Are All Smoke Alarms The Same?

OSD-01 Smoke Alarm With Normal Alarm

That is like asking are all red wines the same. Before we go further into details, let’s first understand the types of smoke alarms.

Broadly speaking, there’s 2 types of smoke alarms, namely:

  • ionization
  • photoelectric

And a hybrid type which consists of both ionization and photoelectric alarms built into a single unit.

Heat Detectors

First things first, heat detectors are different from smoke alarms, in that a heat detector activates when it reaches a predetermined fixed temperature or when a specific increase in temperature has occurred. They are best suited for environments where rapidly burning, high heat fires are anticipated. In our context of homes, heat detectors typically don’t work as well since residential fires usually start small.

Ionization Smoke Alarms

Ionization smoke alarms contain a small amount of radioactive material. The radiation passes through an ionization chamber which is an air-filled space between two electrodes and permits a small, constant current between the electrodes. Any smoke that enters the chamber absorbs the alpha particles, which reduces the ionization and interrupts this current, setting off the alarm. This type of alarm responds best to fast raging fires.

Courtesy of

Photoelectric Smoke Alarms

Photoelectric smoke alarms operate using a light source, a light beam collimating system and a photoelectric sensor. When smoke enters the optical chamber and crosses the path of the light beam, some light is scattered by the smoke particles, directing it at the sensor and thus activating the alarm. This type of alarm responds best to slow smoldering fires.

This type of alarm responds best to slow smoldering fires.

Courtesy of

Which One Is Suitable For Home Use?

Photoelectric smoke alarms are the recommended ones for home use as they respond better to slow smoldering fires than ionization smoke alarms and residential fires usually start small and many a times, the first tell-tale sign being that significant amount of smoke being generated.

In tests, ionization smoke alarms typically respond about 30 to 90 seconds faster to “fast-flame” fires than photoelectric smoke alarms. However, in smoldering fires, ionization smoke alarms respond an average of 15 to 50 minutes slower than photoelectric alarms. Several studies indicate that they will outright fail to activate up to 20-25% of the time.

The vast majority of residential fire fatalities are due to smoke inhalation, not from the actual flames and almost two-thirds of fire fatalities occur at night while we sleep.

How About The Hybrid Type With Both Photoelectric and Ionization Sensors?

Ironically, such hybrid smoke alarms seem to have a worser performance in tests. The main reason being the design approach:

  • An approach is where both photoelectric and ionization sensors must be triggered for the alarm to sound off
    This is actually the worse type since the photoelectric portion will pick up the smoldering fires first, but will not sound until the ionization sensor triggers. Since a smoldering fire usually pose the greatest danger, this is a problem. The family is often fast asleep while the alarm waits for the ionization sensor that may never respond or responds too slow.
  • The other approach is where alarm to sound off as long as one of photoelectric or ionization sensors gets triggered
    In effect, this type of combination alarm performs similarly to a photoelectric only alarm but you are paying additional for the ionization sensor. Reason being the photoelectric sensor will pick up the smoldering fires so the ionization sensor does not become a factor. However, the ion portion is still susceptible to nuisance tripping. The manufacturers do not want the customer to disable the alarm. So to combat nuisance tripping, they often reduce (desensitize) the smoke sensitivity/response of ionization portion of these units.