The flammability of a substance is a measure of its ability to burn. It would be convenient if all substances could be categorized as either flammable or non-flammable but, unfortunately, a significant proportion of substances fall into the category of burning under certain conditions. By burning, one generally means self sustained combustion. Thus, the heat generated when a substance is oxidized must be sufficient to overcome any heat losses and heat up fresh fuel to its ignition temperature. The ability of a substance to do this will depend upon its condition, geometry, and environment. For example, flammability is normally considered with reference to burning in fresh air, but many substances which are not normally flammable will burn easily in an environment of pure oxygen.
Gases are the easiest substances to define in terms of flammability. For a gas/air mixture, if it is flammable at all, then there exists a lower limit of fuel concentration below which self-sustained burning does not occur. This is called the lower flammability or lower explosion limit (LFL or LEL). For hydrogen, for example, the LFL is about 4% by volume. Even this basic limit can be affected by geometry with the LFL for downward propagating flames, for example, being different to that for upward propagating flames.
The flammable gas needs air to burn and, as the fuel concentration increases, the concentration of air is decreased until a point is reached where burning again cannot be sustained. This point is termed the upper flammability or upper explosion limit (UFL or UEL). It should be noted that these various limits are normally quoted for a gas mixed with pure air. The presence of other additional gases or particulates or aerosols (such as water sprays) can significantly affect the flammability of the fuel.
It is normally the vapor given off from liquids which burns rather than the liquids themselves. For a fire to be initially ignited, the concentration of the vapor needs to be above the LFL. This is strongly affected by the temperature of the liquid (hence the rate at which it is evaporating), as well as the ambient conditions and geometry. The liquid temperature necessary to produce ignition is termed the flash point. Whether a fire, once started, can be sustained will depend upon the rate of feedback of the heat from the flames to the liquid fuel. The liquid temperature required to generate sufficient vapor for this is termed the fire point.
Solids burn by the heat from the flames causing volatile gases to be given off from the surface of the material. Whether a fire can be sustained is therefore very dependant upon the geometry and conditions. In general, situations where heat losses are minimized and the ratio of surface area to solid mass is greatest are the most favorable to sustained burning. Various standards and tests exist to demonstrate whether burning will be sustained in various geometries, e.g., propagation up a vertical wall, propagation from a lighted cigarette on a seat.
The SFPE Handbook of Fire Protection Engineering, NPPA, (1988).