The present disclosure relates to a device (100) for monitoring a biomarker in an exhaled breath, the device (100) utilising a combination of a sensing element (110) having a thermochemical reactant (120) that undergoes a thermochemical reaction with the biomarker and a thermal sensor (140) positioned to measure a rate of change in temperature caused by the thermochemical reaction. A user interface (170) is provided for indicating to a user an indicated measure of the biomarker in the exhaled breath, wherein the indicated measure of the biomarker in the exhaled breath is determined from the measured rate of change in temperature.