Any kitchen produces enormous amount of heat and fumes. The fumes include suspending particles like oil and spices. This is normally carried along with the fumes, due to the higher velocity of the fumes. When the velocity drops in the larger volume of the kitchen the fumes drop the suspended particles and spread a layer of this suspended particle all across the kitchen and surroundings. This makes the place very unhygienic. The excessive heat generated also makes it tough to work inside. Continuous burning of gas will deplete the oxygen level inside the kitchen and make it hazardous to the kitchen staff.
A exhaust system basically stream lines the fumes to the hood with a certain air speed so as to carry the suspended particles till the filter. At the hood, the air speed is dropped across the filters to collect all the suspended particles. If the kitchen has got enough air vents the exhaust system will induce an automatic suction from out side to maintain the air quality. In the absence of such air vents a fresh air system is maintained to compensate the air volume inside the kitchen