Is nitrox worth it for deeper rec dives?

[MichaelMc]

On O2 toxicity time limits. Apparently the time limits were added down to 0.6 PPO2 by NOAA third edition (1991), for CNS. I do not have the NOAA manual. Sawatzky (2009), which PfcAJ linked, describes CNS toxicity as over 1.6 PP, and mentions NOAA and Navy limits. US Navy (2017) describes CNS toxicity as unlikely below 1.3. My Nitrox manual does not specify the time limit source but the CNS discussion emphasized depth and buoyancy control. I see why the ones quoted would be CNS numbers. I don't understand why that time limit table goes down to 0.6 as CNS based.

I do not find a Navy (2017) time based CNS limit below 1.3 ATA. Making the low 0.6 PP end of those time limit tables confusing for CNS. Maybe it is due to the rapid onset and severity of CNS vs pulmonary toxicity. Bolds below are mine.

You can see below that the Navy 24 hour 1.3 PP02 limit of 4 hours for pulmonary/visual is very similar to the NOAA 210 minute limit. Now, visual is part of our CNS, but visual is a third type of effect listed by Sawatzky.

The latest Navy Dive Manual v7 (21017) says:
3-9.2.2 Central Nervous System (CNS) Oxygen Toxicity
Central nervous system (CNS) oxygen toxicity, sometimes called high pressure oxygen poisoning, can occur whenever the oxygen partial pressure exceeds 1.3 ata in a wet diver or 2.4 ata in a dry diver. The reason for the marked increase in susceptibility in a wet diver is not completely understood. At partial pressures above the respective 1.3 ata wet and 2.4 ata dry thresholds, the risk of CNS toxicity is dependent on the oxygen partial pressure and the exposure time. The higher the partial pressure and the longer the exposure time, the more likely CNS symptoms will occur. This gives rise to partial pressure of oxygen exposure time limits for various types of diving.

10-3 OXYGEN TOXICITY
The NITROX Equivalent Air Depth (EAD) Decompression Selection Table (Table 10-1) was developed considering both CNS and pulmonary oxygen toxicity. Normal working dives that exceed a ppO2 of 1.4 ata are not permitted, principally to avoid the risk of CNS oxygen toxicity. Dives with a ppO2 less than 1.4 ata, however, can be conducted using the full range of bottom times allowed by the air tables without concern for CNS or pulmonary oxygen toxicity.

Supervisors must keep in mind that pulmonary oxygen toxicity may become an issue with frequent, repetitive diving. The effects of pulmonary oxygen toxicity can be cumulative and can reduce the underwater work performance of susceptible individuals after a long series of repetitive daily exposures. Fatigue, headache, flu- like symptoms, and numbness of the fingers and toes may also be experienced with repetitive exposures. Table 10-1 takes these repetitive exposures into account, and therefore problems with oxygen toxicity should not be encountered with its use. If symptoms are experienced, the diver should stop diving NITROX until they resolve.

15-9: MULTI-DAY DIVING FOR 1.3 ATA PPO2 EC-UBA
Repetitive exposure to an oxygen partial pressure of over 1.0 ata over a multi-day period may result in the gradual development of pulmonary oxygen toxicity and/ or changes in visual acuity. ...
- Limit total 1.3 ata ppO2 dive time to a maximum of 4 hours per day
- Limit total 1.3 ata ppO2 dive time to a maximum of 16 hours per week

16-2.1: CNS oxygen toxicity is not likely to occur at oxygen partial pressures below 1.3 ata

Sawatzky (2009), Oxygen Toxicity - Signs and Symptoms | Dive Rite, starts:
" As shown in the diagram, the toxic effects of oxygen at partial pressures between 0.45 ATA and 1.6 ATA are primarily on the lungs while the toxic effect at pO2s over 1.6 ATA are primarily on the brain. "

Sawatzky describes tracking pulmonary toxicity with time units Unit Pulmonary Toxic Dose (UPTD).

