Healthy volunteers will be positioned supine and undergo preoxygenation by tidal volume breathing for 3 minutes using each technique in turn. The sequence of techniques for each volunteer will be randomised in advance. Investigators will be instructed to ensure a good mask seal throughout the testing period. At the end of a 3 minute preoxygenation period, after a brief breath hold, the volunteer will exhale into reservoir tubing, allowing the fractional expired oxygen concentration (FeO2) to be determined (primary outcome) using a calibrated gas analyser.