The present invention provides a sensor (2) for sensing water vapour in exhaled breath, the sensor comprising: a sensing element (8, 40) disposed to be exposed to the exhaled breath, the sensing element comprising: a working electrode (12, 44);a counter electrode (14, 46); and a layer of ion exchange material (48) extending between the working electrode and the counter electrode; whereby, when in use, contact of the ion exchange layer with water vapour in exhaled breath forms an electrical contact between the working and counter electrodes and the output of the sensor can be used to generate a display representative of the change in concentration of water vapour over time in a breath exhalation.