An oven (10) for the thermal conditioning of preforms made of a thermoplastic material, includes at least one cooling circuit (6) with air flow circulation which sequentially extends from the upstream side to the downstream side through at least one cooling air intake area (16a), a heating area (16b) in which a preform heating element (22) and the cooling air flow projected by ventilation elements are provided, and an air extraction area (16c) to which at least one air extraction duct (42) is connected, characterized in that the oven includes a confining housing that extends from the air extraction duct to at least the heating area, and is capable of insulating at least the heating area from the outside of the oven in order to prevent the pollution of the inside of the oven by airborne pollution particles.