易燃和可燃液体表面以及少数易燃固体表面，都有一定的可燃气体与空气形成的混合物，此种混合物迎火源会发生燃烧。如果可燃气体挥发速度低，一经燃烧，新的蒸气来不及补充，则此种燃烧一闪即灭，称为闪燃。能引起闪燃的温度叫闪点。
闪点越低，危险性越大。闪点是可能引起火灾的最低温度(涂装作业场所常见的危险物品的闪点数据，可参考表5-4- 2所列)。
影响闪点的因素很多。对于同类液体，相对分子质量越大，则闪点越高，如甲醇、乙醇、丙醇和丁醇，相对分子质量分别为32、46、60和74,其闪点分别为17℃、11℃、20℃和35℃。汽油的闪点则随分馏温度的增高而增高，于50~60℃时分馏者，闪点为- 58℃，140~ 150℃分馏者，闪点为10℃，至于混合液体的闪点一般低于每种液体的闪点的平均值。
The flash point has practicality for fire protection technology and is a main technical data. According to the flash point of the substance, the fire hazard of various flammable liquids can be distinguished. For example, the flash point of kerosene is 40. It is close to an open flame at a room temperature of 15-20 and cannot burn immediately, because this temperature is lower than the flash point, and the evaporated oil vapor is very small, and it cannot flash or burn. Only when kerosene is heated to 40C can it flash. That is to say, when the temperature is lower than the flash point, it will not burn when it encounters the instantaneous action of the fire , and there is a danger of fire only when the temperature is above the flash point.
The flammability classification of flammable and combustible liquids is also determined according to the flash point, which can be divided into four classes. The first and second grades are called flammable liquids, and the third and fourth grades are called flammable liquids. The classification standards are shown in Table 5-4-2.
For organic solvents whose flash point is lower than room temperature, it is necessary to strictly control the exposure operation of these organic solvents . The gasoline commonly used in degreasing before painting has a flash point of -20C. It is strictly forbidden to use it to wash hands, wash work clothes, and wipe the floor during operation , because this is a common cause of fire accidents.
