Which type of smoke detector is activated by the invisible products of combustion?

The correct answer is ionization smoke detectors, which are designed to be activated by the invisible products of combustion, particularly by the smaller particles produced during flaming fires.

Ionization smoke detectors operate using a small amount of radioactive material that creates ions in the air within a sensing chamber. When smoke enters this chamber, it disrupts the flow of ions and reduces the current that goes between the electrodes. This drop in current triggers the alarm. These detectors are particularly sensitive to fast, hot fires—which typically produce small smoke particles—making them effective for early detection of such hazards.

Photoelectric smoke detectors, in contrast, are more responsive to smoldering fires that produce larger smoke particles. Thermal detectors, on the other hand, respond to changes in temperature rather than specific combustion byproducts. Each type of detector has its unique strengths and ideal use scenarios, but for detecting the invisible products produced during flaming combustion, ionization detectors are the most appropriate choice.