Sawatzky describes CNS toxicity as over 1.6 PP for shorter times, and has a guideline of 2.0 ATA while resting. Which would make having a CNS time limit at say 0.6 rather confusing, as is found on the PPO2 time limit tables. He talks about limiting time of exposure, but it is for something he describes as occurring above 1.6ATA. He ends noting that NOAA and US Navy and other forces have pO2 limits and times.

 

[tarponchik]

Supervisors must keep in mind that pulmonary oxygen toxicity may become an issue with frequent, repetitive diving. The effects of pulmonary oxygen toxicity can be cumulative and can reduce the underwater work performance of susceptible individuals after a long series of repetitive daily exposures...
Repetitive exposure to an oxygen partial pressure of over 1.0 ata over a multi-day period may result in the gradual development of pulmonary oxygen toxicity and/ or changes in visual acuity.

Good timing, thank you.

 

[tursiops]

On O2 toxicity time limits. Apparently the time limits were added down to 0.6 PPO2 by NOAA third edition (1991), for CNS. I do not have the NOAA manual. Sawatzky (2009), which PfcAJ linked, describes CNS toxicity as over 1.6 PP, and mentions NOAA and Navy limits. US Navy (2017) describes CNS toxicity as unlikely below 1.3. My Nitrox manual does not specify the time limit source but the CNS discussion emphasized depth and buoyancy control. I see why the ones quoted would be CNS numbers. I don't understand why that time limit table goes down to 0.6 as CNS based.

I do not find a Navy (2017) time based CNS limit below 1.3 ATA. Making the low 0.6 PP end of those time limit tables confusing for CNS. Maybe it is due to the rapid onset and severity of CNS vs pulmonary toxicity. Bolds below are mine.

You can see below that the Navy 24 hour 1.3 PP02 limit of 4 hours for pulmonary/visual is very similar to the NOAA 210 minute limit. Now, visual is part of our CNS, but visual is a third type of effect listed by Sawatzky.

The latest Navy Dive Manual v7 (21017) says:
3-9.2.2 Central Nervous System (CNS) Oxygen Toxicity
Central nervous system (CNS) oxygen toxicity, sometimes called high pressure oxygen poisoning, can occur whenever the oxygen partial pressure exceeds 1.3 ata in a wet diver or 2.4 ata in a dry diver. The reason for the marked increase in susceptibility in a wet diver is not completely understood. At partial pressures above the respective 1.3 ata wet and 2.4 ata dry thresholds, the risk of CNS toxicity is dependent on the oxygen partial pressure and the exposure time. The higher the partial pressure and the longer the exposure time, the more likely CNS symptoms will occur. This gives rise to partial pressure of oxygen exposure time limits for various types of diving.

10-3 OXYGEN TOXICITY
The NITROX Equivalent Air Depth (EAD) Decompression Selection Table (Table 10-1) was developed considering both CNS and pulmonary oxygen toxicity. Normal working dives that exceed a ppO2 of 1.4 ata are not permitted, principally to avoid the risk of CNS oxygen toxicity. Dives with a ppO2 less than 1.4 ata, however, can be conducted using the full range of bottom times allowed by the air tables without concern for CNS or pulmonary oxygen toxicity.

Supervisors must keep in mind that pulmonary oxygen toxicity may become an issue with frequent, repetitive diving. The effects of pulmonary oxygen toxicity can be cumulative and can reduce the underwater work performance of susceptible individuals after a long series of repetitive daily exposures. Fatigue, headache, flu- like symptoms, and numbness of the fingers and toes may also be experienced with repetitive exposures. Table 10-1 takes these repetitive exposures into account, and therefore problems with oxygen toxicity should not be encountered with its use. If symptoms are experienced, the diver should stop diving NITROX until they resolve.

