Most fume hoods have adjustable rear baffles. The total air volume exhausted by the hood will not change when adjusting the baffles. Use of heavier than air fumes are best exhausted with the lower baffles opened. Use of fumes lighter than air or when high loads of heated fumes are generated, it is best to exhaust with the upper baffles opened. Select proper exhaust speed setting.
Some fume hoods have a two speed exhaust fan with a local control at the hood.
The LOW or SLOW exhaust setting is designed to provide air exhaust for chemical storage within the hood. The hood should be left running to provide adequate room ventilation along with chemical fume ventilation. Work with the hood sash partially or completely closed. Move work at least six inches inside the face of the fume hood. This minimizes the effect of cross-drafts and eddies created by the hood operator or by occupants walking by the hood.