# 87% of divers with prior hypercapnia exposure bailed out in a later test, but a 40-person trial could not confirm a training benefit

> Eighty-seven percent of divers who had previously felt the effects of raised carbon dioxide chose to bail out during a later blinded exposure. Among divers who had only read an information leaflet, 67% took the same action. The difference pointed toward a...

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Published: 2026-08-23T00:40:02+00:00
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Eighty-seven percent of divers who had previously felt the effects of raised carbon dioxide chose to bail out during a later blinded exposure. Among divers who had only read an information leaflet, 67% took the same action. The difference pointed toward a possible training benefit, although the small trial could not establish that the earlier experience caused the higher bailout rate.

The [randomized study](https://doi.org/10.28920/dhm56.2.115-124), published on June 30, 2026, in **Diving and Hyperbaric Medicine**, involved 40 divers. Thalia Babbage and colleagues at the **University of Auckland** examined whether a controlled encounter with **hypercapnia**, the medical term for excessive carbon dioxide in the body, could help divers recognize the danger later.

Early recognition can be critical for people using rebreathers because rising carbon dioxide may impair thought, produce severe breathlessness and increase the risk of oxygen toxicity. A diver who notices the warning signs may switch to an independent breathing supply, a safety response known as bailout.

## How carbon dioxide threatens rebreather divers

**Closed-circuit rebreathers** recycle breathing gas instead of releasing most exhaled gas as bubbles. Oxygen used by the body is replaced, while a chemical material removes carbon dioxide before the gas returns to the diver. The design allows long and quiet dives, but it depends on correct assembly and reliable gas treatment.

A damaged, exhausted, or poorly prepared **carbon dioxide scrubber** may allow the waste gas to build up in the breathing loop. Carbon dioxide can also accumulate within the diver when dense gas and resistance in the breathing system make each breath harder. The body may then struggle to move enough gas through the lungs, especially during heavy work at depth.

An [Undersea and Hyperbaric Medical Society review](https://www.uhms.org/uhm-search/uhm-journal-volume-44/number-3/hypercapnia-in-diving-a-review-of-co2-retention-in-submersed-exercise-at-depth.html) describes several routes to carbon dioxide retention during diving. Increased breathing effort, extra unused space in the breathing pathway and changes in respiratory control can all contribute. Individual divers also differ in how strongly their breathing responds as carbon dioxide rises.

The danger can accelerate because the brain and muscles produce more carbon dioxide during exertion. A struggling diver may breathe harder while facing resistance from the equipment and dense gas. Confusion can then interfere with the decision to stop work, change breathing supplies, or end the dive.

## How researchers tested hazard recognition

The researchers recruited 40 divers and randomly assigned them to one of two training conditions. One group underwent an open, supervised exposure in which they knew they were breathing a mixture that would raise their carbon dioxide. The other group received a leaflet describing common hypercapnia symptoms.

During the controlled exposure, the team raised the partial pressure of **end-tidal carbon dioxide** to 8.5 kilopascals. End-tidal measurements use the gas at the end of an exhaled breath as a practical indicator of carbon dioxide in the lungs. The exposure allowed participants to feel their own response while researchers watched them under controlled conditions.

At least one month later, members of both groups returned for a blinded session. Researchers further randomized them to receive either raised carbon dioxide or a normal carbon dioxide condition, using a three-to-one allocation. Participants therefore did not know whether the danger they had been taught to identify was actually present.

The main outcome was whether a diver began **self-initiated bailout** before the end-tidal carbon dioxide level reached 8.5 kilopascals. Researchers also recorded breathing volume and breathing rate. Heart rate and blood pressure were monitored, along with oxygen and carbon dioxide values.

## When divers chose to bail out

Among the participants exposed to hypercapnia during the blinded test, 13 of 15 divers with prior experience initiated bailout before reaching the target level. Ten of 15 divers trained with the information leaflet also bailed out in time.

The rates, 87% and 67%, produced a visible difference of 20 percentage points. The statistical analysis returned a P value of 0.149, which left substantial uncertainty about whether the training method caused the difference. With only 15 hypercapnia-tested participants from each training group, a small change in the number responding could strongly affect the percentages.

Physiological measurements at bailout were similar between the two groups. Divers with previous exposure did not appear to act at a clearly different breathing rate, heart rate, blood pressure, or measured carbon dioxide level compared with those who had read the leaflet.

The result suggests that several divers may have used their sensations to make the bailout decision, although the measurements did not reveal a single physical threshold that separated the groups. Some people could recognize discomfort early, while others may have waited for stronger symptoms or interpreted the sensations differently.

## The symptoms divers noticed most

**Shortness of breath** was among the strongest sensations reported during raised carbon dioxide. Participants also described feeling light-headed and disoriented, experiences that can become especially dangerous underwater because a diver must continue breathing, manage equipment and follow an escape plan.

