Ionization vs photoelectric

Published on February 26, 2014

Task Group reports
On July 1, 2009, an NFPA task group issued a follow-up report on ionization vs. photoelectric smoke alarms (PDF, 9.2 MB). This report builds on the work of a 2008 task group report on ionization vs. photoelectric smoke alarms (PDF, 1.3 MB). Both task groups were convened to determine the best methods and practices for detecting smoke and to provide information to the technical committee to help determine if changes should be made to the 2010 edition of NFPA 72®, National Fire Alarm and Signaling Code.

ChrisDubayChris Dubay, NFPA Vice President of Codes & Standards, talks about smoke alarms:

 What are the different kinds of smoke alarms?

What kind of smoke alarm does NFPA suggest?

 The most important message about smoke alarms.

The two most commonly recognized smoke detection technologies are ionization smoke detection and photoelectric smoke detection.

Ionization smoke alarmsIonization smoke alarms are generally more responsive to flaming fires.
How they work: Ionization-type smoke alarms have a small amount of radioactive material between two electrically charged plates, which ionizes the air and causes current to flow between the plates. When smoke enters the chamber, it disrupts the flow of ions, thus reducing the flow of current and activating the alarm. Download this chart on ionization smoke alarms (PDF, 943 KB).

Photoelectric smoke alarmsPhotoelectric smoke alarms are generally more responsive to fires that begin with a long period of smoldering (called “smoldering fires”).
How they work: Photoelectric-type alarms aim a light source into a sensing chamber at an angle away from the sensor. Smoke enters the chamber, reflecting light onto the light sensor; triggering the alarm. Download this chart on photoelectric smoke alarms (PDF, 782 KB).

Source: NFPA Ebook: Rescue Dogs, Firefighting Heroes, and Fire Science Facts

For each type of smoke alarm, the advantage it provides may be critical to life safety in some fire situations. Home fatal fires, day or night, include a large number of smoldering fires and a large number of flaming fires. You can not predict the type of fire you may have in your home or when it will occur. Any smoke alarm technology, to be acceptable, must perform acceptably for both types of fires in order to provide early warning of fire at all times of the day or night and whether you are asleep or awake.

For best protection, use both types of smoke alarm technologies

For best protection, it is recommended both (ionization and photoelectric) technologies be used in homes. In addition to individual ionization and photoelectric alarms, combination alarms that include both technologies in a single device are available.