This chapter outlines the properties of flammable liquids, solids, and gases, the proper storage and use of flammable substances, and the properties of fire extinguishers.

Flammable substances are among the most common hazardous materials found in laboratories. The main objective in working safely with flammable liquids is to avoid accumulation of vapors and to control sources of ignition. However, the ability of a material to vaporize, ignite or explode varies with the type or class of substance. Prevention of fires and explosions requires knowledge of the flammability characteristics (e.g. upper and lower flammability limits, ignition requirements, and burning rates) of materials encountered in the laboratory.
Flammable liquids have a flash point below 100°F (37.7°C) and a vapor pressure not exceeding 40 psi (276 kPa). By contrast, combustible liquids have a flash point at, or above, 100°F (37.7°C). Classes of flammable and combustible liquids are further defined in Appendix 10-A.

The flash point is the lowest temperature, as determined by standard tests, at which a liquid gives off vapor in sufficient concentration to form an ignitable mixture with air near the surface of the liquid within the test vessel. Many common laboratory solvents and chemicals have flash points that are lower than room temperature.

Actually, the vapor, not the liquid, burns. The rate at which different liquids produce flammable vapors depends on their vapor pressure. The degree of fire hazard depends also on the ability to form combustible or explosive mixtures with air.

The United States Department of Transportation (DOT) defines and classifies flammable solids in one of three categories (source: 49 CFR 173.124):

  1. Class 4, Division 4.1: Flammable Solid
    Self-reactive materials that are thermally unstable and can undergo a strongly exothermic decomposition even without participation of oxygen; desensitized explosives also fall within this category.
  2. Class 4, Division 4.2: Spontaneously Combustible Material
    Pyrophoric (air-reactive) materials or self-heating materials, likely to self-heat when in contact with air and without energy supply.
  3. Class 4, Division 4.3: Dangerous when wet material
    Liable to spontaneously combust or give off flammable/toxic gas when in contact with water.

Flammable, pyrophoric, self-heating, or dangerous when wet solids will have the following DOT placards on the shipping container when transporting more than 1,001 pounds of Division 4.1 or 4.2 materials, or any quantity of Division 4.3 material. Smaller versions of these placard labels are frequently on the substance container as well.

Figure 10.1
Figure 10.1. DOT placards for Class 4 materials (Flammable Solids)
The DOT defines flammable gases in 49 CFR 173.115 as materials that are:

  • Gases at 20° C (68° F) or less and 101.3 kPa (14.7 psi) of pressure, and:
    • Are ignitable at 101.3 kPa (14.7 psi) when in a mixture of 13 percent or less by volume with air; or
    • Have a flammable range at 101.3 kPa (14.7 psi) with air of at least 12 percent regardless of the lower limit.

The DOT classifies flammable gases as Class 2, Division 2.1 materials.

The auto-ignition temperature of a substance, whether solid, liquid or gaseous, is the minimum temperature required to initiate self-sustained combustion independent of the heat source. A steam line or a glowing light bulb may ignite carbon disulfide (ignition temperature 80°C [176°F]). Diethyl ether (ignition temperature 160°C [320°F]) can be ignited by the surface of a hot plate. Silane gas (ignition temperature 21°C [69.8°F]) can spontaneously ignite at or near room temperature.

Spontaneous ignition or combustion takes place when a substance reaches its ignition temperature without the application of external heat. Consider the possibility of spontaneous combustion, especially when materials are stored or disposed. Materials susceptible to spontaneous combustion include oily rags, dust accumulation, organic materials mixed with strong oxidizing agents (such as nitric acid, chlorates, permanganates, peroxides and persulfates), the alkali metals (lithium, sodium, potassium, rubidium, and cesium), finely divided metal powders, and phosphorus.

Many potential sources of spark, flame, or heat in laboratories can ignite flammable substances, such as open flames, static electricity, lighted matches and hot surfaces. When flammable materials are in use, pay close attention to all potential ignition sources in the vicinity. The vapors of flammable liquids are heavier than air, and can travel considerable distances. Recognize this possibility and take special note of ignition sources at a lower level than the level of flammable liquid use.

