This week’s article will center on a brief explanation as to how the two most popular smoke alarms work to detect smoke.  But first, I want to take a moment to once again emphasize the importance of smoke alarms in saving lives.

As the fire marshal for the City of Murray, I cannot stress enough the importance of having “working” smoke detectors in your home, camper, trailer … anywhere sleeping is taking place within a structurally enclosed environment. No matter who you are or where you live, fire has the ability to take your life! The only chance any of us have in surviving a fire – especially at night while we’re sleeping – is to be warned of the presence of smoke and fire by a properly working smoke detector.

DON’T BE A VICTIM! In the United States, a person dies in a fire, or from a fire related injury every two hours. According to National Fire Protection Association data, most people (and indoor animals) who survived house fires were able to do so because properly working smoke detectors sounded a life-saving alarm that was heard!

The two most popular, most widely used smoke alarms/detectors are: the photoelectric smoke detector and the ionization smoke detector. And now I will attempt to explain how they both operate to detect smoke in the simplest of terms. The hope here is that you will gain a greater respect for, and understanding of, one of the most least expensive yet highly effective life-saving devices you’ll ever own.

The Photoelectric Smoke Detector

Inside this device is an enclosed space where, on one end of the space, a beam of infrared light (or light from a light emitting diode/LED) travels unblocked toward the other end of the space where a photodiode (a tiny, tubular component that turns light into electric current) is mounted. The light beam does not hit the photodiode; it is directed slightly away from it.

However, when smoke is present in the area, it enters the device and gets into the space where the light beam and light detector (the photodiode) are mounted. Smoke particles scatter the previously straight light beam and cause some of it to hit the photodiode. The photodiode will then convert the light into an electrical pulse that sound the alarm.

The Ionization (Smoke) Detector

Ionization refers to the process where molecules become either positively or negatively charged. Inside an ionization detector is an air-filled compartment where two electrodes – in this case, small thin wires that conduct electricity – are installed. A constant electric current passes between these two electrodes in the absence of smoke. When smoke enters the compartment, however, its particles become ionized disrupting the constant electrical current between the two electrodes. This sudden change (caused by smoke) triggers the alarm mechanism in the device.To summarize, either device can save your life. The greater issue to consider is to understand how best to maintain your detector. Simply stated, test your device(s) monthly; dust and/or blow-out your device(s) at least annually; change the battery annually and change out the entire device every 10 years.

The City of Murray has a very aggressive program – facilitated by the fire department – to install and service smoke detectors in the residences of certain qualifying citizens within the City of Murray who either lack the monetary resources or physical means to install and service smoke detectors in their homes.  In the City of Murray, landlords who rent single-family dwellings, multi-family dwellings such as a duplex, triplex, etc., apartments or any other dwellings where people sleep, MUST provide smoke detectors in every room where people sleep, and in the common space leading to those rooms, e.g. hallways.  

Thank you for being vigilant and fire safe. For questions or further information, contact the City’s website, and/or the Office of the Fire Marshal at

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