In the past, grain elevators sometimes experienced silo explosions. Fine powder from the millions of grains passing through the facility would accumulate and mix with the oxygen in the air. A spark could spread from one floating particle to the other, creating a chain reaction that would destroy the entire structure. (This dispersed-fuel explosion is the mechanism behind fuel-air bombs.) To prevent this, elevators have very rigorous rules against smoking or any other open flame. Many elevators also have various devices installed to maximize ventilation, safeguards against overheating in belt conveyors, legs, bearing, and explosion-proof electrical devices such as electric motors, switches, and lighting.