• Welcome to the DeeperBlue.com Forums, the largest online community dedicated to Freediving, Scuba Diving and Spearfishing. To gain full access to the DeeperBlue.com Forums you must register for a free account. As a registered member you will be able to:

    • Join over 44,280+ fellow diving enthusiasts from around the world on this forum
    • Participate in and browse from over 516,210+ posts.
    • Communicate privately with other divers from around the world.
    • Post your own photos or view from 7,441+ user submitted images.
    • All this and much more...

    You can gain access to all this absolutely free when you register for an account, so sign up today!

Diving on 100% Oxygen

Thread Status: Hello , There was no answer in this thread for more than 60 days.
It can take a long time to get an up-to-date response or contact with relevant users.

J Campbell

Well-Known Member
Sep 17, 2001
I was wondering if anyone has every taken a dive after inhaling a lungfull of 100% oxygen? Does this increase bottom time? Do you still get contractions and the urge to breath because of CO2 buildup, even thuogh you may still have plenty of oxygen in your lungs? Where would one get a small tank (pony bottle?) of 100% oxygen? Comments?

100% oxygen is good for medical purposes on land..
But as pressure grows oxygen becames "poisonous".
Taking a full breath of 100% oxygen before descending would very likely cause one to blackout of oxygen toxicity.
If you wish to take any bottles of air or what ever whit u should start scuba diving.
I have read about people breathing 02 on the surface and doing really long statics in the pool- I thought that Umberto did 19 minutes this way once.
Of course 02 is toxic below 20' and its dangerous side effects can be intensified by cold and medication. This would not be a good idea to try. It is more of a "controlled experiment" that some people have subjected themselves to.
I do remember Eric Fattah talking about the possibilities of a 20 minute reef dive in 15' feet of water on one breath of 02. I just don't know of anyone who would want to take the risk of a black out or a seizure.


100% O2 would be dangerous below 10 metres, but your post made me think that it would be interesting to do all the breathups, negative dives, and be fully kicked in, then take a full breath of say 40% Nitrox. You could still dive to something above 17 metres or so and be reasonably free of O2 convulsion risk. Of course this is not breathing from a reg underwater, just the final breath at the surface.
Erik Y.
I remember hearing that Umberto stopped his breath hold after 20 min or so, not because he couldn't keep on going, but because he was afraid of what he might be doing to his body. :duh

Reminds me of when Cousteau started diving with the first prototype SCUBA tanks and everyone was getting bent and no one knew what was happening. "Gee, Jean-Luc, you look kinda funny... you alright?"

I'll leave the 02 experiments to someone else, thanks.

Oxygen Toxicicty

Yes, oxygen used on the surface could extend your static apnea, but underwater, it is very dangerous.

At 40 feet, under pressure, oxygen is very toxic to the human body. It causes the body to convulse, it can cause vomitting, and can cause the diver to pass out or worse.

The Navy studied this in depth during early developments of Rebreather sytems. Because the rebreather systems used oxygen with a chemical scrubber to remove CO2, it limited diving to under 40 feet. Because of their testing, and the number of injuries (of course they won't say fatalities), the Navy has set standards of diving w/in 40 feet on non-tecnical/salvage diving.

This is also why recreational scuba tanks only use an approximate 20% oxygen w/ 80% nitrogen.

All of this information, as well as the oxygen toxicity information is available in the US Navy Dive Manual.

Remember this. The tests done were performed while swimming and breathing from oxygen! No true PUBLISHED studies have been presented with holding oxygen in your lungs at ANY depth! I don't know if I'd want to truly be the guinea pig on that one!
Actually, there is a study about this available at:

They compared inhaling air, 50% O2, and 100% O2 at the surface and 30 feet. Also with exercise and at rest.
I'm not sure that this has any application to recreational diving. Lots of graphs though.
Well, speaking as a SCUBA diver (OC and CCR) your comments about O2 really baffle me.

I regularly use up to 80% Helium, and 100% Oxygen. On my rebreather the Oxygen mix varies between 100% and 21%. If I start doing deeper than 50m oxygen content will drop below 21%.

If you are going to put forward comments about any subject, please check your information before posting.


