Commercial aircraft cabins are not pressurised to sea level. Regulators allow the air inside to thin to the equivalent of a moderate mountain altitude, a compromise between structural engineering and passenger comfort that has stood largely unchallenged for decades. A study published in 2026 by researchers at the German Aerospace Center’s Institute of Aerospace Medicine in Cologne tested a genuinely counterintuitive fix for the low blood oxygen that results, adding carbon dioxide back into the air rather than trying to remove it.

A Six Hour Chamber Test at Cabin Altitude

The study, led by Titiaan Post and Daniel Aeschbach alongside colleagues including Jens Jordan, director of the DLR Institute of Aerospace Medicine, and published in the Journal of Applied Physiology in 2026, put 17 healthy adults, eight of them women, aged between 18 and 40, through a randomised double-blind crossover trial in a hypobaric chamber. Each participant spent six hours at a simulated pressure of around 565 millimetres of mercury, equivalent to roughly 2,438 metres or 8,000 feet, the same cabin altitude regulators permit on a real flight, once with ambient air containing a background 0.1 per cent carbon dioxide and once with the air enriched to 1.0 per cent carbon dioxide, with blood gases, tissue oxygenation and cognitive performance measured throughout.

The results were substantial for what amounts to a single percentage point change in gas composition. The proportion of time participants spent with blood oxygen saturation below 90 per cent fell from 18.8 per cent of the six hour exposure under normal cabin air to just 2.5 per cent with the carbon dioxide enriched air. Arterial oxygen tension rose from an average of 60.9 to 68.3 millimetres of mercury, and near infrared spectroscopy measurements showed brain tissue oxygenation increased from 62.8 to 63.9 per cent, though muscle tissue oxygenation showed no comparable change. Minute ventilation, the total volume of air breathed per minute, rose modestly from 9.7 to 10.4 litres, and cognitive performance on tests of attention, memory and coordination was unaffected either way.

Why Adding a Waste Gas Helps

The logic runs against the instinct that carbon dioxide is simply something the body needs to expel. In practice, a rise in ambient carbon dioxide is one of the strongest natural triggers for breathing itself, stimulating the chemoreceptors that drive ventilation rate more powerfully than a fall in oxygen does on its own. A person breathing slightly more, in response to slightly more carbon dioxide in the air, draws in more of the already thin oxygen available at altitude with each breath, which is reflected in the modest rise in minute ventilation the DLR team recorded alongside the improved blood oxygen figures. The authors’ own conclusion, stated directly in the paper, is that enrichment of cabin air with carbon dioxide during hypobaric hypoxia may improve blood and brain oxygenation, a mechanism distinct from the more familiar options of raising cabin pressure itself or supplying supplemental oxygen directly.

Set Against What Cabin Air Already Allows

The altitude the DLR team chose to test was not arbitrary. UK Civil Aviation Authority guidance for health professionals states that cabin altitude in commercial aircraft should not exceed 8,000 feet in normal operations, with cruising cabins typically sitting somewhat lower, between 5,000 and 7,500 feet. At the regulatory ceiling, the same guidance notes, arterial oxygen saturation in a healthy traveller falls to around 90 per cent, a level it describes as well tolerated, while flagging that passengers with respiratory disease, cardiac conditions or severe anaemia may not tolerate the reduction in barometric pressure as comfortably. The DLR study’s finding that even healthy young adults spent close to a fifth of a six hour exposure below that same 90 per cent threshold under ordinary cabin air conditions puts a number on what “well tolerated” actually looks like moment to moment, rather than as an average, and suggests there is meaningful room to improve on it without the far larger engineering cost of raising cabin pressure across an entire aircraft.

An Early Result, Not Yet a Redesign

The researchers describe their own work as a feasibility study, and the label is an accurate one. Seventeen healthy adults in a ground based chamber is a proof of concept rather than evidence ready for cabin design, and the participants were younger and fitter than the full range of passengers an aircraft actually carries, precisely the older and less healthy travellers the CAA’s own guidance flags as most vulnerable to reduced cabin pressure in the first place. Whether raising ambient carbon dioxide is practical, safe or even desirable across an aircraft full of several hundred people, rather than a small group in a research chamber, remains untested. What the study does establish is that a widely tolerated and long unquestioned feature of commercial flight, the amount of oxygen available to a passenger at cruising altitude, has more room for improvement than the current regulatory ceiling suggests, and that the fix need not come from the two options aviation medicine has traditionally reached for, more cabin pressure or bottled oxygen.

Sources: Journal of Applied Physiology (Post, Aeschbach et al, 2026); German Aerospace Center (DLR) Institute of Aerospace Medicine; UK Civil Aviation Authority (Physiology of Flight guidance for health professionals).