-2 minutes. I have never felt any while freediving, but have wondered if some effect wasn't present. I've also seen divers do things at those depths that sure looked like mild narcosis was a factor.

I guess that if I dug up a copy of PADI's Nitrox books then I could figure this out, but is the rate of transfer of Nitrogen into the blood stream directly related the the difference between the patrial pressure of the gas in the blood and the partial pressure of the gas in the lungs?

I have done ~40 dives in the range 40-64m this summer and only on the first few dives I noticed narcosis.

On my first 3 dives this season to 41, 43, 45m I remember clearly beeing narked on the first 10-15m on the ascent. I was affected on more dives but this dives it was very strong.

I have done all my dives in cold and dark conditions, mainly in a lake but 5 dives in saltwater.

Maybe the narcosis is there every dive but one gets "used" to it and don't notice it??
It might be a complex issue.

Not only Nitrogen has narcotic effect bust also Carbon dioxide [CO2] has a narcotic effect and they are also thought to interact with each other.

Technical divers learn that nitrogen narcosis hits them the hardest when they descend fast. The speed itself of the diffusion of nitrogen in the body is thought to be the cause of this. When they stay at a depth after a fast descent narcosis and its effects in technical divers “drop to a normal level”. Technical divers however usually descend slower than freedivers even if they try to descend fast.

What complicates things more is that noticing nitrogen narcosis in yourself is difficult. Practical exercises are often done during deepdive courses to illustrate this to participants. Even heavy narcosis is often not noticed by the affected scuba diver itself.
Also in scuba diving there is supposed to be an adaptation to depth narcosis: you get less susceptible to it if you dive deep often. This effect however is said to diminish after only a few days of not diving.

Scuba divers learn that exercise increases nitrogen absorption. Are freedivers too relaxed (trying to save energy=oxygen) for this?

Several effects could be involved. Maybe freedivers descend so fast that in contradiction to scuba divers for them a relative slow descend or a stay at depth might increase narcosis. It would be nice to let freedivers do narcosis tests at depth. Problem is however that as far as I know the tests used in scuba divers are all time consuming.

I am afraid I cannot add practical answers but I hope it helps to see possible effects involved and thereby helps to develop good theories on this.
Nitrogen Narcosis for scuba is dangerous because you can become accustom to it but you are still just as mentally diminished, much like observing a alcoholic, where the booze does not seem to affect them early on in the evening, but still legally intoxicated. That you can deal with nitrogen narcosis is the fallacy of the deep air technical divers that are afraid of helium. It is not only nitrogen though, oxygen is narcotic at depth also. There is no set depth where narcosis can “hit”, every dive is different and the factors that can increase the affects of narcosis are cold, increased CO2 build up, hydration, fatigue, medication and over exertion (see CO2) just to name the obvious. It was always fun to dive with someone who believes they are good on air and I would be on mix (my mix for any dive from 140’ to 200’ was 18/45) and watch them check their pressure gauge over and over again because they would forget what they had just looked at, I also did plenty of deep air diving (dumb!) in the early 90’s and have personal experience in the matter. Obviously this is not the form to be going into detail about trimix, but the free diver does not stay at depth long enough to really get him/herself in to serious trouble as can the tech diver doing a 30 min. dive in 200’ in side a wreck.
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