CNS Oxygen Toxicity Over Multiple Dives: A Nitrox Liveaboard Week

What CNS % and OTU measure

Both numbers track oxygen dose, for two different kinds of toxicity. CNS % tracks the risk to the brain from short, high exposures, with a convulsion as the end point. OTU tracks the slower effect on the lungs and the rest of the body from long or repeated exposures. Our CNS / OTU calculator gives both for any mix, depth and time.

CNS % is the time spent at a ppO₂ divided by the NOAA time limit for that ppO₂. Breathe a ppO₂ of 1.0 for 30 minutes and you have used 10% of the 300-minute limit. The fractions from each part of a dive are added up, and 100% means the limit has been reached.

One OTU (oxygen tolerance unit, the same unit as the older UPTD) is one minute of 100% oxygen at 1 bar. For any other exposure, OTU = minutes × ((ppO₂ − 0.5) ÷ 0.5)^0.83. Nothing counts below a ppO₂ of 0.5, which Hamilton describes as the threshold below which no significant symptoms develop. The units add up across dives and days, and that is what makes them useful on a liveaboard.

NOAA oxygen limits for one dive and for 24 hours

NOAA gives two time limits for each ppO₂: one for a single exposure and one for the total in a 24-hour day. Above a ppO₂ of 1.0 the daily limit is longer than the single one. At 1.0 and below the two are the same.

When these limits were proposed for the NOAA Diving Manual, Hamilton noted that they combine CNS and whole-body limits, but that they do not state a method for exposures at several different ppO₂ levels. Summing CNS fractions segment by segment became the common way to fill that gap. Our calculator does it, and Shearwater computers add up exposure the same way during a dive.

DAN splits the range into three zones for open-circuit scuba. A ppO₂ of 1.4 or less is the green light, 1.4-1.6 is yellow, and above 1.6 is red: recreational divers should not exceed it. Our calculator will not plan anything above 1.6.

Worked example: four EAN32 dives in one day

Take a full liveaboard day on EAN32 with four dives, each split into a deeper and a shallower part. Every part goes through the calculator on its own: ppO₂ = 0.32 × (depth ÷ 10 + 1), rounded to 0.01, then rounded up to the next NOAA bracket to find the CNS limit.

The day comes to 55.4% CNS and 154.4 OTU. The safety stops add nothing, because EAN32 at 5 m gives a ppO₂ of 0.48, below the 0.5 threshold where both counts start.

The method leans conservative in two places. The bracket rounding charges the 1.28 bar at 30 m against the 1.3 limit of 180 minutes. And entering a dive at its maximum depth for the whole time overstates it further: dive 1 as 40 minutes at 30 m gives 22.2% and 57.9 OTU instead of 16.7% and 45.5.

Against the 24-hour column the day is also comfortable. The most time at any single ppO₂ is 70 minutes at 0.9 bar (the second half of dive 1 plus dive 3), against a daily limit of 360 minutes. Before any of this, check the mix against the planned depth: EAN32 reaches a ppO₂ of 1.4 at 33.7 m in our nitrox calculator.

Does CNS recover between dives?

Our calculator does not model surface-interval recovery. It works on one depth and time at a time, and its guidance is to add the results together when you dive more than once a day. The 55.4% above is therefore a ceiling: the number you would reach if no recovery happened at all.

Dive computers such as Shearwater's do give credit. Shearwater explains its method this way: CNS never decreases during a dive, and at the surface it falls with a 90-minute half-life, so 80% becomes 40% after 90 minutes and is close to zero after about six half-lives, or 9 hours. Erik Baker calls the 90-minute half-time a common practice in technical diving. Shearwater also says plainly that the method does not guarantee safety.

Applied to the same day with 3-hour surface intervals (two half-lives, so a quarter remains), the running total peaks at about 19% after dive 2 instead of climbing to 55%. Expect your computer to show different figures again, because it follows the real profile rather than flat segments.

OTU has no half-life within the day. REPEX handles recovery through its multi-day limits instead.

OTU limits for a week of diving (REPEX)

The REPEX limits, developed for NOAA habitat diving, allow 850 OTU in a single day, and the allowance per day shrinks as the days add up: 420 a day over six days, 380 over seven, 300 from 11 days on. Six days at the pace of our example is 926 OTU, about 37% of the six-day total of 2,520.

Even a harder day, four dives of 30 minutes at 30 m, comes to about 174 OTU. In both examples, the CNS number and the ppO₂ at depth are the limits you would reach first. Baker notes that even in technical diving CNS is usually the limit of most concern, and that only a few very long, deep dives have pushed the pulmonary limits.

Four REPEX points matter on a longer trip:

• Compare your running total with the total column. The averages cannot be stacked: 850 on day one and 700 on day two is 1,550, above the two-day total of 1,400.
• Oxygen exposure in the days before the trip counts, so a technical course or another dive trip just before belongs in the tally.
• Five days a week with weekends off counts as continuous exposure.
• The dose covers the whole time the ppO₂ is above 0.5, not only bottom time.

CNS toxicity vs pulmonary toxicity: symptoms

CNS toxicity happens at depth, within minutes at a high ppO₂, and can end in a convulsion. Pulmonary toxicity builds over hours to days and shows up mainly in the chest.

NOAA teaches CNS symptoms with the mnemonic VENTID-C: vision (tunnel vision), ears (ringing), nausea, twitching, irritability, dizziness and convulsions. Its training slides call twitching of the face, lips or limbs the most frequent and clearest sign, and warn that a convulsion may come with no warning at all. Susceptibility varies between people and from one moment to the next. StatPearls lists carbon dioxide retention, immersion, cold and exercise as factors that raise seizure risk, and DAN adds that even mild exercise may put nitrox divers at higher risk.

A convulsion is not harmful in itself under normal conditions, Hamilton writes, but underwater it can lead to drowning. DAN's first-aid guidance is to get behind the diver, leave the regulator in the mouth if it is still there (and not try to replace it if it isn't), and make a controlled ascent with slight pressure on the chest to help exhalation.

Pulmonary toxicity shows up as a tickle or burning when breathing in, coughing, chest discomfort and trouble taking a deep breath. Hamilton adds that after longer exposures it can reach beyond the lungs, with numb fingertips and toes, headache and nausea. On a recreational trip it is unlikely: DAN says the most common cause is very long recompression treatments. For scale, Hamilton puts a US Navy Table 6 treatment at about 600 OTU, roughly four days of our example in a single session.

Practical rules for a week of nitrox diving

The limits are generous for recreational nitrox, so most of the work is keeping the ppO₂ at depth in check and paying attention to your body:

• Plan the deepest part of every dive at a ppO₂ of 1.4 or less. On EAN32 that is 33.7 m; check other mixes in the nitrox calculator before the briefing, not at depth.
• Set your computer to the analysed mix of each tank. Its CNS and OTU are only as good as the oxygen fraction you give it.
• Watch the CNS figure along with no-stop time. The deep part of the dive drives it: 30 minutes at 33 m on EAN32 uses 20% of the limit, the same time at 30 m uses 16.7%.
• Keep work and carbon dioxide down at depth. Hamilton advises low-resistance breathing equipment and avoiding heavy exercise, carbon dioxide build-up and temperature extremes.
• Count the whole trip, including any oxygen-rich diving in the days before it.
• Treat twitching, visual changes or ringing ears at depth as a reason to ascend and end the dive. A cough or chest tightness after several days of diving is a reason to stop and get it checked.