Which type of foam is known for having no ability to suppress vapors?

Class A foam is specifically formulated primarily for combating ordinary combustible materials like wood, paper, and textiles. It is designed to form a thin, aqueous film that enhances the wetting properties of water, allowing it to penetrate and cool down these materials more effectively. However, Class A foam does not possess the capability to suppress vapors, which can be critical when dealing with flammable liquids or gases.

In contrast, Class B foam is tailored to address flammable liquid fires (liquids that create vapors) by creating a blanket over the surface of the liquid, thereby suppressing vapors and preventing ignition. Alcohol-resistant foam is designed to extinguish fires involving polar solvents, which also generates vapors that need to be suppressed. Protein foam, while used for its effectiveness in certain situations, also works by creating a vapor-sealing layer.

So, the defining characteristic of Class A foam is its lack of vapor suppression capabilities, making it unsuitable for scenarios that entail the risk of vapor ignition.