Abstract: A security grille for a security institution has an air inlet face, an air outlet face, and zigzag channels extending from a point adjacent the inlet face toward the outlet face. The zigzag channels are defined by slats which are bent at least twice in alternating directions. The zigzag channels are divided into segments each having a cross-sectional width. The segments adjacent the air outlet face have a cross-sectional width less than any of the other segments along the channel. The slats are bent at certain angles with are larger than those of conventional security grilles. Corners are created along each slat as a result of bending the slats in alternating directions. The corners are rounded.