Abstract: The device comprises a first dry seal and an admission chamber for controlled leakage of a neutral barrier gas at a pressure of slightly higher value than the pressure of the hazardous gas which is thus prevented from passing through the seal. The pressure difference across the first seal is maintained at a low and constant value by an external regulator. A second seal closes the admission chamber and is supplied with barrier gas at a pressure of higher value than the pressure of the external atmosphere. Leakage of the barrier gas from the second seal is discharged along with any leakage of hazardous gas through the first seal into an external recovery circuit. The leakage of barrier gas from the admission chamber is into an adjacent compartment defined by the second seal and by a floating ring which produces a pressure drop so as to prevent the external atmosphere from penetrating into the compartment.