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If fire extinguishers are available for employee use, it is the employer's responsibility to educate employees on the principles and practices of using a fire extinguisher and the hazards associated with fighting small or developing fires.
To understand how fire extinguishers work, you need to understand a little about fire. Fire is a very rapid chemical reaction between oxygen and a combustible material, which results in the release of heat, light, flames, and smoke.
For fire to exist, the following three elements must be present at the same time:
- Enough oxygen to sustain combustion,
- Enough heat to raise the material to its ignition temperature,
- Some sort of fuel or combustible material, and
This is referred to as the fire triangle. Additionally, there must be a chemical chain reaction between the three elements
Portable fire extinguishers apply an extinguishing agent that will cool burning fuel, displace or remove oxygen, or stop the chemical reaction so a fire cannot continue to burn. When the handle of an extinguisher is compressed, agent is expelled out the nozzle. A fire extinguisher works much like a can of hair spray.
 All portable fire extinguishers must be approved by a nationally recognized testing laboratory to verify compliance with applicable standards [29 CFR 1910.157(c)(2)]. Equipment that passes the laboratory's tests are labeled and given an alpha-numeric classification based on the type and size of fire it will extinguish.
Let's take a look at the label pictured. The classification is: 1-A:10-BC.  The letters (A, B, and C) represent the type(s) of fire for which the extinguisher has been approved. The number in front of the A rating indicates how much water the extinguisher is equal to and represents 1.25 gallons of water for every unit of one. For example, a 4-A rated extinguisher would be equal to five (4 x 1.25) gallons of water. The number in front of the B rating represents the area in square feet of a class B fire that a non-expert user should be able to extinguish. Using the above example, a non-expert user should be able to put out a flammable liquid fire that is as large as 10 square feet.
Different types of fire extinguishers are designed to fight different types of fire. The three most common types of fire extinguishers are: air pressurized water, CO2 (carbon dioxide), and dry chemical. The following table provides information regarding the type of fire and which fire extinguisher should be used.
| Extinguisher Type |
Type of Fire |
| Water |
Ordinary Combustibles
Fires in paper, cloth, wood, rubber, and many plastics require a water type extinguisher labeled A.  |
| CO2 or dry chemical |
Flammable Liquids
Fires in oils, gasoline, some paints, lacquers, grease, solvents, and other flammable liquids require an extinguisher labeled B.  |
| CO2 or dry chemical |
Electrical Equipment
Fires in wiring, fuse boxes, energized electrical equipment, computers, and other electrical sources require an extinguisher labeled C. |
| Multipurpose Dry Chemical |
Ordinary Combustibles, Flammable Liquids, or Electrical Equipment
Multi-purpose dry chemical is suitable for use on class A, B, and C. |
| Class D |
Metals
Fires involving powders, flakes or shavings of combustible metals such as magnesium, titanium, potassium, and sodium require special extinguishers labeled D. |
| Class K |
Kitchen Fires
Fires involving combustible cooking liquids such as oils and fats.
Note: Your present fire extinguishing equipment may not put out a fire involving vegetable oil in your deep fat fryer. |
Use the PASS method when using a fire extinguisher:
P Pull the pin
A Aim hose at the base of the fire
S Squeeze the handle
S Sweep back and forth with the extinguisher
Remember these firefighting tips:
- Most fire extinguishers are emptied in less than a minute.
- Do not attempt to fight a large fire.
- Always call the fire department when a fire breaks out.
- Always leave yourself a way out--keep your back to an exit.
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