Oxygen was used to treat several diseases as early as the 1790s, and the belief that “purified air” was better than ordinary air persisted among doctors and the general public for the next two centuries.1 In the early 1960s, Moran Campbell recognised the danger of hypercapnic respiratory failure (carbon dioxide retention) caused by high concentrations of oxygen in the treatment of acute exacerbations of chronic obstructive pulmonary disease.2 Subsequent research has consistently reported adverse outcomes after the use of uncontrolled oxygen treatment in this condition.3 4 Evidence on the benefits and harms of the use of oxygen in acute exacerbations of chronic obstructive pulmonary disease has led all specialties in the United Kingdom to recommend controlled oxygen treatment with a target saturation range (usually 88-92%) in this condition.5 However, this recommendation was limited by the lack of level 1 evidence, because no randomised controlled trials were available.6

The linked cluster randomised trial by Austin and colleagues (doi:10.1136/bmj.c5462) finally fills this …