What triggers activation of heat detectors?

Heat detectors are specifically designed to respond to significant changes in temperature, making a notable increase in temperature the primary trigger for their activation. These devices are often used in fire detection systems, where the onset of a fire typically results in a rapid rise in ambient temperature.

When the temperature exceeds a predetermined threshold, the heat detector activates an alarm to signal the presence of heat that is likely indicative of a fire. This function is crucial for early detection and can ultimately help in minimizing damage and ensuring safety in a building.

The other options concern factors that do not directly relate to the operational purpose of heat detectors. While an increase in humidity, a decrease in temperature, or a change in air pressure may affect environmental conditions, they do not typically signal a fire or hazardous situation that would necessitate the activation of the detector. Hence, the correct choice is the significant increase in temperature, as it directly correlates with the detector's functionality in fire detection.