Vertical sashes offer greater safety than a full open-faced hood. The lowered sash offers the operator "a quick place to hide" in the event of a mishap. Typically, the sash is framed and moves in tracks in the hood's wall. It is counterbalanced to facilitate raising and lowering. The glazing material is normally safety glass; however, rigid plastics compatible with laboratory procedures are acceptable. Sash stops that limit the travel of the sash can aid in solving pressure balance problems sometimes encountered when hoods are retrofitted during renovations. [Saunders, 1993]
