Gases under pressure: nitrogen narcosis, oxygen toxicity and CO₂ retention

Mechanism
Exam priority: medium

At depth, normal air becomes harmful: nitrogen acts like an anesthetic, oxygen turns toxic and CO₂ builds up because dense gas is hard to breathe.

Updated Nov 13, 2026

Also called: Inert gas narcosis, Hyperoxia, Heliox

Key points

Nitrogen narcosis starts around 30 m (≈ 4 atm) and worsens with depth, like progressive intoxication; it resolves on ascent.
CNS oxygen toxicity: twitching, visual and hearing changes, then seizures, with inspired PO₂ above about 1.4–1.6 atm; risk rises with exercise, cold and CO₂.
Pulmonary oxygen toxicity: prolonged inspired PO₂ above about 0.5 atm causes tracheobronchitis, falling vital capacity and diffuse alveolar damage (ARDS-like).
Mechanism: excess reactive oxygen species overwhelm antioxidants (glutathione, superoxide dismutase, catalase) and damage lipids, proteins and DNA.
Other oxygen hazards: absorption atelectasis at high FiO₂ and retinopathy of prematurity in preterm infants.
Helium is far less narcotic and much less dense; it permits deep dives but causes heat loss, voice distortion and, very deep, high-pressure nervous syndrome.

What it is

Every 10 m of seawater adds about 1 atm, so the partial pressure of each breathed gas rises in proportion. Nitrogen at high partial pressure dissolves in nerve membranes and causes narcosis. Oxygen above about 1.4–1.6 atm can cause seizures, and long exposure to more than about 0.5 atm injures the lung. Dense gas raises the work of breathing and lets CO₂ accumulate, which worsens both problems. Helium–oxygen mixtures and saturation techniques allow deeper dives.

How it works, step by step

Descent raises ambient pressure; inspired partial pressures of N₂, O₂ and CO₂ rise in proportion (Dalton's law).
N₂ is fat-soluble; at high pressure it dissolves in neuronal membranes and alters ion channels, slowing reasoning and coordination.
With high inspired PO₂, dissolved O₂ alone meets tissue demand, hemoglobin stays saturated and can no longer cap tissue PO₂.
Tissue PO₂ rises, mitochondria and enzymes produce excess superoxide, hydrogen peroxide and hydroxyl radicals.
When antioxidant systems are overwhelmed, oxidation of membrane lipids and enzymes disturbs neurons (seizures) and lung lining cells (edema, inflammation).
Gas density rises with depth, so airway resistance and the work of breathing increase and alveolar ventilation can fall.
Equipment dead space, exertion and a blunted CO₂ response let PaCO₂ rise; CO₂ dilates cerebral vessels, delivering more O₂ and N₂ to the brain.

Pressure and gas thresholds in diving (approximate)

Parameter	Value	Note
Ambient pressure	+1 atm per ≈ 10 m of seawater	2 atm at 10 m, 4 atm at 30 m
Narcosis with air	noticeable from ≈ 30 m	severe beyond ≈ 60–90 m
CNS oxygen toxicity	inspired PO₂ > ≈ 1.4–1.6 atm
Pulmonary oxygen toxicity	inspired PO₂ > ≈ 0.5 atm for many hours or days
High-pressure nervous syndrome	helium dives beyond ≈ 150–200 m

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

Breathing mixtures at depth

Mixture	Advantage	Limitation
Air	Simple, available	Narcosis and O₂ toxicity limit depth
Enriched-air nitrox	Less nitrogen: longer no-decompression times	O₂ toxicity at shallower depth
Heliox (He–O₂)	Little narcosis; low density eases breathing	Heat loss, voice distortion, high-pressure nervous syndrome
Saturation diving	Tissues fully saturated: one long decompression after days or weeks at depth	Decompression takes days; living in a pressurized chamber

Clinical notes

Hospital oxygen also has limits: long exposure to high FiO₂ adds lung injury in ventilated patients; preterm infants need tightly targeted saturations to lower the risk of retinopathy of prematurity.
A diver who seizes underwater may drown or, if surfacing during the seizure with a closed glottis, suffer lung barotrauma.

Test yourself

Think of the answer, then tap the question to see it.

Why can hemoglobin no longer protect tissues from high PO₂ when breathing O₂ at 3 atm?

Dissolved O₂ alone then meets tissue needs, so hemoglobin does not unload and cannot hold tissue PO₂ in its normal range; tissue PO₂ rises freely.

A diver breathing air at 40 m becomes overconfident and makes errors. What is the cause and how does it resolve?

Nitrogen narcosis from high N₂ partial pressure; it resolves quickly on ascent to shallower depth.

Why does CO₂ retention make oxygen toxicity seizures more likely?

CO₂ dilates cerebral vessels, raising cerebral blood flow and delivering more O₂ to the brain, so brain PO₂ climbs higher.

Why is helium used instead of nitrogen for very deep dives?

It is much less narcotic and less dense, lowering the work of breathing; its drawbacks are heat loss, voice distortion and high-pressure nervous syndrome.

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: Moving in extreme environments: inert gas narcosis and underwater activities (Extreme Physiology & Medicine (PMC), 2015); Oxygen toxicity: cellular mechanisms in normobaric hyperoxia (Cell Biology and Toxicology (PMC), 2023); Retinopathy of prematurity: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2024); Scuba Diving: Decompression Illness and Other Dive-Related Injuries (CDC Yellow Book, 2025).

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