An exhaled breath collection device comprising a sensor unit configured to measure a biomarker in exhaled breath, wherein the biomarker is hydrogen peroxide (H 2 O 2), a cooling device configured to reduce a temperature of exhaled breath, a temperature control unit, and a mouthpiece configured to direct exhaled breath towards the cooling device; wherein the temperature control unit can, in use, control the cooling device to reach a target temperature greater than or equal to 0 °C and less than or equal to 30 °C; wherein the exhaled breath collection device further comprises a breath temperature sensor configured to measure the breath temperature characterised in that the breath temperature sensor is disposed downstream of the mouthpiece unit in a direction of flow of breath and upstream of the cooling device in a direction of flow of breath; and in that the exhaled breath collection device further comprises an exit temperature sensor disposed at a device exhaust to measure the temperature of breath leaving the device; and a flow sensor configured to measure the breath flow rate is disposed downstream of the mouthpiece unit in a direction of flow of breath and upstream of the cooling device in a direction of flow of breath; wherein the temperature control unit is configured to calculate the specific humidity upstream of the cooling device (ω (in,m)) in a direction of flow of breath using breath temperature measured by the breath temperature sensor, calculate the specific humidity at the exhaust (ω (out,m)) using exhaust temperature (T (breath,m)) measured by the exit temperature sensor, and determine the amount of condensate collected in the device based on the specific humidity of breath, the specific humidity at the exhaust and the breath flow rate (V (breath,m)) using flow rate measured by the flow sensor. a sensor unit configured to measure a biomarker in exhaled breath, wherein the biomarker is hydrogen peroxide (H 2 O 2), a cooling device configured to reduce a temperature of exhaled breath, a temperature control unit, and a mouthpiece configured to direct exhaled breath towards the cooling device; wherein the temperature control unit can, in use, control the cooling device to reach a target temperature greater than or equal to 0 °C and less than or equal to 30 °C; wherein the exhaled breath collection device further comprises a breath temperature sensor configured to measure the breath temperature characterised in that the breath temperature sensor is disposed downstream of the mouthpiece unit in a direction of flow of breath and upstream of the cooling device in a direction of flow of breath; and in that the exhaled breath collection device further comprises an exit temperature sensor disposed at a device exhaust to measure the temperature of breath leaving the device; and a flow sensor configured to measure the breath flow rate is disposed downstream of the mouthpiece unit in a direction of flow of breath and upstream of the cooling device in a direction of flow of breath; wherein the temperature control unit is configured to calculate the specific humidity upstream of the cooling device (ω (in,m)) in a direction of flow of breath using breath temperature measured by the breath temperature sensor, calculate the specific humidity at the exhaust (ω (out,m)) using exhaust temperature (T (breath,m)) measured by the exit temperature sensor, and determine the amount of condensate collected in the device based on the specific humidity of breath, the specific humidity at the exhaust and the breath flow rate (V (breath,m)) using flow rate measured by the flow sensor. calculate the specific humidity upstream of the cooling device (ω (in,m)) in a direction of flow of breath using breath temperature measured by the breath temperature sensor, calculate the specific humidity at the exhaust (ω (out,m)) using exhaust temperature (T (breath,m)) measured by the exit temperature sensor, and determine the amount of condensate collected in the device based on the specific humidity of breath, the specific humidity at the exhaust and the breath flow rate (V (breath,m)) using flow rate measured by the flow sensor.