Flammable Materials

Flammable and Combustible Liquids

Flammable and combustible liquids vaporize and form flammable mixtures with air when in open containers, when leaks occur, or when heated. To control these potential hazards, several properties of these materials, such as volatility, flashpoint, flammable range and autoignition temperatures must be understood.  Information on the properties of a specific liquid can be found in that liquid’s safety data sheet (SDS), or other reference material.

Flammable Aerosols

Flammable liquids in pressurized containers may rupture and aerosolize when exposed to heat, creating a highly flammable vapor cloud. As with flammable liquids, these should be stored in a flammable storage cabinet.

Flammable and Combustible Solids

Flammable solids often encountered in the laboratory include alkali metals, magnesium metal, metallic hydrides, some organometallic compounds, and sulfur. Many flammable solids react with water and cannot be extinguished with conventional dry chemical or carbon dioxide extinguishers. See Anecdotes for descriptions of incidents involving such materials.

  • Ensure Class D extinguishers, e.g., Met-L-X, are available where flammable solids are used or stored.
  • Sand can usually be used to smother a fire involving flammable solids. Keep a container of sand near the work area.
  • If a flammable, water-reactive solid is spilled onto skin, brush off as much as possible, then flush with copious amounts of water.
  • NEVER use a carbon dioxide fire extinguisher for fires involving lithium aluminum hydride (LAH). LAH reacts explosively with carbon dioxide.

Catalyst Ignition

Some hydrogenated catalysts, such as palladium, platinum oxide, and Raney nickel, when recovered from hydrogenation reactions, may become saturated with hydrogen and present a fire or explosion hazard.

  • Carefully filter the catalyst.
  • Do not allow the filter cake to become dry.
  • Place the funnel containing moist catalyst into a water bath immediately.

Purge gases, such as nitrogen or argon, may be used so that the catalyst can be filtered and handled in an inert atmosphere.

Flammable Liquid Handling Precautions

Handling Precautions

  • Avoid accumulation of vapors and to control sources of ignition including:
    • open flames
    • electrical equipment
    • sources of static electricity
  • Accounts of a few of the fires that have occurred in our laboratories may be found in Anecdotes.
  • Pouring flammable liquids can generate static electricity. The development of static electricity is related to the humidity levels in the area. Cold, dry atmospheres are more likely to facilitate static electricity. Bonding or using ground straps for metallic or non-metallic containers can prevent static generation.
  • Whenever possible use plastic or metal containers or safety cans.
  • When working with open containers, use a laboratory fume hood to control the accumulation of flammable vapor.
  • Use bottle carriers for transporting glass containers.
  • Use equipment with spark-free, intrinsically safe induction motors or air motors to avoid producing sparks. These motors must meet National Electric Safety Code (US DOC, 1993) Class 1, Division 2, Group C-D explosion resistance specifications. Many stirrers, Variacs, outlet strips, ovens, heat tape, hot plates and heat guns do not conform to these code requirements.
  • Avoid using equipment with series-wound motors, since they are likely to produce sparks.
  • Do not heat flammable liquids with an open flame. Steam baths, salt and sand baths, oil and wax baths, heating mantles and hot air or nitrogen baths are preferable.
  • Minimize the production of vapors and the associated risk of ignition by flashback. Vapors from flammable liquids are denser than air and tend to sink to the floor level where they can spread over a large area.
  • Electrically bond metal containers when transferring flammable liquids from one to another. Bonding can be direct, as a wire attached to both containers, or indirect, as through a common ground system.
  • When grounding non-metallic containers, contact must be made directly to the liquid, rather than to the container.
  • In the rare circumstance that static cannot be avoided, proceed slowly to give the charge time to disperse or conduct the procedure in an inert atmosphere.

Flammable Liquid Properties


Volatility is the tendency or ability of a liquid to vaporize.

Vapor pressure is a measure of a liquid’s volatility. A high vapor pressure usually is an indication of a volatile liquid, or one that readily vaporizes.

Boiling point is the temperature at which the vapor pressure equals atmospheric pressure, such that the pressure of the atmosphere can no longer hold the liquid in a liquid state and bubbles begin to form. In general, a low boiling point indicates a high vapor pressure and, possibly, an increased fire hazard.

Flashpoint is the minimum temperature at which the vapor concentration near the surface of the liquid is high enough to form an ignitable mixture. Any liquid with a flashpoint less than 100oF is considered to be a flammable liquid. A liquid with a flashpoint between 100oF and 200oF is combustible. In general, the relative hazard of a flammable liquid increase as the flashpoint decreases.