Pete S
Pure Oxygen diving

You said in your post that you are using a helium mix as well. This information was posted as to diving on 100% pure oxygen, no mix. Meaning that you can not safely dive while breathing off of an o2 bottle as the original poster was asking.

The information is laid out in the US Navy Dive Manual Volume 2. I recommend you examine it and see that it is clearly documented there.

Here is the exact quote from part of it. It is several pages long, so I have only inserted the important pieces:

"From the air we breathe, oxygen is the only air used by the body. The other 79% is used to dilute the oxygen entering the body. Sometimes 100% oxygen is used for shallow diving operations and certain phases of mixed gas diving operations. However, breathing pure oxygen under pressure may induce the serious problems of oxygen toxicity."

Examine it for yourself. If I am wrong let me know so I can stop teaching my classes wrong information, and I can correct my instructors who taught me.
:duh :naughty
Your both arguing about non-issues.
The original navy rebreather tests were pure 02 units. They have tables that specify 20- 25' max operating depths. They also have "extreme exposure" tables that will allow short excursions down to 50'- I guess anytime your in a combat situation I would call it an extreme exposure.
The reason your heliox rebreather has a PO2 drop past 50m is to keep your toxicity levels in check. That is why you have a "mixed gas" rebreather- and not a pure 02 unit like the original tests were done on.
We know that the deep sled divers are going way into the toxic range, but seem to be safe due to their very short exposures at their target depth. Still, some are getting deep enough to get bent and I wonder how long it will be before someone gets deep enough to oxtox?
But back to your discussion at hand, it seems that your both right, but talking about totally different untis and circumstances.

Sled Divers


Several sled (freedivers) have already blacked out from O2 toxicity, including Pipin and Audrey. Usually they black out on the ascent, still attached to the lift bag, then regain consciousness shortly after, still on their way up, when O2 levels drop.

Not to mention that at the bottom, often they can't see anything (vision tunnels then vanishes completely). Either narcosis or O2 toxicity, or CO2 narcosis, the debate hasn't been resolved.

OK OK. To deal with one post at a time

Effattah, CO2 narcosis? You surely mean CO2 build up causing shallow water blackout. Narcosis is normally caused by Nitrogen levels being high, hence using Helium mixes on deep stuff. These days, O2 is also regarded as slightly narcotic.

Jon, You are correct in that pure O2 can get interesting below 6m, as the PPO2 has risen to over 1.6 bar. But, my rebreather runs my PPO2 at two set points, 0.7 for shallow, and 1.3 for depth, and varies the O2% according to depth.

Stoddelle, You CAN breath pure O2 underwater, provided you monitor the following. PPO2 levels (max 1.4 bar for depth, 1.6 bar for decompression), CNS (central nervous system) exposure, and OTU's (Oxygen toxicity units).

All the above adds up to:- Any O2 mix can be used, provided that you monitor PPO2 levels, CNS and OTU's, and times.

More information about the above can be obtained from IANTD, TDI, and these days, even PADI.
CO2 Narcosis

No, I mean CO2 narcosis. I've been researching it for a while now, it's not well known, but it has happened to me several times. It is totally different from N2 narcosis, and it has nothing to do with shallow-water anything, because it only happens deep. When PaCO2 goes over 70mmHg, CO2 narcosis begins, after which you have approximately 100 seconds (1:40) before you either suffer CO2-syncope or narcolepsy (sudding falling asleep). Symptoms of CO2 narcosis are vaguely similar to N2 narcosis.

Only deep freedivers get CO2 narcosis (i.e. 80m+), ordinary scuba divers cannot tolerate enough CO2 to allow their CO2 levels to go over 70mmHg, with a couple of exceptions--I heard a couple of people with very high CO2 tolerance blacked out from CO2 narcosis while scuba diving at 40m and 69m on air dives (or so the medic claim after examination). I'm not sure how the medic discovered that the 69m blackout was from CO2 syncope and not O2 toxicity.