The study abstract states, "Shortness of breath, light-headedness and disorientation were the most intensely reported symptoms." Such warnings can overlap with exertion, anxiety, equipment resistance, or the effects of depth, which may make their cause difficult to identify during a real dive.

The [Divers Alert Network](https://dan.org/alert-diver/article/your-lungs-and-diving/) explains that severe hypercapnia can lead to intense breathlessness, panic and loss of consciousness. Raised carbon dioxide can also increase the danger of central nervous system oxygen toxicity, a serious concern because a convulsion underwater can prevent a diver from keeping a mouthpiece in place.

Cognitive effects add another layer of risk. A diver needs clear thought to recognize a failing breathing loop, locate a bailout regulator and leave the dangerous environment. Disorientation may delay those steps even when the body is already signaling distress.

## Why personal warning signs varied

Training based on a personal **symptom signature** assumes that someone will feel a similar set of warning signs during later exposure. The trial found limited consistency. About 47% of participants who received the supervised experience showed a correlated symptom response during the later hypercapnia test.

In practical terms, roughly half experienced a recognizable relationship between the two sessions, while the remaining participants did not show the same degree of symptom agreement. A diver might feel pronounced breathlessness during one exposure and notice disorientation more strongly during another.

Carbon dioxide responses can depend on workload, breathing resistance, gas density and the speed at which the level rises. Stress and attention may influence which sensations a diver notices first. A laboratory exposure also takes place in a controlled setting, while a real underwater failure may occur during movement, low visibility, or another demanding task.

An [NCBI medical overview](https://www.ncbi.nlm.nih.gov/books/NBK500012/) describes rebreathers as complex devices that require careful monitoring and management. Their risks include excessive carbon dioxide as well as problems involving oxygen levels. Sensations alone therefore provide one part of a wider safety system that includes equipment preparation, instrument checks and practiced emergency procedures.

## What supervised exposure could offer

A **supervised hypercapnia exposure** gives divers a controlled opportunity to feel how rising carbon dioxide affects breathing and thought. Familiarity may help some people connect a strange underwater sensation with the need for immediate action, especially when a warning develops gradually.

The trial's 20-point difference in bailout rates supports further investigation, even though the result remained statistically uncertain. A larger study could determine whether the apparent advantage persists across a wider group and whether particular divers benefit more than others.

Any exposure would require appropriate medical oversight because high carbon dioxide can impair judgment and produce severe physical distress. The researchers presented supervised familiarization as a possible addition for recreational and occupational rebreather divers, rather than evidence for divers to create their own exposure exercises.

Training could also focus on rapid use of bailout gas whenever unexplained breathing difficulty or mental impairment develops. Repeated practice may reduce the number of decisions required during an emergency, allowing a diver to follow a rehearsed sequence while leaving the hazardous situation.

## Limits of the 40-diver trial

The small sample was the clearest limitation. Although 40 divers entered the research, the key bailout comparison involved 30 participants exposed to hypercapnia in the later blinded session. Thirteen successful responses compared with ten produced encouraging percentages, yet the difference fell short of conventional **statistical significance**.

A larger sample would provide a more precise estimate of the training effect. It could also help researchers examine experience level, rebreather background, age and individual sensitivity without dividing participants into groups too small for dependable comparisons.

The test measured recognition under controlled experimental conditions. Real dives include depth, cold water, task demands and equipment-related breathing resistance. An actual scrubber problem may also develop along a different timeline from the controlled exposure used in the trial.

Researchers found no difference in cardiorespiratory measurements at bailout, which limits efforts to identify a simple physiological marker for action. Future work could examine whether combining symptom training with clearer equipment warnings provides more dependable protection than either approach used alone.

## What the results mean for diver training

The study gives instructors a cautious reason to examine experience-based teaching, while preserving established safeguards. Prior exposure was followed by more self-initiated bailouts, but the trial could not confirm a reliable training advantage. Symptom consistency was also modest, with a correlated response found in about half of those who received the practical experience.

Rebreather safety therefore depends on several connected defenses. Divers need careful assembly procedures and scrubber management. They also need suitable bailout gas, regular instrument checks and emergency actions that can be performed quickly under stress.

Equipment monitoring may strengthen those defenses. Under specified alternative U.S. workplace rules for recreational diving instructors and guides, [OSHA requirements](https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910SubpartTAppC) call for a continuously functioning carbon dioxide sensor in each rebreather, with an alarm that the diver can detect under operating conditions. Such rules apply to a defined regulatory setting, yet they illustrate the value of warning systems that do not depend solely on symptoms.

For researchers, the next step is a larger trial that can test whether familiarization produces a repeatable improvement in **bailout training**. For divers, the present finding offers a narrower lesson: carbon dioxide can affect people differently, warning sensations may change between events and a practiced response can become vital when breathing or thinking begins to deteriorate underwater.