What characteristic defines high expansion foam?

High expansion foam is characterized by its ability to create a vast volume of foam with a high concentration of air, resulting in the formation of large bubbles. The foam expands significantly upon application, which is crucial for various firefighting scenarios, particularly in confined spaces or where high-volume coverage is needed. This expansive quality allows high expansion foam to blanket and suffocate fires, especially those involving flammable liquids or solids, by displacing oxygen and providing a barrier that protects against heat.

While other characteristics are relevant to firefighting and foam types, the distinctive trait of high expansion foam is its production of a great amount of air mixed with the foam concentrate, resulting in the formation of the large bubbles that define its effectiveness. This characteristic enables it to cover substantial areas and protect structures from potential fires.