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Smoke Kills! Fire Progression
How do they work? Are they all the same?
Which is better? How are they powered?
Are more alarms desirable? Does it need maintenance?
Here are some facts! How can I prevent deaths?
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Smoke kills people

Smoke causes the majority of fire-related deaths. Hot flames are low on the list of killers during a fire. Smoke contains deadly killer gases such as carbon monoxide and, when smoke is produced, life-sustaining oxygen is consumed. Most fire victims die from these factors, frequently before they awaken. In many fires, extinguished in early stages, people have been found dead of smoke inhalation without having suffered any skin burns.

A smouldering fire may go undetected for hours, especially when people are asleep. Such a fire will continue developing, giving off smoke and gases which may not be visible but which consume oxygen and replace it with toxic gases.

In addition to the deadly killer carbon monoxide, smoke carries poisons such as hydrogen cyanide plus irritants such as formaldehyde and acetic acid. Added to this deadly environment are the toxic substances that come from burning synthetic materials commonly found in the home, especially plastics. Oxides of nitrogen, sulphur dioxide, and ammonia are just a few examples.

These killer agents can take effect before a sleeper is disturbed. An early warning system - a smoke alarm - is considered one of the most effective defences against this situation.

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Progression of a residential fire

Understanding the sequence of events in a residential fire makes it easier to appreciate how smoke alarms can provide a time margin that can save your life. Most fires progress through four distinct stages- first the incipient stage, next the smoke stage, next the flame stage, and finally the high-heat stage as shown in figure 1.

Figure 1

Smoke alarms can alert the occupants in these early stages of a fire while conditions are such that a safe evacuation is still possible.

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How do smoke detectors work?

Smoke detectors work by sensing the rising smoke from a fire and sounding an alarm. They can detect smoke far from the origin of the fire. Smoke detectors are most valuable at night, alerting family members to the presence of fire when they are asleep.

There are at present two types of smoke detectors on the market - the photo-electric smoke detector and the ionization chamber smoke detector.

The IONIZATION DETECTOR contains a radioactive source in a smoke chamber that emits radiation, resulting in a weak flow of electric current. When particles such as those produced by fire enter the smoke chamber, they reduce the current and trigger the alarm. Figure 2 illustrates this type of detector.

Figure 2

The PHOTO-ELECTRIC DETECTOR contains a chamber with either overlapping or porous covers which prevent the entrance of outside sources of light but which allow the entry of smoke. The unit contains a light source and a special photosensitive cell in the darkened chamber. The cell and light are positioned within the alarm in one of the two manners indicated in figure 3 so that either the light beam is interrupted by the smoke as in the obscuration type or the beam is deflected into the cell as in the light scattering type.

Figure 3

Smoke Alarms are now being marketed that provide dual detection. These units contain both an ionization detection chamber and a photo-electric detection chamber and operate on the same principle as either a single photo-electric or a single ionization alarm. Dual detection smoke alarms offer the advantages of both types in a single unit.

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Which detector is better, the ionization chamber smoke detector or the photo-electric smoke detector?

Both types of detectors are equally effective in the home. If properly installed, they can provide adequate warning for the family. Some differences exist between the two when they operate close to the origin of the fire. Ionization detectors will respond more quickly to flaming fires. Photo-electric detectors will generally respond faster to smoldering fires. These differences, however, are not critical.

The detector you buy should be listed by a recognized testing laboratory such as Underwriters Laboratories of Canada (ULC), Underwriters Laboratories (UL) or by any other recognized testing laboratory.

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How are smoke detectors powered and installed?

Detectors are powered two ways - by batteries or by household electric current. Battery-operated detectors are the easiest to install. They require no connections to household wiring. However, the batteries must be replaced approximately once a year to keep the detector operating properly.

All ULC (Underwriters Laboratories of Canada) listed battery operated smoke detectors are required to sound a trouble signal when the battery needs to be replaced. This "chirp" signal usually lasts approx. seven days. If you are away from home for an extended period of time, it is advisable when you return to check your detector, according to manufacturer's instructions, to make sure the battery has not lost power.

Smoke detectors that operate on household current are wired permanently into your home's electrical system. It is better to have them on a circuit that is hooked with other unit sources such as lighting because people have tendancy to swith the power off when the detector sounds for no apparent reason. That way when it's connected on a multiple links circuit they don't want to go without lighting so they have it checked or change them a.s.a.p.. That way the smoke detector is always under power and working.

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Are more alarms desirable?

The location of the basic smoke alarm in the vicinity of sleeping areas does not provide sufficient protection for the occupants from a fire starting in their bedroom with the door closed. It may be desirable, therefore, particularly if you smoke, to have additional alarms within each bedroom itself, and in other areas such as basements, family rooms, etc.

Research indicates that substantial increases in warning time can be obtained with each, properly installed, additional alarm.

A closed door can delay the movement of smoke, preventing it from reaching a smoke alarm, and fire confined in a large room often develops great intensity before it breaks out of the room. The basement is one area that can be large, remote from the alarm, and isolated by some type of closure at the top of the stairs such as a door. Basements also account for a significant number of fires. For these reasons, it is recommended that the smoke alarm be placed at the head of the stairs from the basement.

A smoke alarm is also recommended at the head of each stairway leading to an occupied area. This location may also serve the purpose of protecting a sleeping area in a two storey house if the room layout is suitable. Whether or not you choose to add the additional units, at least install the minimum required as per your local bylaw or at least one per household if there is no bylaw. Remember, a life is irreplaceable!

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Smoke alarms should be cleaned periodically (usually once every six months) as recommended by the manufacturers. This can usually be done by opening the cover and gently vacuuming out the interior of the alarm. The cover can usually be wiped clean with a damp cloth.

The operation of the smoke alarm should be checked by activating the alarm with smoke at the intervals recommended by the manufacturer to ensure that the detector is functioning.

If the smoke alarm is battery operated, replace the battery when it's low (don't just remove it). The general rule that a fire dept. will give out is to change the battery at least once a year whether the battery is good or not. With no battery a smoke alarm is powerless to save your life.

False nuisance alarms indicate a poor location. Don't disconnect or cover up the detector while cooking move it to a better location (you may forget to plug in or uncover the detector and lose your life).

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  • Careless use of smoking materials is the greatest single cause of deaths by fire.
  • Over 60 per cent of fire deaths occur at home.
  • Over 40 per cent of accidental fire victims are asleep at the time of fire. Most people die from SMOKE and toxic gases rather than fire itself.

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Protect your family from fire
  1. PURCHASE one or more smoke detectors.
  2. INSTALL your detectors properly.
  3. IDENTIFY escape routes and PRACTISE ESCAPING
  4. MAINTAIN your detectors.

Contact your local fire department for any special aspect of fire protection.

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