A wrong response at depth can place a diver in immediate danger, especially when pressure, heavy breathing gas and a demanding task compete for attention. A chamber experiment has found that people breathing heliox made fewer mistakes under pressure than they did while breathing compressed air or trimix.
The study in Biology examined healthy volunteers at 4 ATA, a pressure equal to roughly 30 meters of seawater. Researchers tested reaction speed, accuracy, attention and memory before the pressure exposure, while participants were inside the chamber and after decompression.
Heliox produced the clearest benefit during a task that required people to resist an automatic response. Compressed air was linked with more errors under pressure, while trimix produced a smaller change. The breathing gases had similar effects on the short-term memory scores measured during the experiment.
How pressure can affect a diver’s thinking
At sea level, the human body experiences pressure close to 1 atmosphere absolute, or 1 ATA. Pressure rises as a diver descends because the surrounding water presses against the body. At 30 meters below the surface, a diver experiences about four times the pressure found at sea level.
Higher pressure changes how breathing gases behave inside the body. Nitrogen becomes more likely to affect the nervous system, producing a condition called nitrogen narcosis. A diver may feel unusually confident, slow to react, or less able to judge a situation. The strength of the effect varies among people and can change with depth, workload, cold, stress and breathing effort.
Gas density also rises under pressure. Each breath requires more effort because the gas is thicker and harder to move through the airways and breathing equipment. Greater breathing effort can promote carbon dioxide buildup, especially when a diver is working hard or breathing too shallowly.
Elevated carbon dioxide can interfere with attention and decision-making. It may also increase the effects associated with nitrogen narcosis. A diver can therefore face several pressures on mental performance at once, even during a dive that remains within common recreational depth limits.
Inside the 4 ATA chamber experiment
Researchers from the Medical University of Gdańsk carried out the experiment with healthy adult volunteers in a hyperbaric chamber. The author team included Rita I. Sharma, Natalia D. Mankowska, Anna B. Marcinkowska, Pawel J. Winklewski and Jacek Kot. Their affiliations included the university’s National Centre for Hyperbaric Medicine.
The research used a randomized crossover design, meaning participants returned for separate pressure sessions involving different breathing gases. The order was randomized and at least 24 hours separated the exposures. Participants were unaware of which mixture they received during each session, while the chamber staff knew the gas composition for safety and control.
Compressed air contained about 21 percent oxygen and 78 percent nitrogen. Heliox contained about 21 percent oxygen, with helium making up almost all the remainder. Trimix used a similar oxygen level while combining nitrogen with helium.
Cognitive testing took place at three stages. Volunteers first completed tests at normal pressure. They repeated the tasks at 4 ATA and then performed them again after decompression. Trial totals varied slightly, with 40 people completing the air condition, 36 completing heliox and 37 completing trimix. The publication is also listed in the Medical University of Gdańsk’s research database.
Heliox produced the lowest error rate
The strongest result came from the Simon task, which tests how well a person can control a response when a signal contains conflicting information. Participants must follow the meaning of a signal while ignoring its position. Some trials place the signal where the expected response feels natural, while other trials create a conflict that must be overcome.
Among participants breathing heliox, the error rate during conflicting trials fell from 1.89 percent at normal pressure to 1.12 percent at 4 ATA. The decrease suggested that volunteers adopted a careful response style while breathing the helium and oxygen mixture.
Reaction times with heliox rose modestly, although the main practical result involved accuracy. In diving operations, a slightly slower correct action can carry less risk than a quick mistake involving buoyancy, navigation, breathing equipment, or communication.
The study abstract summarizes the result directly: “Breathing heliox minimises errors under cognitive conflict.” The finding applies to the specific chamber conditions, cognitive task, pressure level and short exposure used in the experiment.
Compressed air increased mistakes
Air produced a different pattern during conflicting Simon task trials. The error rate rose from 2.3 percent before pressure exposure to 4.38 percent at 4 ATA, nearly doubling during the chamber session. After decompression, it fell to 2.77 percent.
Participants maintained their response speed while breathing air, yet their accuracy declined. The researchers interpreted the combination as a greater willingness to respond quickly under pressure, even when the task required extra mental control.
