Oxygen Toxicity Calculations: OTUs and CNS Clock

Recommended resources

The freely available resources this lesson will direct you to.

Hello! Welcome back to your course.

In our last two lessons, we explored the two major types of oxygen toxicity: the acute, convulsive risk to the Central Nervous System (CNS) and the slower, inflammatory damage to the lungs (pulmonary toxicity). We established that both are "dose-dependent," meaning the risk is a function of both the partial pressure of oxygen ( $P O_{2}$ ) you breathe and the duration of the exposure.

This brings us to a critical practical question: How do we track and limit this dose?

This lesson directly addresses that question and fulfills the learning outcome: Calculate oxygen exposure using Oxygen Toxicity Units (OTUs) and the CNS Oxygen Toxicity Clock.

We will cover two essential tools used in technical dive planning to manage oxygen exposure:

The CNS Oxygen Toxicity Clock: A method for tracking your cumulative exposure to high $P O_{2}$ to manage the risk of acute, neurological toxicity.
Oxygen Toxicity Units (OTUs): A system for quantifying your oxygen "dose" to manage the risk of long-term, pulmonary toxicity.

By the end of this lesson, you will be able to perform the fundamental calculations that underpin safe technical diving.

1. The CNS Oxygen Toxicity Clock

The CNS Clock is a way of tracking your exposure as a percentage of a maximum allowable limit. The higher your $P O_{2}$ , the faster the "clock" ticks and the quicker you accumulate percentage points. The goal is to keep your total exposure for a single dive and across multiple dives within accepted limits (typically, not exceeding 80% on a single dive and 100% in a 24-hour period are common guidelines).

The foundation for this system is the table of single exposure limits published by the National Oceanic and Atmospheric Administration (NOAA).

Shearwater and the CNS Oxygen Clock

The article 'Shearwater and the CNS Oxygen Clock' from dive computer manufacturer Shearwater provides a clear explanation of how the CNS clock works, based on the NOAA limits. It's an excellent resource for understanding the principles your dive computer uses.

Please read the beginning of the article, up to the end of the paragraph that describes the 90-minute half-life. Focus on the NOAA table and the example calculation provided: CNS% = (Time at PO2 / Max Time at PO2) * 100.

As the article explains, the calculation is straightforward. For a given segment of a dive at a stable $P O_{2}$ :

CNS% = \frac{Exposure Time}{NOAA Single Exposure Limit} \times 100

Example:
If you spend 30 minutes at a $P O_{2}$ of 1.4 ATA:

The NOAA table shows the maximum single exposure limit for a $P O_{2}$ of 1.4 ATA is 150 minutes.
Your CNS% loading for that segment is: $\frac{30 min}{150 min} \times 100 = 20%$ .

If you then ascended to a depth where your $P O_{2}$ dropped to 1.2 ATA and spent 60 minutes there:

The NOAA limit for 1.2 ATA is 210 minutes.
Your CNS% loading for that segment is: $\frac{60 min}{210 min} \times 100 \approx 28.6%$ .
Your total CNS% for the dive would be the sum of the segments: $20% + 28.6% = 48.6%$ .

Surface Intervals and Recovery

A key feature of the CNS clock model is that your body "recovers." The accumulated CNS% is not permanent. The model uses a 90-minute half-life. This means that for every 90 minutes you spend on the surface breathing air, your accumulated CNS% value is reduced by half.

If you surface with a CNS of 80%, after 90 minutes it will be 40%.
After another 90 minutes (180 minutes total), it will be 20%.
And so on, until it returns to near zero.

This is crucial for planning repetitive dives within a single day.

2. Oxygen Toxicity Units (OTUs)

While the CNS clock manages acute neurological risk, we need a different tool for the chronic pulmonary risk we discussed in the last lesson. This tool is the Oxygen Toxicity Unit (OTU), also known as the Unit Pulmonary Toxic Dose (UPTD).

The concept is to quantify the total dose of oxygen that contributes to lung irritation and damage.

Definition: One OTU is defined as the pulmonary toxic effect of breathing 100% oxygen at 1 atmosphere absolute (ATA) for one minute.
Threshold: Pulmonary effects are considered negligible at a $P O_{2}$ of 0.5 ATA or below. Therefore, OTUs only begin to accumulate when your $P O_{2}$ exceeds 0.5 ATA.

For a practical guide on how to calculate OTUs, we'll turn to a resource that walks through a full example.

How to calculate your CNS and OTU exposure | Jump - Sail

The article 'How to calculate your CNS and OTU exposure' from the Jump-Sail-Dive blog provides a step-by-step guide for calculating both metrics for a typical recreational nitrox dive. We will focus on its explanation of OTUs and its worked example.

Please read the sections 'What is the Oxygen Tolerance Unit (OTU)?' and 'Example'. Pay close attention to the table provided, which gives OTU values per minute at different PO2 levels. Follow the example calculation for a 25-minute dive to 30m on 32% Nitrox.

