A fume hood sash is the sliding glass door at the front of a laboratory fume hood. In a chemical hood, the sash forms the open side that the user can raise or lower. It is usually made of safety glass and serves as the interface between the user and the hood's workspace. The sash controls airflow into the hood and protects the operator: it acts as a transparent barrier that blocks splashes or explosions and directs fumes into the exhaust vent. By adjusting the sash height, the user manages the hood's face velocity to capture contaminants.
The sash is therefore a critical safety component. Most sash windows are constructed from tempered or laminated glass so they can safely absorb impact or heat. Tempered glass resists high shocks and, if broken, shatters into small blunt pieces. Laminated glass has an interlayer that holds shards together. In either case, the sash is designed to crack rather than fly apart; in a fire or explosion it will "spider-web" instead of shattering fully. This way the sash contains chemicals and debris inside the hood, protecting lab personnel. The sash also influences ventilation: as it moves, it changes the airflow path through the hood, pushing harmful vapors toward the exhaust and away from the room.
Types of Fume Hood Sash
Vertical sash: A single large glass panel that moves up and down in vertical tracks. This is the most common sash design. Raising the sash opens a large window directly overhead, giving maximal access to the interior. In use, the sash is typically raised to about eighteen inches above the work surface, and is then closed when work is done.
Horizontal sash: One or more glass panels that slide left and right. Often a two-pane system, this sash lets the user stand behind it and work with arms through the opening. It gives excellent access to the top of the hood but cannot open the full width all at once. Like all hoods, a horizontal sash should be closed completely when not in use.
Combination sash: This integrates vertical and sliding panes. For example, a vertical sash frame may house sliding horizontal panels. In such hoods, the user can either raise the entire sash or move the horizontal panels side-to-side. This design offers the advantages of both types. During work, one direction is used at a time, and all panels are kept closed when the hood is idle.
Split sash: The front is divided into two separate vertical sash windows. Each sash slides up and down independently. Split sashes are used in very wide hoods to create two side-by-side work openings. This allows two users or two processes to operate simultaneously, or lets one user open only half the hood at a time. For containment, each side is closed when its part of the hood is not in use.
Should the Fume Hood Sash Be Open or Closed When Not in Use?
When the fume hood is idle, the sash should be closed. The sash whenever the hood is unattended or finished with a task provides the greatest protection. A closed sash maximizes containment of any residual vapors, fumes or spilled chemicals and acts as an effective shield against accidents.
Ideal Height for Vertical Fume Hood Sash During Use
For vertical sashes, standard guidance sets a recommended "working height" that balances access with containment. In many labs, inspectors mark the sash at about 18 inches above the bench as the normal operating height. During a procedure, users should raise the vertical sash only up to that marked point. Working with the sash at this moderate level ensures that the face opening remains small enough to maintain a strong inward airflow. In practice, safety instructions often emphasize keeping the sash as low as possible while still allowing hand access.
Maintaining a lower sash height is important for containment. A higher opening greatly reduces the hood's face velocity (the speed of air drawn in through the opening) and makes it easier for fumes to spill out. Studies note that each increment of opening decreases the average inflow, so a raised sash may allow contaminated air to escape. Conversely, keeping the sash at or below the 18-inch mark increases capture efficiency. Most protocols therefore advise users to keep the sash below eye level and to never place their head inside or above the plane of the open sash. In summary, the ideal vertical sash height is the one that is just high enough for safe work (often around eighteen inches), since this preserves airflow and safety.
7 Ways to Keep the Fume Hood Sash Safe
Close the sash when idle
Always lower the sash fully when you finish working or leave the hood. A closed sash provides maximum containment and conserves energy. Do not leave the sash open more than needed for adjustments.
Use the correct operating height
Raise the vertical sash only up to its marked or recommended position (roughly 18″). Working at this height maintains proper face velocity. Do not raise the sash above the indicated stop line or above your face. For horizontal sashes, slide panels only enough to access the needed area without creating a large opening.
Keep yourself behind the sash
Position the sash between you and the hood's interior contents at all times. Never lean into the hood or place your head inside the opening. Horizontally sliding sashes can be used as a physical shield by keeping panels in front of your body while working.
Inspect the sash regularly
Before use, check that the sash glides smoothly on its tracks and seals properly when closed. The sash's glass should be free of cracks or chips, and any counterweight or gas spring should hold it without drifting. A smoothly operating, intact sash is essential for protection, so report any stiff movement or damage immediately.
Maintain clear airflow
Keep materials at least six inches back from the sash opening to avoid blocking airflow. Do not place unused chemicals or clutter in the front of the hood, since this can disrupt ventilation. Likewise, do not rely on the sash as a place to rest equipment or weigh down notebooks or towels. Ensure the sash can close fully without obstruction.
Handle the sash gently
Open and close the sash in a controlled manner. Do not slam it or allow it to fall under its own weight, as this can damage the mechanisms. Do not lean on the sash or place heavy objects on it – most sashes are not load-bearing. Treat the sash mechanism (rails, cables, pulleys) carefully to avoid misalignment.
Pay attention to alarms and servcing
If the hood has a flow alarm or auto-close feature, verify they work. If the sash does not stay in position, drifts down, or if a face-velocity alarm sounds, close the sash and notify maintenance. Never operate a hood with a malfunctioning sash or alarm. Proper maintenance and timely repairs ensure that the sash remains an effective barrier and airflow controller.