What is the key action to take when extinguishing Class D fires?

Class D fires involve combustible metals such as magnesium, sodium, or potassium, which react violently with water and can cause explosions. The key action to effectively extinguish these types of fires is to use special agents specifically designed for metal fires, such as dry powder agents that smother the burning material and prevent the chemical reaction that fuels the fire. These agents create a barrier between the metal and the oxygen, effectively cutting off the fire's fuel supply without triggering a dangerous reaction.

Using water to cool Class D fires is not appropriate as it can worsen the situation, potentially leading to violent reactions. Similarly, while sand can be used in some scenarios to smother fires, it is not the most effective or safe method for combustible metals. Evacuating the area might be necessary in certain situations, but it does not provide a solution for extinguishing the fire directly. Therefore, the most critical and effective action is to utilize the proper smothering agents designed for Class D fires to manage the hazard safely.