Close the Sash for Safety
The sash on a fume hood serves many purposes, but the most important one is to protect persons working in the laboratory. When the sash is closed it prevents any "leakage" of chemical fumes from the hood.
A closed sash also protects you from "escapes" caused by accidents. Shattered glass, chemical spills, and vapors are contained in the hood if the sash is closed and an "event" occurs.
Closing the sash improves overall hood performance for other hoods in the lab and within the building. Also, in case of a power outage or hood ventilation failure, chemical vapors will not back up into the laboratory. Closing the Sash for Safety is a very healthy habit to develop!