We thank Dr Fox for the important point he makes regarding the use of fixed performance devices to deliver a predetermined concentration of oxygen. However, we consider that his concerns are not central to the theme of the commentary. In our case example, oxygen was administered by nasal prongs and a Hudson mask to describe two different approaches to oxygen therapy commonly used in the management of pneumonia. As discussed in our commentary, we contend that, in this clinical situation, the crucial issue is the titration of oxygen therapy to relieve hypoxaemia without causing hyperoxia, with continuous monitoring of the response by measurement of the oxygen saturation.