A method of controlling gas composition within a container containing respiring produce is described. The container includes at least one gas outlet and at least one gas inlet and the method includes the steps of； drawing a selected gas from within the container through a selective membrane element and through the at least one gas outlet； detecting ambient pressure inside the container； and introducing ambient air under selected control conditions from outside the container into the container through the at least one gas inlet in response to the detected ambient pressure to control the relative composition of gases inside the container.
