Selectronix now provide Vents manufactured by Japan based Nitto Denko. The vents are used typically on sealed enclosures to allow them to breathe. Their job is to allow the pressure inside your device to equalise with the outside world, to help reduce condensation build-up and to allow potential harmful gases escape from your device (say a battery compartment), without letting dust or water to ingress.
The membrane Vents are supplied in various forms:
A 'patch' or 'plaster' - available in various sizes with differing air permeability. These are typically installed on the inside wall over a hole in your device.
For harsh/tough environments the vent is protected within a plastic body and it is inserted externally into a hole in your device.
They are IP67 rated and hose proof.