Contains breakage and small explosions. Maintain constant face velocity independent of sash position. Prevents dead space at front of hood, helps protect user from small spills. Protection from airborne toxic materials. Be sure air is moving into the hood.
Some hoods have meters or alarms.
Always use a tissue to verify that air is flowing into the hood. Maintain air flow pathways front to back. Elevate large items so that air can flow under them. Align items from front to back instead of across the back of the hood. Hood ventilation is more effective with a smaller sash opening.