How to Ship UN3373 (Category B) with P650 Packaging

UN3373, "Biological substance, Category B", covers most diagnostic and clinical samples moved between labs, hospitals and research sites. To transport it legally you must use P650 triple packaging: a leak-proof primary receptacle, a leak-proof secondary packaging with absorbent material, and a rigid outer bearing the UN3373 diamond mark. This guide walks through P650 layer by layer, the performance tests, marking, and the differences between air and road under IATA DGR 67th edition and ADR 2025.

What is UN3373, "Biological substance, Category B"?

UN3373 is the UN number for a Category B infectious substance — biological material (Division 6.2 dangerous goods) that is capable of causing infection but does not meet the criteria for the more hazardous Category A. In practice this is the bracket most routine samples fall into: blood and serum, tissue, swabs, cultures for routine diagnosis, and similar specimens being shipped for testing.

The proper shipping name is "Biological substance, Category B" and the package must display both that text and the UN3373 mark. Specimens with only a minimal likelihood of pathogens, and exempt human/animal specimens, may sit outside dangerous-goods rules entirely — but that is a classification decision, not an assumption.

Diagnostic and clinical labs, hospitals and blood services, life-science and pharma companies, veterinary labs and academic R&D groups all move UN3373 routinely — and volume never lowers the standard. One non-compliant Category B box can mean a rejection, a delay to a time-critical result, or a reportable incident.

Category A vs Category B — and why the distinction matters

Getting the category right is the single most important compliance step, because it changes the packaging, marks and paperwork:

• Category A — an infectious substance able to cause permanent disability, life-threatening or fatal disease in otherwise healthy humans or animals. It ships as UN2814 (affecting humans) or UN2900 (affecting animals only), under the far stricter packing instruction P620 with tested/certified packaging.
• Category B — an infectious substance that does not meet Category A criteria. It ships as UN3373 under P650.

If a sample is misclassified as Category B when it is actually Category A, the packaging is inadequate and the shipment is non-compliant. When in doubt, confirm the UN number and classification before you pack — our free ADR substance lookup and the tools in the MYDG compliance hub are a fast first check, and a DGSA can confirm borderline cases.

P650 triple packaging, layer by layer

P650 is a three-part "triple packaging" system. Each layer has a job, and all three are mandatory.

1. Primary receptacle (leak-proof)

The tube, vial or container that actually holds the specimen. It must be leak-proof, and for air transport a liquid primary receptacle must not contain more than 1 L. Fragile receptacles should be individually wrapped or separated so they cannot touch.

2. Absorbent material

Between the primary receptacle(s) and the secondary packaging you must place absorbent material — cotton wool, absorbent pads or similar — in a quantity sufficient to absorb the entire contents of all primary receptacles, so that any leak is contained before it reaches the outer box.

3. Secondary packaging (leak-proof)

A second leak-proof, sealed layer (for example a sealed pouch or a screw-top canister) that encloses the primary receptacle(s) and the absorbent. For multiple primaries, keep them from contacting one another.

4. Rigid outer packaging

A rigid outer of strength appropriate to its capacity and use. At least one surface must have minimum dimensions of 100 mm × 100 mm so the marks fit and stay legible. An itemised list of contents must be enclosed between the secondary packaging and the outer.

The performance requirement: 95 kPa and leak resistance

P650 is a performance-based instruction. The completed package must be built and tested so that:

• the primary receptacle or the secondary packaging can withstand, without leakage, an internal pressure differential of 95 kPa across a temperature range of −40 °C to +55 °C; and
• the complete package passes a 1.2 m drop test with no release from the primary receptacle and no compromise to protection.

This is why pre-qualified P650 kits matter: they are engineered and documented to meet these thresholds, so you are not improvising with unrated boxes.

Marking, labelling and documentation

The mark for UN3373 is not a Class 6.2 hazard label — it is a dedicated mark:

• The UN3373 mark is a square set at 45° (a diamond) with a minimum side length of 50 mm, showing "UN3373" in characters at least 6 mm high.
• The text "Biological substance, Category B" must appear adjacent to the diamond, in letters at least 6 mm high.
• The name, address and telephone number of a responsible person must be provided (on the package or in accompanying documents), and an itemised list of contents goes inside the outer.

For most Category B moves, a full Shipper's Declaration for Dangerous Goods is generally not required, but carrier air waybill entries and operator-specific paperwork still apply. If P650 preparation is new to your team, structured dangerous goods training pays for itself the first time it prevents a rejected shipment.

Air vs road: IATA DGR 67th edition and ADR 2025

The triple-packaging concept is the same, but quantity limits differ by mode:

• Air — IATA DGR 67th edition: liquid primary receptacles max 1 L; the outer packaging must not contain more than 4 L (liquids) or 4 kg (solids), excluding ice, dry ice or liquid nitrogen used for cooling.
• Road — ADR 2025: the primary receptacle limit is 1 L / 1 kg, and there is no stated maximum quantity per outer package for Category B, though carrier and vehicle rules still apply.

Air is the stricter regime, so a package built to satisfy IATA DGR will normally also satisfy ADR 2025 for the same contents. Confirm the current edition text before shipping.

Temperature control: gel packs and dry ice (UN1845)

Many samples must stay cold. Coolant sits outside the secondary packaging but inside the rigid outer, which must never be a sealed, airtight box when a subliming coolant is used.

• Gel packs / refrigerated (not frozen with dry ice): generally not a dangerous good in themselves. Use qualified temperature-controlled packaging to hold your target range.
• Dry ice is itself a dangerous good — UN1845, Class 9. When used with UN3373 you have a combination shipment: the package must vent CO₂ to prevent pressure build-up, and the UN1845 mark plus the net mass of dry ice must be shown alongside the UN3373 mark. By air, dry ice is always fully regulated; by road it is handled under the lighter ADR coolant provisions.

Work out how much you need with our free dry ice calculator, and mark the box correctly with compliant dry ice / UN1845 labels. Never seal dry ice in an airtight primary or secondary layer.

Frequently asked questions

Does UN3373 need a Shipper's Declaration for Dangerous Goods?

For most Category B shipments under P650, a full Shipper's Declaration is generally not required for road or air. You still need the UN3373 mark, the text "Biological substance, Category B", responsible-person contact details, an itemised list of contents inside the outer, and any carrier-specific documentation. Adding dry ice (UN1845) introduces extra marking and, by air, additional requirements.

What is the difference between UN3373 and Category A infectious substances?

UN3373 is a Category B infectious substance, packed under P650. Category A substances are more hazardous, ship as UN2814 (humans) or UN2900 (animals only), and require the stricter, tested P620 packaging. Classification must be confirmed before packing — misclassifying Category A as Category B means the packaging is not adequate and the shipment is non-compliant.

Can I reuse a P650 box, and does the packaging need certification?

P650 is a performance standard, not a UN-marked, batch-certified packaging like P620 — but the completed package must still pass the 95 kPa pressure and 1.2 m drop performance. Use packaging documented to meet P650 and follow the maker's reuse guidance; damaged or contaminated components must not be reused.

Ready to ship UN3373 correctly? Explore MYDG.SHOP temperature-controlled and P650 packaging, check quantities with the free tools in our compliance hub, and if a classification or shipment is borderline, talk to our DGSA team before it leaves the door.

By the MYDG.SHOP Dangerous Goods Team — DGSA-certified