What is the UN Number for an Infectious Substance Classified as a Category B?

The safe and compliant transport of biological samples is not merely a logistical task; it is a paramount imperative for healthcare, research, and diagnostic industries worldwide. Every shipment carries the responsibility of ensuring sample integrity, preventing environmental contamination, and, most critically, safeguarding personnel and the public from potential hazards. This intricate process demands strict adherence to a complex web of international and national regulations, which meticulously classify substances based on their inherent risk level and assign specific identification numbers.

One of the most frequently encountered classifications for laboratories, clinics, and research facilities is “Category B infectious substances.” This report aims to demystify the specific United Nations (UN) identification number associated with these substances, explain their definition and classification, outline the essential packaging and labeling requirements, and demonstrate how cutting-edge solutions like the ai650 specimen bag play a vital role in achieving compliant and secure transport.

Understanding Infectious Substances: Category A vs. Category B
To properly classify and transport biological materials, it is essential to understand the distinctions between different categories of infectious substances. Accurate classification is the foundational step in ensuring compliance and safety, as it dictates all subsequent packaging, labeling, and documentation requirements.

What Defines an Infectious Substance?
An infectious substance is officially defined as a material known or reasonably expected to contain a pathogen. A pathogen, in this context, refers to any microorganism (including bacteria, viruses, rickettsiae, parasites, and fungi) or other agent, such as a proteinaceous infectious particle (prion), that has the capacity to cause disease in humans or animals. These materials are rigorously regulated as hazardous materials under various authoritative frameworks, including the U.S. Department of Transportation’s (DOT) Hazardous Materials Regulations (HMR) for domestic transport and the International Air Transport Association (IATA) Dangerous Goods Regulations (DGR) for international air shipments.

Category A: High Risk, Strict Rules (UN 2814, UN 2900)
Category A infectious substances represent the highest risk level. These are defined as substances capable of causing permanent disability, life-threatening, or fatal disease in otherwise healthy humans or animals when exposure to them occurs. Due to their severe potential for harm, these substances are assigned specific and distinct UN identification numbers:

• UN 2814: For infectious substances affecting humans (or both humans and animals).  
• UN 2900: For infectious substances which cause disease only in animals.  

Examples of Category A substances include cultures of highly virulent pathogens such as Ebola virus, Bacillus anthracis, and concentrated laboratory cultures of Human Immunodeficiency Virus (HIV) or Mycobacterium tuberculosis. The distinction that applies to “cultures only” for many Category A pathogens is a crucial nuance in risk assessment. This indicates that the concentration or form of the pathogen, rather than just its presence, is a key determinant of classification. For instance, a clinical specimen from an HIV-positive patient (typically lower concentration) would fall under Category B, whereas a pure, highly concentrated laboratory culture of HIV would be Category A. This distinction prevents the over-regulation of routine clinical samples while ensuring that genuinely high-risk laboratory cultures are handled with the utmost stringency.

Category B: The “Biological Substance, Category B” (UN 3373)
Category B infectious substances are defined as those that do not meet the stringent criteria for Category A. Exposure to these substances does not generally cause permanent disability or life-threatening or fatal disease in otherwise healthy humans or animals. Consequently, the potential for harm from Category B substances is considered moderate to low.This is the direct answer to the central question: All Category B infectious substances are assigned the identification number UN 3373. They must be identified by the proper shipping name “Biological substance, Category B” on their outer packaging.Common examples include clinical specimens containing Mycobacterium tuberculosis, HIV clinical specimens (when in clinical specimen form, not cultures), Arbovirus, and hepatitis A, B, and C. A significant portion of materials transported for routine diagnostic or investigational purposes falls into this category.

Distinction from Category A and “Exempt” Substances
It is crucial to differentiate UN 3373 from UN 2814/2900 (Category A) due to the significant difference in packaging and transport requirements. Additionally, the regulatory framework includes an “Exempt” category for certain biological materials. These include substances with a low probability of containing an infectious substance, or non-infectious biological materials such as routine drug/alcohol testing samples, cancer biopsies, or non-pathogenic microorganisms. Exempt specimens are not considered dangerous goods and are not assigned a UN identification number. This pragmatic approach prevents the over-regulation of genuinely low-risk samples, thereby streamlining logistics and allowing regulatory resources to be focused on materials that pose a more significant hazard.Another important distinction is Regulated Medical Waste (RMW). While RMW often contains Category B substances, it is assigned a different UN number, UN 3291, and has its own specific packaging requirements (49 CFR 173.197), distinct from UN 3373. This separation highlights that the form factor and end-use of the material, not just the underlying pathogen, can dictate its regulatory pathway. RMW, by its nature as waste from medical treatment, often presents unique challenges such as sharps or bulk quantities that necessitate different packaging standards than individual diagnostic specimens. This distinction ensures specialized safety measures for high-volume waste streams.

