Any reported cases of Ox Tox between 1.4 and 1.6?

[crofrog]

I don't need an analysis, I just know your statement doesn't apply to any of the many active divers I know.

I could make a similar "qualified" statement like: "Most divers probably shouldn't be below 60 ft", not sure how that is helpful or relevant to the discussion, but it is at least as valid as yours.

I appreciate the rigor you apply to discussions you engage in.

 

[flymolo]

I'm talking about how Nitrox is actually used, not how it is marketed. There is no requirement to dive up to your NDL, and most don't while using Nitrox. That means less N2 uptake compared to doing the same dive on air.

i am fairly certain recreational divers use nitrox to extend their NDL times. Less N2 uptake is in fact another way of saying, increased NDL

 

[boulderjohn]

The most common use of recreational Nitrox is to uptake less nitrogen, not to try and extend the bottom time. On a repetitive-dive day, less N2 is good.

I'm talking about how Nitrox is actually used, not how it is marketed. There is no requirement to dive up to your NDL, and most don't while using Nitrox. That means less N2 uptake compared to doing the same dive on air.

I have never heard this argument before. I dive nitrox quite a bit, and my primary purpose is to extend bottom time. No, there is no rule that says you have to dive to the nitrox limits, but if you dive longer than the air limits, you have extended bottom time.

Less O2 is also a benefit. If you dive longer than air time and shorter than the limits, you get both benefits.

 

[tursiops]

@flymolo @boulderjohn
Perhaps we see different recreational divers. The ones I see mostly are on liveaboards, boat dives, and shore dives with resorts, almost always with a dive guide in the water. Also BHB.
The profiles are multilevel, not square (except BHB), and almost always limited by gas available or "maximum dive time" allowable (like 60 mins), NOT by NDL. Air and Nitrox divers are often mixed in the group. Yes, Nitrox give you more bottom time, but for a square profile that is 80 ft or less, you are gas limited (with an AL80 at an RMV of 0.5 cuft/min). I see people surfacing based on gas reserves, not NDL limits.

 

[boulderjohn]

The accident report seems a little mangled or incomplete. It states, "Tanks were filled with 24/25 trimix". Was that based on a direct gas analysis or tank labels or dive computer gas setting?

Since this was an IUCRR report, I think we can be sure the gas was checked. That is a standard part of their procedures.

The description of the victim's symptoms seems consistent with a seizure. But assuming it was a seizure, was it caused by CNS oxygen toxicity or something else?

That is true. Jared Hires died after a seizure, and it was determined it was not related to toxicity.

This illustrates the problem with making any decisions on oxygen toxicity at these PO2s. You cannot diagnose it after the fact. This is the only case I know of in which a possible seizure was observed with someone who was not in very serious violation of MODs.

A couple decades ago DAN had an article related to the question of the potential for Sudafed to trigger toxicity. The article I found on that now is newer, so I cannot cite the older article. The older article spelled out this problem. There are (or or at least were) pretty much no known cases in recreational diving on which to make a judgement. Every year a few divers are found with no apparent cause of death--no one saw what happened. The entire research was based on a handful of such divers who were found deceased with no apparent reason. If they had been using nitrox and Sudafed on the dive, then maybe (who knows?) it might have been a case of toxicity. They had no cases in which a recreational diver was clearly observed having what was deemed to be a toxing event, with or without Sudafed.

 

[inquis]

The most common use of recreational Nitrox is to uptake less nitrogen, not to try and extend the bottom time.

Aren't you implicitly assuming being gas limited with this statement?
Nevermind, your post above indicates you are, based on the reasonable assumption of multilevel profiles or 70 ft max depths. I have to agree, that is probably "most".

 

[Capt Jim Wyatt]

The original question:

I know of a woman who toxed and died at 1.4 here in cave country.

 

[crofrog]

The profiles are multilevel, not square (except BHB), and almost always limited by gas available or "maximum dive time" allowable (like 60 mins), NOT by NDL. Air and Nitrox divers are often mixed in the group. Yes, Nitrox give you more bottom time, but for a square profile that is 80 ft or less, you are gas limited (with an AL80 at an RMV of 0.5 cuft/min). I see people surfacing based on gas reserves, not NDL limits.

That's basically exactly what I said, no reason to dive higher than a 1.2 for single tank recreational diving, because the gas is the limiting factor.

In managing partial pressures of oxygen (ppO2) during dives, there’s a balancing act between minimizing decompression sickness (DCS) risk by maintaining a higher ppO2 and reducing the probability of central nervous system (CNS) oxygen toxicity by running a lower ppO2. To evaluate these choices, it’s helpful to remember that risk is defined by both probability and impact.

Running a lower ppO2 increases the probability of DCS due to greater nitrogen loading, since a lower oxygen fraction means a higher nitrogen fraction in the breathing gas. This elevated nitrogen fraction leads to additional inert gas uptake in tissues, increasing the risk of DCS. The impact of DCS varies widely, ranging from minor symptoms that resolve with surface oxygen to more severe cases that require recompression treatment, drowning from DCS is not common.

