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Oxygen Toxicity



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.

To find out more about International Training, visit

From its humble beginning in 1994 to today, the group of training agencies Scuba Diving International (SDI), Technical Diving International (TDI), and Emergency Response Diving International (ERDI) form one of the largest diving certification agencies in the World – International Training. With 24 Regional Offices servicing more than 100 countries, the company today far exceeds the original vision the founders had when they conceived the idea on a napkin, sitting at a kitchen table in the early 1990’s.

Dive Training Blogs

Quick Scuba Tips #12: Pimp Your GoPro for Amazing Underwater Video Colors (Watch Video)



We’re back with the quick tips series and my secret weapon for getting the best colort out of your GoPro underwater!

Introducing the Flip 8 from A color correction system that mounts straight onto the dive housing of your GoPro. Check them out below!

As always, thanks for watching!

D.S.D.O James

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Dive Training Blogs

What you need to know about SMBs!



Ok, so not the most exciting of topics… but an important one nonetheless. Especially as many of us will be starting to enjoy the UK dive season and heading out to explore our beautiful coastline. Some of you may even be heading into the UK waters for the first time due to the travel restrictions… welcome, you will wish that you had done it sooner! 

Surface marker buoys. SMB’s are an invaluable piece of equipment. To demonstrate your position in the water, to fend off boats, to show off your buoyancy to your dive buddy when you can inflate it without moving an inch in the water… or to un-intentionally make your buddy laugh when you forget to attach your reel and send it up like a lost rocket… A must have skill and piece of equipment for all divers. But, how do you choose which one is right for you, and how do you use it correctly? 

Choosing a colour, we all know to look cool as a diver, its all about co-ordinating, but not so much with SMB’s I’m afraid. The standard colour is orange and is what you will typically see being used, and yellow due to it’s higher ability to be seen at night time is just for an emergency…. Not because it is your favourite colour…sorry yellow lovers! If you are wanting to personalise it though you could put your name down the SMB, that way the surface cover knows who it is underneath. 

Next, inflation. Here we have the option of open bottom or direct inflation. An open bottom means that you will need to use your alternate to inflate the SMB, direct inflation you would use your inflator hose. Either of these are sufficient and is generally down to preference. If you are not sure which you prefer, or how to use them, there is a course that you can take to learn all of the skills and offer some helpful tips of how to inflate it and control your buoyancy too. I happen to know an instructor that teaches it… so just drop me a message and I can help…!

So, we have the SMB, next we need a line or spool. So many decisions with a basic piece of kit! Most SMB’s will come with a line, which is great as you can use the equipment straight away. The only down side is, with gloves it can become annoying, especially if you are changing depths quite often as typical on a shore dive here. You may wish to look at a spool instead… They also come in more colours, and this time you can choose which ever you want… even yellow, result! 

Having got to the point of choosing you SMB and line/spool, where are we now going to keep it? Clipping it onto your BCD, keeping it in your pocket. Anywhere is sufficient as long as its easily accessible… like not in your car once you have entered the water…. So be sure to add you SMB to your buddy check! Happy diving! 

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Explore the amazing triangle of Red Sea Reefs - The Brothers, Daedalus and Elphinstone on board the brand new liveaboard Big Blue.  With an option to add on a week at Roots Red Sea before or after. 

Strong currents and deep blue water are the catalysts that bring the pelagic species flocking to these reefs. The reefs themselves provide exquisite homes for a multitude of marine life.  The wafting soft corals are adorned with thousands of colourful fish. The gorgonian fans and hard corals provide magnificent back drops, all being patrolled by the reef’s predatory species.

£1475 per person based on double occupancy.  Soft all inclusive board basis, buffet meals with snacks, tea and coffee always available.  Add a week on at Roots Red Sea Resort before or after the liveaboard for just £725pp.  Flights and transfers are included.  See our brochure linked above for the full itinerary.

This trip will be hosted by The Scuba Place.  Come Dive with Us!

Call 020 3515 9955 or email

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