10.4 Flammables and Combustibles

Flammable/combustible materials can generate sufficient vapor to cause a fire in the presence of an ignition source. They are categorized based on flash point--the minimum temperature at which a liquid gives off vapor in sufficient concentrations to allow the substance to ignite.

As of 2012, OSHA’s Hazard Communication Standard revised the criteria to determine the flammability hazard and does not use the term “combustible” (however, the term flammable and combustible are both used in other OSHA regulations).  These categories are what will be listed on chemical labels and safety data sheets to convey hazard information. According to these criteria below, the lower the liquid’s flash point, the higher the degree of flammability.  This was the case before 2012 as well but the new criteria creates the 4 new categories of flammability shown below.  You can see that Category 1 has the highest degree of flammability (and therefore hazard) and Category 4 the least.

Category Criteria
1 Flash point < 23˚C (73.4˚F) & initial boiling point ≤ 35˚C (95˚F)
2 Flash point < 23˚C (73.4˚F) & initial boiling point > 35˚C (95˚F)
3 Flash point ≥ 23˚C (73.4˚F) & ≤ 60˚C (140˚F)
4 Flash point > 60˚C (140˚F) & initial boiling point ≤ 93˚C (199.4˚F

OSHA Category 1 flammable liquids, with the highest degree of flammability, are ignitable at concentrations of <13% by volume at ambient temperatures OR have wide flammable concentration ranges of at least 12 percentage points. For comparison, these chemicals will also be rated with a red 3 or 4 on the NFPA hazard diamond.


The liquid does not itself burn; it is vapors from the liquid that burn. The rate of vapor generation depends upon the liquid's vapor pressure, which increases with temperature. The degree of fire hazard depends upon the ability of vapors to mix with air to form combustible or explosive mixtures and the ease of ignition of these mixtures.

Category 1 Flammables are more hazardous because they are more volatile than the other Categories. Safe handling procedures are based upon controlling one or more of the elements necessary to initiate a fire: fuel, ignition source, and oxygen.

Flammable solids may cause fire by friction, absorbing moisture, spontaneous chemical change, retention of heat, or being easily ignited and burning in such a way to cause a serious hazard.


  1. Eliminate ignition sources such as open flames, smoking materials, hot surfaces, sparks from welding or cutting, operation of electrical equipment, and static electricity.
  2. Minimize the quantity kept in work area.
  3. Store in approved flammable liquid containers (safety cans) and storage cabinets or in a special storage room designed for that purpose. Store away from oxidizers.
  4. Flammable liquids stored in glass containers shall not exceed one quart unless chemical purity must be protected. In that case one gallon is permissible.
  5. Refrigerators and freezers used for storage of flammables shall be explosion safe and labeled as such.
  6. Assure proper bonding and grounding when transferring or dispensing flammable liquid from a large container.
  7. Assure appropriate sprinklers or fire extinguishers are in the area.