Flammable range is the proportion of vapor to air mixture that is ignitable and is expressed in terms of percentage of vapor in air by volume. The flammable range is bounded by the Lower Flammable Limit (LFL) and the Upper Flammable Limit (UFL). The LFL is the minimum concentration of flammable liquid vapor in air that will support the propagation of flame, or spread of flame through the entire volume of vapor-air mixture, upon contact with an ignition source. The UFL is the maximum concentration of vapor in air that will support the propagation of flame. It is important to note that vapor-air mixtures below the LFL may burn at the ignition source without propagating away from the point of ignition.

Auto ignition temperature is the minimum temperature at which a vapor-air mixture will spontaneously ignite, without the necessity of a spark or flame.

Vapor density is a measure of a vapor’s weight when compared to air. Air is assigned a value of 1. Heavier, or denser, vapors tend to sink to floor level while lighter, less dense vapors tend to rise to ceiling level. This property must be taken into account when working with flammable or combustible liquids outside of fume hoods. Most flammable liquid vapors are heavier than air. These vapors can travel some distance and encounter ignition sources remote from the workstation.

Flammable and Combustible Liquid Hazard Classifications

Hazard classifications

National Fire Protection Association (NFPA) hazard classifications for flammable and combustible liquids are listed below:

Hazard classification for flammable liquids      
Class Flash point Boiling point Examples
I-A below 73°F (23°C) below 100°F (38°C) diethyl ether, pentane, ligroin, petroleum ether
I-B below 73°F (23°C) at or above 100°F (38°C) acetone, benzene, cyclohexane, ethanol
I-C 73-100°F (24-38°C) ---- p-xylene
Hazard classification for combustible liquids      
II 101-140°F (39-60°C) ---- diesel fuel, motor oil, kerosene, cleaning solvents
III-A 141-199°F (61-93°C) ---- paints (oil base), linseed oil, mineral oil
III-B 200°F (93°C) or above ---- paints (oil base), neatsfoot oil


Flammable and Combustible Liquid Storage Considerations

Storage Considerations


Maximum type and size of container allowed based on the liquid's hazard classification.

Maximum container size by hazard class

Container Type Class I-A Class I-B Class I-C Class II Class III-A – B
Glass1 1 pint (0.47L) 1 quart (0.94L) 1 gallon (3.79L) 1 gallon (3.79L) 1 gallon (3.79L)
Metal or listed, approved plastic 1 gallon (3.79L) 5 gallon (18.95L) 5 gallon (18.95L) 5 gallon (18.95L) 5 gallon (18.95L)
Approved plastic 0 gallon 0 gallon 0 gallon 0 gallon 5 gallon (18.95L)
Safety cans 2 gallon (7.58L) 2 gallon (7.58L) 2 gallon (7.58L) 5 gallon (18.95L) 5 gallon (18.95L)
Polyethylene 0 gallon 0 gallon 0 gallon 60 gallon (227.4L) 60 gallon (227.4L)
Metal drums For storage requirements: contact EHS representatives 60 gallon (227.4L) 60 gallon (227.4L)

1Exceptions may be made to this requirement for storage of Class I-A and I-B liquids when stored in orginal glass container from the manufacturer (i.e. A solvent bottle from Sigma Aldrich).  Quantity should not exceed 4L. 


A flammable liquid storage cabinet is an approved cabinet that has been designed and constructed to protect the contents from external fires. Storage cabinets are usually equipped with vents, which are plugged by the cabinet manufacturer. Venting is not required by any code or the local municipalities and may actually prevent the cabinet from protecting its contents. Therefore, vents should remain plugged at all times. Storage cabinets must also be conspicuously labeled “FLAMMABLE – KEEP FIRE AWAY”.


Use only those refrigerators that have been designed and manufactured for flammable liquid storage. Standard household refrigerators must not be used for flammable storage. Refrigerators must be prominently labeled as to whether or not they are suitable for flammable liquid storage.

Other Storage Considerations

  • Use approved containers
  • Quantities should be limited to the amount necessary for the work in progress.
  • No more than 10 gallons of flammable and combustible liquids, combined, should be stored outside of a flammable storage cabinet unless safety cans are used. When safety cans are used up to 25 gallons may be stored without using a flammable storage cabinet.
  • Storage of flammable liquids must not obstruct any exit.
  • Flammable liquids should be stored separately from strong oxidizers, shielded from direct sunlight, and away from heat sources.