According to the National Fire Protection Association, ionization alarms have a small amount of radioactive material between two electrically charged plates that ionizes the air and causes current to flow between the plates.
When smoke enters a chamber, it disrupts the flow of ions, reducing the flow of current, activating the alarm. Though such alarms respond best to “flming fires,” the group says, photoelectric smoke detection is more responsive to fires that begin with smoldering.
Photoelectric-type alarms aim a light source into a sensing chamber. Smoke enters the chamber, reflecting light onto the sensor, triggering the alarm.
A fire-protection association study found that ionization alarms produced far more nuisance alarms than photoelectric alarms, making it more likely people will disable them.