The air is 'thinner' because the pressure is less and, as a result, there is less oxygen. In fact, the concentration of oxygen falls from 21% at sea level, to about 15%. Most people will tolerate this, but, if you already have problems breathing, this will become worse.

Yes, flying can significantly worsen shortness of breath (dyspnea) due to the lower oxygen levels in a pressurized cabin. While an airplane is flying at 35,000 feet, the interior air pressure is equivalent to being at an altitude of 6,000 to 8,000 feet. This "relative hypoxia" results in a 10–15% drop in blood oxygen saturation, which is harmless for healthy people but can cause severe distress for those with COPD, asthma, or heart conditions. Additionally, the extremely dry air (less than 20% humidity) can irritate airways and cause mucus buildup, further complicating breathing. In 2026, medical experts recommend that anyone who experiences breathlessness after walking 50 meters on level ground should undergo a "Hypoxia Altitude Simulation Test" or consult a pulmonologist for a "fit-to-fly" certificate and potential in-flight supplemental oxygen before booking a journey.