Category B Infectious Substances (UN3373): Complete Guide

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Category B infectious substances are patient or animal specimens—such as blood, urine, swabs or tissue—that may contain pathogens but are not generally capable of causing life‑threatening or fatal disease if exposure occurs. When shipped, they are assigned to UN3373 and, for most modes, must be packed to a defined “triple packaging” standard to prevent leaks and protect handlers.

This guide explains exactly what UN3373 covers and how to ship it compliantly and confidently. You’ll learn how to classify specimens (Category B vs Category A vs exempt), the key rules under IATA, ADR/RID, IMDG and UK law, the UN numbers and proper shipping names to use, how to meet Packing Instruction P650, marking and labelling requirements, documents and responsibilities, using dry ice and other refrigerants, UK courier/postal acceptance, a step‑by‑step packing checklist, common pitfalls, and the training you need next.

Regulatory framework: IATA, ADR/RID, IMDG and UK requirements

Category B infectious substances are Dangerous Goods Class 6.2 and, when shipped as UN3373, are governed by the mode you use: IATA DGR for air, ADR/RID for road/rail, and the IMDG Code for sea. Across all modes, UN3373 must follow Packing Instruction P650: triple packaging with leak‑proof primary and secondary receptacles, absorbent material, and a strong outer, with demonstrated performance (e.g., 1.2 m drop test) and an outer surface ≥ 100 × 100 mm. You must mark the proper shipping name “Biological substance, Category B” and apply the UN3373 mark; air shipments must also meet any IATA operator requirements.

Category B vs Category A vs exempt specimens: how to classify

Accurate classification drives everything else. Start by asking: is there a realistic risk of severe disease on exposure, or is the likelihood of pathogens minimal? From there, you’ll decide between Category A, Category B (UN3373), or an exempt specimen with simplified controls.

  • Exempt specimen: Minimal likelihood pathogens are present. Pack to the exempt conditions (leak‑proof primary and secondary, absorbent for liquids, strong outer) and mark “Exempt human specimen” or “Exempt animal specimen.” Typical examples include routine monitoring (e.g., cholesterol, glucose, pregnancy tests) where infection isn’t suspected.
  • Category A: If exposure in transport could cause permanent disability, life‑threatening or fatal disease in otherwise healthy persons/animals, classify as Category A (UN2814 for humans, UN2900 for animals). This includes sources known to have serious, readily transmissible diseases without effective treatment or prevention.
  • Category B (UN3373): Infectious substances that do not meet Category A criteria. Most clinical diagnostic specimens fall here and must follow PI 650.

Use professional judgement based on medical history, symptoms and local epidemiology; when in doubt, classify conservatively and seek competent advice.

What’s included (and excluded): common examples and edge cases

In practice, most routine clinical and veterinary specimens are Category B infectious substances and shipped as UN3373, provided they don’t meet Category A criteria. Think of patient samples sent for investigation: they may contain pathogens, but not in a form generally expected to cause life‑threatening disease on exposure. Equally important is knowing what’s not UN3373, because many specimens are specifically exempt from the dangerous goods rules.

  • Typically included as UN3373: Human or animal diagnostic specimens (excreta, blood and its components, tissues and other body fluids), swabs and similar samples for investigation where Category A criteria are not met.
  • Specifically excluded (not subject): Dried blood spots; faecal occult blood screening tests; blood or blood components collected for transfusion; tissues or organs for transplantation; environmental samples (food, water) with no significant infection risk; substances containing only non‑pathogenic micro‑organisms; specimens where pathogens have been neutralised/inactivated.
  • Exempt patient specimens: Where there is minimal likelihood pathogens are present, pack to the exempt conditions and mark “Exempt human specimen” or “Exempt animal specimen.”
  • Not UN3373 by definition: Live infected animals are not diagnostic specimens; medical waste is usually UN3291; equipment contaminated with Category A falls under UN2814/UN2900.
  • Edge case—upgrade to Category A: If the source is known to have a serious, readily transmissible disease and effective treatment/prevention is not usually available, consign as UN2814 (humans) or UN2900 (animals).

