Chapter 5 was about the inert gas in your breath. The oxygen in it can also harm you. Your body needs oxygen, but at a high enough pressure it becomes a poison to the brain and nerves. The worst result is a convulsion, a fit, under water.
This is central nervous system (CNS) oxygen toxicity. The central nervous system is your brain and spinal cord. It is the more dangerous of the 2 forms of oxygen toxicity. The other form harms the lungs over long exposures, and chapter 7 covers it.
Paul Bert, who also explained DCS, first saw it in 1878 in animals breathing oxygen at high pressure. It is still called the Paul Bert effect.
Divers control CNS exposure in 2 ways:
- A PO2 limit: the highest PO2 (partial pressure of oxygen, also written ppO2) they will breathe. The partial pressure is the oxygen’s share of the total pressure. It is measured in ATA (atmospheres absolute, about the same as bar).
- The CNS clock: a running total of how much of the allowed exposure they have used so far, as a percentage of the NOAA limit.
Think of the CNS clock like a fuel gauge: it shows how much of your oxygen allowance you have used, and how much is left.
- Air
Planned with DiveLogicFind the maximum depth for a mix
Partial pressure and depth
Air has an FO2 (fraction of oxygen) of 0.21 at any depth. But the PO2 rises with depth. This is Dalton’s law (chapter 2): PO2 is the FO2 times the ambient pressure, the total pressure around you.
At the surface, air gives a PO2 of 0.21 × 1 = 0.21 ATA. At 10 m the ambient pressure has doubled to 2 ATA, so the PO2 doubles too: 0.21 × 2 = 0.42 ATA. Your body responds to the PO2, not to the percentage on the cylinder label.
The working PO2 of 1.4
Nitrox has more oxygen than air, so it reaches a given PO2 at a shallower depth. Most divers keep their bottom gas, the gas they swim and work on, at a working PO2 of 1.4 ATA or less.
The depth at which a mix reaches that limit is its maximum operating depth (MOD). Each dot on the chart marks one. Richer mixes let you stay longer without decompression stops, but have a shallower MOD.
The deco PO2 of 1.6
During decompression stops you are resting and breathing easily. So divers accept a higher contingency or deco PO2 of 1.6 ATA on their deco gas. This is the highest value in the NOAA table below. It is why EAN50 is used from 21 m and pure oxygen from 6 m.
Hard work and hypercapnia (a build-up of carbon dioxide, CO2) both raise the risk of oxygen toxicity. So the limit on the bottom is lower than the limit on a stop.
Symptoms
Symptoms of CNS oxygen toxicity can include:
- blurred or narrowed vision (tunnel vision);
- ringing or other sounds in the ears;
- feeling sick;
- twitching, often of the lips or face;
- irritability, restlessness or anxiety;
- dizziness.
The most serious sign is a convulsion, which can come with no warning at all. Under water, a convulsion can make you lose your mouthpiece and drown. Do not rely on warning signs. If you notice any of these symptoms, lower your PO2 at once, in the way your training sets out.
What raises the risk
In the 1940s Kenneth Donald tested oxygen exposures on volunteer divers for the Royal Navy. Tolerance varied widely between people, and in the same person from day to day. It was lower in water than in a dry chamber.
Hard work and hypercapnia (CO2 build-up) raise the risk. Work raises the carbon dioxide in your blood. Carbon dioxide widens the blood vessels of the brain, so more oxygen reaches it. Dense gas at depth has the same effect, and so does a rebreather whose scrubber no longer removes carbon dioxide. The limits below are for a diver at rest or working lightly. None of them is a personal guarantee.
Working out the CNS clock
The US National Oceanic and Atmospheric Administration (NOAA) sets a time limit for each PO2. The CNS clock is simply how much of that time you have used.
A dive has parts at different PO2s. Work out the share for each part and add them up. The 2 parts in the example make 13.3 + 22.2 = about 36 %. At 100 % you have reached the NOAA limit.
- Minutes allowed
Planned with DiveLogicFind the time limit for a ppO₂
NOAA single-exposure limits
NOAA publishes a table of how long you may breathe each PO2 in one exposure. The time falls steeply as the PO2 rises: 150 minutes at 1.4 ATA and 45 minutes at 1.6 ATA.
Below 0.5 ATA there is no limit. At the top of the table, 0.1 ATA more oxygen cuts the allowed time by more than half.
The CNS clock on a planned dive
The CNS clock is the percentage of the NOAA limit used so far. Each part of the dive adds its share, as worked out above. Between the rows of the NOAA table, DiveLogic fills in the values in a straight line.
The figure shows the clock through a dive to 40 m for 25 minutes on air, with EAN50 as deco gas from 21 m. On the bottom, air at 40 m (5 ATA) gives a PO2 of about 0.21 × 5 = 1.05, and the clock rises slowly. At the gas switch to EAN50 at 21 m (3.1 ATA), the PO2 jumps to about 0.5 × 3.1 = 1.6. About half of the CNS exposure on this dive comes after the switch.
Deco gas and oxygen exposure
The grey line is the same dive with no decompression gas. On air the stops take much longer. Less oxygen in the mix means more nitrogen, so the tissues (chapter 10) release nitrogen more slowly.
But the CNS clock finishes lower. A rich deco gas shortens decompression and adds oxygen exposure. Divers choose their deco gases and switch depths to spend that exposure where it shortens decompression most.
Recovery at the surface
At the surface the CNS clock falls. DiveLogic, like most planners and dive computers, halves it every 90 minutes. So the 21 % from this dive falls to about 11 % after 90 minutes, and to about 5 % after 3 hours.
A second dive starts with what is left. On a day of several dives, the clock at the start of each dive depends on how long you spent at the surface before it.
Try it
Set a PO2 and a time. The gauge shows the CNS clock for that single exposure. Its marks show where the DiveLogic planner gives advice and where it warns. Compare 1.4 ATA for 60 minutes with 1.6 ATA for 60 minutes.
Planned with DiveLogicOpen the CNS and OTU calculator
What to remember
- CNS oxygen toxicity depends on PO2: the FO2 of the gas times the ambient pressure at which you breathe it.
- It can cause a convulsion without warning. Vision, hearing and other symptoms may come first, but often do not.
- Most divers keep a working PO2 of 1.4 or less and use a deco PO2 of 1.6 only while resting on decompression.
- The CNS clock is the share of NOAA’s single-exposure limit you have used. It halves every 90 minutes at the surface.
- Hard work, hypercapnia (CO2 build-up) and being in water raise the risk. Tolerance varies between people and from day to day.
Sources
- Bert P. (1878). La pression barométrique: recherches de physiologie expérimentale. Masson, Paris.
- Donald K. W. (1947). Oxygen poisoning in man. British Medical Journal 1: 667-672 and 712-717.
- NOAA (2001). NOAA Diving Manual: Diving for Science and Technology, 4th edition. Best Publishing, Flagstaff, AZ. (Oxygen exposure limits.)