Do Ionisation Smoke Alarms Work?

Updated: Apr 26

Fire alarms are familiar to us all. Chances are, we will all have hazy recollections of safety talks from school, and apart from when alarms sound at the sleepy break of dawn because we have burnt our toast, we probably never think of them again.

We almost certainly don’t consider how they work. We just assume they always will.

However, some companies are moving away from using solely ionisation alarms altogether, due to the risks presented in fire tests and in domestic housefires.

So, do ionisation smoke alarms work? What smoke alarm types are there?


Properties will have either,

  • ionisation,

  • Photoelectric, which can also be known as ‘optic,’

  • Or dual detector alarms, which combines ionisation and photoelectric technologies.

Ionisation smoke alarms contain an incredibly small quantity of americium, totalling ‘around 0.29 microgram, which is roughly one thousandth the weight of a grain of salt.’​1 This radioactive material sits 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.’​2

Americium element infographic

Photoelectric smoke alarms however, detect smoke by aiming ‘a light source into a sensing chamber at an angle away from the sensor. Smoke enters the chamber, reflecting light onto the light sensor and triggering the alarm.’​3

Dual detector alarms are a combination of ionisation and photoelectric alarms, utilising both technologies that make both kinds of alarms work. This option is increasingly encouraged by smoke alarm manufacturers, as they allow for the sensing of both types of fires.

Do different smoke alarms work best for different fire types?

Recent fire tests are demonstrating that increasingly, Ionisation smoke alarms work best with fast, flaming fires and can struggle to pick up on smoke particles from slow, smouldering fires.

In independent tests carried out by news channel WCCO for NBS Minnesota, a smouldering fire test was started by the fire brigade. WCCO’s Jennifer Mayerle and the emergency services found that the photoelectric alarm sounded at 32 -minutes, 50-seconds, while the ionisation alarm did not sound until 1- hour, 28-minutes. Long after the fire brigade had left the scene due to the dangerous levels of harmful smoke. ​4

This issue with ionisation alarms has led to states such as Iowa, Massachusetts, and Vermont banning the use of ionisation only smoke alarms in domestic properties, while Australia and New Zealand are still in some debate about the effectiveness of ionisation vs. photoelectric smoke alarms.​5


Information posters for ionisation and photoelectric smoke alarms

Photoelectric, or optic, smoke alarms on the other hand, will sound fastest for slow, smoky, smouldering fires. These fires emit more smoke and are the most common type of fires in domestic properties, including fires from,

  • electrical faults,

  • cigarettes on soft furnishings,

  • candles, and

  • tumble dryer malfunctions.

These release large quantities of smoke and a vast cornucopia of toxic, life-ending gases, which were accountable for 30% of domestic fire fatalities in 2019/20 in the UK.​6

Photoelectric smoke alarms, in specific smouldering fire tests have been demonstrated to sound at least 45 minutes before the call of an ionisation smoke alarm, giving families precious extra time to escape.

However, this does not mean that a photoelectric alarm will be best overall.

“The main issues with ionisation alarms is their susceptibility to false alarms and they are simply not as good at detecting some of the typical house fire as optical alarms… there is also the obvious issue of residents not wanting to have radioactive materials in their homes and difficult disposal.”​7 James Hall, Technical Director at FireAngel

Due to an increased likelihood of false alarms when using ionisation smoke alarms, homeowners are more likely to remove the batteries, or turn off smoke alarms while cooking or smoking. This can then lead to the individual forgetting to reinstall the batteries, or turn the smoke alarm back on, and leaves their home unprotected. This is not said to be an issue with photoelectric, dual detector, or multi-sensor alarms.

As such, UK limited company FireAngel have opted to phase out ionisation-only technology from their product catalogue, opting instead for dual-detectors, or multi-sensor alarms.

Example of a Multi-Sensor Smoke Alarm

Example of a Multi-Sensor Smoke Alarm.​8

Dual detectors contain both ionisation, and photoelectric technologies, so can offer the protection of both types of alarms, without the limitations of either. This allows them to detect slow, smouldering fires, and fast, flaming fires, with plenty of advanced warning, giving them precious minutes to evacuate.

Alongside dual detectors, homeowners should consider the installation of multi-sensor alarms. Multi-sensor alarms combine two or more sensor types, most commonly optical and heat, to accomplish: decreased reaction time to a fire event; decreased occurrence of false alarms; or both of these factors. Subject to compliance with either BS EN 54-7 or BS EN 14604: 2005, […] multi-sensor alarms can be used in place of smoke alarms when following the Code.​9


The type of alarm system you decide to install in your home is very much up to you if you are the homeowner.

Until a final decision is made regarding the effectiveness of ionisation smoke alarms, however, one thing all experts agree on is the important of the interconnected alarm system. This means that no matter the location of the fire, all alarms, be they ionisation, photo-electric, dual-detector, or multi-sensor, will sound, giving building users increased time to exit the property and call the emergency services.

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1 McGill, Office for Science and Society, 'Smoke Alarms are not created equal,' by Jonathan Jarry M.Sc, published 08.01.2022, <https://www.mcgill.ca/oss/article/technology-general-science/smoke-alarms-are-not-created-equal>

2 Robert Robillard, ‘Smoke Alarm Facts: "Ionization vs Photoelectric,"’ posted to YouTube on 02.02.2019, <https://www.youtube.com/watch?v=GART34Xbszg>

Americium image from Encyclopædia Britannica, ‘Americium,’ published by Lester Morss, < https://www.britannica.com/science/americium#/media/1/20405/64372> 22.02.2022


3 Robert Robillard, ‘Smoke Alarm Facts: "Ionization vs Photoelectric,"’ posted to YouTube on 02.02.2019, <https://www.youtube.com/watch?v=GART34Xbszg>

4 Quote from ‘Rossen Reports: Popular smoke alarms may go off too late, experts warn,’ broadcast on The Today Show, on NBC, 03.10.2012, <https://www.youtube.com/watch?v=zrOLxh8GuCU>

5 Smoke alarm posters from National Fire Protection Association, ‘Ionization vs photoelectric’ public education posters, <https://www.nfpa.org/Public-Education/Staying-safe/Safety-equipment/Smoke-alarms/Ionization-vs-photoelectric> 25.02.2022

6 UK Government statistics, ‘Detailed analysis of fires attended by fire and rescue services,’ published 01.10.2020, <https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/923072/detailed-analysis-fires-attended-fire-rescue-england-1920-hosb2820.pdf>

7 FireAngel.co.uk, 'Why FireAngel Is Moving Away From Ionisation Alarms,' <https://www.fireangel.co.uk/why-fireangel-is-moving-away-from-ionisation- alarms/#:~:text=The%20British%20Standard%205839%2D6,a%20minimum%20level%20of%20protection. > 28.02.2022

8 Example of a multi-sensor alarm from FireAngels, part code: FS2126-T, <https://www.cef.co.uk/catalogue/products/4834321-multi-sensor-smoke-alarm-with-10-year-lithium-battery> 25.02.2022 9 BS 5839-6: 2019 Summary, from Safelincs.co.uk, reviewed 16/01/2020 <https://www.safelincs.co.uk/a- summary-of-the-bs5839-6-2019/>


Cover image from Pexels.com, courtesy of Tetyana Kovyrina

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