The Risks of Oxygen at Increased Depth

Having been engaged in a discussion with a number of readers on Undercurrent’s bulletin board, I’ve become aware of many misconceptions about the real risks related to central nervous system (CNS) oxygen toxicity and the rather benign effects of longer-term “low dose” exposure. Because so many divers use Nitrox these days and therefore are exposed to higher oxygen partial pressures than they would be with regular compressed air, it’s important they understand the basic elements of oxygen physiology. There are real risks if limits are not observed, but they are relatively small and difficult to attain within normal diving ranges and practice. More often than not, unwarranted panic over slightly exceeding a depth can lead to excessive ascent rates, buddy abandonment, or other bad behavior when little risk will actually manifest. It’s a confusing subject and bears some more in-depth discussion.

As divers, we must be concerned primarily with the effects of elevated partial pressures of oxygen that occur as we descend. It’s the partial pressure of oxygen (PO2) that is most critical, not the percentage of oxygen in a mix.

Dalton’s Law of Partial Pressures

The total pressure exerted by a gas mixture is equal to the sum of the partial pressures of the components of the mixture (oxygen and nitrogen in the case of air or Nitrox), i.e., P = P1+P2+P3 (“P” stands for each individual gas in the total mix), etc. Put simply, as your depth increases, there is a corresponding increase in the partial pressure of oxygen. At the surface we are naturally adapted to PO2 at .21 atmospheres absolute (ATA).

For air, the PO2 at a 66-foot depth in the ocean is expressed as .63 ATA of O2. This is derived from multiplying .21 (the percentage of O2 in air) by the pressure in ATAs: .21 X 3 = .63 ATAs of O2. Though the percentage of O2 in the air we breathe will remain constant, the PO2 will increase with depth. Therefore, when breathing compressed air at 66 feet, we are breathing in three times as much oxygen as we did on the surface.

The Hazard

The CNS is primarily affected in the acute phase, meaning a relatively but high PO2 exposure. Predictable results will follow if oxygen limits are exceeded. You can use the acronym VENTID to help remember the CNS O2 toxicity symptoms

* Vision: any disturbance including “tunnel vision,” etc.
* Ears: any changes in normal hearing function
* Nausea: severity may vary and be intermittent
* Twitching: classically manifest in facial muscles
* Irritability: personality shifts, anxiety, confusion, etc.
* Dizziness: vertigo, disorientation

Even a cursory examination of these effects should illustrate the seriousness of a CNS O2 hit in deep water. Onset and severity of symptoms do not follow any particular pattern, and may vary daily in an individual diver. There may be no warning from less serious symptoms before a full convulsion is precipitated.

Oxygen convulsions, per se, are not inherently harmful but imagine the implications for an untended diver, or even one with a buddy nearby. Management of a patent airway and subsequent rescue in such an extreme situation is nearly impossible, and the diver will almost certainly drown.

Managing Oxygen Exposure

Back in 1971, when I worked on Navy diving projects, the P02 limit was commonly accepted to be 2.0 ATA. Over the years, this was backed off to nearly universal recommendation now of 1.6 ATA, which is the equivalent of 132 feet of depth if you are using Nitrox-32. Yes, you have probably read conservative recommendations to keep your PO2 under 1.4, or even 1.3, but there have been no incidents of oxygen toxicity at 1.6 as long as the time limits are properly observed. The DAN Nitrox Workshop held in November 2000 (I was on the faculty along with other industry experts) universally concluded that a PO2 of 1.6 was an appropriate operational limit for sport divers, thus ending an ongoing controversy.

However, understand that the partial pressure of oxygen only makes up part of the equation for oxygen “dose.” The other variable is time, usually expressed in minutes at a particular PO2. NOAA has published a table (above) that allows quick reference for divers to plan exposures.

While the potential hazard of CNS oxygen toxicity cannot be underestimated, the good news is that the risk to sport divers is almost nonexistent if the NOAA limits are observed since there has never been a sport diving oxygen incident within the NOAA limits.

The “oxygen dose” is sometimes referred to as the “oxygen clock,” which implies the time limits with the PO2. Your Nitrox dive computer stores this information in its memory (along with changeable PO2 settings), and will calculate your exposure. This is usually expressed as a percentage of the maximum dose rather than in a minute “count down” like remaining bottom time. If your diving practice is to avoid decompression, you will never approach the CNS dose limits because your no-deco time limit will always occur first. Because most divers tend to dive in multi-level profiles and don’t spend the entire dive at the maximum PO2, the actual “oxygen clock” rarely will even reach 20 percent of the dose limit.

Note that there is no more danger with a 50-percent exposure to oxygen at 1.6 than there is with a 50-percent exposure at 1.4 or 1.3. It’s the total dose, not the PO2, that determines your risk factor. It’s this distinction that seems to lead to a lot of the confusion and rather absurd suggestions for increased conservatism.

Other Considerations

Susceptibility to oxygen toxicity is increased by other factors. These include elevated carbon dioxide levels caused by hard working conditions or prolonged swimming efforts. Sport divers typically do not approach the exertion levels of actual working divers for which the NOAA/Navy limits were defined.

In fact, most divers swim lazily around the reef or wall, stopping to take photos or simply take in the sights. The most active part of the dive usually occurs at the beginning or end, where some higher swimming exertion happens descending against current, traveling to the starting point, or swimming back to the boat or shore. And this is typically in shallower depths where the PO2 is so low as to be inconsequential. Divers, as a population, really don’t work very hard. A lot of overly shrill cautions about reducing PO2s came from those who had an incomplete understanding of how divers actually dive and what the Navy and NOAA limits were designed for in their original applications.

PO2s will obviously need to be lowered if your dive plan will exceed 45 minutes at 1.6 ATA. But for you folks on single-cylinder, open circuit scuba, whether breathing air or Nitrox, it is virtually impossible to reach the “dose” time limits.

Breathe Easy

Oxygen has certain well-defined risk windows. But the hazards are easily avoidable by ensuring that your dive profiles observe the NOAA limits. Set your PO2 at 1.6; watch your computer display your “dose” accumulation, and do not exceed the maximum depth limit for your Nitrox mix. The depth limit for a 1.6 PO2 exposure on 32-percent Nitrox is 132 feet. If you go deeper, you will not spontaneously combust or go into seizures. But your time limit at increased depths will reduce.

As a general rule, I do suggest observing the 1.6 level for PO2, but don’t panic if you briefly go deeper. Your computer will account for it. And most importantly: breathe in, breathe out, repeat as necessary.

You may also have heard divers refer to tracking their OTUs (oxygen tolerance unit). This refers to another form of oxygen toxicity that occurs on very long exposures at relatively low PO2s. This is primarily a consideration for saturation divers or dealing with patients in recompression chambers. It is impossible for open circuit divers to attain sufficient OTU dose to serve any practical discussion. If you observe CNS limits, OTUs take care of themselves.

You don’t have to take a day off from diving midweek to allow for “oxygen out-gassing,” as one reader was told. As Tony Soprano might say, “Fuggitabout it.”

Bret Gilliam is a 40-year veteran of the professional diving industry. He founded Technical Diving International (TDI) and crafted the standards and procedures for training nitrox divers for that agency. He is extensively published on the subjects of nitrox, mixed gas, rebreathers, technical diving, oxygen physiology, and emergency treatment for divers in recompression chambers and in remote areas where evacuation is not an option. He is credential as a Recompression Chamber Supervisor and Diver Medical Technician.