Oxygen toxicity and air diving depth records

[Sandtiger Steve]

Hello -- something has been driving me crazy for some time and so i hope someone can answer this. My instructors (c-card, nitrox, etc) have always been very clear about the dangers of oxygen toxicity, and that compared to DCS or narcosis, it's far worse because you don't recover by just ascending.

So, i always have in the back of my mind that going to 20ft or so on 100% O2 means instant death. So does going to 172 ft (1.4 ATA) on Air. However, i keep reading about these guys setting records, diving to 400+ feet on air. that seems to me to be impossible (i didn't do the partial pressure calculation but it's obviously a lot!).

Any help?
thanks

[cerich]

Each individual will has a different susceptibility to oxygen toxicity that actually will varies for that person.

In recreational diving using Nitrox it is generally accepted that 1.4 is the max PPO2 to plan for (sometimes you'll hear 1.6) These limits were chosen because for the average person on a average day keeping your exposure to those limits are very unlikely to cause oxygen toxicity.

Just like sometimes exceeding a PPO2 of 1.4 can cause trouble, sometimes you can exceed 1.4 and not have problems. Once you get above 1.6 it really is like playing Russian Roulette.

By the way CNS O2 tox, will go away when you ascend. The problem is if you have convulsed there is a large likelihood that your regulator came out of your mouth and you drowned. Thats hard to fix underwater.

It's not impossible just well on the crazy side.

[ianr33]

20 feet (1.6ATA O2) is the normal depth for doing deco on pure O2. Normal accepted maximum time for 1.6ATA O2 is 45 minutes.

30 feet ,or even 40 feet on pure O2 for a few minutes would PROBABLY not be a problem,but no informed diver would choose to do that.

There is evidence that the narcotic affect of N2 tends to counteract the affects of high pressures of O2,so if you are going to breathe,say, 3 ATM O2 you are better doing it on air at depth than you would be on pure O2 at 66 feet. Again,no sane diver is going to rely on that.

The pO2 of oxygen at 400 feet is "only" 2.8ATM. How do people do that? A high personal tolerance,very brief time at depth,very low exertion (to reduce CO2 production) and an awful lot of luck/stupidity.

I have always liked this article about diving deep on air
http://www.subaqua.co.uk/reference/wah-wah.shtml

[*Floater*]

Hello -- something has been driving me crazy for some time and so i hope someone can answer this. My instructors (c-card, nitrox, etc) have always been very clear about the dangers of oxygen toxicity, and that compared to DCS or narcosis, it's far worse because you don't recover by just ascending.

So, i always have in the back of my mind that going to 20ft or so on 100% O2 means instant death. So does going to 172 ft (1.4 ATA) on Air. However, i keep reading about these guys setting records, diving to 400+ feet on air. that seems to me to be impossible (i didn't do the partial pressure calculation but it's obviously a lot!).

Any help?
thanks

I think the standards were different then. I think the Navy ppO2 limit was 2.0 at one point. Also, it depends on the person and the activity and how long they stay at depth. If you just float down quickly and then up again, you are less likely to tox than if you are actively finning and fighting currents for extended periods of time. 1.4 is very conservative for sport divers that accounts for poor conditions, poor conditioning and longer dives. Also from reading one of Sheck Exley's books, Caverns Measureless to Man, it seems that he could feel the onset of OxTox symptoms and would abort the dive if he did. There's one part where he has descended down to about 460 ft (on air) to rescue his friends who had blacked out another 20 ft further down, but has to call it because he's about to black out himself.

[johnny_bravo]

Read "Deep Diving" by Brett Gilliam. Great book and it explains his rationale and methods he used to set depth records on air.

Cheers.

-J.-

[Sandtiger Steve]

Thanks. interesting -- i had no idea the tables were that conservative. i thought my head would spontaneously explode if i went below the nitrox MODs . In addition, i always thought oxygen tox (either type) was very insidious, and you may not know you are getting it until too late. But as you all say, a lot of that is ultra conservative sport diving rules.

great board
SS

[cerich]

Thanks. interesting -- i had no idea the tables were that conservative. i thought my head would spontaneously explode if i went below the nitrox MODs . In addition, i always thought oxygen tox (either type) was very insidious, and you may not know you are getting it until too late. But as you all say, a lot of that is ultra conservative sport diving rules.

great board
SS

no, it's not ULTRA conservative, it just conservative, many do have O2 tox without any warning (which is why Navy/commercial test the divers for O2 tolerance). Pulminary O2 tox is almost impossible to hve in recreational diving, CNS tox VERY possible1

[Charlie99]

So, i always have in the back of my mind that going to 20ft or so on 100% O2 means instant death.

The probability of CNS oxtox is a function of both ppO2 and TIME. That's why your nitrox class had probably had that table where you combine ppO2 and time to figure out % of oxtox allowance that you have used up.

Standard NOAA exposure limit is 45 minutes at 1.6ppO2 -- short enough that one can easily exceed it. The NOAA exposure limit at 1.4 ata ppO2 is 150 minutes -- long enough that a recreational diver is not likely to exceed it. Somehow this gets translated in many divers minds to "<1.4ata ppO2 = safe" and ">1.4ata ppO2 = unsafe" or ">1.4ata ppO2 = instant death".

Many of the standard hyperbaric chamber treatment protocols use pure O2 at 60', or 2.8ata. (IIRC, generally for 20 minute blocks with 5 minute air breaks in between). While a few patients do have convulsions, most do not. Convulsion in a chamber are generally no big deal, even if they do occur.

Convulsions underwater can easily result in drowning, and oxtox sensitivity varies widely from person-to-person and from day-to-day in the same person, so it is wise to use conservative limits when diving.

Charlie Allen

[*Floater*]

There is evidence that the narcotic affect of N2 tends to counteract the affects of high pressures of O2,so if you are going to breathe,say, 3 ATM O2 you are better doing it on air at depth than you would be on pure O2 at 66 feet. Again,no sane diver is going to rely on that.

The pO2 of oxygen at 400 feet is "only" 2.8ATM. How do people do that? A high personal tolerance,very brief time at depth,very low exertion (to reduce CO2 production) and an awful lot of luck/stupidity.

In one experiment they had a diver breathe 100% O2 at rest at 70 ft (ppO2 of 3 ATA) twice a week for 3 months, 65 deg F water. It took him anywhere from 7 to 148 min to tox. I don't have a reference for the study, but it's recounted in Recreational Nitrox Diving by Robert N. Rossier.

[Thalassamania]

In the old days (and maybe still today) prior to training with oxygen rebreathers you did a standard oxygen tolerance test, which was 20 minutes (?) at 60 feet on pure oxygen. If you convulsed, you were out of the training. In a 1986 paper it is noted that, “The U.S. Navy standard Oxygen Tolerance Test was not sensitive enough to produce symptoms in the three oxygen-susceptible divers identified ...”

In any case, 60 FSW is 2.82 ATA of oxygen which is like 410 feet on air.

BTW: The Navy Transit with Excursion Limits for poure oxygen are 20 FSW with an excursion to 40 FSW for 15 min or 50 FSW for 5 min for a total dive not to exceed 240 min.