What does a UN number identify and why is it important?

• A. An identifying weight code
• B. A packaging code
• C. A flight route code
• D. A four-digit code identifying a specific dangerous substance or article for safe handling and emergency response

Answer: (D)

UN numbers are four-digit identifiers assigned to hazardous substances and articles to name the exact material in transport. This unique code ties to the approved shipping name and its hazards, so anyone handling or responding to an incident can quickly know what material is involved and what actions are required. The UN number stays constant across modes and regions, enabling consistent labeling, documentation, and regulatory compliance, and it directs responders to the correct emergency guidance and safety information. The other options refer to weight, packaging, or route details, which don’t identify a specific dangerous substance and thus don’t provide the essential, material-specific information needed for safe handling and effective emergency response.