Oxygen Toxicity & ROS

What Oxygen Does to a Diver's Body

Oxygen is essential to life, but at elevated partial pressures it becomes a source of cellular damage. The mechanism runs through reactive oxygen species (ROS) — highly reactive molecules generated as a byproduct of normal oxygen metabolism, which become pathological when production outpaces the body's ability to neutralise them. Understanding this mechanism explains why PO₂ limits exist, why CO₂ makes things worse, and why exercise at depth carries more risk than the same effort at the surface.

Reactive Oxygen Species

ROS are partially reduced or activated forms of oxygen produced continuously during cellular respiration. The three species most relevant to oxygen toxicity: superoxide (O₂•⁻) — the primary product of electron leakage from the mitochondrial electron transport chain; hydrogen peroxide (H₂O₂) — produced from superoxide by the enzyme superoxide dismutase (SOD); and hydroxyl radical (•OH) — produced from H₂O₂ via the Fenton reaction with iron (Fe²⁺), the most reactive and damaging of the three.

Under normal atmospheric conditions, mitochondria leak approximately 1–2% of electron flow as superoxide. The body's antioxidant systems handle this without difficulty.

How High PO₂ Breaks the Balance

At elevated PO₂ — such as breathing enriched gas at depth — the mitochondrial electron transport chain (ETC) becomes saturated with oxygen. More O₂ molecules are available to accept leaked electrons, dramatically increasing superoxide generation. The cascade: elevated PO₂ drives increased electron leakage at ETC complexes I and III, generating excess superoxide; SOD converts superoxide to H₂O₂ faster than it can be cleared; H₂O₂ reacts with free iron via the Fenton reaction to produce hydroxyl radicals; hydroxyl radicals attack lipid membranes, proteins, and DNA.

At PO₂ levels above 1.4–1.6 bar, ROS production in the brain can outpace antioxidant clearance within minutes to hours, depending on individual tolerance and concurrent risk factors.

For a detailed review of ROS biology, see Auten & Davis (2009) in Pediatric Research (doi:10.1038/pr.2009.174).

The Neurological Cascade — How ROS Cause Seizures

The CNS is particularly vulnerable to oxidative stress because neurons have high metabolic rates, limited antioxidant reserves compared to other tissues, and are densely packed with polyunsaturated fatty acids — a preferred target for lipid peroxidation.

The sequence leading to hyperoxic seizure: ROS accumulation suppresses GABA (gamma-aminobutyric acid), the brain's principal inhibitory neurotransmitter; with GABA inhibition reduced, NMDA glutamate receptors become hyperactivated; hyperactivated NMDA receptors drive excessive neuronal depolarisation; sodium/potassium pump function fails under oxidative stress, compounding the electrical instability; uncontrolled neuronal firing propagates as a grand mal seizure.

This pathway explains why the convulsion threshold is not fixed: anything that increases ROS production or impairs GABA function lowers it. CO₂ retention is the most important cofactor — hypercapnia causes cerebral vasodilation, increasing O₂ delivery to brain tissue and accelerating ROS generation locally.

The Antioxidant Defence System

The body operates three primary enzymatic defences against ROS. Superoxide dismutase (SOD) converts superoxide to H₂O₂ and is the first line of defence, present in mitochondria (Mn-SOD) and cytoplasm (Cu/Zn-SOD). Catalase converts H₂O₂ to water and O₂, concentrated in peroxisomes. Glutathione peroxidase (GPx) converts H₂O₂ and lipid peroxides to harmless products using glutathione (GSH) as the electron donor.

Non-enzymatic support comes from glutathione, vitamin C (ascorbate), and vitamin E (tocopherol). The glutathione-ascorbate cycle allows these to regenerate each other, extending their functional capacity.

Physical exertion increases mitochondrial oxygen consumption and ROS production. At the surface, this is well within the system's capacity. At elevated PO₂ — already pushing antioxidant defences toward saturation — the additional oxidative load from exercise can tip the balance. This is why exercise is the single most significant modifiable risk factor for CNS oxygen toxicity during a dive, not just an additive one.

CO₂ Retention — The Synergistic Risk

CO₂ retention and elevated PO₂ interact in two ways. Hypercapnia causes cerebral vasodilation, delivering more blood and more O₂ to brain tissue, increasing local ROS production. CO₂ also modifies acid-base balance in neurons, altering membrane excitability independently of ROS.

The Gur et al. (2024) retrospective of 47 years of Israeli military O₂ rebreather dive records found hypercarbia documented in 11 of 75 confirmed CNS oxygen toxicity cases — 14.7%. This is likely an undercount, since CO₂ monitoring was not universal and hypercarbia is transient and difficult to measure retrospectively.

Sources of CO₂ retention in diving: skip breathing, high gas density at depth causing increased work of breathing, CO₂ absorbent bypass or channelling in a rebreather, and high-intensity exercise.

Methylphenidate and COT Risk

Gur et al. (2024) addressed whether methylphenidate (Ritalin), prescribed for ADHD, increases seizure risk under oxygen. From 75 COT cases and a parallel mouse experiment: methylphenidate use in the preceding 3 months showed no increased COT risk (OR 0.72, 95% CI 0.16–3.32). In mice exposed to 5 atm absolute of 100% O₂, methylphenidate dose-dependently prolonged time to seizure onset (placebo: 877 s; 5 mg/kg: 1,500 s; p = 0.014). Proposed mechanism: dopamine reuptake inhibition providing a degree of neuroprotection.

This does not mean methylphenidate is protective in human divers. The data come from pure-O₂ military rebreathers at shallow depths; generalisability to recreational or technical CCR divers at lower PO₂ exposures requires caution. But it addresses a frequently raised concern: divers with ADHD on methylphenidate are not at demonstrably increased COT risk.

Why This Matters for Divers

PO₂ limits are not arbitrary — they reflect the PO₂ range at which ROS production approaches the capacity of the antioxidant system for an average individual under moderate conditions. CO₂ management is not optional: hypercarbia lowers the seizure threshold through a direct biochemical pathway. Exercise at depth is not the same as exercise at the surface — at 1.3–1.4 bar PO₂, the antioxidant margin is already reduced, and added ROS from exertion has less buffer to absorb. Individual variability is real: the same diver can have different tolerance on different days depending on fatigue, illness, hydration, and prior oxygen exposure. No PO₂ limit guarantees safety for every individual at every time.

Related Reading

• CNS Oxygen Toxicity — When Oxygen Turns Against You
• Pulmonary Oxygen Toxicity — What Long Exposure Does to Your Lungs
• Gas Density & CO₂ — The Silent Risk in Deep Diving

References

• Auten RL, Davis JM. Oxygen toxicity and reactive oxygen species: the devil is in the details. Pediatric Research. 2009;66(2):121–127. doi:10.1038/pr.2009.174
• Gur I, Arieli Y, Matsliah Y. Methylphenidate and the risk of acute central nervous system oxygen toxicity: a rodent model and observational data in human divers. Diving Hyperbaric Med. 2024;54(3):168–175. doi:10.28920/dhm54.3.168-175
• Halliwell B, Gutteridge JMC. Free Radicals in Biology and Medicine. 5th ed. Oxford University Press; 2015.
• Torbati D, Parolla D, Lavy S. Organ blood flows, cardiac output, arterial blood pressure, and vascular resistance in rats exposed to various oxygen pressures. Aviat Space Environ Med. 1979;50(8):839–845.
• Bitterman N. CNS oxygen toxicity. Undersea Hyperb Med. 2004;31(1):63–72.

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