UN numbers and proper shipping names for UN3373

For Category B infectious substances there is one entry: consign as UN3373 with the proper shipping name “Biological substance, Category B”. This applies across air (IATA), road/rail (ADR/RID) and sea (IMDG). Exempt specimens use no UN number; mark “Exempt human specimen” or “Exempt animal specimen”. Don’t confuse UN3373 with other Class 6.2 entries: UN2814 (affecting humans) and UN2900 (affecting animals) for Category A, and UN3291 for medical/clinical waste; UN3549 covers Category A medical waste solids (not by air).

Packing instruction P650: triple packaging explained

Packing Instruction P650 is the simple, robust recipe for moving UN3373 safely. It standardises “triple packaging” across IATA, ADR/RID and IMDG so leaks are contained even if a primary receptacle fails. Category B does not require UN‑approved outer packaging, but it must meet performance criteria, including withstanding a 1.2 m drop test and having at least one outer surface of 100 × 100 mm. The core idea: keep liquid inside leak‑proof layers, absorb any release, and stop fragile items touching.

  • Primary receptacle(s): Leak‑proof containers holding the specimen.
  • Absorbent (liquids): Enough to absorb the entire contents, placed between primary and secondary.
  • Secondary packaging: Leak‑proof, enclosing the primary receptacle(s) and absorbent.
  • Separation/cushioning: If using multiple fragile primaries, individually wrap or separate to prevent contact.
  • Rigid outer packaging: Strong enough for its mass and use, with ≥ 100 × 100 mm surface and proven performance (e.g., 1.2 m drop).
  • Assembly discipline: Pack so any release cannot reach the outer and cannot compromise cushioning; follow supplier instructions for tested packaging sets.

Marking and labelling: UN3373 mark, PSN and orientation arrows

Clear, durable marking is what tells handlers exactly what they’re dealing with and how to keep it upright. For Category B infectious substances, the outer packaging must be big enough to carry the required marks and they must be easy to see and not obscured by other labels.

  • UN3373 mark: Apply the diamond‑shaped UN3373 mark on the outer packaging on a contrasting background.
  • Proper shipping name (PSN): Mark “Biological substance, Category B” adjacent to the UN3373 mark.
  • Orientation arrows (liquids): For liquid specimens, apply “This Way Up” orientation arrows on the outer packaging as required by IATA/PI 650.
  • Space for marks: Ensure at least one outer surface is ≥ 100 × 100 mm so marks fit and are legible.
  • Exempt specimens (for comparison): Use only “Exempt human specimen” or “Exempt animal specimen” (no UN3373 mark) when the minimal‑likelihood criteria are met.

Documentation and mode-specific responsibilities

Your responsibilities don’t stop at packing. For UN3373, the consignor must classify correctly, use PI 650, apply the required marks, and ensure the shipment is accepted for the intended route. Documentation is mode- and operator-driven: always check carrier instructions in addition to the core IATA/ADR/IMDG rules and keep records of your classification and packing checks.

  • All modes: Ensure correct classification, PI 650 compliance, legible UN3373 mark and “Biological substance, Category B”, sender/receiver details, and any required emergency contact information. Retain packing and inspection records.
  • Air (IATA): Comply with PI 650 and any operator variations; orientation arrows for liquids. Some carriers require pre-approval or special commodities agreements for international movements.
  • Road/Rail (ADR/RID): Follow PI 650 and marking; provide any transport information your carrier requests to accompany the consignment.
  • Sea (IMDG): Use PI 650; confirm acceptance and any stowage or paperwork specifics required by the shipping line before booking.

Refrigerants and temperature control: dry ice, gel packs and liquid nitrogen

You can control temperature and still comply with PI 650—provided the triple packaging remains intact and its performance isn’t compromised. Place any refrigerant outside the leak‑proof secondary, ensure liquids still have sufficient absorbent, and keep marks visible on the outer. For air, operator and IATA requirements may add conditions when using refrigerants that are themselves regulated; confirm acceptance before you ship. Orientation arrows remain required for liquids regardless of how you cool them.

