# Firefighter divers spent five minutes at a simulated 50-meter depth and made more balance errors after surfacing, while an oxygen stop preserved improvement on a bead test, suggesting nitrogen narcosis can briefly linger after pressure returns to normal

> Eighty-six professional firefighter divers entered a hyperbaric chamber for a short but deep pressure exposure, then repeated simple balance and hand-skill tests after surfacing. The most striking change did not appear in their fingers first. It appeared in how steadily they could...

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Byline: ARGO.net Editorial Team
Published: 2026-08-07T13:25:04+00:00
Categories: Explainer, Humans

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Eighty-six professional **firefighter divers** entered a hyperbaric chamber for a short but deep pressure exposure, then repeated simple balance and hand-skill tests after surfacing. The most striking change did not appear in their fingers first. It appeared in how steadily they could hold a demanding heel-to-toe stance with their eyes closed.

The 2024 [Medicina study](https://pmc.ncbi.nlm.nih.gov/articles/PMC11278881/) asked whether breathing oxygen near the end of decompression might reduce the after-effects of **inert gas narcosis**. Fifty-eight divers decompressed on air, while 28 switched to pure oxygen during the last 30 minutes. After a five-minute stay at a **50-meter simulated dive**, the air group showed more failed balance trials, while the oxygen group did not worsen on that measure.

The result is interesting because it lands in perceptual-motor territory, not only in abstract cognition. Divers have to orient themselves, manage posture, read instruments, move with control and respond fast when something changes. This study does not prove that a post-dive oxygen stop protects every aspect of performance, but it does suggest that some balance-related impairment may outlast ascent for a short time and that difference matters most when work or safety decisions depend on stable movement right after surfacing.

## What the chamber dive tested

The research team studied 86 male firefighter divers from different German cities, with an average age of 36.4 years. All of them were already qualified to work in compressed air and were completing chamber exposure as part of ongoing training, not as a separate experimental dive arranged only for research. That practical setup gives the paper a real-world training context, but it also means the groups were not built through random assignment.

Each diver spent five minutes at 50 meters inside a 12-person hyperbaric chamber. After that, one group followed decompression on air. The other group used **oxygen during decompression** for the final 30 minutes, including time at 1.6 and 1.3 bar before returning to surface pressure. The authors framed that switch as a way to increase the gradient for nitrogen washout rather than as a broad performance treatment.

Before and after the chamber session, investigators ran several neurological tasks. The paper treats the ordinary finger-to-nose check mostly as a familiarization step. The more important measures were the **Sharpened Romberg test**, which challenges **postural control** by placing a person heel-to-toe with eyes closed and the **modified tweezers test**, which counts how many small beads someone can move in 60 seconds. Together, those tasks probe balance, sensory integration and fine motor execution in a way that is easy to repeat around a dive.

## Where balance slipped after surfacing

The strongest signal came from the balance task. In the air group, positive results on the Sharpened Romberg test rose from 47 percent before the dive to 67 percent after it, which the authors interpreted as worse balance performance. In the oxygen group, the proportion stayed at 68 percent before and after the dive. Because a positive result here meant visible movement, irregular swaying, or loss of the position, the post-dive change points toward a temporary disruption in the systems that keep the body upright when visual guidance is removed.

That pattern fits the study's main psychological angle. A tandem stance with closed eyes forces the brain to lean harder on vestibular and body-position signals instead of easy visual correction. When those signals are less stable, the body starts making small compensations that an observer can see on video long before a person necessarily reports feeling impaired. The paper therefore places the post-dive effect in the **vestibular system** and related sensorimotor control rather than in a vague idea of feeling groggy.

Another detail matters just as much as the post-dive increase: the oxygen group already started with poorer SRT performance at baseline. The groups also differed in age, with the oxygen group older on average. Those baseline differences limit any clean claim that oxygen was the sole reason the two groups diverged after decompression. What the study can say more safely is narrower: within the air group, balance errors increased after the exposure, while the oxygen group's already-high error rate did not climb further.

## What the bead test says about fine motor learning

The bead-transfer task told a subtler story. The air group moved about the same number of beads before and after the dive, averaging 42 before and 41 after. The oxygen group started lower, at 36 and improved to 42 after the dive. The authors read that split as evidence for a blocked **learning effect** in the air group rather than as direct proof that nitrogen narcosis made hand control collapse.

