AFFF's vapor-suppressing film is primarily due to which property?

• A. High viscosity.
• B. Color.
• C. Low viscosity.
• D. Density.

Answer: (C)

The ability of AFFF to suppress vapors comes from its low viscosity, which allows the foam solution to spread rapidly across the fuel surface and form a thin, continuous film. This quick spread blankets the liquid, creating a barrier that reduces vapor release and slows evaporation by limiting heat transfer at the interface. If the liquid were highly viscous, spreading would be slow and uneven, leaving gaps where vapors can escape. Color and density don’t influence film formation or vapor suppression; they don’t affect how well the film covers the surface. The key is that low viscosity enables fast, complete surface coverage, producing an effective vapor barrier during firefighting.