PURPOSE: To increase the draw of gas along a work surface and to improve uniformity of the draw under various operating conditions by incorporating a baffle mechanism comprising at least three nearly rectangular baffle plates arranged in tiers in a vertical series.

CONSTITUTION: Under an ordinary operating condition, air flowing out from a lower slot 26, an intermediate slot 27 and an upper slot 28 is averaged. In the case when fumes heavier than the air are produced, the upper slot 28 is closed. A large quantity of air so sweeps a work surface as to traverse it. Whirling actions increase in the upper portion of a hood and thereby close and swift whirls in the lateral direction are generated, which promotes evacuation of the fumes from the upper portion of the hood. At the time when gases are lighter than the air, an intermediate baffle plate 21 is regulated so that the intermediate slot 28 be opened to its full-open extent. A large rate of fresh air sweeps across the central part and it is discharged through the slot 28.