It sounds like deep water blackout that your describing. It is most common amongst deep air divers who don't ventilate properly while breathing- either due to cheap regulators, improper breathing patterns, or just diving too deep on air. It causes co2 build ups in the body which result in the loss of consiousness.
Longtime scuba divers are more prone to this because they are conditioned to be co2 retainers and may also not breath enough, or are working too hard at depth. There have been cases were divers black out from co2 on deep air dives and are revived by their buddies on the ascent.
The ox-tox that I am talking about are more like epileptic seizures. Divers that switch to the wrong gas at depth can go into flailing convulsions that result in loss of consiousness, regulator mouth piece, and life. Who knows how deep a freediver has to get, or how long his exposure is at that depth, before we start to this happen.
Like I said before, a controlled experiment.

I have been doing some calculations, and from my preliminary results it seems that a deep black out during no-limits (such as Makula, Pipin and Audrey have had) seems to be from O2 toxicity--only because they don't use any energy during the dive, so their CO2 levels are low, despite the huge pressure.

Variable weight dives result in higher CO2 because of the effort during the ascent, but even then, the diver is quite buoyant, because he/she wears a 7mm wetsuit with no weight belt.

My calculations show that the highest CO2 ever encountered will on the ascent of an 80+m constant weight dive, and, that same dangerous CO2 level will start around 70m and peak between 50m and 40m, and it definitely goes WAY over the critical 70mmHg, but there is the question of time. It seems on my 'accident' dive I was truly approaching that time limit and just about fell asleep at 53m. Come to think of it, it feels like I'm on the edge of falling asleep during the entire ascent on the 80+m dives, a classic symptom of CO2 narcosis/narcolepsy.

Yup, the biggest threat to techy divers is Ox Tox. Onset is sudden, and even if you get a warning it's probably too late. Warning apparently take the form of facial twitches, metallic taste in mouth, plus others I cannot remember. Me and a friend of mine were in a recompression chamber for 5.5 hours after getting bent. We were on Pure O2 at a depth of 18m. That's a PPO2 of 2.8 bar!!!!! Double the normal scuba limit of 1.4 bar. He got facial twitches and very fast reactions from the chamber operators. I got nothing at all. Ox tox, like narcosis, is very personal, and no one can tell you why.

I'm told that the process is as follows:- You blackout, you fit, you relax, and still blacked out, you let the mouthpeice out and drown. Easy really.

CO2 retention is usually caused by divers "skip breathing" to try and extend gas usage. Not a problem on a CCR.


Pete S

Seriously? I always get a metallic taste in my mouth when ascending from more than 73-74m...I thought it was a part of the nitrogen narcosis, now I'm worried it's O2 toxicity...The taste goes away by the time I reach about 55m.

Eric, 80+ meter constant- your a machine!

Do you feel the facial twitches that Pete was talking about? There are a series of other symptoms I believe that VENTID is the acronym I was taught to remeber them. visual disturbances are another key sign.

Pete was right about skip breathing. That is a way to build up some serious co2. using a hacksaw at 170' on air is another. With the rebreather it usually only happens when the scrubber goes bad.

Pete, it sounds like your using an Insperation. Is that the one you have?
More symptoms

When I'm ascending from a deep dive in cold water, in addition to the vertigo, dizziness, confusion, sleepiness, loss of concentration, metallic taste, and 'buzzing feeling', my vision definitely gets screwed up, everything is a bit faded, less clear, I hesitate to say blurry--it's like all the images are there, but my mind is paying no attention to them, as if they are far away in a distant tunnel, but right there at the same time, as if in a vague dream.

ok this is just out of curiosity cause i think might make a good school project

here is what i know (or think i know)
1 ata=14.7psi=1013mmHg

how in the world do you calculate PPO2and PPC02 this is my guess but im probably wrong

depth(m)*0.1(ata)*(% of gas)

for example: dive to 50 meters on nitrox 36% (i know it should never be done)

1.8 PP02

thanks ,vince
DeeperBlue.com - The Worlds Largest Community Dedicated To Freediving, Scuba Diving and Spearfishing


ISSN 1469-865X | Copyright © 1996 - 2024 deeperblue.net limited.

DeeperBlue.com is the World's Largest Community dedicated to Freediving, Scuba Diving, Ocean Advocacy and Diving Travel.

We've been dedicated to bringing you the freshest news, features and discussions from around the underwater world since 1996.