What does a '4' indicate in the NFPA 704 marking system?

In the NFPA 704 marking system, a '4' indicates a deadly hazard. This system utilizes a color-coded diamond, where each section represents a specific type of hazard: health, flammability, reactivity, and specific hazards.

The numerical scale ranges from 0 to 4, with 0 representing minimal hazards and 4 indicating the highest level of danger. A '4' in the health hazard section signifies that exposure to this material can result in serious or fatal injury. This understanding is crucial for emergency responders, as it helps assess the risks associated with various substances during fire and rescue operations, ensuring safety protocols are followed effectively. The focus on 'deadly' hazards underscored by the rating helps prioritize the necessary precautions when handling or being near such materials.