As the article demonstrates, the process involves a few steps:

Calculate the ambient pressure (ATA) for your depth.
Calculate the partial pressure of oxygen ( $P O_{2}$ ) for your breathing gas at that depth.
Use a table to find the OTU accumulation rate (OTUs per minute) for that $P O_{2}$ .
Multiply the rate by the time spent at that depth.

Let's re-examine the example from the article:

Dive: 25 minutes at 30 meters on 32% Nitrox.
Step 1 (ATA): $ATA = (\frac{Depth in meters}{10}) + 1 = (\frac{30}{10}) + 1 = 4 ATA$
Step 2 ( $P O_{2}$ ): $P O_{2} = Fraction of O_{2} \times ATA = 0.32 \times 4 = 1.28 ATA$
Step 3 (Table Lookup): The table in the article doesn't have 1.28. As per standard practice, we round up to the next highest value, which is 1.3 ATA. The table shows that at a $P O_{2}$ of 1.3, you accumulate 1.477 OTUs per minute.
Step 4 (Total OTUs): $Total OTUs = 1.477 OTU/min \times 25 min = 36.93 OTUs$

The article uses a slightly different method of adding pre-calculated blocks of time from the table, but the result is the same. Just like with CNS%, you would calculate the OTUs for each segment of your dive and add them together. A common daily limit recommended by training agencies is 300 OTUs.

3. Practice Calculation: A Multi-Stage Dive

Now, let's apply both concepts to a more complex dive profile. This is what you would do when planning a technical dive with decompression stops.

Dive Profile:

Bottom Phase: 20 minutes at 40 meters, breathing Trimix 21/35 (21% oxygen, 35% helium).
Decompression Stop: 15 minutes at 6 meters, breathing EANx50 (50% oxygen).

We will use the tables from the resources you've just reviewed.

NOAA Single Exposure Limits Table

OTU/CNS Table

Part 1: Bottom Phase (20 mins @ 40m on Trimix 21/35)

Calculate ATA:
$ATA = \frac{40}{10} + 1 = 5.0 ATA$
Calculate $P O_{2}$ :
$P O_{2} = 0.21 \times 5.0 = 1.05 ATA$
For our calculations, we'll round this up to 1.1 ATA.
Calculate CNS%:
- From the NOAA table, the limit for 1.1 ATA is 240 minutes.
- $CNS% = \frac{20 min}{240 min} \times 100 = 8.3%$
Calculate OTUs:
- From the second table, the OTU rate at 1.1 ATA is 1.118 OTU/min.
- $OTUs = 1.118 \times 20 min = 22.36 OTUs$

Part 2: Decompression Stop (15 mins @ 6m on EANx50)

Calculate ATA:
$ATA = \frac{6}{10} + 1 = 1.6 ATA$
Calculate $P O_{2}$ :
$P O_{2} = 0.50 \times 1.6 = 0.8 ATA$
Calculate CNS%:
- From the NOAA table, the limit for 0.8 ATA is 360 minutes.
- $CNS% = \frac{15 min}{360 min} \times 100 = 4.2%$
Calculate OTUs:
- From the second table, the OTU rate at 0.8 ATA is 0.535 OTU/min.
- $OTUs = 0.535 \times 15 min = 8.03 OTUs$

Total Exposure for the Dive

Finally, we sum the values from each phase to get the total for the dive.

Total CNS%: $8.3% + 4.2% = 12.5%$
Total OTUs: $22.36 + 8.03 = 30.39 OTUs$

Both of these values are well within typical single-dive limits. This process of breaking a dive into segments, calculating the exposure for each, and summing the results is the core of manual oxygen exposure tracking.

Conclusion

In this lesson, we have moved from the theory of oxygen toxicity to the practical methods for managing it. You now understand the two primary tracking tools used by divers and how to perform the essential calculations for a dive plan.

Key Takeaways:

The CNS Oxygen Toxicity Clock tracks cumulative exposure to high $P O_{2}$ as a percentage of NOAA-defined time limits to manage acute neurological risk.
Oxygen Toxicity Units (OTUs) track the cumulative dose of oxygen to manage chronic pulmonary risk, with 1 OTU equaling the effect of breathing 100% O2 at 1 ATA for 1 minute.
Both calculations depend on the $P O_{2}$ and the duration of exposure.
For a multi-stage dive, you must calculate the exposure for each segment (bottom time, travel, decompression stops) and sum the results to find the total for the dive.
While dive computers automate these calculations, understanding the underlying principles is fundamental to safe dive planning and for evaluating the plan your computer generates.

Preview of the Next Lesson:

Knowing how to calculate your exposure is one part of the puzzle. But is a CNS of 50% equally risky for every person on every day? Not necessarily. In the next lesson, we will explore the nuances of oxygen risk by examining the learning outcome: Analyze the factors that increase susceptibility to oxygen toxicity during diving. We'll look at how factors like exercise, cold, and even certain medications can alter your personal risk profile.