this invention teaches an improved exhaust system to be used with a paint spray booth and having a continuously operating power exhaust fan . the exhaust system has a damper that is opened when spraying actively is taking place in the spray booth and that is closed when there is no spraying actively taking place in the spray booth . a control switch is mounted in any of several locations to determine when the spray gun is in use or is not in use . thus , the control switch is actuated when the spray gun is hung up . . . indicating a nonuse condition of the spray system ; whereupon use of the spray gun lifts the gun from the hook to indicate spraying activity . a motor drives the damper to the opened position ; and a spring biases the damper to the closed position . a timer is also used to delay the closing of the damper until between twenty and possibly two hundred seconds have lapsed after spraying activity has terminated . this purges the spray booth while yet allows for automatic damper control under normal spraying sequences of use and nonuse . a fire sensing switch is used to deenergize the damper motor to allow the damper to be closed in the event of a fire .