What is the Difference Between Ionization and Photoelectric Smoke Alarms?

Posted March 10, 2020 by Koorsen Fire & Security


According to the NFPA, there are over 354,000 residential fires a year, causing, on average, around 2,600 deaths and over 11,000 injuries. The majority of fire-related deaths occur at night while people sleep.

The important role of well-placed, quality smoke alarms is easy to see. There are two main types of smoke alarms – ionization and photoelectric. Understanding the difference between the two can help you make the best decision regarding the smoke alarms protecting your home or business.

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How They Work

Ionization smoke alarms and photoelectric alarms rely on drastically different mechanisms to detect fires:

  • Ionization Smoke Alarms

    Ionization smoke alarms are sophisticated in design. They consist of a chamber formed by two electrically charged plates and a radioactive material that ionizes the air moving between the plates.

    Electronic circuitry within the plates actively measures the ionization current created by this design.

    In the event of combustion, combustion particles enter into the ionization chamber, repeatedly colliding and combining with the ionized air molecules and decreasing their number.

    The electronic circuitry within the plates senses this change in the chamber and, when a predetermined threshold is crossed, triggers the alarm.
  • Photoelectric Smoke Alarms

    Photoelectric smoke alarms are designed based on how smoke from a fire changes the intensity of light passing through the air:
    • Light scattering: most photoelectric smoke detectors operate on the principle of light scattering. They possess both an LED light beam and a photosensitive element. The beam is directed towards an area that cannot be detected by the photosensitive element. But when smoke particles from a fire enter into the beam’s path, the light beam hits the smoke particles and deflects into the photosensitive element, triggering the alarm.
    • Light obscuration: the other type of photoelectric alarms are designed around light obscuration. These alarms are also constructed with both a light source and a photosensitive element. In this instance, though, the light beam is sent directly to the element. When smoke particles partially block the light beam, the photosensitive device’s output is altered by the reduction in light. This reduction in light is detected by the alarm’s circuitry and triggers the alarm.
  • Combination Alarms: additionally, there are a variety of combination alarms. Many combination smoke alarms join the ionization and photoelectric technologies in the hopes of increasing effectiveness.

    Other combinations add additional sensors, such as infrared, carbon monoxide, and heat sensors, to help accurately detect a real fire and decrease false alarms due to things like smoke from a toaster, steam from a shower, etc.

Key Differences Between Ionization and Photoelectric Smoke Alarms

Entities such as the Underwriters Laboratories (UL), the National Fire Protection Association (NFPA), and others have conducted many studies in to determine key differences in performance between these two main types of smoke detectors.

The findings from these studies and tests generally reveal the following:

  • Photoelectric smoke alarms respond much faster (15 to 50 minutes faster) to smoldering fires – fires which move more slowly but produce the most smoke, the element of residential fires most responsible for fatalities – than ionization alarms.
  • Ionization smoke alarms generally perform slightly faster (30-90 seconds) to fast flame fires – fires with flames which spread quickly – than photoelectric alarms. The NFPA acknowledges that a well-designed photoelectric alarm will usually outperform ionization alarms in all fire situations, regardless of type and material.
  • During smoldering fires, ionization alarms failed to give sufficient egress time more frequently than photoelectric alarms failed to do so.
  • During fast-flame fires, ionization alarms failed to provide sufficient egress time more frequently than photoelectric alarms failed to do so.
  • Ionization alarms are responsible for 97% of “nuisance alarms” – false alarms – and are thus much more likely to be disabled all together than other types of smoke alarms. The NFPA acknowledges the significant superiority of photoelectric smoke alarms over ionization alarms with regards to false alarm susceptibility.

Which Smoke Alarm is Best?

The majority of fatalities caused by fires are not from the flames, but from smoke inhalation, which is why most fire-related fatalities - nearly two-thirds - happen during the night while people sleep.

Since this is the case, it is apparent that having a smoke alarm that can quickly and accurately detect smoldering fires, fires that produce the most smoke, is extremely important. In this category, photoelectric smoke alarms clearly outperform ionization alarms.

Additionally, in fast-flame fires, the differences between ionization and photoelectric alarms proved slight, and the NFPA concludes that a high-quality photoelectric alarm will still likely outperform an ionization alarm.

Finally, since nuisance alarms lead people to disable smoke detectors, rendering them useless, photoelectric alarms demonstrate superiority in this realm as well, being far less susceptible to false alarms and thus less likely to be disabled.

Clearly, photoelectric smoke alarms are the most accurate, reliable, and therefore safe option, a conclusion supported by the NFPA and observable in trends among manufacturers and fire safety organizations as well.

With regards to combination alarms, there were no apparent or significant advantages observed. The NFPA concludes that the test-results do not justify requirements to install two-technology or photoelectric-ionization smoke alarms, though they do not necessarily hurt either.

However, the NFPA concludes that photoelectric alarms with additional sensors, such as CO or heat sensors, does improve fire detection and reduce false alarms even more.

Which Fire Alarms Do Experts Suggest?

While photoelectric smoke alarms generally prove superior, and while many commercial entities are moving to photo-technology as their sole smoke alarm technology, many expert authorities such as the NFPA, the NASFM (National Association of State Fire Marshals), USFA (United States Fire Administration), and the Home Safety Council still strongly advise the use of both types of smoke alarms in your home.

The main argument behind the recommendation is that since ionization alarms typically respond faster to fast-fires and photoelectric alarms respond faster to smoldering fires, the presence of both types will be most effective in protecting a home.

What Type of Smoke Alarm do You Have?

Determining which type of smoke alarm you currently have can be difficult. If you find anything mentioning radioactive materials, a model number with an “I” or any mention of Americium-241 on the alarm’s label, it is an ionization alarm. If you find a “P,” it is a photoelectric alarm.

If you are not sure what type of alarms you have, it is likely high time that you replaced them anyway. Be sure to select both ionization and photoelectric alarms for your home and place them appropriately – inside each bedroom, just outside every sleeping space, and one on every level of the house. Placing detectors near stairways and in the living room of the main floor (especially if it is a floor without bedrooms) is also advisable.

Need Help?

Not sure what types of smoke detectors you have or should get to best protect your home or business? Give the experts at Koorsen Fire and Security a call today to learn how they can help protect your property against fire. Don’t let tragedy catch you unprepared – protect your home and business today.

Fire Alarm System Questions? Click to Contact Us Now!


Topics: Fire Protection, Fire Safety, Fire Alarm Systems, Fire Safety & Security

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