--- title: "CNS Oxygen Toxicity" description: "Why high oxygen pressure can trigger a seizure underwater, the PO2 limits that keep you safe, and why the famous warning signs don't warn you." canonical: "https://donarundas.com/depth-notes/physiology/cns-oxygen-toxicity" author: "Donarun Das — TDI Trimix Instructor, KISS Sidewinder CCR Instructor" category: "physiology" published: "2025-05-31" tags: ["oxygen-toxicity", "cns", "ppo2"] disclaimer: "Dive theory published by a working instructor for education. It is not medical advice and not a substitute for training. Decompression, gas, and depth decisions belong with your own training, your dive computer, and a dive medical officer." license: "© Donarun Das. Quote with attribution and a link to the canonical URL." --- # CNS Oxygen Toxicity ## The poison you can't live without Every diver gets taught that running out of air is what kills you. Almost nobody gets taught that too much oxygen can do it just as easily. Breathe oxygen at a high enough pressure and it can set off a seizure underwater, with a regulator still in your mouth and little or no warning. The gas itself hasn't changed at all. What's changed is the *pressure* you're breathing it at, and past a certain point oxygen stops being fuel and starts acting like a poison. For a diver, the scary version of that is **CNS oxygen toxicity**: a sudden seizure that can hit with no warning, while you have a regulator in your mouth and water all around you. The seizure itself almost never kills anyone. Drowning during the seizure does. The good news is that this is one of the most *preventable* problems in diving. You don't avoid it by being brave or experienced. You avoid it with arithmetic: know your oxygen pressure, and keep it inside limits we already know. The rest of this article is where those limits come from and how to stay on the right side of them. One quick calibration before we go further. If you dive recreational nitrox, this is mostly a planning discipline you'll rarely bump up against (EAN32 at 30 m is only a PO₂ of 1.28). The sharp end of this article, where seizures actually happen, is rebreather and decompression diving. The arithmetic is the same either way, so it's worth owning now. ## Why oxygen turns toxic Your body runs on oxygen, but it also has to *defend* against it. Burning oxygen for energy throws off a small amount of chemical shrapnel: unstable molecules called **reactive oxygen species (ROS)**. Think of them as sparks coming off a grinding wheel. At normal pressure your body's antioxidant systems sweep those sparks up as fast as they appear. Now raise the pressure. The deeper you go, the more oxygen dissolves into your blood and tissues, far more than your body was ever tuned for. The sparks start flying faster than the cleanup crew can handle, and the leftovers begin damaging cells. That's **oxidative stress**, and it's the root of every form of oxygen toxicity. One leading explanation of the seizure itself is that, in the brain, this oxidative stress tips the chemistry toward over-excitement. It eats into **GABA**, the signal your neurons use to calm each other down, while nudging up the excitatory side. GABA is the brakes. Ease the brakes and press the accelerator at the same time and the neurons start firing together, faster and faster, until the whole system runs away. That runaway is the seizure. The exact mechanism is still argued over, but the practical lesson holds either way. ### A whole dive, end to end Put both tables together on one dive. Say you plan **EAN32 to 32 m for 30 minutes**, then switch to **EAN50 at 18 m for 10 minutes** of deco. - **Gas check.** EAN32's working MOD is about 34 m, so 32 m is fine (PO₂ there is 0.32 × 4.2 = 1.34 bar). EAN50's working MOD is about 18 m, so you switch right at its limit (PO₂ 0.50 × 2.8 = 1.40 bar). Both legal, nothing over 1.4. - **CNS clock.** Thirty minutes at about 1.34 bar spends roughly 30 ÷ 168 ≈ **18%**. Ten minutes at 1.40 bar spends 10 ÷ 150 ≈ **7%**. Total ≈ **25%**, comfortably under the 80% you'd plan to. So this dive is limited by your gas supply and your decompression, not by oxygen, which is the usual story on a single nitrox dive. Stack four or five of these on a liveaboard day, though, and the clock starts to matter, which is exactly what the tool above lets you feel. ## The warning signs that don't warn you You'll be taught a mnemonic, **VENTID-C**, for the symptoms that can come before a seizure: - **V** is for Vision (tunnel vision, blurring, flashes) - **E** is for Ears (ringing) - **N** is for Nausea - **T** is for Twitching, classically of the lips and face - **I** is for Irritability or sudden anxiety - **D** is for Dizziness - **C** is for Convulsion Learn it, but do not trust it. This is the single most important sentence in the article. > **VENTID is not an early-warning system** > > In real dives the convulsion is very often the **first** sign, with no twitching or dizziness beforehand. The classic Royal Navy experiments showed the time-to-seizure varies wildly between people and even in the *same* person from one day to the next [2]. You cannot dive up to a high oxygen dose planning to "feel it coming and bail." There may be nothing to feel. If any of those first six signs *do* show up, treat it as an emergency: signal, ascend, and get onto a lower-oxygen gas. Don't wait to see if it settles. ## Carbon dioxide: the hidden accelerator If you remember one risk factor, make it this one. **Carbon dioxide is the master switch that makes oxygen far more dangerous.