Class A foam fire extinguishing agent refers to the fire suppressant that mixes with water and generates fire-extinguishing foam via chemical reaction or mechanical means. As a poor heat conductor, foam forms a floating layer on liquid surfaces to put out fires through cooling, smothering and blocking effects.

-Isolation: Separate liquid combustibles from combustion gas.
-Cooling: Foam and water vaporize and absorb heat. Meanwhile, foam blocks thermal radiation and suppresses combustion chain reactions.
- Smothering: Water vapor reduces oxygen concentration in the burning zone.

Class A foam fire extinguishing agents have diverse categories, classified by fire-extinguishing mechanism, foaming method, drainage performance, application and base material.

1. Classification by Fire-extinguishing Mechanism
It is divided into chemical foam agent and air foam agent. Chemical foam agent is rarely used nowadays, and current fire protection codes mainly apply air foam agent.
(1) Chemical Foam Fire Extinguishing Agent
Mixing foaming agent, stabilizer and acid-base salts triggers chemical reactions, releasing massive carbon dioxide and forming low-density viscous foam. The foam covers combustible surfaces and isolates oxygen to extinguish fires.
It contains acid-base salts, so it cannot be used for fires involving acid-resistant, alkali-resistant and water-resistant chemical equipment.

 (2) Air Foam Fire Extinguishing Agent
It generates air foam to cover liquid combustibles and isolate substances from air.

2. Classification by Mixing Ratio
Mixing ratio means the volume percentage of foam concentrate in foam solution.
Common ratios are 3% and 6%. Special types include 8% fluoroprotein foam and Class A foam agent with ratio below 1%.

3. Classification by Foam Expansion Ratio
Expansion ratio refers to the volume expansion multiple after foam solution turns into fire-extinguishing foam.
1. Low-expansion foam agent: Expansion ratio 1~20
2. Medium-expansion foam agent: Expansion ratio 21~200
3. High-expansion foam agent: Expansion ratio above 200

4. Classification by Base Material
Class A foam agents are split into protein-based foam agent and synthetic foam agent.