Safety in the Chemistry Laboratory
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Fire Safety, Part 2: Putting Out Fires

Types of Fire Extinguishers

Fire extinguishers are categorized according to the substances that they contain and by class of fire. Extinguishers are rated for use against only certain kinds of fires, and will carry an ABCD marking to indicate the classes of fire against which they may properly be used. For example, an extinguisher rated for class B and C fires is called (and marked) a type BC extinguisher. A fire extinguisher should never be used on a class of fire for which it is not rated -- see notes following the table.

Type Mechanism Effective Range Discharge Duration Classes of Fire
A B C D
water reduces temperature 30-40 ft 60 sec [Yes] [No] 1 [No] 2 [No] 1
CO2 displaces oxygen 3-8 ft 8-30 sec [No] [Yes] [Yes] [No] 1
dry chemical binds oxygen 5-20 ft 10-40 sec [Yes] 3 [Yes] [Yes] [No]
halon binds oxygen 4-10 ft 8-24 sec [No] [Yes] [Yes] [No]
Met-L-X smothers See Note 4 NA [No] [No] [No] [Yes]
[Yes]  Extinguisher rated for this type of fire
[No]  Extinguisher not rated for this type of fire
                                            Discharge    Classes of Fire
 Type      Mechanism     Effective Range    Duration      A   B   C   D
 ----      ---------     ---------------    ---------    ---------------
 water     reduces          30-40 ft          60 sec      Y   N   N   N
           temperature
   
 CO2       displaces         3-8 ft          8-30 sec     N   Y   Y   N
           oxygen
           
 dry       binds oxygen      5-20 ft         10-40 sec    Y   Y   Y   N
 chemical
 
 halon     binds oxygen      4-10 ft          8-24 sec    N   Y   Y   N
 
 Met-L-X   smothers        see note 4           NA        N   N   N   Y
 -----------------------------------------------------------------------
   Y => extinguisher rated for this type of fire
   N => extinguisher not rated for this type of fire
Notes:
  1. Fire extinguishers should never be used on classes of fires for which they are not rated. In some cases (eg. water used on a Class D fire), the extinguisher can actually make the fire worse!
  2. Never use water on a class C fire -- shock hazard.
  3. Some dry-chemical extinguishers are rated BC, others are rated ABC. Those rated BC contain sodium or potassium bicarbonate; those rated ABC contain ammonium phosphate.
  4. Extinguishing agents for class D fires are usually applied with a scoop or shovel.

Fire Extinguisher Operation

The information given here describes how a fire extinguisher should be used to fight a fire. However, do not believe, because you have read this, that you know how to use a fire extinguisher! If you really want to learn how to use a fire extinguisher properly (and you should), you should seek out a hands-on training class in which you will have the opportunity to put out some real fires using portable extinguishers. Such classes are offered free of charge through the Fire Marshal's office in many cities in the US, including the city of Eugene.

To remember how to use a fire extinguisher, think of putting out the fire as a test you must PASS.
P Pull the locking pin.
A Aim the nozzle at the base of the fire.
S Squeeze the trigger all the way closed.
S Sweep the extinguisher discharge side to side over the area of the fire.

When should you fight a fire?

In the event of a fire, your personal safety is your most important concern. You are not required to fight a fire. If all of the following conditions are met, then you may choose to use a fire extinguisher against the fire. If any of the conditions is not met, or you have even the slightest doubt about your personal safety, do not fight the fire.

Attempt to use a fire extinguisher if and only if...

  • The fire alarm has been pulled and fire department has been called. The on-campus emergency number at the University of Oregon is 6-6666. The off-campus emergency number in the Eugene-Springfield area is 911.
  • The fire is small and contained.
  • You know your escape route and can fight the fire with your back to the exit.
  • You know what kind of extinguisher is required.
  • The correct extinguisher is immediately at hand.
  • You have been trained in how to use the extinguisher.

Fire Safety, Part 1: About Fires and Fire Types