A mask 100 for measuring a composition of exhaled and/or inhaled breath, the mask comprising a first sensing means for sensing one or more constituents in the exhaled/inhaled breath to measure metabolism and other responses to physical activity. The first sensing means may comprise an oxygen sensor 135A, a carbon dioxide sensor 135B, an air flow meter 135D or be operable to sense ammonia, acetone, nitric oxide 135C or isoprene concentration. The mask may comprise a second sensing means 145 wherein the second sensing means may comprise an air flow meter, a temperature sensor or a humidity sensor. The first sensing means may be mounted on a panel 140 which may be mounted adjacent to a flow of exhaled air A. The mask may comprise one or more membranes 150, 155 and contain a consumable insert for facilitating hygienic use by multiple subjects.
