Improvements relating to respiratory masks

A respiratory apparatus comprises a sensor 100 having a transmitter and a receiver of electromagnetic radiation and a respiratory mask 10 having an enclosing wall 25A, 25B that defines an interior cavity, the enclosing wall including first and second windows (85, fig.3) (second window not visible) between which a portion (90) of interior cavity exists. The sensor 100 is mounted to the respiratory mask 10 such that electromagnetic radiation from the transmitter, for example infrared light, can be transmitted through the first window (85), through the interior cavity portion (90), through the second window, and into the receiver. Based on the infrared light that is detected, the amount of amount of carbon dioxide in the expiratory gas may be determined. Preferably, the interior cavity portion (90) is spaced from the inspiratory and expiratory gas inlet ports 55, located at the intersection between expiratory flow from a wearer's nose and mouth.