In What is a Hazardous Material?, safety engineering expert James L. Unmack, P.E., C.I.H., C.S.P., describes what makes a hazardous material:

Consider the characteristic of flammability, or more generally, the characteristic of being capable of supporting combustion. How may this characteristic be quantified in a meaningful way? What point on the scale represents a hazardous material?

Organizations such as the National Fire Protection Association and the American Society for Testing Materials have developed standard methods to test and criteria for labeling combustible materials. A flammable material is considered more hazardous than a combustible material. To simplify the quantification, solids, liquids, and gases are addressed separately. The flammability of a gas is easily described by the minimum concentration needed to support combustion, expressed as the lower flammability limit (LFL) or the lower explosive limit (LEL). The flammability of a liquid is measured by the flash point (FP), the minimum temperature at which the vapors, above the surface of the liquid, form an explosive mixture. The flammability of solids is much more complex and is usually addressed on a case by case basis.