15-9: MULTI-DAY DIVING FOR 1.3 ATA PPO2 EC-UBA
Repetitive exposure to an oxygen partial pressure of over 1.0 ata over a multi-day period may result in the gradual development of pulmonary oxygen toxicity and/ or changes in visual acuity. ...
- Limit total 1.3 ata ppO2 dive time to a maximum of 4 hours per day
- Limit total 1.3 ata ppO2 dive time to a maximum of 16 hours per week

16-2.1: CNS oxygen toxicity is not likely to occur at oxygen partial pressures below 1.3 ata

Sawatzky (2009), Oxygen Toxicity - Signs and Symptoms | Dive Rite, starts:
" As shown in the diagram, the toxic effects of oxygen at partial pressures between 0.45 ATA and 1.6 ATA are primarily on the lungs while the toxic effect at pO2s over 1.6 ATA are primarily on the brain. "

Sawatzky describes tracking pulmonary toxicity with time units Unit Pulmonary Toxic Dose (UPTD).

Sawatzky describes CNS toxicity as over 1.6 PP for shorter times, and has a guideline of 2.0 ATA while resting. Which would make having a CNS time limit at say 0.6 rather confusing, as is found on the PPO2 time limit tables. He talks about limiting time of exposure, but it is for something he describes as occurring above 1.6ATA. He ends noting that NOAA and US Navy and other forces have pO2 limits and times.

I don’t have the very latest edition (Vers 6) of the NOAA Diving Manual, dated 2017, but I do have the 5th Edition, dated 2013. The material quoted above is from the 3rd edition, dated 1991, so is more than a quarter-century out of date.

Summarizing the Oxygen Toxicity material in section 4.3.3, pages 4-24 through 4-29:

• CNS oxygen toxicity (or “oxygen poisoning”) can develop at high PPO2 exposures (around 1.6 and above) after a few minutes
• Whole-Body oxygen toxicity is slow developing after longer exposures at lower PPO2 levels. A classical symptom is pulmonary irritation.
• There are wide variations in tolerance.
• Prevention of CNS toxicity is managed by time limits related to PPO2, for example 45 mins at PPO2=1.6 for a single exposure, and 120 mins at 1.5 for s single exposure.
• For multiple exposures (repetitive dives), the OxygenClock concept allows calculation of a percentage of allowable maximum on each dive, and thee are added up to a 100% maximum.
  • Example: a 30 min dive to PPO2=1.5 allows 120 mins maximum, so 30 mins would accumulate 25% of the maximum. Four such dives could be done before hitting the 100% limit. NOAA assumes a 12 hour flushing time: after 12 hours of no high oxygen input, you can start over with a zero clock.
  • Note: four limits must be simultaneously monitored: Gas, Nitrogen, and Oxygen. The smallest limit applies.
  • It is presumed that no CNS oxygen clock accumulation occurs for PPO2 less than 0.6.
• Prevention of pulmonary (Whole-Body) toxicity is unlikely in open-circuit diving operations, but extended rebreather operations may involve this risk.

 

[MichaelMc]

Thanks for posting more of the NOAA details.

I was wrong in thinking the 1.4 clock was pulmonary. I think I assumed that based on Sawatzky saying pulmonary is the main effect below 1.6 and how my manual read.

In an earlier thread, CNS %, Dr Mitchell links to a cool 2010 proceedings that discusses O2 toxicity on pages 38-66. On page 59 it notes that
"the NOAA limits apply not just to CNS oxygen toxicity but also to pulmonary toxicity and to symptoms such as finger numbness that have been described as 'whole-body' oxygen toxicity"
Though they go on to refer to them as CNS time clock.

Some other threads, mostly saying its fussy:
Article: Daily Limits for CNS Oxygen Toxicity
ppO2 for nitrox, why so low?
https://www.scubaboard.com/community/threads/cns.566321/

 

[MrVegas]

This has been a very interesting thread. I took the SDI nitrox class, and the book really focuses on 1.6 as the max safe limit and indicated that 32% was the "optimum" mix because it gave you a PPO of 1.6 at 132 ft -- right around the same recommended max depth for recreational diving. The instructor did say in class, though, that 1.4 was the most commonly used number, and the times I have used nitrox, the tank has always been marked with the MOD based on 1.4, and I have never even really been close to the MOD for any nitrox dive I have done.

I guess one question I have is how many documented oxygen toxicity seizures have there been for nitrox divers staying under a PPO of 1.6 and within the exposure time limits?

