Buzz
The smoke detectors in my apartment building are both overly sensitive and deafeningly loud. They frequently activate even in the absence of smoke, and I often find myself climbing onto a chair to reset them when something like steam from a pot of boiling water sets them off. If I stand in the hallway just before dinner, I can hear alarms ringing from different apartments, followed by the frustrated curses and hurried footsteps of my neighbors. This made me curious: How exactly do these devices work?
Where there's smoke...
There are two primary types of smoke detectors found in homes: ionization and photoelectric. The ionization models contain an ionization chamber with two plates and a source of ionizing radiation. The device's battery applies voltage to the plates, charging one plate positively and the other negatively. The radiation source—Americium-241, a small amount (about 1/5000th of a gram)—emits alpha particles, which are subatomic particles made of two protons and two neutrons, at a steady rate. As these particles pass through the chamber, they ionize, meaning they knock an electron off oxygen and nitrogen atoms in the air passing through the chamber.
The free electrons, now negatively charged, are drawn to the positively charged plate, while the positively charged atoms are attracted to the negative plate. This creates a small, consistent current between the plates. When smoke enters the chamber, it disrupts the ionization process, reducing or eliminating the current between the plates, and that triggers the alarm.
Now, if the thought of radioactive isotopes hanging from the ceiling in your home is a bit unsettling, and you'd prefer a quick fiery demise over a slow radiation-induced one, don't worry. Alpha particles have minimal penetrative ability. They can't pass through the plastic of the detector, and if they somehow escaped, they wouldn't travel far in normal air. Given the tiny amount of Americium in the detector and its design, there’s no health risk unless you interfere with the chamber and directly expose yourself to the particles (such as inhaling or ingesting them).
The main issues with ionization detectors are that the radioactive material needs to be properly disposed of when the detectors are no longer in use, to avoid creating a hazard, and that their design makes them highly sensitive (able to detect fast, hot fires with little smoke). This high sensitivity, as I can confirm, leads to false alarms from dust, steam, and other vapors.
Ray of Light
The other popular type of detector, a photoelectric detector, uses a light-emitting diode to send a beam of light across the top of a T-shaped chamber. At the bottom of the T is a photocell that detects light. When smoke enters the chamber, the light hits the particles, gets scattered into the base of the T, and strikes the photocell.
When enough light reaches the photocell, it triggers an electrical current that activates the alarm. These detectors aren’t as sensitive as the ionization models and are better suited to detecting slow, smoldering fires that produce more smoke.
