What does the letter "W" signify when shown on the NFPA Diamond?

The letter "W" in the NFPA (National Fire Protection Association) Diamond signifies that the material will react with water. This important safety indicator warns first responders and personnel that the substance may become hazardous or more reactive upon contact with water, potentially leading to dangerous chemical reactions, explosions, or the release of toxic gases. For instance, some alkali metals and certain acids are known to react vigorously with water, and thus, they require specific handling and extinguishing methods that do not include water. Recognizing this symbol is crucial for ensuring the safety and effective management of hazardous materials during emergencies.