What Sets off a Smoke Detector?

Smoke detectors have been used in homes in the U.S. for around four decades. In many countries, having at least one smoke detector in a home is required by law. What exactly sets off a smoke detector and how it works depends on the type of smoke detector in question.

Two main types of smoke detectors are used.

Blocked Light

One type of smoke detector is the photoelectric smoke detector. These smoke detectors each have a light and a sensor. At all times, the light shines on the sensor. When something thick like smoke rises through the air and blocks the light from the sensor, it sets off the smoke alarm.

Air Particle Size

Another type of smoke detector is the ionization smoke detector. These smoke detectors use an electrical current and a radioactive source. This source creates particles that react with air and become negatively charged ions. The smoke detector then contains both negative and positive ions, which keep the current steady. Smoke particles are bigger than the ions, which creates an imbalance in the current. When this imbalance is detected, the smoke alarm is set off.


Both existing types of smoke detectors are better suited to certain types of fires. For example, a photoelectric smoke alarm will be set off faster by a smoky fire, rather than a fast-burning, flame-intense fire, which would set off an ionization smoke detector the fastest. Because of this, many smoke detectors use both methods to have a better chance of detecting smoke faster. This type of smoke detector is called a dual detector, and it is recommended by the United States Fire Administration.

Carbon Monoxide

Many smoke detectors are also carbon monoxide detectors. Carbon monoxide is an odorless, invisible gas that can be deadly if too much is inhaled. If a smoke detector is also a carbon monoxide detector, carbon monoxide gas will set it off.