Ionization and Photoelectric Smoke Alarms - Bulletins
Technical Information Bulletin No. 2000-1
Background
All smoke alarms must comply with Underwriters Laboratories of Canada (ULC) Standard CAN/ULC-S531 which includes thirty performance and twenty construction categories involving approximately one hundred individual tests.
Ionization Type Smoke Alarms
Ionization Smoke Alarms contain a minuscule amount of a material called Americium 241, which emits alpha particles that collide with the oxygen and nitrogen in the air and to create ions. These ions conduct electricity and allow a steady electrical current to flow between two electrodes. Smoke enters the detection chamber and the alpha particles are absorbed by the larger smoke particles, causing a drop in the current, and the alarm is triggered.
All ionization smoke alarms are regulated under the Hazardous Products Act and are required to be certified by the Canadian Nuclear Safety Commission (formerly the Atomic Energy Control Board or AECB). The specific requirements for the certification of ionization type smoke alarms can be found on the Canadian Nuclear Safety Commission web site.
In ULC tests, ionization alarms have performed more effectively with fast, flaming fires which rapidly consume combustibles and spread quickly emitting tiny particles. These types of fires include flammable liquids, loosely packed light combustibles, and kitchen grease.
Photoelectric Type Smoke Alarms
Photoelectric Smoke Alarms employ a light emitting diode (LED) to send a beam of light across a chamber. When smoke enters the chamber, the particles scatter the photons. A photocell installed at an angle to the diode senses the light, and subsequently sets off the alarm.
In ULC tests, photoelectric alarms have performed more effectively with slow burning, smoldering types of fires that can burn for hours before bursting into flame and emit large smoke particles. These types of fires include cigarettes burning in furnishings and bedding.
Recommendations
In homes, over 70% of fires tend to be of the flaming type, more suited to an ionization smoke alarm. However, that leaves an additional 30% of fires which are the smouldering type, which are more suited to a photoelectric smoke alarm. The risk of either occurring is very real.
Since the type of fire likely to occur in any specific environment is impossible to accurately predict, it is recommended that a dual technology fire alarm is purchased. There are many such fire alarms on the market today, and all conform to ULC Standards as long as the ULC mark is found on the packaging.
Glossary of Terms
Americium is potentially dangerous if taken in the body in soluble form, however, swallowing of the Americium will not lead to significant absorbance of Am-241. The dioxide is insoluble, thus there is no significant radiation dose as it passes through the digestive tract. The small amount of radioactive Am-241 used in smoke detectors is not a health hazard. It amounts to a radiation level less than that of everyday background radiation.
An alpha particle is a positively charged particle consisting of two protons and two neutrons bound together. It is spontaneously emitted by some radioactive substances.
An electrode is an electric conductor which is usually metal. Two electrodes form the terminals of an electrically condusting medium, thus conducting current through the medium.
Any atom or group of atoms that has a positive or negative charge. This charge is created by the removal or addition of one or more electrons.
A type of semiconductor device that conducts current in one direction only and emits light.
A Photoelectric cell emits electrons when illuminated causing a current to flow.
A small amount of electromagnetic radiation; essential a light carrying particle in this case.