What Is the UN 3373 Mark and When Is It Required?

UN 3373 Mark refers to the diamond-shaped marking applied to packages containing Biological Substance, Category B for transport. The mark communicates the UN number (UN 3373) and the classification so carriers, handlers, and regulatory authorities can identify the shipment’s hazard class during ground, air, and multimodal transport.

What The Mark Represents

The UN 3373 Mark is a standardized label used globally to identify shipments of clinical specimens, diagnostic samples, cultures, and other materials classified as Biological Substance, Category B. Category B covers infectious substances that do not meet the criteria for Category A (which are capable of causing permanent disability, life‑threatening or fatal disease in otherwise healthy humans or animals). The marking indicates to handlers that the content requires special handling and must comply with applicable packaging, documentation, and carrier rules.

Why The Mark Matters

Regulatory compliance: The mark is a regulatory signal required by modal and national regulations (for example, air transport rules and hazardous materials regulations). Proper marking helps avoid shipment delays, refusals, fines, and civil liability.

Safety and chain-of-custody: For carriers and receiving labs, the mark communicates that personnel should use appropriate PPE and follow biological handling procedures. It also supports traceability and incident response when spills or exposure events occur.

How The Mark Is Used In Practice

• Label Content: The diamond typically shows the UN number "UN 3373" and the words "Biological Substance, Category B" or an equivalent description required by the regulation or carrier.
• Placement: The mark must be placed on the outer packaging where it is clearly visible and not obscured by other labels or text.
• Visibility: Labels should be proportionate to package size to remain legible throughout the transport chain.

When It’s Required

Use the UN 3373 Mark whenever you are shipping biological material that has been classified as Category B. Examples include routine clinical specimens (blood, urine) sent to a reference lab for testing, or environmental samples for diagnostic work. If the material meets Category A criteria, a different marking and placarding system applies and Category A regulations must be followed.

How It Relates To Documentation And Carrier Rules

Most carriers and modal rules require the UN 3373 mark plus a clear description on the transport document (air waybill, ground bill of lading) such as "Biological Substance, Category B, UN 3373." Air carriers follow IATA Dangerous Goods Regulations; ground carriers and domestic shipments in the United States follow DOT and PHMSA guidance. For Category B shipments, a full Shipper’s Declaration is generally not required, but accurate description and contact details are mandatory.

Practical Example

A clinic sending blood samples to an external lab places their primary leak-proof receptacle inside secondary absorbent material and a rigid outer package. The shipper affixes the diamond-shaped UN 3373 Mark on the outer package and records "Biological Substance, Category B, UN 3373" on the shipping paperwork. The carrier accepts the package, the label remains visible through transit, and the receiving lab is notified and prepared to handle the samples safely.

Tips For Avoiding Common Problems

• Label Legibility: Use printed labels not handwritten ones; ensure contrast and font size that remain legible after rough handling.
• Follow Carrier Requirements: Check the carrier’s acceptance rules — some airlines and couriers have specific placement or size requirements beyond regulatory minimums.
• Training: Ensure staff who prepare Category B shipments are trained on classification, packaging, marking, and documentation.

In short, the UN 3373 Mark is the standardized diamond-shaped label that identifies Biological Substance, Category B shipments. Proper marking, combined with compliant packaging and accurate paperwork, reduces safety risks and prevents transport delays.