Abstract: A system for use in preventing a flow of gas from a gas-containing region (20) to an adjacent region (22) and/or for collecting or dispersing gas at the interface between the regions, comprising a composite barrier (10) installed so as to separate the two regions (20, 22), the barrier (10) comprising a gas-impermeable layer (11) and a gas-permeable membrane (12) interconnected so as to define an interspace (16) between them and arranged with the permeable layer (12) facing the region containing gas (20) to be dispersed or collected. A method is also described comprising the steps of connecting an aperture (36) or aperture of the barrier communicating with the interspace (16) thereof to pumping means (37) which, in operation, act to pump gas into or out from the interspace (16) by creating a pressure differential across the permeable membrane such as to cause gas to flow therethrough across a major proportion of its surface area.