LEV is a system that: 

• uses extract ventilation to prevent or reduce the level of airborne hazardous substances from being breathed by people in the workplace;
   

• draws pollutants away from a process or operation that is likely to release a hazardous substance into the workroom air; and
   

• consists of an inlet such as a hood, slot, booth, or cabinet placed around or close to the point of release of the substance.  This device is connected via ducting to the inlet of a fan or air mover, The extracted air is usually discharged to the atmosphere or returned elsewhere in the workplace, having first been cleaned to make it safe for release (see HSG37 An introduction to local exhaust ventilation¹).

• Inlets such as a booth, hood, slot, canopy, cabinet or enclosure.
   

• Ducting which may contain bends, junctions, changes of section and dampers may be circular or rectangular in cross-section and be rigid or flexible.
   

• Fan or occasionally some other type of air mover such as a compressed air venturi.
   

• Air cleaner (where necessary) such as a dust filter, wet scrubber, or solvent recovery device.
   

• Discharge to the atmosphere or a room via a stack, diffuser, grille or just an open duct.