It is important to keep in mind that at temperatures over 200°C, cooking oils give off flammable vapours and that spontaneous combustion can occur at temperatures of 310°C to 360°C. It can thus be said that it might just take a moment for a safe situation to become dangerous, if thermostats are no longer working properly.
It is also worth mentioning that the flash point of cooking oils that are re-used is lower due to progressive oxidation. In the case of some blends, such as chicken fat and vegetable oil, the residues can ignite especially easily.
The primary risk of fire in kitchen ventilation systems is caused by build-up of cooking oil residues. Various forms of food preparation inevitably create various grease deposits. For example, frying creates grease similar to transparent creosote oil, while frozen foods with a high water content result in a hard, glossy layer of grease. Oriental methods of food preparation create a very sticky, syrup-like grease that can adhere very well to metal surfaces. Meat cooked on a solid fuel stove or over coals will give off a large quantity of rendered fat. The first layer of grease adheres to metal surfaces, followed by layers of black carbon created in the food preparation process by fat, and ash.