What does the 4 digit number on a placard indicate?

The four-digit number on a placard is known as the UN product number, which is a unique identifier assigned to hazardous materials by the United Nations. This number is crucial because it helps first responders and transport personnel quickly identify the specific substance involved in a hazardous material incident, ensuring appropriate measures are taken for safety and handling. It facilitates effective communication about the material's hazards, as the number corresponds to detailed safety information and guidelines in the Emergency Response Guidebook and other resources, enabling a rapid and effective response to emergencies.

In contrast, the NFPA hazard rating system is a separate system used to indicate the level of health, flammability, and reactivity hazards, but it does not provide the standard unique identification that the UN number does. Emergency response plans are comprehensive strategies outlining procedures during incidents but do not utilize four-digit numbers on placards. The transport category refers to classifications that might relate to regulations or guidelines but does not specifically assign a four-digit number to an individual hazardous material.