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Confusion about the CNS Clock October 14, 2008

Posted by Chris Sullivan in Training.
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In 1988 National Oceanic and Atmospheric Administration (NOAA), published guidelines develop Drs. Christian J. Lambertson, Russell S. Peterson, F. K . Butler and Edward Thalmann regarding the maximum recommended exposure to high (0.5 to 1.6) partial pressures of Oxygen (PPO2). This is important in diving if you want to use Enriched Air (a.k.a. Nitrox) for extended bottom times or accelerated decompression.

This table specifies an ever shorter number of minutes allowable as the Oxygen partial pressure being breathed increases, in terms of both single dive and daily maxima. In the versions I’ve seen, no mention is made of whether this minimizes the risk of Central Nervous System (CNS) toxicity (a potentially deadly problem underwater), or Pulmonary (sometimes called “whole body”) toxicity. However, Dick Rutkowski, one of the founders of IANTD, adapted the table to provide limits for repetitive dives and exposures to various O2 levels in a single day, naming this adaptation the “CNS Clock”.

In recreational enriched air training, we learn not to exceed a PPO2 of 1.4 atmospheres, or 1.6 as a “contingency”. The NOAA tables are also taught, with the footnote that most of the time the recreational diver will not come close to the limits. The PADI Enriched Air Diver course uses Rutkowski’s adaptation based on percentages of the NOOA limits, but adds conservatism by not giving credit for surface intervals, using the single dive limits as the full day maximum. The limits are printed on one of the slates PADI gives you with the course.

Subsequent courses, including the IANTD Advanced Nitrox Course and the DSAT Tec Deep Course, delve deeper into the concept but are a little vague on whether it applies to CNS or Pulmonary toxicity, despite being called “CNS Clock” by IANTD. The Tec Deep course mentions that it should mostly be used with regard to Pulmonary Toxicity, which is to say the least confusing. Both IANTD and DSAT discuss Oxygen Tolerance Units, or OTU, that are specifically designed for Pulmonary (a.k.a. Whole Body) Toxicity planning. IANTD also discusses the older UPTD (Unit Pulmonary Toxicity Dose) method, which has been virtually replaced by OTU. Both courses recommend 1.4 PPO2 as the limit for the working phase of the dive, and 1.6 PPO2 for decompression. IANTD also allows 1.5 PPO2 in the working phase as long except when there is heavy exertion or the water is cold. Some studies I’ve read show that very warm water is no better than cold water.

After a lot of investigation I gathered that the NOAA table was intended to serve both needs. Below 1.2-1.3 PPO2 research studies show little or no evidence of CNS Toxicity, so the tables are only useful in managing Pulmonary Toxicity. At 1.3 and above, the length of exposure increases the risk of CNS Toxicity, so the table’s exposure limits are a useful addition to the fixed limits of 1.4 PPO2 during the working phase of the dive and 1.6 PPO2 during decompression. This concept also points out the utility of taking “air breaks” to manage exposure, although it has been pointed out that while it is has been observed that the time limits can be extended by air breaks, no studies have been done to show to quantify the effect.

The NOAA table has withstood the test of time, even though it was put together in the absence of an underlying theoretical model, relying instead on the wisdom and experience of its creators. A good explanation is in Chapter 10.2 (by Hamilton and Thalmann) of the book Physiology & Medicine of Diving.



1. deepstop - October 26, 2008

On page 5-19 of “The Encyclopedia of Recreational Diving” by PADI it says “NOAA limits aren’t based on the prevention of CNS toxicity, but rather pulmonary toxicity”. Despite this I still think based on other references that it is useful for both.


2. Contingency Depth « Chronicle of an older diver - April 25, 2011

[…] longer you are exposed the more likely you are to have a problem, so we also learn about the “CNS clock” to manage exposure over time. For what it’s worth my old IANTD Advance Nitrox manual […]


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