(Safety) Ejection of indoor air to the outdoors fast enough to keep the concentration of toxic vapor in the air inside a work area within safe limits. Note: A local exhaust system is used to carry off an air contaminant by trapping it near its source, as contrasted with general ventilation, that lets the contaminant spread throughout the workrocm, later to be diluted by exhausting quantities of air from the room. A local exhaust is generally preferred to general ventilation because it provides a cleaner and healthier work environment and because a local exhaust system handles a relatively smaller volume of air with less attendant heat loss and with a smaller fan and arrester. A local exhaust system consists of four parts: hoods, into which the airborne contaminant is drawn; ducts, for carrying the contaminated air to a central point; an air cleaning device, such as a dust arrester, for purifying the air before it is discharged; and a fan and motor to create the required air flow through the system. Also called: Ventilation