Every laboratory that ships a patient specimen, a bacterial culture, a diagnostic sample, or an animal isolate by air must first answer one question: is this material a Biological Substance, Category A or Category B? The answer determines which UN number applies, which packaging is legally required, whether a formal Shipper’s Declaration for Dangerous Goods is needed, and who at the institution is even allowed to sign off on the shipment. Getting the classification wrong is not a paperwork technicality — shipping an infectious substance under the wrong category is a violation of both the International Air Transport Association’s Dangerous Goods Regulations (IATA DGR) and, in the United States, the Department of Transportation’s Hazardous Materials Regulations (49 CFR Parts 171–180), and it can result in a rejected shipment, a fine, or a dangerous mishandling incident.
This guide explains how Category A and Category B are defined, which UN numbers and packing instructions apply to each, how the two differ in packaging, marking, and documentation, and where common exemptions and adjacent rules (dry ice, exempt specimens, permits) fit in.
Category A vs. Category B: The Core Distinction
Both categories fall under UN Hazard Class 6.2 (Infectious Substances), but the classification test is about the consequence of exposure, not the substance’s identity alone.
- Category A — An infectious substance that, when exposure to it occurs, is capable of causing permanent disability, life-threatening disease, or fatal disease in otherwise healthy humans or animals. This includes cultures of known Category A pathogens and any material reasonably expected to be a Category A pathogen based on the patient’s or animal’s known medical history, symptoms, or local endemic conditions.
- Category B — An infectious substance that does not meet the criteria for Category A. Most routine diagnostic and clinical specimens — blood, urine, tissue, or swab samples being sent for testing where a specific high-consequence pathogen is not suspected — fall here by default.
A useful way to hold the distinction: Category A is defined by the pathogen and its consequences (would exposure to this specific organism, in this form, plausibly kill or permanently disable someone?), while Category B is the default classification for everything infectious that doesn’t clear that bar. The same organism can be Category A in one context and Category B in another — a concentrated laboratory culture of a pathogen is judged more severely than a small, dilute clinical specimen that may or may not contain it, because a culture is a purified, amplified quantity of the organism itself.
Category A Examples
Category A assignment covers cultures (not necessarily raw patient specimens) of organisms including Bacillus anthracis, Burkholderia mallei and pseudomallei, Brucella species, Ebola and Marburg viruses, Lassa virus, Mycobacterium tuberculosis, Yersinia pestis, and variola virus, among others. IATA publishes the full reference list in DGR Table 3.6.D; institutional biosafety programs typically cross-reference it against the CDC/USDA Federal Select Agent Program list, since many Category A organisms are also select agents subject to separate possession and transfer permitting.
Category B: The Default Classification
Category B is assigned the single proper shipping name “Biological Substance, Category B” and a single UN number regardless of the specific pathogen suspected or confirmed. This is the classification used for the vast majority of clinical and research specimen shipments — diagnostic samples sent to a reference lab, biobank specimens, environmental or veterinary samples for routine testing, and cultures of organisms that don’t meet the Category A severity threshold.
UN Numbers and Proper Shipping Names
| Classification | UN Number | Proper Shipping Name | Packing Instruction (Air) |
|---|---|---|---|
| Category A — affecting humans | UN 2814 | Infectious substance, affecting humans | PI 620 |
| Category A — affecting animals only | UN 2900 | Infectious substance, affecting animals only | PI 620 |
| Category B | UN 3373 | Biological substance, Category B | PI 650 |
A related but separate UN number, UN 1845 (carbon dioxide, solid, or “dry ice”), applies whenever dry ice is used as a refrigerant/coolant for either category — see the dry ice section below.
Packaging Requirements
Both categories use the same conceptual triple-packaging system — a leak-proof primary receptacle, a secondary packaging, and a rigid outer packaging — but the performance standard each must meet is very different.
