Judicious elevation of ambient carbon dioxide during hypobaric hypoxia to improve oxygenation in airline passengers:...

Despite pressurization of airliner cabins, some passengers experience in-flight hypobaric hypoxia with blood oxygen saturation dropping below 90%, potentially causing discomfort and increasing the risk of medical events. Enrichment of the cabin air with carbon dioxide (CO₂) may augment passengers' blood and tissue oxygenation by stimulating respiratory drive, thereby increasing health and safety during air travel. In a randomized double-blind crossover study, we exposed 17 healthy adults (8 women; age range: 18-40 yr) on separate days to two ambient CO₂ levels (0.1% vs. 1.0% indoor sea-level equivalents; 0.76 vs. 7.60 mmHg partial pressure) during 6 h of hypobaric hypoxia (∼565 mmHg total barometric pressure; corresponding to 2,438 m altitude) in an altitude chamber simulating long-haul flight conditions. We measured oxygen saturation of the blood ([Formula: see text]), brain, and muscle (tissue saturation index derived from near-infrared spectroscopy), respiration, and cognitive function (sustained attention, working memory, and hand-eye coordination) hourly. In addition, we conducted capillary blood gas analyses at baseline, 15 min, and 6 h after hypoxia onset. During hypobaric hypoxia, ambient CO₂ enrichment on average increased Pco₂ from 36.3 to 38.3 mmHg, Po₂ from 60.9 to 68.3 mmHg, and minute ventilation from 9.7 to 10.4 L/min, whereas reducing the time fraction of [Formula: see text] < 90% from 18.8% to 2.5%. Tissue oxygenation increased in the brain from 62.8% to 63.9%, whereas no change was found in the muscle. High ambient CO₂ had no effect on cognitive performance. Taken together, enrichment of cabin air with CO₂ during hypobaric hypoxia may improve blood and brain oxygenation.NEW & NOTEWORTHY Elevating ambient CO₂ can improve blood and brain oxygenation in healthy individuals during moderate hypoxia as experienced in an airliner cabin. The findings offer a new approach potentially benefiting the health and safety of airline passengers and support investigating CO₂ enrichment in broader passenger populations, especially in those at risk of hypoxemia. Exploration of CO₂-based interventions to enhance passenger oxygenation may also prompt regulatory reassessment of cabin CO₂ limits and technological innovations in ventilation systems.