Flashpoint

The flash point of a substance is defined as the lowest temperature at which enough vapour evaporates from a combustible liquid to form an explosive mixture with air. In practice, this signifies that the vapour pressure of the liquid at this temperature is sufficient to create with air a mixture corresponding to the LEL.
Flashpoint represents the change point from safe to risk. High vapour pressure results usually in high flammability and a low flash point.

Flammable Limits

Flammable limits apply generally to vapours and are defined as the concentration range in which a flammable substance can produce a fire or explosion when an ignition source (such as a spark or open flame) is present. The concentration is generally expressed as percent fuel by volume.
Above the upper flammable limit (UFL) the mixture of substance and air is too rich in fuel (deficient in oxygen) to burn. This is sometimes called the upper explosive limit (UEL).
Below the lower flammable limit (LFL) the mixture of substance and air lacks sufficient fuel (substance) to burn. This is sometimes called the lower explosive limit (LEL).
Any concentration between these limits can ignite — use extreme caution!
The Flammability limits can be changed by addition of inert gas.

Flammability

In the process of burning, hydrocarbon gases react with the oxygen in the air to produce carbon dioxide and water. The reaction gives sufficient heat to form a flame, which travels through the mixture of hydrocarbon gas and air. When the gas above a liquid hydrocarbon is ignited, the heat produced is usually enough to evaporate sufficient fresh gas to maintain the flame, and the liquid is said to burn. In fact, it is the gas that is burning and is being continuously replenished from the liquid.