Free Survey Call 078 43889270. No matter how efficient the extraction system, a build up of grease and other flammable substances in ductwork can become a significant fire hazard. Furthermore the liquid grease that "pools" in the bottom of the ductwork presents a real fire risk.Once set alight the ductwork acts as a huge chimney and fire spreads rapidly. The devastating effects of any subsequent fire on a business are enormous.Regardless of the size of your business, cleanliness of the extraction system is too important to ignore.When the build-up of grease becomes heavy, a fire hazard exists. Approximately one of three restaurant fires is caused by grease. A common scenario of how a kitchen exhaust fire starts is this:A flame flares up on the stove.