Fume hoods are used to contain various levels of airborne toxic or hazardous materials. Fume hoods are available in numerous types and sizes, depending on the required function. Specific examples include; standard, "constant-volume" bypass air hoods, radioisotope hoods, walk-in hoods, distillation hoods, and perchloric acid hoods. Fume hoods are generally available in four-, five-, six-, eight-, ten-, and twelve-foot widths. [Ruys, 1990]
Saunders (1993) emphasizes, that "Constant-volume bypass hoods are NOT constant face velocity hoods... [When the vertical] sash is raised from the closed to the open position, the velocity increases by a factor of from three to four depending on the clear open size of the bypass."
Hoods incorporating variable air volume (VAV) control systems have nearly constant face velocity. [Saunders, 1993]
According to Monsen (1989), "Auxiliary-air fume hoods [can be] less expensive to operate than conventional or bypass hoods. [However, their use has] significant trade-offs and potential problems." Air flow at the hood face may not be uniform. Temperature or humidity control can be influenced by drafts caused by the auxiliary-air fume hood. The auxiliary air can create a potential venturi effect, drawing contaminated air from inside the hood into the laboratory. Consequently, laboratory balancing and pressurization control can be difficult. [Monsen, 1989]