Flammable vapors from substantial sources such as spills can descend into stairwells and elevator shafts and ignite on a lower story. If the path of vapor is continuous, the flame can propagate itself from the point of ignition back to its source.

Properly bond and ground all metal lines and vessels dispensing flammable substances to discharge static electricity. When nonmetallic containers (especially plastic) are used, the bonding can be made to the liquid rather than to the container.

The basic precautions for safe handling of flammable materials include the following:

  • Handle flammable substances only in areas free of ignition sources.
  • Do not heat flammable substances with an open flame. Preferred heat sources include steam baths, water baths, oil baths, heating mantles and hot air baths.
  • When you transfer flammable liquids in metal equipment, avoid static-generated sparks by bonding, and the use of ground straps.
  • Ventilation is one of the most effective ways to prevent the formation of flammable mixtures. Use an exhaust hood when you handle appreciable quantities of flammable substances (e.g. transferring between containers or in an open container, especially if you are heating it).
  • When withdrawing a flammable liquid from a drum, or filling a drum, both the drum and other equipment must be individually, electrically grounded and bonded to each other.
  • Containers of flammable liquids shall not be drawn from or filled within buildings without provisions to prevent the accumulation of flammable vapors in hazardous concentrations.
Basic rules for safe storage of flammable materials include the following:

  • Store flammable and combustible liquids only in approved containers. Approval for containers is based on specifications developed by organizations such as DOT, the Occupational Safety and Health Administration (OSHA), the National Fire Protection Association (NFPA), or the American National Standards Institute (ANSI). Containers used by the manufacturers of flammable and combustible liquids generally meet these specifications.
  • Flammables stored in the open in the laboratory work area shall be kept to the minimum necessary for the work being done.
  • Do not store flammable liquids in domestic type refrigerators. Domestic type refrigerators are not recommended for laboratory use, even if flammable storage is not contemplated, since future research needs may require the use of flammables. Existing domestic refrigerators in labs must have a posting (similar to Figure 10.2) stating that no flammable storage is permitted. “Safety” refrigerators (Figure 10.3) are recommended for laboratories. These have the electrical contacts (door switch, light, thermostat, etc.) removed or exteriorized. “Explosion-proof” refrigerators (Figure 10.4) are not necessary except in unusual circumstances, such as within an inside storage room (for flammables) or other potentially hazardous atmospheres. Note that each of these is clearly labeled if it is for flammable material storage or explosion-proof.
  • Flammable liquids must not block laboratory aisles or exits. Do not locate flammable storage cabinets near an exit or in the hallway.
  • Keep flammable liquids away from heat and direct sunlight.
  • Store flammable liquids in a way that prevents accidental contact with strong oxidizing agents (such as permanganates or chlorates).
  • University policy prohibits smoking in all University buildings; remove other sources of ignition from areas where flammable liquids are stored.
  • Maximum allowable size of containers for flammable and combustible liquids shall be in accordance with Appendix 10-A.
  • The potential fire hazard also depends on the total quantity of flammable and combustible liquids present within a laboratory unit (room) and the type of containers in which the liquids are stored. The maximum quantity allowed per laboratory unit is as follows:
    1. Shelf or open storage/use
      1. glass, approved plastic or metal: 10 gallons (37.9 liters)
      2. safety cans: 25 gallons (94.7 liters)
    2. Approved storage cabinets
      1. Class I: 30 gallons (113.6 liters)
      2. Class I, II, & III: 60 gallons (227.2 liters)
    3. Inside storage room (meeting NFPA Code recommendations)
      1. with sprinkler: 4-10 gal/ft2
      2. without sprinkler: 2-4 gal/ft2
    4. For laboratories located on upper floors within new or remodeled buildings, the limits for flammable and combustible liquids might be more restrictive due to the North Carolina Fire Prevention Code. Refer to Chapter 4 for more details, or contact EHS at 919-962-5507 if you have questions.
Figure 10.2
Figure 10.2. Labels for lab refrigerators are available on the EHS website.
Figure 10.3
Figure 10.3. Safety refrigerator.
Figure 10.4
Figure 10.4. Explosion-proof refrigerators.
For ease of identification, labels A, B, C, D, and K (and, more recently, pictograms) indicate the type of fire on which one can use an extinguisher.

