Abstract: A CO.sub.2 monitor which has a reusable portion and a disposable portion is disclosed. The disposable portion includes an airway sensor for connecting between a ventilator output and an endotracheal tube. The airway sensor has ports on opposite sides. In one port a disposable infrared light source is inserted with wire contacts extending to the exterior of the airway sensor body. The reusable portion is a detector module which includes a detector and an amplifier. The detector module attaches to the airway sensor so that the detector is disposed in the second port and so that contacts in the detector module communicate with the wire contacts of the light source. The detector module may be removed from the airway sensor without removing the light source from the airway sensor.