Reduced air volume (RAV) fume hoods are suitable for low toxic emissions. The opening of the bypass is partially obstructed in these ductless units, reducing overall airflow required to maintain a negative pressure environment.

Constant air volume (CAV) fume hoods provide a constant quantity of airflow, yet air velocity increases as the sash is closed and decreases when the sash is opened wider. Designed to overcome the drawbacks of non‑bypass hoods. They have an additional opening above the sash that adjusts itself according to the movement of the sash. So even when the sash is opened widely or completely closed, the bypass opening adjusts accordingly to prevent flow velocity from increasing or decreasing, providing better protection against toxic gases, designed to save energy and reduce operational costs.