Professional ventilation is one of the many costs faced by a new cafe or restaurant. Without it, staff can't work in the kitchen, and the customer areas would be unbearable.
A large canopy is needed above open flames (eg on a stove or grill) to extract the hot, greasy air and smoke. One may also be needed above a large dishwasher to remove steam, depending on the capacity. If the exhaust extraction is too powerful, it will suck up chilled air produced by the front of house air-conditioning. If it's too weak, heat and fumes from the kitchen will affect the air-quality of the front-of-house. In a good installation, air-conditioning techs will 'balance' the extraction by providing a source of make-up air directly into the kitchen.
Extraction systems and hoods get greasy and dirty very quickly, so proper cleaning is essential - for filter panels in the hood, and for the inside of the ductwork, which should be installed with inspection hatches. There's a lot to plan, install and maintain - seek professional advice early.
Mechanical exhaust ventilation is required above all gas and electric cooking appliances. It sucks the grease and dirty air produced by cooking and filters it through hoods. The hood filters collect the grease and finally the exhaust transfers the grease free air outside the building.