A capnometer comprising: an air flow region through which said gas passes to and/or from said patient's lung; a mid-IR semiconductor emitter configured to provide IR light at a wavelength in the range 3-5 μm; a mid-IR semiconductor detector to detect said IR light; a reflector to reflect said IR light emitted by said emitter; wherein said emitter, said detector and said reflector are arranged such that said IR light emitted by said emitter passes through said air flow region via said reflector to said detector; the airflow region of the capnometer further comprising: a first air-flow path connecting an inhaling/exhaling portion of the capnometer at which a user of the capnometer inhales/exhales air with an intake portion of said capnometer through which air enters said capnometer, and characterised by a second air-flow path connecting said inhaling/exhaling portion with an exit portion of said capnometer at which air exits said capnometer; and a diversion device for blocking one of said first and second air-flow paths in the capnometer at a time, wherein said diversion device comprises a diverting valve which is controlled by said inhalation/exhalation of said air by said user, whereby during inhalation the diverting valve opens the first air-flow path between the inhalation/exhalation portion and the intake portion while it closes the second air-flow path between the inhalation/exhalation portion and the exit portion and whereby during exhalation the diverting valve opens the second air-flow path between the inhalation/exhalation portion and the exit portion while it closes the first air-flow path between the inhalation/exhalation portion and the intake portion. an air flow region through which said gas passes to and/or from said patient's lung; a mid-IR semiconductor emitter configured to provide IR light at a wavelength in the range 3-5 μm; a mid-IR semiconductor detector to detect said IR light; a reflector to reflect said IR light emitted by said emitter; wherein said emitter, said detector and said reflector are arranged such that said IR light emitted by said emitter passes through said air flow region via said reflector to said detector; the airflow region of the capnometer further comprising: a first air-flow path connecting an inhaling/exhaling portion of the capnometer at which a user of the capnometer inhales/exhales air with an intake portion of said capnometer through which air enters said capnometer, and characterised by a second air-flow path connecting said inhaling/exhaling portion with an exit portion of said capnometer at which air exits said capnometer; and a diversion device for blocking one of said first and second air-flow paths in the capnometer at a time, wherein said diversion device comprises a diverting valve which is controlled by said inhalation/exhalation of said air by said user, whereby during inhalation the diverting valve opens the first air-flow path between the inhalation/exhalation portion and the intake portion while it closes the second air-flow path between the inhalation/exhalation portion and the exit portion and whereby during exhalation the diverting valve opens the second air-flow path between the inhalation/exhalation portion and the exit portion while it closes the first air-flow path between the inhalation/exhalation portion and the intake portion. a first air-flow path connecting an inhaling/exhaling portion of the capnometer at which a user of the capnometer inhales/exhales air with an intake portion of said capnometer through which air enters said capnometer, and characterised by a second air-flow path connecting said inhaling/exhaling portion with an exit portion of said capnometer at which air exits said capnometer; and a diversion device for blocking one of said first and second air-flow paths in the capnometer at a time, wherein said diversion device comprises a diverting valve which is controlled by said inhalation/exhalation of said air by said user, whereby during inhalation the diverting valve opens the first air-flow path between the inhalation/exhalation portion and the intake portion while it closes the second air-flow path between the inhalation/exhalation portion and the exit portion and whereby during exhalation the diverting valve opens the second air-flow path between the inhalation/exhalation portion and the exit portion while it closes the first air-flow path between the inhalation/exhalation portion and the intake portion.