Heat carbon dioxide and water vapour three components of exhaled breath need to be exhausted from the mask in readiness for the next breath of inhaled air if this process of evacuation fails or is impeded then the mask may become uncomfortable with poor airflow and may result in the wearer not using the mask we therefore believe that exhalation valves are an essential component which allows the mask to perform well under the conditions described above the valves in respro174 masks are