By comparison, CNS oxygen toxicity events are low-probability occurrences within recommended ppO2 limits (such as 1.2 atm during working phases and up to 1.6 atm in decompression). However, the impact of a CNS oxygen toxicity event is likely catastrophic if it occurs underwater, as a seizure would likely incapacitate the diver, leading to a high risk of drowning before assistance can be provided.

Given the much higher impact of CNS oxygen toxicity (possible drowning) versus DCS (generally treatable) diving practices should emphasize conservative ppO2 limits. This approach accepts a slight increase in DCS probability due to higher nitrogen tissue loading in favor of reducing the chance of a low-probability, high-impact CNS oxygen toxicity event.

Risk Type	Condition	Probability	Impact	Overall Risk Level
DCS	Lower ppO₂ (e.g., 1.2 ATA)	Moderate	Moderate - Variable symptoms; may require recompression but rarely life-threatening	Moderate
	Higher ppO₂ (e.g., 1.4-1.6 ATA)	Low	Moderate - Reduced inert gas loading, lowering probability of DCS	Low to Moderate
CNS Oxygen Toxicity	Lower ppO₂ (e.g., 1.2 ATA)	Very Low	Catastrophic - Seizure can result in drowning	Low
	Higher ppO₂ (e.g., >1.2 ATA)	Low	Catastrophic - Greater ppO₂ increases seizure risk, leading to drowning if it occurs	High

 

[davehicks]

That's basically exactly what I said, no reason to dive higher than a 1.2 for single tank recreational diving, because the gas is the limiting factor.

In managing partial pressures of oxygen (ppO2) during dives, there’s a balancing act between minimizing decompression sickness (DCS) risk by maintaining a higher ppO2 and reducing the probability of central nervous system (CNS) oxygen toxicity by running a lower ppO2. To evaluate these choices, it’s helpful to remember that risk is defined by both probability and impact.

Running a lower ppO2 increases the probability of DCS due to greater nitrogen loading, since a lower oxygen fraction means a higher nitrogen fraction in the breathing gas. This elevated nitrogen fraction leads to additional inert gas uptake in tissues, increasing the risk of DCS. The impact of DCS varies widely, ranging from minor symptoms that resolve with surface oxygen to more severe cases that require recompression treatment, drowning from DCS is not common.

By comparison, CNS oxygen toxicity events are low-probability occurrences within recommended ppO2 limits (such as 1.2 atm during working phases and up to 1.6 atm in decompression). However, the impact of a CNS oxygen toxicity event is likely catastrophic if it occurs underwater, as a seizure would likely incapacitate the diver, leading to a high risk of drowning before assistance can be provided.

Given the much higher impact of CNS oxygen toxicity (possible drowning) versus DCS (generally treatable) diving practices should emphasize conservative ppO2 limits. This approach accepts a slight increase in DCS probability due to higher nitrogen tissue loading in favor of reducing the chance of a low-probability, high-impact CNS oxygen toxicity event.

Risk Type	Condition	Probability	Impact	Overall Risk Level
DCS	Lower ppO₂ (e.g., 1.2 ATA)	Moderate	Moderate - Variable symptoms; may require recompression but rarely life-threatening	Moderate
	Higher ppO₂ (e.g., 1.4-1.6 ATA)	Low	Moderate - Reduced inert gas loading, lowering probability of DCS	Low to Moderate
CNS Oxygen Toxicity	Lower ppO₂ (e.g., 1.2 ATA)	Very Low	Catastrophic - Seizure can result in drowning	Low
	Higher ppO₂ (e.g., >1.2 ATA)	Low	Catastrophic - Greater ppO₂ increases seizure risk, leading to drowning if it occurs	High

The logical conclusion you came to based on an "overall risk level" is extremely misguided.

Example:

Chance of death in car crash: Low - most car crashes are minor and don't involve significant injuries.
Chance of death in an airplane crash: High - airplanes tend to crash at high speed and kill everyone.

Conclusion: Traveling my Car is safer than by Plane.

Reality: Airplanes crashes are vanishingly rare, and car crashes kill ~10,000 times more people than airplane crashes each year.

Oxygen toxicity events in Scuba diving are vanishing rare. DCS occurs far more frequently resulting in hundreds or thousands of injuries every year. Even finding documented examples of OxTox in Scuba are difficult as highlighted by this thread.

 

[Nick_Radov]

The original question:

I know of a woman who toxed and died at 1.4 here in cave country.

Are you referring to the 14 July 2007 incident or something else? Based on the limited public information available it seems unclear whether she actually toxed, or seized for some other reason.

https://caves.org/wp-content/uploads/2021/10/February_10_ACA0708.pdf

2007-07-14 Incident – IUCRR

iucrr.org