In the second article, Kelly and colleagues25 (see page 920) point out that the HAST is performed in a stable environment and that it may not fully represent the variability in aircraft cabin pressures and the in-flight physical activity of passengers with COPD, especially during long flights. In order to investigate these issues, Kelly and colleagues25 conducted an interesting and well-designed study that compared the results of the HAST with what actually occurred in 13 COPD patients during commercial air travel. Using a wrist altimeter, they demonstrated that the cabin pressures at cruising altitude in four different types of commercial aircraft were all above the allowed minimum of 565 mm Hg, with a mean of 587 ± 13 mm Hg. They also report that the mean in-flight Spo2 was 86 ± 4%, compared to the mean preflight Spo2 of 95 ± 1%. The in-flight Spo2 further decreased with activity, such as walking to the lavatory, with a mean nadir of 78 ± 6%. During the HAST, nine subjects had a Pao2 < 50 mm Hg and three subjects had a Pao2 from 50 to 55 mm Hg. Of importance is the fact that none of these 12 subjects had any significant medical events during flight. From the observed data, the authors conclude that the higher-than-minimum aircraft cabin pressures may be a factor in the lack of adverse events, that a baseline Pao2 of > 70 mm Hg does not ensure an in-flight Pao2 > 55 mm Hg, that a Pao2 < 55 mm Hg on the HAST should not be an automatic indication for in-flight supplemental oxygen in COPD patients, and that the HAST should include an exercise component for more accurate estimation of in-flight oxygenation.