We thank Burns et al 1 for their comments on the updated British Thoracic Society recommendations for managing passengers with respiratory disease planning air travel.2 In particular, a central aim of the recommendations was to promote research in this field, and we therefore commend their recent study investigating hypoxaemia in patients with pulmonary arterial hypertension (PAH) during simulated air travel.3 The method of assessment which best serves a clinician in judging which patients with PAH require in-flight oxygen remains a challenge due to a lack of evidence. Indeed, the current recommendation that those PAH patients in New York Heart Association (NYHA) functional class III or IV should receive in-flight oxygen has only a grade D evidence base. Importantly, NYHA functional class does provide a key measure of the impact of the disease on patients and is a strong predictor of survival in PAH.4 In this context, the successful completion of a hypoxic challenge test (HCT) alone does not always translate to an absence of symptoms in flight for these patients.3 The question of whether NYHA functional class III–IV or HCT better predicts the requirement for in-flight oxygen in PAH, clearly remains open to debate, and we hope this will only serve to encourage further high-quality research in this area to strengthen the evidence base for future air travel guidelines.