Smoke Alarm Sensor Types

Ionization Sensors & Photoelectric Sensors

There are two common types of smoke alarm sensors used in residential applications in the US; ionization sensors and photoelectric sensors. Each has its pros and cons..

Ionization Sensors

The smoke alarms installed in most homes in the US use ionization sensors. Ionization sensors are pretty good at detecting small particle smoke. Small particle smoke is primarily generated by an open flame fire rather than a smoldering, smokey fire. 

Ionization sensors are also much more prone to false alarms. You know how a smoke alarm near a kitchen will go off when you burn the toast? Ionization sensors can see common things in your home like dust, water vapor from showers, or normal cooking smoke as a problem and cause a false alarm.

Ionization smoke detectors use a small amount of americium (a slightly radioactive substance) as a source of alpha particles. Alpha particles from the americium source ionize air molecules, creating some positively charged and some negatively charged particles. Two charged plates inside the ionization smoke detector create a flow of positively and negatively charged ions. When smoke enters the ionization chamber, it disrupts this flow of ions, and the smoke particles attach to the ions and neutralize them. This drop in current between the plates triggers the alarm.

Ionization detectors are more responsive to flaming fires that result from flammable liquids, wood, or paper starting on fire. This type of fire produces a lot of flames with a limited amount of smoke. 

Photoelectric Sensors

Photoelectric smoke detectors on the other hand are designed to detect smoke from smoldering fires. They work by using a light source and a light sensor. The light source emits a beam of light into the sensing chamber, and the light sensor is positioned at an angle to detect the light reflected off the walls of the chamber. When smoke enters the chamber, it scatters the light beam, causing some of the light to hit the sensor and trigger the alarm.

Most attended fires tend to be open flame fires from cooking or candles and such. Most unattended fires tend to be smoldering fires. These are caused by things like electrical issues or furniture lit by burning cigarettes.

Up until recently ionization sensors were the minimum standard that could be installed in homes, but that is changing in many places across the US. 

So, which should you choose? Surprisingly, not that many people in the fire safety industry are willing to recommend one over the other. This is probably because, let’s face it, you don’t know what type of fire is going to break out in your home, so almost no matter what they recommend, they will be wrong some percentage of the time.

Latest Smoke Alarm Standards

Make sure your smoke alarms meet the very latest standards for smoke alarms. The UL 217 Eight Edition went into effect in 2025.  This new standard requires smoke alarms in jurisdictions that adopt this standard to be less sensitive to false alarms caused by things like cooking smoke.

Nuisance alarms are more than just annoying — they’re also dangerous, as they are often the reason why people disable the power sources from their smoke detectors. According to a 2024 NFPA research report, smoke alarms were present in three-quarters of reported home fires from 2018 to 2022, but in 16% of cases, they failed to operate. In these cases, 35% of non-functioning alarms had missing or disconnected batteries, and 6% had a hardwired power failure, shutoff, or disconnect.

Smoke alarms provide the earliest warning of a home fire, and with only three minutes or less to escape (according to research findings by the Fire Safety Research Institute), every second matters. Quite simply, if a smoke alarm or detector has been disabled due to a nuisance alarm, it cannot alert residents in the first moments of a fire — when escape is critical.

Last updated: 1/29/2025