** When CO₂ builds up in your blood, it widens the blood vessels in your brain, pushing more blood, and therefore more oxygen, into exactly the tissue you're trying to protect. The effective oxygen dose to your neurons goes up even though your PO₂ on the computer hasn't changed. CO₂ builds up when you work hard, when you breathe a dense gas that's tiring to move, when you "skip-breathe" (deliberately pause between breaths) to stretch your gas, and, on a rebreather, when the scrubber that removes CO₂ starts to fail. Scrubber breakthrough has triggered seizures at a PO₂ as low as 1.2 to 1.3 bar [3]. On a rebreather, managing CO₂ *is* managing oxygen toxicity. There's a whole article on that side of it: [gas density and CO₂ buildup](https://donarundas.com/depth-notes/physiology/gas-density-and-co2-buildup). Everything else that raises your risk works through a similar door. Hard exertion, cold, and simply being immersed in water all lower the seizure threshold. Susceptibility varies between people too, and the same diver isn't equally tolerant every day. > **How to actually stay safe** > > - Keep your **working PO₂ at or below 1.4 bar**; 1.6 only at rest on deco. > - Run the **four-step gas check** above before every dive. A wrong, unanalysed number is the most common cause of trouble. > - **Don't fight current at depth.** Exertion at high PO₂ is the dangerous combination, so stop, hover, rethink. > - **Manage CO₂**: maintain your scrubber, never skip-breathe, keep gas density down (helium helps on deep mixes). > - **Watch the CNS clock** through the dive, not just while planning. ## If your buddy convulses You cannot stop the seizure, so your whole job is to prevent a drowning and a burst lung: 1. **Hold them at depth and stop the ascent.** A clenched, breath-holding body wants to float up, and a held breath on the way up can rupture a lung. 2. **Leave the regulator alone during the convulsion.** If it's in the mouth, hold it there. If it has already come out, do not try to force it back against a clenched jaw; you can't, and you may cause injury. 3. **Don't try to pin the limbs.** You won't stop the convulsion and you'll just hurt yourself. 4. **Wait for the shaking to stop.** Do not ascend during the active convulsion. 5. **Once it passes and they're breathing**, replace the regulator if it came out, make a slow, controlled ascent together, then get them onto oxygen at the surface and to emergency medical care. ## The honest summary CNS oxygen toxicity sounds terrifying, and the consequence genuinely is. But the cause is boringly mechanical: too much oxygen pressure, often pushed over the edge by carbon dioxide. You control the first with a mix and a depth limit you can calculate before you ever get wet, and the second with fitness, good gear, and calm breathing. Do both and the risk drops to a tiny fraction of a percent [4]. Skip the arithmetic and trust your symptoms to save you, and you're betting your life on a warning system that frequently doesn't fire. > **One more thing** > > This is education, not a substitute for proper training, your dive computer and agency tables, or a dive physician. The numbers here move (the CNS limits were revised in 2025), so treat your current training and a properly set computer as the authority, never a web article. ### Keep reading - [Oxygen Toxicity & ROS: what the damage actually is](https://donarundas.com/depth-notes/physiology/oxygen-toxicity-ros) - [Pulmonary Oxygen Toxicity: the slow lung version](https://donarundas.com/depth-notes/physiology/pulmonary-oxygen-toxicity) - [Oxygen on a Multi-Day Trip: what the 1.3 rule means over a week](https://donarundas.com/depth-notes/physiology/oxygen-multi-day-diving) - [Gas Density & CO₂ Buildup: the hidden accelerator, in depth](https://donarundas.com/depth-notes/physiology/gas-density-and-co2-buildup) ## References 1. Hoyt J, Murphy FG, Mitchell SJ, et al. *Revised guideline for central nervous system oxygen toxicity exposure limits when using an inspired PO₂ of 1.3 atmospheres* (≈1.3 bar). Diving and Hyperbaric Medicine. 2025;55(3). PMID 40986922. 2. Donald KW. *Oxygen and the Diver.* SPA Ltd, 1992. Summarising the Royal Navy oxygen-tolerance experiments, 1942–1945. 3. Pollock NW. *Hazards in Rebreather Diving.* 2024. 4. Shearwater Research. *Oxygen Seizures at PO₂ ≤ 1.6 bar: how rare?* A review putting reported CNS-toxicity incidence in clean conditions well under 1 in 1,000 dives. 5. Arieli R, et al. *CNS oxygen toxicity in closed-circuit diving: symptoms reported from 2,527 dives.* Aviat Space Environ Med. 2006. PMID 16708533. 6. NOAA. *NOAA Diving Manual* (source of the CNS oxygen-exposure limits and the 1.4/1.6 conventions). ## Common questions ### What is the safe PO2 limit for diving? 1.4 bar is the accepted working limit while actively diving, and 1.6 bar is the absolute ceiling, used only briefly at rest on a decompression stop. These are deliberately conservative margins, not a hard line where a seizure suddenly happens. ### Can you feel oxygen toxicity coming on? Usually not reliably. The classic warning signs often do not appear before a convulsion, so you cannot count on them. The real defence is keeping your oxygen partial pressure within limits, not watching for symptoms. ### How do you calculate maximum operating depth (MOD)? MOD in metres equals (the PO2 limit divided by the oxygen fraction, minus 1) times 10. For EAN32 at a 1.4 bar limit that is about 33 m. --- **Source:** [https://donarundas.com/depth-notes/physiology/cns-oxygen-toxicity](https://donarundas.com/depth-notes/physiology/cns-oxygen-toxicity) **About the author:** Donarun Das is a TDI Trimix and KISS Sidewinder CCR instructor teaching technical and rebreather diving in the Maldives, Sri Lanka, and India. He holds the deepest verified civilian dive in India (103 m, open-circuit trimix, Lakshadweep, March 2026). Courses: https://donarundas.com/courses — enquiries: https://donarundas.com/contact > Dive theory published by a working instructor for education. It is not medical advice and not a substitute for training. Decompression, gas, and depth decisions belong with your own training, your dive computer, and a dive medical officer. ## Sitemap See the full [sitemap](https://donarundas.com/sitemap.md) for all pages available as Markdown.