A cargo storage unit comprising: a cargo storage space (12, 112); and a controlled atmosphere system (16, 116) comprising: a separation area (22, 122) configured to receive atmospheric gas from the cargo storage space (12, 112); a gas moving device arranged to move atmospheric gas from the cargo storage space (12, 112) into the separation area (22, 122) to thereby increase a pressure within the separation area (22, 122); and a molecular sieve (20, 120), the molecular sieve being a membrane which is permeable to selected molecules and being arranged in communication with the separation area (22, 122) such that, when the separation area (22, 122) is at an overpressure, the selected molecules are continuously vented out of the separation area (22, 122) through the molecular sieve (20, 120) at a removal rate that is substantially proportional to a pressure gradient across the molecular sieve (20, 120); wherein the separation area (22, 122) further comprises a return outlet (26, 30, 126) for flowing recirculated gas from the separation area (22, 122) to the cargo storage space (12, 122), the return outlet (26, 30, 126) provided with a throttling valve for throttling flow from the separation area (22, 122) to the cargo storage space (12, 112), wherein the throttling valve and the gas moving device are controllable to thereby alter the pressure within the separation area (22, 122). a cargo storage space (12, 112); and a controlled atmosphere system (16, 116) comprising: a separation area (22, 122) configured to receive atmospheric gas from the cargo storage space (12, 112); a gas moving device arranged to move atmospheric gas from the cargo storage space (12, 112) into the separation area (22, 122) to thereby increase a pressure within the separation area (22, 122); and a molecular sieve (20, 120), the molecular sieve being a membrane which is permeable to selected molecules and being arranged in communication with the separation area (22, 122) such that, when the separation area (22, 122) is at an overpressure, the selected molecules are continuously vented out of the separation area (22, 122) through the molecular sieve (20, 120) at a removal rate that is substantially proportional to a pressure gradient across the molecular sieve (20, 120); wherein the separation area (22, 122) further comprises a return outlet (26, 30, 126) for flowing recirculated gas from the separation area (22, 122) to the cargo storage space (12, 122), the return outlet (26, 30, 126) provided with a throttling valve for throttling flow from the separation area (22, 122) to the cargo storage space (12, 112), wherein the throttling valve and the gas moving device are controllable to thereby alter the pressure within the separation area (22, 122). a separation area (22, 122) configured to receive atmospheric gas from the cargo storage space (12, 112); a gas moving device arranged to move atmospheric gas from the cargo storage space (12, 112) into the separation area (22, 122) to thereby increase a pressure within the separation area (22, 122); and a molecular sieve (20, 120), the molecular sieve being a membrane which is permeable to selected molecules and being arranged in communication with the separation area (22, 122) such that, when the separation area (22, 122) is at an overpressure, the selected molecules are continuously vented out of the separation area (22, 122) through the molecular sieve (20, 120) at a removal rate that is substantially proportional to a pressure gradient across the molecular sieve (20, 120); wherein the separation area (22, 122) further comprises a return outlet (26, 30, 126) for flowing recirculated gas from the separation area (22, 122) to the cargo storage space (12, 122), the return outlet (26, 30, 126) provided with a throttling valve for throttling flow from the separation area (22, 122) to the cargo storage space (12, 112), wherein the throttling valve and the gas moving device are controllable to thereby alter the pressure within the separation area (22, 122).