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What is Diesel?

Diesel fuel is a liquid fuel for use in motor vehicles and other compression ignition engines. Diesel fuel poses potential hazards for fire or explosion, individuals health and the environment. As a highly flammable liquid, it will easily be ignited by heat, sparks or flames. Ignition sources such as open flames or smoking should be kept away from flammable and combustible liquids. Diesel is toxic if ingestion, inhalation and/or skin exposure occurs.

Fire Protection Guide to Hazardous Materials - Edition 14th

Chemical Name / CAS No. NFPA 30 / OSHA Class Flash Point °F (°C) Health Flammability Instability
Diesel Fuel Oil No. 1-D
II 100°F (38°C) Min. or Legal 1 2 0
Diesel Fuel Oil No. 2-D
II 125°F (52°C) Min. or Legal 1 2 0
Diesel Fuel Oil No. 4-D II 130°F (54°C) Min. or Legal 1 2 0

Combustible Liquid:

Any liquid that has a closed-cup flash point at or above 100°F (37.8°C), as determined by the test procedures and apparatus set forth in NFPA 30. Combustible liquids are classified as Class II or Class III combustible liquids.

  • Class II Liquid: Any liquid that has a flash point at or above 100°F (37.8°C) and below 140°F (60°C).

Other Hazard Ratings

The other ratings that appear on the Diesel signs page besides those listed in NFPA's Fire Protection Guide to Hazardous Materials - Edition 14th above, where found in other documentation.

NFPA 704

NFPA 704 is the stardard used for the hazard ratings on the table above (health, flammability and instability)

This system of markings should identify the hazards of a material in terms of the following three principal categories:

  1. Health
  2. Flammability
  3. Instability

The system should indicate the degree of severity by a numerical rating that ranges from 4, indicating severe hazard, to 0, indicating minimal hazard.

Signs should be in locations approved by the authority having jurisdiction and as a minimum shall be posted at the following locations:

  1. Two exterior walls or enclosures containing a means of access to a building or facility
  2. Each access to a room or area
  3. Each principal means of access to an exterior storage area.


These FAQ's were obtained from

The definitions of "flammable liquid" and "combustible liquid" in NFPA 30 differ from those used by the U.S. Department of Transportation. Why?

Definition and classification of flammable and combustible liquids are addressed in Subsection 3.3.33 and Chapter 4 of NFPA 30. A flammable liquid is defined as a liquid whose flash point does not exceed 100°F, when tested by closed‐cup test methods, while a combustible liquid is one whose flash point is 100°F or higher, also when tested by closed‐cup methods. These broad groups are further classified as follows:

  • Class IA ‐ Flash Point less than 73°F; Boiling Point less than 100°F
  • Class IB ‐ Flash Point less than 73°F; Boiling Point equal to or greater than 100°F
  • Class IC ‐ Flash Point equal to or greater than 73°F, but less than 100°F
  • Class II ‐ Flash Point equal to or greater than 100°F, but less than 140°F
  • Class IIIA ‐ Flash Point equal to or greater than 140°F, but less than 200°F
  • Class IIIB ‐ Flash Point equal to or greater than 200°F

Notice that boiling point is only used to distinguish between Class IA and Class IB. Class IA liquids are extremely volatile, but there are few liquids that are so classed. Note also that, theoretically, there is no upper limit to Class IIIB.

These definitions and classifications were agreed to years ago by NFPA, the U. S. Department of Transportation (DOT), and the U. S. Occupational Safety and Health Administration (OSHA) in an attempt to remove inconsistencies in the definitions used at the time. Since then, DOT has changed its definition of "flammable liquid" by raising the upper limit to 141°F (60.5°C). This was done because the United States is a partner to a world‐wide set of hazardous materials regulations sponsored by the United Nations and must use the UN definitions, at least for international transportation. Note, however, that DOT regulations include a so‐called "domestic exemption" that allows a shipper to redesignate as a combustible liquid any liquid whose flash point is in the NFPA Class II range and which does not meet any other hazardous material definition.

What are common examples of the various flammable and combustible liquids classified by NFPA 30?

  • Class IA ‐ Diethyl Ether, Ethylene Oxide, some light crude oils
  • Class IB ‐ Motor and Aviation Gasolines, Toluene, Lacquers, Lacquer Thinner
  • Class IC ‐ Xylene, some paints, some solvent‐based cements
  • Class II ‐ Diesel Fuel, Paint Thinner
  • Class IIIA ‐ Home Heating Oil
  • Class IIIB ‐ Cooking Oils, Lubricating Oils, Motor Oil

Transport Information

Hazmat Placards are required for the transportation of diesel fuel. As described above, different hazard classifications may apply depending on the chemicals flash point and also the mode of transport.

Classification UN Number Hazard Class Packing Class
US Department of Transportation
NA1993 3 III
International Air Transport Association
UN1202 3 III
International Maritime Dangerous Goods
UN1202 3 III

In the U.S, DOT classification allows diesel fuel to be re-classified as a combustible liquid – permiting the use of a combustible liquids placard for transport. If diesel fuel is transported by vessel, aircraft or internationally, a flammable liquids placard is applicable as further explained in 49 CFR:

This code was obtained from 49 CFR

49 CFR §173.150 Exceptions for Class 3 (flammable and combustible liquids)

(f) Combustible liquids. (1) A flammable liquid with a flash point at or above 38 °C (100 °F) that does not meet the definition of any other hazard class may be reclassed as a combustible liquid. This provision does not apply to transportation by vessel or aircraft, except where other means of transportation is impracticable.


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