Category A Infectious Substances: Classification, List, and Transport Requirements

A category A substance (pathogen or agent) is “an infectious substance which is transported in a form that, when exposure to it occurs, is capable of causing permanent disability or life-threatening or fatal disease to otherwise healthy humans or animals.” Deciding if an infectious substance is a Category A substance is relatively easy because there are relatively few Category A substances and because Category A substances are specifically designated and listed by IATA and DOT. The list of Category A substances is not all-inclusive, and a thorough risk assessment must be performed before assigning a substance to Category A.

What is Category A, and what is it not

"Category A" is a term used in three different ways in microbiology, and mixing them up is the most common error in this topic. Distinguish them:

• IATA/DOT Transport Category A: The classification we cover here. It is a designation for shipping and transport purposes, assigned to infectious substances (whether cultures, patient specimens, or waste) that are capable of causing permanent disability or life-threatening or fatal disease when exposure occurs. Assigned the UN numbers UN2814 (affects humans or both humans and animals) or UN2900 (affects animals only). These are based on a risk assessment, not on a fixed list of organisms.
• CDC Bioterrorism Category A: A separate list of agents of concern for bioterrorism response (Bacillus anthracis, Yersinia pestis, variola virus, toxins, etc.). This list is used by public health agencies and biosafety committees, not by transport authorities. An organism on the CDC bioterrorism list may or may not be a transport Category A, and vice versa.
• Biosafety level (BSL1–BSL4): A laboratory containment classification based on the hazard posed by an organism and the route of transmission. A BSL2 organism like Salmonella or S. aureus is not necessarily a transport Category A. Many BSL3 organisms are. Category A substances in the transport sense can sit at several different biosafety levels.

This article focuses on IATA/DOT transport Category A only.

What defines a Category A infectious substance?

A Category A infectious substance is "an infectious substance which, in a form that, when exposure to it occurs, is capable of causing permanent disability, life-threatening or fatal disease in otherwise healthy humans or animals." Category A substances are specifically designated by IATA and DOT, and are assigned to one of two UN numbers:

• UN2814: Infectious substance, affecting humans (or both humans and animals)
• UN2900: Infectious substance, affecting animals only

The official list of Category A agents is maintained by IATA (International Air Transport Association) and published in the IATA Dangerous Goods Regulations. The list is not all-inclusive; a thorough risk assessment must be performed on any organism not listed to determine whether it meets the Category A definition. Once classified as Category A, a substance must be shipped under strict packing and transport rules regardless of the form in which it is being sent.

The cultures-only distinction: Why it matters

The most common error in Category A classification is not understanding what "(cultures only)" means. Many organisms in the Category A list carry this notation, and it changes the shipping rules entirely.

When an organism is marked "(cultures only)":

• A culture of that organism (grown in a flask, plate, or tube in a laboratory) is a Category A substance and must be shipped as UN2814 or UN2900 with full triple-packaging and labeling.
• A patient specimen or clinical sample containing that same organism (sputum sample from a TB patient, blood culture with Mycobacterium tuberculosis) is NOT automatically Category A. Instead, it is typically classified as Category B (UN3373) or may be exempt if it meets certain diagnostic criteria.

Why the difference? A culture is an intentionally propagated, high-concentration preparation of the organism, posing greater risk if released. A diagnostic specimen is a single sample from a patient, and the transport risk is considered differently under IATA regulations.

The practical consequence: A microbiology lab that routinely receives patient specimens with Mycobacterium tuberculosis (cultures only) does NOT package those samples as UN2814. The specimens are received and handled under standard biosafety protocols appropriate to the organism's BSL. But if that lab cultures the TB organism from the specimen and then ships a culture tube to a reference lab, that culture must be shipped as UN2814 with triple packaging.

This distinction is the single largest source of shipping compliance errors. Many labs incorrectly assume that because an organism is Category A, all forms of it require Category A shipping. That is wrong. The rules apply only to the form specified in the list.

