Abstract: An air filtration system for removing smoke and grease from cooking exhaust of ducted range hoods is disclosed. In particular, the system includes a coarse filter device fine filter device which substantially remove all of the cooking effluents from the cooking exhaust drawn into the system by a fan powered by a variable speed motor. A sail switch device is connected to the motor and is positioned in the system to determine the flow quantity of the air drawn into the system and to maintain this flow quantity at a substantially constant volume by controlling the speed of the motor device. A maximum speed of the motor is determined by an indicator device signaling an operator to change filters clogged with cooking particles.