Connect with us

Dive Training Blogs

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

Jeff chats to… Richard Corner from Mares about liveaboards and training with SSI (Watch Video)



In this exclusive Zoom interview, Jeff Goodman chats to Richard Corner, UK Watersports Category Manager at Mares, about the complete experience for all aspects of diving with Mares, their liveaboard adventures with the purchase of, and the Mares/SSI combination.

Last in a series of four videos.

Rather listen to a podcast? Listen to the audio HERE on the new Scubaverse podcast channel at Anchor FM.

For more information, visit, and

Continue Reading

Dive Training Blogs

Reef Rescue Network launches new interactive map



The Reef Rescue Network (RRN) was established in 2017 by the Perry Institute for Marine Science (PIMS) as a network of non-profit organizations and for-profit businesses committed to improving the condition of coral reefs by restoring populations of corals and other species that will build coral reef resilience. Since then the RRN has grown to include nearly 30 coral restoration sites in partnership with 25 local partners from 9 islands within The Bahamas as well as Aruba and St. Lucia. Through this partnership between coral reef scientist’s local conservation and education organizations and private businesses in the dive industry, the RRN is making significant advances in restoring coral and building reef resilience.

Visitors and locals can now immerse themselves in coral restoration activities at a partner location within the Reef Rescue Network. The network has coral nurseries that offer coral restoration experiences throughout The Bahamas, Aruba & St. Lucia. PIMS has developed a PADI Reef Rescue Diver Specialty Course that dive shops throughout the Reef Rescue Network are teaching. To participate, you must be a certified open water diver and at least 12 years old. The course takes one day and consists of knowledge development and two open water dives at a coral nursery.

You can learn how to assist with maintaining the nursery and get a hands-on experience or you can just scuba or snorkel the coral nursery as a fun dive to just observe and enjoy the nursery and marine life that it attracts. Another option is to scuba or snorkel one of the many restoration sites to view the corals that have been outplanted and witness for yourselves this habitat restoration and the marine life it has welcomed.

To find out more about the Reef Rescue Network, watch this video:

To visit the new Reef Rescue Network Interactive Map click here.

To learn more about the Reef Rescue Network visit their website by clicking here.

Continue Reading

E-Newsletter Sign up!


Sharks Bay Umbi Diving Village is a Bedouin-owned resort with stunning views and a lovely private beach. It is ideal for divers as everything is onsite including the resort's jetty, dive centre and house reef. The warm hospitality makes for a diving holiday like no other. There is an excellent seafood restaurent and beach bar onsite, and with the enormous diversity of the Sharm El Sheikh dive sites and the surrounding areas of the South Sinai, there really is something for every level of diver to enjoy.

More Less

Instagram Feed

Facebook Feed

Facebook Pagelike Widget