What is the primary condition that leads to smoldering combustion?

Smoldering combustion primarily occurs under low oxygen levels, which allows for the combustion of organic materials without reaching the high temperatures typical of flaming combustion. In a smoldering fire, the materials burn slowly and often glow without producing flames. This type of combustion can occur in porous materials like wood or charcoal where there is enough heat to keep the material ignited, but not enough oxygen to support a more intense flaming fire.

In contrast, high oxygen levels are generally conducive to flaming combustion rather than smoldering. The presence of flammable liquids contributes to quick ignition and flaming combustion rather than the slow, extended process associated with smoldering. High heat exposure can indeed ignite materials, but it is the availability of oxygen that plays a critical role in the transition from smoldering to flaming combustion.