A. Liquids

Flammable liquids have a flash point below 100°F (37.7°C) and a vapor pressure not exceeding 40 psi (276 kPa). By contrast, combustible liquids have a flash point at, or above, 100°F (37.7°C). Classes of flammable and combustible liquids are further defined in Appendix 10-A.

The flash point is the lowest temperature, as determined by standard tests, at which a liquid gives off vapor in sufficient concentration to form an ignitable mixture with air near the surface of the liquid within the test vessel. Many common laboratory solvents and chemicals have flash points that are lower than room temperature.

Actually, the vapor, not the liquid, burns. The rate at which different liquids produce flammable vapors depends on their vapor pressure. The degree of fire hazard depends also on the ability to form combustible or explosive mixtures with air.

B. Solids

The United States Department of Transportation (DOT) defines and classifies flammable solids in one of three categories (source: 49 CFR 173.124):

  1. Class 4, Division 4.1: Flammable Solid
    Self-reactive materials that are thermally unstable and can undergo a strongly exothermic decomposition even without participation of oxygen; desensitized explosives also fall within this category.
  2. Class 4, Division 4.2: Spontaneously Combustible Material
    Pyrophoric (air-reactive) materials or self-heating materials, likely to self-heat when in contact with air and without energy supply.
  3. Class 4, Division 4.3: Dangerous when wet material
    Liable to spontaneously combust or give off flammable/toxic gas when in contact with water.

Flammable, pyrophoric, self-heating, or dangerous when wet solids will have the following DOT placards on the shipping container when transporting more than 1,001 pounds of Division 4.1 or 4.2 materials, or any quantity of Division 4.3 material. Smaller versions of these placard labels are frequently on the substance container as well.

Figure 10.1
DOT placards for Class 4 materials (Flammable Solids)

C. Gases

The DOT defines flammable gases in 49 CFR 173.115 as materials that are:

  • Gases at 20° C (68° F) or less and 101.3 kPa (14.7 psi) of pressure, and:
    • Are ignitable at 101.3 kPa (14.7 psi) when in a mixture of 13 percent or less by volume with air; or
    • Have a flammable range at 101.3 kPa (14.7 psi) with air of at least 12 percent regardless of the lower limit.

The DOT classifies flammable gases as Class 2, Division 2.1 materials.

D. Ignitability

The auto-ignition temperature of a substance, whether solid, liquid or gaseous, is the minimum temperature required to initiate self-sustained combustion independent of the heat source. A steam line or a glowing light bulb may ignite carbon disulfide (ignition temperature 80°C [176°F]). Diethyl ether (ignition temperature 160°C [320°F]) can be ignited by the surface of a hot plate. Silane gas (ignition temperature 21°C [69.8°F]) can spontaneously ignite at or near room temperature.

Spontaneous ignition or combustion takes place when a substance reaches its ignition temperature without the application of external heat. Consider the possibility of spontaneous combustion, especially when materials are stored or disposed. Materials susceptible to spontaneous combustion include oily rags, dust accumulation, organic materials mixed with strong oxidizing agents (such as nitric acid, chlorates, permanganates, peroxides and persulfates), the alkali metals (lithium, sodium, potassium, rubidium, and cesium), finely divided metal powders, and phosphorus.