By: Gemma Smith

The existence, development, and continuing survival of human life is nothing short of incredible. This is especially true when we consider how many different factors had to come together to make life here on earth possible. There is no more obvious example of this ideal melding of variables than in the air around us. We all remember our basic science from school. Here we learnt the air around us is about 21% oxygen and 79% nitrogen (give or take the odd trace gasses). As far as our human body is concerned, we can take or leave the gas nitrogen. On the other hand, oxygen is absolutely critical to our survival.

Without this tasteless, odourless, colourless gas, we simply wouldn’t be here. We all know that too little oxygen (hypoxia) is a BAD thing. Less commonly known is the fact that too much oxygen (hyperoxia) is equally bad. Yes, in excess, oxygen is, in fact, toxic to us. Now, this wouldn’t be so much of an issue if we kept our feet firmly on terra firma…but we are divers. We journey into a new dimension, with new rules, and therefore new risks.

Partial Pressure

In your initial scuba diving class, you will have learnt about Dalton’s Law. This Law states that the total pressure exerted by a gas mixture is equal to the sum of the partial pressures of the different gases in the mix. So, at the surface, the partial pressure of oxygen is 0.21 (21% oxygen breathed at 1-atmosphere pressure). This changes when we dive. The increasing depth when we submerge causes an increase in the pressure around us. This means an increase in partial pressure as well. The partial pressure of the oxygen in our breathing gas will of course rise. This can potentially cause all kinds of issues.

These issues are especially pertinent for divers today. There has been an increase in the use of Enriched Air Nitrox (a gas mixture with a higher than 21% oxygen content). Deep diving beyond recreational limits (most often set at depth of 40m/130ft) is now commonplace. This type of diving requires the carrying of various gas mixtures. Some of these gasses will have a higher than normal oxygen content in certain designated cylinders. This is to aid and accelerate any decompression that may be incurred while at depth. Finally, the growing use and popularity of Closed Circuit Rebreathers also has implications. Many of these units utilise pure oxygen as one of the onboard gasses. This means that more than ever we need to be aware of our oxygen levels. Alongside the dangers of insufficient oxygen, it is equally important that we can spot potentially fatal problems that can occur because of too MUCH oxygen.

Types of Oxygen Toxicity

Very broadly speaking, oxygen toxicity splits into two main types. These are Central Nervous System Oxygen Toxicity (CNS), and Pulmonary Oxygen Toxicity. In all but the most extreme dives, the most common issue for divers will stem from CNS toxicity. This usually happens by breathing a gas mixture at the wrong depth. The partial pressure of oxygen (PO2) in the mix should never exceed 1.6 ATA (Atmospheres Absolute). To put this in context, breathing pure oxygen should not happen deeper than 6m/20 ft to ensure the PO2 does not exceed 1.6. Ideally, unless decompression is happening, your PO2 should be at no higher than 1.4. Pulmonary Toxicity, on the other hand, comes from breathing an elevated partial pressure of oxygen over an extended period of time. Oxygen toxicity is a fascinating and important subject. It is worth noting though that it is still not fully understood on all levels. Prevention, as in most things, is better than cure.

Let’s look a bit closer at the signs and symptoms of each of the main types of oxygen toxicity:

Central Nervous System Toxicity

The chance of problems with CNS toxicity is most likely in the kind of diving many sport divers do. These dives are typically short durations at depth, but potentially much higher than normal PO2 exposures. As we have already said, CNS toxicity tends to happen when breathing a gas mix that exceeds a PO2 of 1.6. Dipping below the maximum operating depth for what you are breathing, or breathing the wrong gas, are easy ways to exceed your PO2 limits. To help you remember the key things to look for both in yourself and others, remember the acronym CON-VENTID.

• CON – Convulsions. These convulsions in and of themselves are not lethal. Convulsions underwater increase the likelihood of you losing your regulator. This can be fatal.
• V – Visual disturbances. This can range from tunnel vision to blurring.
• E – Ears. A ringing in the ears or other auditory disturbances.
• N – Nausea. Intermittent or constant, with varying severity.
• T – Twitching. Most commonly noted in the facial muscles, although not always easy to see when wearing a hood and mask.
• I – Irritability. Any kind of character change.
• D – Dizziness. Feeling confused, or a sense of vertigo.

Of course, the first thing to point out is that many of these signs and symptoms overlap with other common diving problems. Nitrogen Narcosis can produce many of the same effects.

Knowing all the possible signs and symptoms to look out for is nonetheless important. It is worth noting though that for many CNS toxicity cases there may be no warning. Convulsions will be the initial, and clearly blatant, indication that something is seriously wrong.

Pulmonary Oxygen Toxicity

Pulmonary toxicity tends to occur in only the most serious of technical dives. Extreme depth or duration can mean breathing an elevated PO2 for many hours. The partial pressure is usually not high enough to cause immediate CNS issues. However, over a significant time period irritation of the lungs may occur. This can cause a burning sensation in the throat, coughing, difficulty breathing or shortness of breath, among other things. Published tables show the maximum amount of time considered safe to stay at a certain PO2. Technical divers planning to do excessive decompression are well advised to implement air breaks. This allows the lungs to have a break from a high PO2 for a period of time. It is also important to consider that Pulmonary oxygen levels do not decrease with a surface interval. This means even recreational divers can be at risk of pulmonary problems when completing multiple dives with nitrox over multiple days.

Preventing Oxygen Toxicity

As with everything related to the human body, nothing is set in stone. Day to day variables will cause your body to react differently to situations. All the limits set in place for avoiding oxygen toxicity are guidelines. It is always prudent to work well within those limits. There is no way to guarantee that oxygen toxicity will not affect you. There are however several steps you can take to minimise your risks:

• Analyse your gas – Never breath anything that you have not personally analysed. Calibrate the analyser, analyse your gas, and then correctly mark that cylinder. This includes writing what percent you analysed, and what the maximum operating depth for that gas is. All divers, without question, must do this for every single cylinder that they take in the water.
• Watch your depth – Even when a diver correctly marks all gasses, if they do not stick to depth limits they are not helping themselves. Most dive computers now have alarms for when you exceed the depth of the gas you have programmed in. Dive computers are also a good way of tracking your oxygen exposure over several dive days. Stay aware, and make sure you maintain good buoyancy control at all times.
• Avoid strenuous activity – Carbon dioxide has been shown to increase the likelihood of oxygen toxicity. Never work hard underwater, or get out of breath. Make sure your regulator is properly serviced to ensure an easy breathe. If you do have to work physically underwater it is worth considering the option of a full face mask. There are many options we as divers can take to limit our risks. It is important that we utilise all that is on offer to keep us safe.

Both types of oxygen toxicity are something all divers need to be aware of. Whether doing shallow or deep dives, recreational or technical, it is important to always know and be aware of what you are breathing, and what depth you are at. It may save your life.

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