Packing requirements for Category A substances (Packing Instruction P620)

A Category A substance shipped as UN2814 or UN2900 must meet the triple-packaging requirement defined in IATA Packing Instruction 620 (PI 620) or the equivalent DOT/ADR Packing Instruction 620 (P620). There are no exceptions.

The three-layer packaging system:

1. Primary receptacle (inner container): Leak-proof, glass or plastic, containing the infectious material. Must hold no more than 100 mL or 100 g per receptacle. All primary receptacles must be identical and properly sealed.
2. Secondary packaging: A leak-proof container (often a plastic bag or second protective case) that encloses and cushions the primary receptacle. If the secondary packaging itself leaks or breaks, the primary must still contain the material.
3. Outer packaging: A rigid container (cardboard box, plastic box, etc.) that protects the secondary packaging from external damage. The outer packaging must:
   • Display the UN hazard class marking for Class 6.2 (infectious substances)
   • Have a "Biological Hazard" label affixed
   • Be marked with the UN number (UN2814 or UN2900) and proper shipping name
   • Include a Shipper's Declaration form completed by the shipper
   • Be able to withstand a drop test and pressure changes without leaking

Common packing errors that trigger regulatory fines:

• Using a single container instead of triple packaging
• Exceeding 100 mL or 100 g per primary receptacle
• Placing the hazard label on the wrong face of the box or upside down
• Missing or incomplete Shipper's Declaration
• Using improper materials for the secondary or outer layers

For detailed diagrams and specifications, consult the current IATA Dangerous Goods Regulations (DGR) or the CDC's Packaging and Transporting Infectious Substances resource.

List of Category A infectious substances

The following organisms and agents are designated as Category A infectious substances by IATA and DOT. When marked "(cultures only)," the classification applies to laboratory cultures, and patient specimens containing these organisms are not automatically Category A (see The cultures-only distinction section above). The complete official list is maintained in the IATA Dangerous Goods Regulations; new organisms and risk assessments may add to this list over time.

Bacillus anthracis

Bacillus anthracis is the etiological agent of anthrax. Bacillus anthracis was intentionally released into the US postal system in October 2001. In this bio-terrorism-related event, various individuals got a letter containing a written note that the person opening the letter had been exposed to spores of Bacillus anthracis. Cultures of Bacillus anthracis are labeled as Category A infectious agents.

Brucella

The culture of three Brucella species, Brucella abortus, Brucella melitensis, and Brucella suisare labeled as Category A infectious agents. Brucellaspecies causes brucellosis (undulant fever). People acquire this zoonotic disease from infected cattle (Brucella abortus), goats and sheep (Brucella melitensis), and pigs (Brucella suis) by ingesting contaminated milk products or through direct contact in occupational settings such as an abattoir.

Other organisms

1. Burkholderia mallei (cultures only)
2. Burkholderia pseudomallei (cultures only)
3. Chalamydia psittaci (avian) (cultures only)
4. Clostridium botulinum (cultures only)
5. Coccidioides immitis (cultures only)
6. Coxiella burnetii (cultures only)
7. Crimean-Congo hemorrhagic fever virus
8. Dengue virus (cultures only)
9. Eastern equine encephalitis virus (culture only)
10. Escherichia coli, verotoxigenic (cultures only)
11. Ebola virus
12. Francisella tularensis(cultures only)
13. Hantavirus causing hemorrhagic fever with renal syndrome
14. Hepatitis B virus (cultures only)
15. Herpes B virus (cultures only)
16. Human Immunodeficiency Virus (cultures only)
17. Lassa virus
18. Marburg virus
19. Monkeypox virus
20. Mycobacterium tuberculosis(cultures only)
21. Poliovirus virus (cultures only)
22. Rabies virus (cultures only)
23. Rickettsia rickettsii (cultures only)
24. Rift Valley fever virus (cultures only)
25. Shigella dysenteriae type 1 (cultures only)
26. Variola virus
27. Venezuelan equine encephalitis virus (cultures only)
28. West Nile virus (cultures only)
29. Yellow fever virus (cultures only)
30. Yersinia pestis (cultures only)
31. Classical swine fever virus (cultures only)
32. Foot and mouth disease virus (cultures only)
33. Goat pox virus (cultures only)
34. Lumpy skin disease virus (cultures only)
35. Newcastle disease virus (cultures only)
36. Sheep pox virus (cultures only)
37. Swine vesicular disease virus (cultures only)
38. Vesicular stomatitis virus (cultures only)

