Particular areas of risk are: the older parts of cities, where buildings tend to be crowded together and it can be difficult to open up clear avenues as a precaution against the spread of fire; in the case of skyscrapers; and where additional storeys have superimposed on an old building. While the building materials may be fireproof, furnishings and other objects can feed a fire for hours and create high temperature levels. Many synthetic substances are readily combustible and, once set alight, are difficult to extinguish. An additional problem is that the insulation afforded by plastic on electrical leads drops sharply at temperatures above 500 deg C. In a fire, vapours and acids attack these leads and cause short-circuits which in turn start secondary fires.
Total annual fire losses in the USA, for example, are estimated at US$ 5,000 million, of which 80-85% are due to urban fires. Buildings are sometimes set on fire deliberately by criminal acts or attacks in war. Towards the end of World War II, 60% of the bombs dropped on towns were incendiary. In major city fires, the rate of damage is in direct proportion to the density of construction. When buildings occupy 20-30% of the surface area, destruction may amount to 65%; when the density is 40%, the rate of destruction may be 80%.