At what stage does flashover occur?

Flashover is a critical stage in fire development that occurs when all combustible materials in a room or space reach their ignition temperature almost simultaneously. This phenomenon is characterized by a rapid transition from a growing fire to a fully developed fire, resulting in a sudden increase in heat release and a dramatic increase in the rate of combustion.

When discussing the correct answer, it's important to recognize that flashover represents a point where the fire transitions from burning specific items to creating conditions where everything in the environment can ignite due to the high temperatures and concentration of flammable gases. This stage often occurs when the room has been filled with smoke and heat, allowing for sufficient thermal feedback to heat surrounding materials to their ignition points.

In contrast, during extinguishing, smoke levels, or just after ignition, conditions are not conducive to flashover. The other scenarios described do not represent the specific condition in which all surfaces uniformly reach their ignition threshold, which is the defining moment of flashover.