How to determine if something needs Category A shipping

Before you ship any infectious specimen or culture, ask these questions in order:

1. Is it cultured or is it a patient specimen?

• If it's a CULTURE of an organism on the Category A list → go to question 2
• If it's a PATIENT SPECIMEN or CLINICAL SAMPLE → check the list for "(cultures only)" notation. If marked "(cultures only)," it may be Category B or exempt; if unmarked, it may be Category A. Ask your biosafety officer. When in doubt, classify higher.

2. Is it on the Category A list?

• If YES → it is Category A; use UN2814 or UN2900 (depends on whether it affects humans or animals only)
• If NO → it may still be Category A if a risk assessment determines it meets the definition. If it is a new, emerging, or genetically modified organism, perform a risk assessment in consultation with your biosafety committee.

3. What is the maximum quantity per primary receptacle?

• No more than 100 mL or 100 g per primary receptacle.
• Multiple receptacles can go in one shipment, but each must be < 100 mL/g.

4. Is it authorized for air transport in your region?

• Some Category A substances cannot be transported by air under any circumstances.
• Check with your shipping authority (IATA) or your institution's dangerous goods coordinator before sending by air.

If at any step you are uncertain, consult:

• Your institution's biosafety officer or dangerous goods coordinator
• The current IATA Dangerous Goods Regulations
• The CDC's Packaging and Transporting Infectious Substances resource
• Your regional health authority or customs service

Misclassification or improper packing can result in fines of thousands of dollars, loss of shipping privileges, and exposure of transport workers. When in doubt, escalate. Never guess.

Key exam facts in one table

Where Students Get Confused

• Thinking all Category A organisms require the same shipping rules. The "(cultures only)" notation changes everything. A culture is Category A; a patient specimen containing the same organism may not be. Most regulatory fines come from this single misunderstanding.
• Conflating transport Category A with CDC bioterrorism Category A. These are two separate lists maintained by different agencies for different purposes. An organism can be one, both, or neither. They use different decision criteria and serve different regulatory frameworks.
• Assuming a BSL3 organism is automatically transport Category A. Biosafety level and transport classification are independent decisions. Some BSL2 organisms are transport Category A; some BSL3 organisms are not. Both must be assessed separately.
• Thinking you can ship a Category A culture without triple packaging. There are no exceptions. IATA P620 triple packaging is mandatory for all Category A substances. A single container, even if labeled, is a violation.
• Missing the Shipper's Declaration or using a typewritten signature. The declaration must be hand-signed. Missing it or signing it with a typewriter is a compliance violation and can result in fines.
• Assuming you can figure out the classification yourself without risk assessment. If an organism is not on the list, a formal risk assessment must be performed. Guessing can classify an organism too low (regulatory violation) or too high (unnecessary cost and hassle). Ask.

References

1. International Air Transport Association (IATA). Dangerous Goods Regulations (DGR). 67th ed. Montreal, Canada: IATA; 2025.
2. U.S. Department of Health and Human Services, CDC and NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL). 6th ed. Atlanta, GA: CDC; 2020.
3. U.S. Centers for Disease Control and Prevention. Packaging and Transporting Infectious Substances. Atlanta, GA: CDC. Available at: https://www.cdc.gov/niosh/topics/emres/chemsafe.html
4. World Health Organization. Laboratory Biosafety Manual. 4th ed. Geneva: WHO; 2020.
5. U.S. Department of Transportation. Hazardous Materials Regulations (HMR). Title 49, Code of Federal Regulations. Washington, DC: DOT;