Mask 10 is positioned over a subjects mouth and nostrils in use so as to collect breath exhaled and comprises a motion detector such as MEMS accelerometer 32. The movement detector may be furthest from the subjects head and detects angular displacement in up to three orthogonal axes. A range of acceptable orientations e.g. up to 20 degrees may be stored and an alarm raised if the head is outside of this range during breath collection, perhaps for a defined period of time e.g. 6s. Sudden movements that do not fall outside this range may also be detected and used to trigger the disregarding of contemporaneous data. The device avoids airway constriction due to excessive head tilt during breath sampling and may also avoid compromise of mask fit at sealing portion 80 (e.g. silicone rubber), to ensure exclusion of ambient air. The mask may be useful during analysis of biomarkers.
