The two primary hazards associated with flammable and combustible liquids are explosion and fire. The property that makes flammable liquids so hazardous is their vapor. The vapor mixes with air and burns quickly when the liquid is heated to its’ flashpoint or above and is ignited.

Safe handling and storage of flammable liquids requires the use of appropriate equipment and practices.

While there is a technical distinction between “flammable” and “combustible” liquids, both burn readily and intensively, are explosive under certain conditions, and if not properly contained can spread fire rapidly and uncontrollably.


flammable liquid is defined by NFPA and OSHA 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 Properties Common examples

Flammable liquid

Class IA Flash Point less than 73°F; Boiling Point less than 100°F Diethyl Ether, Ethylene Oxide, some light crude oils
Class IB Flash Point less than 73°F; Boiling Point equal to or greater than 100°F Motor and Aviation Gasoline, Toluene, Lacquers, Lacquer Thinner
Class IC Flash Point equal to or greater than 73°F, but less than 100°F Xylene, some paints, some solvent‐based cements

Combustible liquid

Class II Flash Point equal to or greater than 100°F, but less than 140°F Diesel Fuel, Paint Thinner
Class IIIA Flash Point equal to or greater than 140°F, but less than 200°F Home Heating Oil
Class IIIB Flash Point equal to or greater than 200°F Cooking Oils, Lubricating Oils, Motor Oil

Keep in mind these are the NFPA and OSHA classifications. Other regulatory schemes define these materials in different ways, so what you are doing with the material (and therefore which regulation applies) is an important consideration when managing them.

Regulations, interpretations and standards


Wisconsin Department of Safety and Professional Services

The Wisconsin Department of Safety and Professional Services (DSPS) adopted OSHA’s general industry and construction standards as the rule for public employees in Wisconsin. No modifications or amendments have been adopted to the standards cited above. Two modifications in related areas were adopted:

  • SPS 332.24 – To the extent that ventilation is used when working with flammable liquids, there are additional rules in place for the workplace ventilation rules at 29 CFR 1910.94.
  • SPS 332.26 – There is a plan submittal requirement for spray finishing using flammable and combustible materials that is an addition to the rules in CFR 1910.107.

Consensus Standards

  • NFPA 30, Flammable and Combustible Liquids Code (about NFPA 30)



This publication was prepared for environmental, health and safety staff at University of Wisconsin System campuses, to assist in finding resources and information for regulatory compliance. It is not intended to render legal advice.

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