Table 1: Key UN Numbers and Infectious Substance Categories
This table provides a concise, visual reference for the different UN numbers associated with infectious substances, their corresponding categories, and risk levels, making complex information easily digestible and serving as a quick answer to the blog post’s core question.

The Significance of UN 3373 for Category B Substances
The assignment of UN 3373 to Category B infectious substances is more than just an identification tag; it is a critical component of a globally recognized standard for hazardous materials transport. This system ensures consistent communication of risks and handling procedures across international borders and various modes of transport.
Why UN 3373 is Assigned
UN 3373 specifically identifies substances that, while infectious, pose a lower risk compared to Category A materials. This classification allows for a standardized, yet less stringent, set of packaging and labeling requirements, thereby streamlining transport processes while maintaining essential safety protocols. This tiered approach is fundamental to hazardous materials safety, balancing necessary protection with practical transport logistics.
Regulatory Bodies Involved (DOT, IATA, ADR)
The classification and transport of UN 3373 substances are governed by various international and national authorities, ensuring a coordinated approach to safety.

• DOT (U.S. Department of Transportation): Regulates hazardous materials transport within the United States via highway, rail, air, and water. Category B substances fall under DOT’s Hazardous Materials Regulations (HMR; 49 C.F.R., Parts 171-180), specifically Section 173.199 for packaging.  
• IATA (International Air Transport Association): Governs air transport of dangerous goods globally. IATA’s Dangerous Goods Regulations (DGR) classify Class 6.2 infectious substances and assign UN 3373 to Category B, detailing specific packaging instruction PI650 for air shipments.  
• ADR (European Agreement concerning the International Carriage of Dangerous Goods by Road): Regulates road transport of dangerous goods across Europe and beyond, also specifying packing instruction P650 for UN 3373.  

These regulations aim to ensure that packages can withstand normal transport conditions, including shocks, pressure changes, humidity, and vibrations, without leakage. The consistency of Packing Instruction 650 (PI 650) across ADR (road) and IATA (air) streamlines compliance for multimodal transport, significantly reducing complexity for shippers. This harmonization is a considerable benefit for global logistics, as a shipper does not need to learn entirely different packaging rules for domestic road versus international air shipments, simplifying training and material procurement. However, it is important to note that while global standards provide a baseline, subtle differences can exist, such as variations in primary receptacle volume limits. Therefore, shippers cannot assume full compliance across all modes or regions by adhering to just one set of rules. They must consult the specific regulations applicable to their chosen mode of transport and destination. This necessitates a proactive approach to regulatory awareness and may require packaging solutions designed to meet the most stringent of these harmonized-yet-nuanced requirements to ensure universal compliance.
Ensuring Safe Transport: The Triple Packaging System for UN 3373
For Category B infectious substances (UN 3373), the cornerstone of safe transport is the “triple packaging” system. This mandatory, multi-layered system is meticulously designed to prevent any release of hazardous material into the environment, even if one layer fails, ensuring safety under normal transport conditions including potential drops, shocks, pressure changes, humidity, and vibrations. The completed package must be capable of successfully passing a drop test from at least 1.2 meters (3.9 feet) without any leakage from the primary receptacle.
Each component of the triple packaging system plays a critical and distinct role:

