What does the symbol W indicate on an NFPA label?

The symbol W on an NFPA (National Fire Protection Association) label signifies that water should not be used on the substance. This warning is essential in handling hazardous materials because certain chemicals can react violently with water, leading to dangerous situations such as explosions or the release of toxic gases. The NFPA label is designed to provide immediate, visual guidelines to responders regarding the proper methods for controlling fire involving hazardous substances.

By highlighting the risk associated with adding water, the label aids in ensuring safety for both the responders and the environment when dealing with specific materials that may be incompatible with water. Understanding this symbol is crucial for anyone involved in emergency response or hazardous material handling, as it directly affects the strategies employed to mitigate fire hazards.