Abstract: A generally closed lab hood having an opening closable by a movable door and connected to a variable-throughput suction source has a vent-control system having a sensor for detecting a velocity of flow of air into the hood and for generating an output corresponding to the detected velocity. A controller connected to the sensor and to the variable-throughput suction source receives the output and operates the suction source to set throughput of the source at a level corresponding to the output so that as the detected velocity increases the throughput of the suction source is increased and vice versa. The controller includes a sensor tube having an inner end opening into the hood and an outer end opening outside the hood and a sensor in the tube for detecting the velocity of air-flow therethrough. This tube is offset from the opening. The controller operates proportionally to set the throughput of the suction source relative to the air-flow velocity.