embodiment of the invention in which an in line vent assembly is provided like numerals are used to indicate like features with previous embodiments in this embodiment the in line vent assembly comprises a generally cylindrical shaped vent frame with u201c windows u201d or u201c ports u201d covered with a membrane as described above the thin air permeable membrane of the present invention may be attached to the mask by any suitable means for example the stainless steel vent described above may be attached to a polycarbonate mask shell by way of hot glue adhesive for example or any other suitable adhesive the durability sought to be achieved will determine the suitable approach for attachment in a further embodiment there is provided a means to indicate the volume of air that has passed through the vent or alternatively the time that the vent assembly has been used when a sufficient volume of air has passed through the vent assembly or the assembly has been used for a sufficient time and may have become blocked the indicator will signal that the vent assembly should be replaced for convenience the thin air permeable membrane can be provided in an insert which is releasably attachable to the mask shell via a push fit mechanism as shown in fig8 preferably on at least the outer surface of the insert there is provided at.