  • Gel/ice packs: Common, effective and simple. Seal them to prevent leakage/condensation affecting cushioning, and position them outside the secondary to avoid chilling or stressing primary receptacles.
  • Dry ice: Releases CO₂ gas as it sublimates. Ensure outer packaging can vent (never airtight), and follow any carrier/IATA requirements that apply to refrigerated dangerous goods. Keep UN3373 and PSN markings unobscured.
  • Liquid nitrogen: Only use purpose‑designed, vented “dry shippers” that retain nitrogen and avoid free liquid. Check carrier acceptance and any additional air transport conditions before consignment.

Courier and postal acceptance in the UK

Acceptance of Category B infectious substances is carrier‑specific. Always confirm service availability, packaging and booking conditions before you ship, especially for exports. In the UK, UN3373 is widely accepted when PI 650 is met, but operators impose additional limits or contracts you must follow.

  • Royal Mail (domestic): Limits any parcel to a total of 50 ml/50 g, requires PI 650 packaging, and the sender’s name and return address visible on the outer.
  • UPS (UK): Accepts UN3373 when PI 650 is used; verify destination against the Approved Country List and obtain an International Special Commodities (ISC) contract for shipments outside the UK. Mark “Biological substance, Category B” with the UN3373 mark.

Step-by-step packing checklist

Use this quick sequence to pack Biological substance, Category B (UN3373) to PI 650. Choose a packaging set meeting performance (e.g., 1.2 m drop test) and confirm carrier acceptance. Work cleanly and keep marks visible.

  1. Confirm classification: Category B (UN3373), not Category A or exempt.
  2. Check packaging: Leak‑proof primary/secondary; rigid outer ≥ 100 × 100 mm.
  3. Prepare primaries: Don’t overfill; close securely; bag if needed.
  4. Add absorbent (liquids): Enough to absorb all contents.
  5. Load and seal secondary: Separate multiple fragile primaries.
  6. Cushion, place in outer, seal: Refrigerant outside secondary; vent if dry ice.
  7. Mark outer: UN3373 and “Biological substance, Category B”; add arrows (liquids).
  8. Address and final checks: Verify acceptance; retain packing record.

Common mistakes to avoid

Small mistakes cause refusals and spills—use this sense‑check to avoid the most common UN3373 errors. Prioritise correct classification, full PI 650 compliance, clear marking, and carrier acceptance before you seal the box; a two‑minute review now can save days of delay and rework.

  • Misclassifying Category A or exempt as UN3373.
  • PI 650 gaps: no absorbent, primaries touching, weak outer.
  • Missing/covered marks: UN3373, PSN, or orientation arrows.
  • Refrigerant inside secondary or dry ice in sealed outers.
  • Ignoring carrier limits (50 ml/50 g; UPS ISC).

Training and competence requirements

UN3373 may be simpler than Category A, but regulations still require anyone who classifies, packs, marks or offers these consignments to be trained and competent for their tasks. Training must be role‑appropriate for the mode used (IATA/ADR/RID/IMDG), documented, and refreshed; carriers may also ask for evidence before accepting shipments.

  • Classification: Category A vs Category B vs exempt decisions.
  • PI 650: Triple packaging, absorbent, separation, performance checks.
  • Marking: UN3373 mark, PSN, orientation arrows for liquids.
  • Temperature control: Gel packs, dry ice/liquid nitrogen placement and carrier acceptance.

Key takeaways

Shipping UN3373 works when you classify correctly and follow PI 650. Most diagnostic specimens are Category B; upgrade to Category A only for severe disease risk, or exempt when likelihood of pathogens is minimal. Pack, mark, and train competently—every time.

  • PSN “Biological substance, Category B” and UN3373 mark; arrows for liquids.
  • Triple packaging to PI 650; absorbent for liquids; outer must withstand a 1.2 m drop.
  • Keep refrigerants outside the secondary; follow carrier rules (incl. dry ice) and confirm acceptance before booking.

Need hands‑on help or training? Speak to the specialists at Logicom Hub.