UN3373 Explained: How to Package and Label Category B Biological Substances
UN3373 identifies Biological substance, Category B. Correct classification, triple packaging, required markings and mode specific conditions matter; the diamond mark alone is not enough.
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Quick answer
UN3373 identifies Biological substance, Category B. Correct classification, triple packaging, required markings and mode specific conditions matter; the diamond mark alone is not enough.
Classify the material before choosing a package
Not every clinical specimen is Category B. A trained shipper must determine whether the material is Category A, Category B, exempt or otherwise regulated using the applicable classification rules. Do not use a UN3373 label to make an uncertain specimen acceptable for transport.
This guide summarizes United States DOT requirements in 49 CFR 173.199. Air carriers, international movements and additional hazards can introduce further conditions. Follow the packaging manufacturer's instructions and obtain current carrier acceptance requirements before tendering the shipment.
Use the required three layers
The DOT rule requires a primary receptacle, secondary packaging and a rigid outer packaging. Secure the contents with suitable cushioning. The completed package must meet the specified performance requirements, including capability to withstand the 1.2 meter drop test. At least one outer surface must measure at least 100 mm by 100 mm.
For liquids, use leakproof primary receptacles and leakproof secondary packaging.
Place enough absorbent material between those layers to absorb the entire liquid contents.
Separate or individually wrap fragile primary receptacles to prevent contact.
For solids, use siftproof primary and secondary packaging; apply the liquid provisions if residual liquid may be present or the material may become liquid.
Apply markings and contact information correctly
The outer package requires the UN3373 square on point mark on a contrasting background. Each side is at least 50 mm, the border at least 2 mm and the letters and numbers at least 6 mm high. The DOT wording adjacent to the mark is “Biological substances, Category B,” in letters at least 6 mm high. Overpacks must leave required marks visible or reproduce them.
Include the name and phone number of a knowledgeable contact, or a person with immediate access to that knowledge, on the outer package or a written document. The number must be monitored during the company's operational business hours under this rule.
Check the air transport and refrigerant conditions
For liquid shipments by aircraft, the primary receptacle or secondary packaging must withstand the specified 95 kPa pressure differential without leaking. The DOT limits are 1 liter per primary receptacle and 4 liters per outer packaging, excluding refrigerants. Solid shipments by aircraft generally have a 4 kilogram outer package limit, with specified exceptions.
Place ice or dry ice outside the secondary packaging or in an overpack as required. Dry ice must vent safely and comply with applicable refrigerant rules. Additional preservatives or hazards require their own assessment. Each person offering or transporting Category B material under this section must know its requirements.
Final acceptance check
Confirm classification, package assembly, quantity, marks, contact information and carrier acceptance. Reject leaking or incorrectly closed packages for correction by authorized staff. This overview is not a substitute for packaging instructions, current regulations or task appropriate training.
Plan your next step
Explore OnWay's medical courier services or request a delivery quote. Share the material, route, handling requirements and receiving hours so the team can assess service suitability. Specialized or regulated shipments require confirmation of capabilities before acceptance.
Sources and scope
49 CFR 173.199: Category B infectious substances
49 CFR 173.217: dry ice transport
This educational guide was prepared on November 2, 2026. It is not legal, clinical, packaging certification or product release advice. Requirements vary by material, transport mode, jurisdiction and contract. Follow current regulations, manufacturer instructions and your facility's approved procedures.
Key Takeaways
- Classify before labeling.
- Use compliant triple packaging and closure instructions.
- Check additional air and refrigerant requirements.
Frequently Asked Questions
Does every blood specimen need a UN3373 mark?
No. Classification depends on the material and applicable rules. A trained shipper must determine the correct category rather than applying the mark to every specimen.
Is a sealed specimen bag enough for Category B transport?
Not by itself under 49 CFR 173.199. The rule requires compliant primary, secondary and rigid outer packaging, plus the applicable performance and marking provisions.