Abstract: An air curtain doorway for preventing cross-filtration of air at a doorway between a relatively warm area and a relatively cool area has duct work and an air mover. The duct work includes a supply air duct and a return air duct at opposite sides of the doorway joined by an intermediate air duct at the top of the doorway. The intermediate air duct houses the air mover, which creates an air stream circulated through the duct work that blows an air curtain from the supply air duct to the return air duct. The return air duct has an enlarged collector chamber upstream of an adjustable return air inlet aperture into which the downstream end of the air curtain is drawn. The supply air outlet aperture is at the base of a nozzle that has an upper portion that directs the air curtain toward the warm side and a lower portion that directs the air curtain toward the cold side of the doorway.