Any substance that will burn, regardless of its autoignition temperature or whether it is a solid, liquid, or gas. Although this definition necessarily includes all flammable materials as well, this fact is disregarded in official classifications. As usually defined, the term combustible refers to solids that are relatively difficult to ignite and that burn relatively slowly and to liquids having a flash point greater than 100F (37.7C). It is difficult to generalize about the combustibility of solids. The rate and ease of combustion may depend as much on their state of subdivision as on their chemical nature. Many metals in powder or flake form will ignite and burn rapidly, whereas most are noncombustible as bulk solids. Cellulose is combustible as a textile fabric or as paper and is flammable as fine fibers (cotton linters). A plastic that burns at flame temperature will be a greater fire hazard as a foam than as a bulk solid because of the large surface area exposed to air and the thinness of the cell walls. Some polymers, such as nylon and polyvinylidene chloride, will melt and burn but not propagate flame; others, e.g., polyvinyl chloride and polyurethane, ignite at high temperature and evolve toxic fumes. Acrylics and cellulose-derived plastics, such as rayon and cellulose acetate, are readily combustible. This may be partially offset by use of fire-retardant chemicals. Glass is noncombustible in all forms.See Flammable Material; Hazardous Material; Combustion.
