In a properly functioning hood, a curtain of air flows in through the open face of the hood and out through the duct work.

Chemical Fume Hoods A chemical fume hood is the primary means of controlling inhalation exposure in the laboratory. If used correctly a hood will retain the vapors and gases released within them, preventing the laboratory employee from inhaling the contaminant. This level of control is made possible by the curtain of air flowing in through the open face of the hood and out through the duct work. To ensure that the air is flowing properly when working in a fume hood, make sure the sash opening does not exceed the maximum height acceptable for that hood. This can be assured by lining up the arrows on the sash door and hood frame. When you are not using the hood, keep the sash open three to six inches to maintain air flowing through the hood; this will remove contaminants that may be present in the hood. Align arrows for correct working height.