Both Flammable and Combustible Liquids are Class 3 hazardous materials defined and described by the USDOT/PHMSA at 49 CFR 173.120.  Too often, this type of hazardous material is assumed to be the same as a USEPA hazardous waste with the characteristic of ignitability (D001).  Though similar in many respects, the terms originate from different federal agencies and have differing determination criteria.  The purpose of this article is to identfiy and describe both a Class 3 Flammable Liquid and a Class 3 Combustible Liquid as defined and regulated by USDOT/PHMSA.

For more information about ignitable hazardous waste read: US EPA Determination of an Ignitable Liquid as a Hazardous Waste.

A Flammable Liquid “means a liquid having a flash point of ≤60°C (140°F), or any material in a liquid phase with a flash point ≥37.8°C (100°F) that is intentionally heated and offered for transportation or transported at or above its flash point in a bulk packaging…”

So, to meet the definition of a Class 3 Flammable Liquid it must be…

1. A liquid with a flash point of ≤60°C (140°F), or;
2. Any material with a flash point ≥37.8°C (100°F), that is…
   • Intentionally heated and offered for transportation, or;
   • Transported at or above its flash point in a bulk packaging.

Even if matching the above description, the following materials are exempt from definition as a Flammable Liquid:

1. Any liquid meeting one of the definitions of a Class 2 (Divisions 2.1, 2.2, & 2.3) compressed gas at 49 CFR 173.115.
2. Any mixture having one or more components with a flash point ≥60°C (140°F), that make up at least 99% of the total volume of the mixture, if the mixture is not offered for transportation or transported at or above its flash point.
3. Any liquid with a flash point >35°C (95°F) that does not sustain combustion according to ASTM D 2406 (IBR, see §171.7 of this subchapter) or the procedure in appendix H of this part.
4. Any liquid with a flash point >35°C (95°F) and with a fire point >100°C (212°F) according to ISO 2592 (IBR, see §171.7 of this subchapter).
5. Any liquid with a flash point >35°C (95°F) which is in a water-miscible solution with a water content of more than 90% by mass.

A Combustible Liquid “means any liquid that does not meet the definition of any other hazard class specified in this subchapter and has a flash point >60°C (140°F) and <93°C (200°F).”

So, to meet the US DOT definition of a Class 3 Combustible Liquid it must…

1. Not meet the definition of any other US DOT hazard class, and;
2. Be a liquid with a flash point >60°C (140°F) and <93°C (200°F).

In some cases, the US DOT will allow you to reclassify a Flammable Liquid as a Combustible Liquid to take advantage of the reduced regulatory burden when transporting Combustible Liquids.

Whether it’s a Flammable Liquid, Combustible Liquid, or liquid Ignitable hazardous waste it is your responsibility to keep them straight and to determine if you are subject to their unique regulations when handling or transporting them.