That interpretation is plausible because repeated manual tasks often improve on the second attempt even when the underlying skill has not changed much. If a diver has just practiced picking up and transferring beads, a later round should usually get smoother. In this study, the oxygen group showed that expected gain, while the air group stayed flat. The gap suggests that residual narcosis may have interfered with the normal short-term improvement that comes from immediate repetition.

Perceptual-motor psychology makes that distinction important. A missed learning boost is not the same thing as gross motor failure, but it still points to altered processing. Fine motor tasks depend on attention, timing, visual guidance and tiny online corrections from the hand and fingers. Other diving research has reported similar concerns at depth, including a 2021 [event-related brain potentials study](https://pubmed.ncbi.nlm.nih.gov/33942846/) that found cognitive performance changes during hyperbaric air exposure and a 2023 [open-water decision-making study](https://pmc.ncbi.nlm.nih.gov/articles/PMC10944662/) showing that narcosis can impair choices even around 30 meters.

## Why residual nitrogen could affect perception and movement

The authors build their case around a simple physical idea: ascent does not instantly remove dissolved nitrogen from tissues. Their introduction notes earlier work showing that post-dive impairment may persist after surfacing and they argue that a diver can feel more normal while some nervous-system effects are still fading. A 2012 [European Journal of Applied Physiology paper](https://pubmed.ncbi.nlm.nih.gov/22476770/) reached a related conclusion after a 33-meter dive, reporting persistent impairment in critical flicker fusion frequency, another measure used to track narcosis-related changes.

For movement control, that lingering phase matters because balance is built from several streams of information at once. The inner ear helps estimate head motion and orientation. Proprioceptive signals from muscles and joints report where the body is in space. Vision can steady the whole system, but the Sharpened Romberg test deliberately removes that crutch. If residual nitrogen briefly disturbs central processing or sensory weighting, the earliest sign may be a wobble, a delayed correction, or a missed chance to refine a repeated action.

Current clinical summaries stay cautious on mechanism. The 2026 [StatPearls review on nitrogen narcosis](https://www.ncbi.nlm.nih.gov/books/NBK470304/) describes the condition as a depth-dependent change in consciousness, neuromuscular function and behavior, notes that meaningful impairment can begin around 30 meters and says objective effects may persist transiently after surfacing. That broader background supports the chamber paper's central idea, but it does not settle the exact pathway behind the balance findings in these firefighters. The present study measured behavior, not brain chemistry.

## Why the oxygen comparison stays limited

The oxygen result is promising, yet the study leaves several reasons to avoid overstatement. The groups were unequal in size, 58 versus 28. The oxygen group was older. Baseline performance also differed before the dive began, with the oxygen group already showing more positive Sharpened Romberg results and fewer transferred beads. Those facts make it harder to treat the comparison like a clean trial of one decompression strategy against another.

The design also cannot isolate nitrogen perfectly from every other influence on balance. The authors note that repeated ear-clearing during compression and venting during decompression might have affected vestibular responses in both groups. They also mention that the bead task may have ceiling concerns in theory, although some participants moved far more than 40 beads, which argues against a tight cap. A third comparison group breathing a helium-based mix would have helped test whether the post-dive shift really tracked nitrogen rather than some other feature of the chamber exposure.

Generalizability stays narrow as well. These were fit, trained, male firefighter divers in a simulated chamber dive, not a mixed civilian sample in open water. The paper did not measure long-term outcomes and it did not show that the observed changes translated into operational mistakes, injuries, or dangerous decisions after surfacing. The safest reading is still the most useful one: this study suggests a brief post-dive balance cost after deep air exposure and it suggests that oxygen near the end of decompression may reduce that cost, but stronger causal claims need better-matched groups and more direct measures.

Even with those limits, the study adds something practical to the psychology of diving. Nitrogen narcosis is often discussed as a feeling, a judgment problem, or a depth-only event that disappears once ascent is complete. Here the clearest signal sits in coordination after the pressure is gone. For divers who must climb ladders, handle gear, or make rapid responses soon after surfacing, that narrow point may be the part worth remembering most.