MichaelMc
Working toward Cenotes
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 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.