How is spontaneous ignition defined?

Spontaneous ignition is defined as the initiation of combustion by an internal chemical or biological reaction. This phenomenon occurs when a material generates enough heat through its own processes, such as oxidation or microbial activity, to reach its ignition temperature without the need for an external flame or spark.

This can happen in various materials, particularly organic substances, that can accumulate heat over time in an environment where airflow is limited. As the internal temperature rises and reaches a critical point, the material ignites, leading to combustion.

In contrast, the other options discuss situations involving external heat sources or characteristics unrelated to spontaneous ignition, such as immediate fire occurrences due to high temperatures or the reaction to exposure to flames. Understanding the distinct nature of spontaneous ignition helps investigators assess fire scenarios more accurately and identify potential dangers in certain materials or conditions during fire investigations.