What typically triggers a smoke explosion?

A smoke explosion typically occurs when a mixture of flammable gases within smoke interacts with an ignition source. As a fire burns, it generates smoke that contains various gases, some of which can be highly flammable or explosive. If these gases accumulate and then come into contact with an ignition source—whether from an open flame, a spark, or even a high surface temperature—that can lead to a rapid combustion event, or explosion. This situation is often exacerbated in enclosed or poorly ventilated spaces where the concentration of flammable gases can reach critical levels.

The other options do not directly relate to the formation and ignition of explosive gas mixtures. A high neutral plane can influence fire behavior and smoke movement, but it does not specifically trigger a smoke explosion. Similarly, suddenly introducing water to a fire can cause steam to form, which can lead to rapid changes in pressure but is not a typical trigger for smoke explosions. Lastly, an increase in air pressure in a structure can affect fire dynamics but does not directly correlate with the presence of flammable gases leading to an explosion.