Category B Packaging (PI 650 / UN 3373)
Packing Instruction 650 requires:
- A leak-proof (or sift-proof, for solids) primary receptacle, containing no more than 1 liter (liquids) or 1 kg (solids) per receptacle. If multiple primary receptacles are packed together, each must be individually wrapped to prevent contact.
- A leak-proof secondary packaging, with enough absorbent material between the primary and secondary layers to absorb the entire contents of the primary receptacle(s) in case of breakage or leakage.
- A rigid outer packaging, with one surface at least 100 mm x 100 mm, since IATA does not permit UN 3373 packages small enough to be easily overlooked in a shipment.
Unlike Category A packaging, PI 650 packaging is not required to be UN performance-tested/certified, though many institutions use certified packaging anyway for consistency and because some carriers require it as an operator variation.
Category A Packaging (PI 620 / UN 2814, UN 2900)
Packing Instruction 620 layers substantially more onto the same triple-packaging concept:
- The primary receptacle, secondary packaging, and outer packaging must together be certified to UN Packing Group I performance standards — the most stringent of the three UN packing groups, involving drop tests, stacking tests, and pressure differential tests specific to Class 6.2 infectious substances.
- A rigid outer packaging is mandatory (no exceptions), with a list of prohibited outer-packaging materials.
- An itemized list of contents must be placed between the secondary and outer packaging.
- A refrigerant (if used) must be positioned so that the secondary packaging is not compromised as it depletes, and dry ice, if used, must be added after the secondary packaging is sealed inside the outer packaging.
Marking, Labeling, and Documentation
| Category A | Category B | |
|---|---|---|
| Package marking | UN number (UN 2814 or UN 2900) and proper shipping name | UN 3373 diamond-shaped mark with “Biological Substance, Category B” text, minimum 50mm x 50mm |
| Hazard label | Class 6.2 Infectious Substance label (skull-and-crossbones style hazard label) | None required beyond the UN 3373 mark |
| Shipper’s contact | Name and 24-hour phone number of a person responsible for the shipment, on the package | Not required on the package itself |
| Shipper’s Declaration for Dangerous Goods | Required | Not required |
| Air Waybill notation | Required, referencing the dangerous goods declaration | Required (“Biological Substance, Category B” noted per operator requirements) |
This documentation gap is the single biggest practical difference researchers notice: Category A shipments require the full, signed Shipper’s Declaration for Dangerous Goods that most dangerous-goods shipments need, while Category B shipments are deliberately streamlined — correct marking and an air waybill notation, without a separate declaration form — because Category B represents a much lower consequence-of-exposure risk profile.
Dry Ice as a Refrigerant
Dry ice (solid carbon dioxide, UN 1845) is itself a Class 9 miscellaneous dangerous good because it sublimates into CO₂ gas and displaces oxygen, and because of the risk of pressure buildup in a sealed compartment. When dry ice is packed as a coolant with either a Category A or Category B shipment:
- The outer packaging must permit release of CO₂ gas (never hermetically sealed).
- The package must be marked with the UN 1845 marking and the net quantity of dry ice in kilograms.
- Quantity limits and additional documentation apply above certain thresholds, and combining dry ice with an infectious substance shipment is itself treated as a “dangerous goods in excepted quantities/combination packaging” scenario under IATA DGR — shippers should confirm current thresholds with their carrier and DGR edition in use, since dry ice quantity limits are revised periodically.
Exempt Human and Animal Specimens
Not every clinical or research specimen is subject to Category A or Category B rules at all. IATA DGR provides an “exempt human specimen” / “exempt animal specimen” exclusion for material where there is a minimal likelihood that pathogens are present — for example, a specimen from a patient with no history, symptoms, or risk factors suggesting an infectious disease, being sent for a routine test unrelated to infection (a metabolic panel, a genetic screen, a non-infectious pathology sample). Exempt specimens must still be packaged to prevent leakage (a basic triple-packaging approach, without the full PI 650 volume limits or marking), but they don’t require the UN 3373 mark or Category B documentation. This exemption is a judgment call made by the shipper or a qualified clinician/veterinarian, not an automatic default — it should be documented, since misapplying it to a specimen that should have been Category B is a compliance finding, not a paperwork nicety.
