A portable device for monitoring of exhaled air composition provided with a mouthpiece (U) and an air duct leading the air to the outlet port (01) through a sensor unit (M), wherein the air duct is provided with a moisture filter (F) and the sensor unit (M) is connected with a control module (S) connected to an external interface system (I). The air duct also includes a pre-concentrator (P) and a carbon dioxide sensor (CO2) installed upstream of the sensor unit (M) and connected to the control unit (S) provided with a measurement triggering module (W) triggering a measurement using the sensor unit (M).A method of exhaled air composition analysis according to the invention is characterised in that the composition analysis starts when the sensor (CO2) indication exceeds a threshold value.