Nitrogen provides one possible explanation. At 4 ATA, the partial pressure of nitrogen is much higher than it is at the surface, allowing more nitrogen to dissolve into body tissues. Its effects on nerve-cell activity may weaken judgment or response control in some people.
Breathing resistance could contribute as well. Dense compressed air requires more work to move through the lungs and equipment. Increased breathing effort may encourage carbon dioxide retention, which can further disturb concentration. The experiment did not directly establish how much each mechanism contributed, so the physiological explanation remains open for further testing.
Memory results remained similar across gases
The researchers also examined short-term memory through digit span tests and the Corsi Block Tapping task. Digit span asks a participant to repeat a sequence of numbers. The Corsi test uses a sequence of locations, requiring the participant to reproduce the order in which blocks were indicated.
Memory performance showed some decline during pressure exposure, although the type of breathing gas did not produce a statistically significant overall advantage. The study conclusion states, “Short-term memory deteriorated at 4 ATA in all trials.”
One comparison involving the forward Corsi task found that participants breathing air reproduced shorter sequences at pressure than they did after decompression. The wider analysis did not show clear differences between air, heliox and trimix across the memory tests.
Heliox therefore showed its strongest measured benefit in controlling errors during a conflicting task. Memory relies on partly different brain processes and a gas mixture that supports careful response selection may produce a smaller or harder-to-detect influence on immediate recall.
Why helium may support accuracy
Helium is much less dense than nitrogen. Replacing most of the nitrogen in a breathing mixture with helium creates a gas that moves more easily through airways and breathing equipment, especially as pressure rises.
Lower gas density can reduce the effort needed for each breath. Easier ventilation may help the lungs clear carbon dioxide more effectively, which could support attention during demanding work. The researchers presented reduced breathing resistance and lower carbon dioxide buildup as possible contributors to the accuracy result.
Helium also lacks the narcotic strength associated with nitrogen at ordinary diving pressures. A heliox mixture greatly lowers nitrogen exposure, reducing one of the main gases linked with cognitive changes at depth.
The oxygen level remained close to 21 percent in all three experimental mixtures. At 4 ATA, each mixture therefore produced a similar oxygen partial pressure. Differences in performance were more closely associated with the balance between helium and nitrogen, along with the physical density of each breathing gas.
What the findings mean for diving safety
Operational diving safety often depends on avoiding a small number of serious errors. Divers must monitor depth, gas supply, ascent speed and their position in the water. They may also need to respond to equipment problems while communicating with a partner.
An accuracy advantage could help in situations where a wrong choice carries immediate consequences. The chamber results suggest that heliox may support an accuracy-first response at pressures reached during some recreational and professional dives.
Hyperbaric medical staff may also work under increased pressure while attending patients inside a chamber. Clear thinking remains important because attendants must follow procedures, operate equipment and respond to medical changes in a confined environment.
Heliox already has recognized physical advantages for deeper diving because its low density eases breathing and its minimal nitrogen content reduces narcosis. The cognitive findings add evidence that those properties may also support fewer mistakes during tasks involving conflicting signals. The paper’s publication details and abstract are available through its PubMed record.
Limits of a short chamber exposure
The experiment recreated pressure inside a dry hyperbaric chamber. Real dives include water immersion, limited visibility, current, cold, heavy equipment and physical work. Each factor can influence breathing and mental performance, so chamber results provide a controlled view of only part of the diving environment.
Exposure at 4 ATA lasted for a short period, with the study discussing changes during sessions of up to about 30 minutes. Longer dives could produce different effects as fatigue, carbon dioxide levels, temperature and gas exposure change over time.
The sample was also modest and the number of completed sessions differed among gas conditions. Practice may have influenced performance because volunteers repeated similar cognitive tests. Randomized gas order and intervals between sessions helped reduce this concern, although repeated testing can still affect speed and accuracy.
Future research could measure breathing effort and carbon dioxide at the same time as cognitive performance. Experiments conducted underwater could also examine whether the lower error rate appears during realistic diving procedures. Larger groups would help researchers identify how experience, fitness, stress and individual sensitivity influence the result.
The current evidence supports a focused conclusion: during short exposure at 4 ATA, heliox was linked with fewer errors in a cognitive conflict task, while memory changes were broadly similar across breathing mixtures. The formal study record is available through its DOI page.