Type Materials Description Label Pictogram
A Ordinary Combustibles Fires in paper, cloth, wood, rubber, and many plastics require a water or dry chemical type extinguisher labeled A. A: Combustibles Pictogram A
B Flammable Liquids Fires in solvents and other flammable liquids require dry chemical, Halon™, or CO² extinguisher labeled B. B: Liquids Pictogram B
C Electrical Equipment Fires in wiring, fuse boxes, energized equipment and other electrical sources require a dry chemical, Halon™, or CO² extinguisher labeled C. C: Electrical Equipment Pictogram C
D Metals Combustible metals such as magnesium and sodium require special extinguishers labeled D. D: Combustible Metals Pictogram D
K Cooking Oils and Fats Wet chemical extinguishers specially designed to put out fires of cooking oils or fats are labeled K; unlikely to be needed in a laboratory setting. K: Combustible Cooking Pictogram K

Most chemical laboratory fire hazards require multipurpose dry chemical extinguishers (ABC) located in hallways. “Gas” extinguishers containing carbon dioxide (CO2) offer a first defense against flammable liquids or electrical fires without leaving a powder residue that could harm electronic equipment. Halon™ 1211 fire extinguishers are no longer to be used on campus. If your lab has one, contact EHS so that it can be replaced.

EHS personnel perform annual inspection and maintenance of campus fire extinguishers. Laboratory personnel are encouraged to check their extinguishers regularly for these items:

  • Accessibility
  • Charge Gauge (CO2 units lack gauges)*
  • Tamper Seal
  • Physical Damage

Report any problems or missing extinguishers to EHS at 919-962-5507.

*EHS recharges extinguishers at no cost to the department or building to which the extinguisher was assigned if one the following applies:

  • If there is evidence of pressure leakage, or
  • If the extinguisher has been used
EHS offers training in the correct use of fire extinguishers and building evacuation. For information and training dates, contact EHS at 919-962-5507. You are NOT required to use a fire extinguisher on campus. However, if you wish to use one on campus you must receive training in its correct use.

Additional information on University Fire Safety policies is found in Chapter 3 of the UNC Environment, Health and Safety Manual.

Class IA:
Liquids having flash points below 73°F (22.8°C) and having a boiling point below 100°F (37.8°C).
Class IB:
Liquids having flash points below 73°F (22.8°C) and having a boiling point at or above 100°F (37.8°C).
Class IC:
Liquids having a flash point at or above 73°F (22.8°C) and below 100°F (37.8°C).
Class II:
Liquids with flash points at or above 100°F (37.8°C) and below 140°F (60°C).
Class III:
Liquids with flash points at or above 140°F (60°C).
Safety Can:
An approved container, of not more than 5-gallons capacity, having a spring-closing lid and spout cover and so designed that it will safely relieve internal pressure when subjected to fire exposure.
Figure 10.5
Figure 10.5. Safety Cans

Many types of containers are required depending on the quantities and classes of flammable or combustible liquids in use.

Class IA Class IB Class IC Class II Class III
Glass 1 pt.
(0.47 L)
1 qt.
(0.95 L)
1 qt.
(0.95 L)
1 gal.
(3.79 L)
1 gal.
(3.79 L)
Metal or
1 gal.
(3.79 L)
5 gal.
(18.94 L)
5 gal.
(18.94 L)
5 gal.
(18.94 L)
5 gal.
(18.94 L)
2 gal.
(7.58 L)
5 gal.
(18.94 L)
5 gal.
(18.94 L)
5 gal.
(18.94 L)
5 gal.
(18.94 L)
Class IA and Class IB liquids shall be permitted to be stored in glass containers of not more than 1.3 gal (5 L) capacity if the required liquid purity (such as American Chemical Society analytical reagent grade or higher) would be affected by storage in metal containers or if the liquid can cause excessive corrosion of a metal container.