Training and Who May Ship
Anyone who classifies, packages, marks, labels, or signs documentation for either category must complete Dangerous Goods training that meets IATA DGR requirements (in the US, this training also satisfies 49 CFR §172.704 for hazmat employees) before shipping, and must repeat it on a recurrent cycle — typically every 24 months, though institutions and specific course providers may set a shorter interval. This applies to Category B shippers, not just Category A: PI 650’s simpler documentation does not exempt the shipper from the training requirement. Most research institutions route infectious substance shipping through an Environmental Health & Safety (EHS) office or a small number of trained departmental shippers rather than allowing any lab member to ship, both to limit liability and because IATA-compliant packaging and paperwork have a real learning curve.
Permits and Import/Export Considerations
Classification under IATA/DOT rules governs how a package is packed and transported, but it is separate from — and does not substitute for — permits that may independently be required to possess, transfer, import, or export the material itself. Depending on the organism and destination, a shipment may also require a CDC import permit (for etiologic agents entering the US), a USDA APHIS permit (for animal or plant pathogens, or material crossing certain borders), or Federal Select Agent Program registration and authorization (42 CFR Part 73) if the organism is on the HHS/USDA Select Agents and Toxins list. International shipments add the destination country’s own import requirements on top of IATA DGR. Shippers should confirm permit status well before packaging, since IATA-compliant packaging does not itself satisfy any of these separate permitting regimes.
Frequently Asked Questions
What’s the difference between IATA Category A and Category B biological substances?
Category A substances are capable of causing permanent disability or fatal/life-threatening disease upon exposure and require UN 2814 or UN 2900 packaging tested to UN Packing Group I standards plus a signed Shipper’s Declaration for Dangerous Goods. Category B is the default classification for infectious substances that don’t meet that severity threshold; it ships under UN 3373 with simpler, non-certified triple packaging and no separate dangerous goods declaration.
Do all patient specimens need to be shipped as Category B?
No. A specimen can qualify as an “exempt human specimen” if there is minimal likelihood that pathogens are present, based on the patient’s history, symptoms, and the reason for testing. Exempt specimens still need leak-proof packaging but not the full UN 3373 marking and documentation. Applying this exemption is a documented judgment, not an automatic default.
Does UN 3373 (Category B) require a Shipper’s Declaration for Dangerous Goods?
No. That is the main documentation difference between the two categories: Category A requires a formal, signed dangerous goods declaration; Category B requires correct package marking and an air waybill notation, but not a separate declaration form.
Do I need dangerous goods training to ship Category B specimens?
Yes. IATA DGR requires anyone who classifies, packages, marks, or documents an infectious substance shipment — Category A or Category B — to complete function-specific dangerous goods training and recurrent refresher training, typically on a roughly two-year cycle.
Can I ship dry ice with a Category B specimen?
Yes, dry ice (UN 1845) is commonly used as a coolant for both categories, but it is itself a Class 9 dangerous good with its own marking, quantity, and ventilation requirements, and adds an additional compliance layer on top of the biological substance classification.
Related CASRAI Resources
- Biosafety Level (BSL) — how laboratories are classified by the containment practices needed for the organisms they work with, a related but distinct classification system from IATA’s shipping categories.
- Institutional Biosafety Committee (IBC) — the institutional body that typically reviews and approves work with, and often the shipment of, infectious and recombinant materials.
- Select Agent List — the separate federal list of high-consequence biological agents subject to possession and transfer permitting on top of any IATA/DOT shipping classification.
- Biosafety Officer (BSO): Role and Responsibilities — the role most often responsible for coordinating infectious substance shipping compliance at a research institution.
- Export Control (EAR/ITAR) and International Research Collaboration — additional controls that can apply when biological materials or associated technical data cross international borders.
