Class A concentrate is simply a surfactant, similar to dish-washing soap, that reduces surface tension. When added to water, the resulting foam solution consists of many smaller droplets with much more surface area, allowing faster heat absorption. An example would be like cooling a glass of water with a single ice cube rather than crushed ice of the same volume. The crushed ice would cool it faster. In addition to creating smaller droplets, reducing the surface tension allows water to penetrate the fuel faster and deeper, dramatically raising the moisture content (see Fig. 3). This isolates the fuel leg of the triangle, increasing the resistance to burn. Finally, foam solution separates the fuel from oxygen with a vapor-securing barrier that provides excellent protection.
The proportion of Class A foam concentrate to water is dictated by use. A common rate for initial attack is 0.5%; overhaul 0.2% and exposure protection 1.0%. Application of the final solution can be achieved through a number of different mediums, including your standard nozzles, or specifically-designed aspirating nozzles.