 

[BRT]

This has been a very interesting thread. I took the SDI nitrox class, and the book really focuses on 1.6 as the max safe limit and indicated that 32% was the "optimum" mix because it gave you a PPO of 1.6 at 132 ft -- right around the same recommended max depth for recreational diving. The instructor did say in class, though, that 1.4 was the most commonly used number, and the times I have used nitrox, the tank has always been marked with the MOD based on 1.4, and I have never even really been close to the MOD for any nitrox dive I have done.

I guess one question I have is how many documented oxygen toxicity seizures have there been for nitrox divers staying under a PPO of 1.6 and within the exposure time limits?

That is going to be a small number on OC scuba.

 

[uncfnp]

There are sooo many threads on oxtox. Here are a few of the more recent/interesting...

Likelihood of getting oxygen toxicity at 1.4 PPO

Oxygen Toxicity Limits & Symptoms

 

[Manatee Diver]

I took the SDI nitrox class, and the book really focuses on 1.6 as the max safe limit and indicated that 32% was the "optimum" mix because it gave you a PPO of 1.6 at 132 ft -- right around the same recommended max depth for recreational diving.

I came to a similar conclusion. EAN32 allows for a significant increase of no stop time particularly at 100ft, which is the depth of main deck of the interesting wrecks, to the point that you get a nice satisfying dive of at least 30 minutes. Also my air usage on my HP100 typically matches up with my no stop time on the first dive of the day.

So unless I need a particular mix for a dive series, I pretty much just fill my steel tanks with EAN32.

 

[Alan Browne]

15-9: MULTI-DAY DIVING FOR 1.3 ATA PPO2 EC-UBA
Repetitive exposure to an oxygen partial pressure of over 1.0 ata over a multi-day period may result in the gradual development of pulmonary oxygen toxicity and/ or changes in visual acuity. ...
- Limit total 1.3 ata ppO2 dive time to a maximum of 4 hours per day
- Limit total 1.3 ata ppO2 dive time to a maximum of 16 hours per week

This section caught my eye. (Yes I read it all). I'll be going on a winter Carrib week later this winter. If all goes right I will do as many as 21 dives on 6 days.

Question: will the computer (I have not yet received) warn me of the integral PPO2 over the course of many days? That said, I doubt very much I will be anywhere close to 1.3 ata PPO2 * 4 hours per day or even 16 hours in the 6 days.

I like to think I'm very aware of my visual acuity. Is that false confidence?

Will loss of visual acuity (if it occurs) due to high PPO2 exposure recover?

 

[Manatee Diver]

This section caught my eye. (Yes I read it all). I'll be going on a winter Carrib week later this winter. If all goes right I will do as many as 21 dives on 6 days.

Question: will the computer (I have not yet received) warn me of the integral PPO2 over the course of many days? That said, I doubt very much I will be anywhere close to 1.3 ata PPO2 * 4 hours per day or even 16 hours in the 6 days.

I like to think I'm very aware of my visual acuity. Is that false confidence?

Will loss of visual acuity (if it occurs) due to high PPO2 exposure recover?

See further down in the post.

Prevention of pulmonary (Whole-Body) toxicity is unlikely in open-circuit diving operations, but extended rebreather operations may involve this risk.

I went back and read how the OTU was calculated, and let's run some numbers. Per the NOAA Manual 5th edition, over 6 days of diving you are allowed up to 420 OTU doses average per a day and a total of 2520. At PPO2 of 1.4 you get 1.63 OTU/min. So even if you spend the entire time at 1.4, you need to dive 257 minutes a day, and a total of 1,546 minutes for the 6 day period. So over 21 dives that would be 73 minutes per a dive, which means unless you are diving EAN40 at 80 feet (using the NOAA Nitrox charts ver 7) for every dive you aren't likely to come close to it.

Whole body toxicity is only really a concern for rebreather divers because of the select able PPO2 set point. They can tell their rebreather to keep the PPO2 at 1.3 for the entire dive, including shallow portions. Which allows them to build up the toxicity. While recreational OC divers only really near their PPO2 max for a small portion of the dive. I am willing to bet if I went through my computer my time above a PPO2 of 1.3 per a dive could be counted on my fingers and toes.