How does a smoke detector actually detect smoke?

Smoke alarms work by sensing small particles in the air. When the particle level passes a set threshold, the device sounds the alarm. Two technologies are most common: ionization and photoelectric. Each reacts to smoke in a different physical way. According to NIST, citing the National Fire Protection Association, working smoke alarms in a home cut the risk of dying in a fire by 55%. Sources: [nist.gov](https://www.nist.gov/how-do-you-measure-it/how-do-smoke-detectors-work). [1]

An ionization detector contains a small, shielded radioactive source. It ionizes the air between two metal plates, which lets a small electric current flow from one plate to the other. When smoke particles enter the chamber, they attract the ions and carry them away, so the current falls. Once it drops below a certain level, the device treats the change as smoke and the alarm sounds. NIST says most of the radiation is blocked inside the device and that the levels involved are much lower than everyday natural background radiation. A local fire department page names the radioactive material as americium. [1] [2]

A photoelectric detector uses a light source and a light sensor, arranged so that the beam normally misses the sensor. When particles drift into the chamber, they scatter some of the light onto the sensor. If the registered light reaches a threshold, the alarm sounds. This works because particles reflect and scatter light from their surfaces. [1] [3]

Fires produce different smoke. NIST describes flaming fires as producing a very large number of small particles, under one micrometer, while smoldering fires produce larger particles, over one micrometer, and release heat slowly. Ionization alarms tend to respond faster to flaming fires. Photoelectric alarms tend to respond faster to smoldering ones, because large particles have more surface area to scatter light. In NIST experiments described in a 2007 statement, ionization detectors responded on average 50 seconds earlier in flaming fires. The same statement notes that ionization detectors sometimes failed to alarm in smoldering fires even when visibility was significantly reduced, and that most photoelectric detectors alarmed substantially sooner in those cases. [4] [1]

Because of these differences, the NFPA recommends both types, or combination alarms with both sensors, for the best protection. Some newer multicriteria alarms add heat sensing and an algorithm. The NFPA says these mainly reduce nuisance alarms, such as those from cooking, and do not necessarily detect fires sooner. [5]

Sensitivity involves a trade-off. NIST notes that a detector sensitive enough to catch almost any smoke would also react to cooking fumes or dust. A less sensitive one would produce fewer nuisance alarms but might sound late in a real fire, or not at all. NIST says researchers are developing new tests and standards to separate wanted from unwanted smoke. The 2007 NIST statement adds that neither detector type responds to carbon monoxide or heat. [1] [4]