What negative effect can occur if a fog stream is applied incorrectly during an interior attack on a confined building fire?

When a fog stream is applied incorrectly during an interior attack on a confined building fire, it can upset the thermal layer. The thermal layer is the distinct temperature gradient or boundary that separates different layers of air, which is usually created by the hot gases and smoke accumulating close to the ceiling. When firefighters use a fog stream improperly—such as directing it into the hot smoke without consideration of the thermal layering—this can cause the cooler water droplets to disrupt the established thermal layer.

This disruption may lead to a sudden redistribution of heat, causing the hot gases to descend rapidly or even flow towards the firefighters. This can drastically alter the thermal conditions, leading to potentially dangerous situations such as flashover or backdraft. Maintaining the integrity of the thermal layers is crucial for both effective firefighting and the safety of personnel inside the structure.