Local exhaust hood is an important part of the local exhaust system. Its effectiveness of the entire exhaust system has a very important impact on the technical economic performance. Well-designed local exhaust hood can be used to obtain the best control effect with a small air volume, to ensure that the concentration of harmful substances in the work area in line with the provisions of national health standards. Exhaust hood according to the degree of closure points, there are closed exhaust hood, semi-closed exhaust hood open exhaust hood. Semi-closed exhaust hood refers to the operational needs, often can produce pollutants in the equipment completely partially closed, but must be open with a larger working hole exhaust hood. Fume hoods are one type of such semi-enclosed exhaust hoods. There are three general forms of fume hoods, the difference being that the location of the vent is different for different densities of contaminants. When the density of pollutants is small, use the upper exhaust; when the density is large, use the lower exhaust; when the density is uncertain, use the upper lower exhaust at the same time, the upper exhaust port can be adjusted. Ventilated cabinet cabinet door up down can be adjusted, under the conditions of operation permit, the cabinet door opening degree the smaller the better, so that in the same amount of exhaust air under the better results. The ability of the fume cupboard to control pollutants mainly depends on the wind speed at the opening, the wind speed at the opening is generally recommended to be 0.3~1.5m/s.