These exhaust gases leave a residue on the inside of the ductwork. This is usually a grease residue of some sort, depending on the type of cooking. Charbroilers commonly leave a heavy black grease. Chinese cooking normally deposits a sticky or rubbery residue. When a charcoal or wood-burning stove is in use, soot and ash residue builds up in the ductwork.
Dishwashers leave heavy lint deposits.
When the buildup of grease becomes heavy, a fire hazard exists. Approximately one of three restaurant fires is caused by grease. The fire contacts the filters above the stove on the kitchen hood. The filters ignite. Since the exhaust fan is on, drawing air into the hood, through the filters, and up the duct, the flame on the filters is pulled into the duct.