1. Primary Receptacle: Leakproof/Siftproof
   This is the innermost container directly holding the infectious substance (e.g., a screw cap tube, vial, or culture dish). It must be leakproof for liquids and siftproof for solids. To prevent breakage or leakage into the secondary packaging, multiple fragile primary receptacles must be individually wrapped or separated (e.g., using tube racks or cushioning) before being placed inside the secondary packaging. For air transport, the maximum quantity for liquids in a primary receptacle is typically 1 L.
2. Secondary Packaging: The Critical Layer (Highlighting the ai650 Specimen Bag)
   The primary receptacle(s) must be placed inside a leakproof secondary packaging. This layer acts as a crucial barrier, containing any leakage from the primary receptacle and preventing it from reaching the outer packaging.
   Absorbent Material: A sufficient quantity of absorbent material must be placed between the primary receptacle(s) and the secondary packaging. This material must be capable of absorbing the entire contents of the primary receptacle(s) in case of leakage. The absorbent material serves a dual purpose: it not only contains any leaks but also provides cushioning, preventing further damage to the packaging system and ensuring the overall integrity of the shipment. This highlights the importance of selecting high-quality, appropriately sized absorbent materials that also provide cushioning, rather than just any absorbent.  
   95 kPa Pressure Differential Test: For liquid Category B infectious substances transported by aircraft, the primary receptacle or the secondary packaging must be capable of withstanding, without leakage, an internal pressure producing a pressure differential of not less than 95 kPa (0.95 bar, 14 psi) in the temperature range of -40°C to +55°C. This specific engineering requirement is a critical safety measure for air transport, directly addressing the significant pressure fluctuations and temperature extremes encountered at altitude. Without this capability, changes in atmospheric pressure could cause primary receptacles to burst or leak, releasing hazardous material.  
   The ai650 Specimen Bag: Our ai650 specimen bag is engineered as a compliant secondary packaging solution for UN 3373 substances. It is designed to be leak-proof and robust, ensuring containment even if the primary receptacle fails. The ai650 specimen bag is rigorously tested and certified to meet this critical 95 kPa standard, providing an essential layer of safety for air transport. This positions the ai650 as a premium, reliable solution that helps users avoid costly non-compliance issues, particularly for sensitive or time-critical samples shipped by air. Furthermore, many compliant secondary bags, including the ai650, feature practical elements such as patented leak-resistant seals and separate pockets for requisition forms, ensuring documentation remains clean and secure, further streamlining workflow. It is crucial to always follow the exact instructions for filling and closing the packaging provided by the manufacturer, as proper use is paramount for achieving and maintaining compliance.  
3. Rigid Outer Packaging: Protection and Labeling
   The secondary packaging must be secured within a rigid outer packaging using suitable cushioning material. This outermost layer provides physical protection and must be strong enough to withstand shocks, drops (e.g., a 1.2-meter drop test), and vibrations encountered during transport. At least one surface of the outer packaging must have a minimum dimension of 100 mm by 100 mm (3.9 inches) to accommodate all required markings.For liquids, the maximum quantity per primary receptacle is typically 1 L, and the maximum quantity per outer packaging is 4 L. It is important to note that a carrier-provided shipping bag or box (e.g., a standard courier envelope) is not a substitute for this rigid outer packaging. This distinction addresses a common misconception and reinforces the strict interpretation of triple packaging. If coolants like ice or dry ice are used, they must be placed outside the secondary packaging, and the outer packaging must be leakproof or have a leakproof liner if ice is used.
   Marking and Labeling Requirements for UN 3373 Packages
   Proper and clear labeling is essential for immediate identification and safe handling throughout the transport chain. The outer packaging must clearly display specific markings on a contrasting background. The detailed specifications for outer packaging dimensions and marking (e.g., 100mm x 100mm minimum surface, 50mm diamond, 6mm text) are designed for immediate visual recognition and universal understanding by handlers. This ensures that critical information is clearly visible, legible, and immediately recognizable regardless of language, minimizing errors and enhancing safety across diverse logistics chains.
   Required Markings:
   The square-on-point mark with the UN number UN 3373. The mark must have sides at least 50 mm in length, and the letters/numbers must be at least 6 mm high. For air transport, the entire mark must appear on one side.  
   The proper shipping name: “Biological Substance, Category B” adjacent to the diamond-shaped mark, in letters at least 6 mm high.  
   The name, address, and telephone number of both the shipper (consignor) and the receiver (consignee).  
   The name and telephone number of a person responsible for providing emergency response information (this can be on the package or in accompanying documentation).  
   Important Notes:
   Unlike Category A substances, Category B shipments generally do not require an Infectious Substance label (the biohazard symbol), a Shipper’s Declaration for Dangerous Goods, or emergency response information directly on the package (though a contact number is required). This absence of certain labeling and documentation requirements signifies a calculated regulatory relief based on lower risk, meaning the physical packaging and clear UN3373 marking become even more critical, as the package itself must convey all necessary information for safe handling.  
   If liquids are present, package orientation labels (arrows pointing up) must be affixed on at least two opposite sides.  
   If dry ice is used as a coolant, a Class 9 label, UN 1845, and the net quantity of dry ice must be indicated.  
   If packages are placed in an overpack (e.g., consolidated in a larger box), all required markings must either be clearly visible or reproduced on the outside of the overpack, along with the word “Overpack”.  

Why Compliance Matters: Safety, Legality, and Reputation
Shipping biological substances non-compliantly carries significant risks and severe consequences that extend far beyond simple logistical hurdles. Adhering to UN 3373 regulations is not merely a bureaucratic requirement; it is fundamental to ensuring safety, maintaining legal standing, and protecting an organization’s reputation.
Safety
The primary concern in transporting infectious substances is public health and safety. Non-compliant packaging can lead to leaks, exposure to pathogens, and potential disease transmission, endangering personnel involved in handling and transport, as well as the general public and the environment. Even Category B substances, while generally not life-threatening, can still cause illness if containment is breached. The triple packaging system and specific performance standards like the 95 kPa test are engineered to prevent such incidents, acting as multiple layers of defense against potential hazards.
Legality and Penalties
Regulatory bodies like DOT and IATA impose strict penalties, including substantial fines and even imprisonment, for violations of hazardous materials transport regulations. Non-compliance can also lead to immediate shipment delays, refusal by carriers, and costly remediation efforts. Furthermore, incidents involving non-compliant shipments must be reported, leading to investigations and potential liabilities that can far exceed the cost of compliant packaging. The financial and operational disruptions caused by non-compliance can be significant, with some organizations facing losses exceeding $5 million due to business interruptions.
Reputation
For organizations, consistently demonstrating a commitment to safety and compliance builds trust with clients, partners, and regulatory bodies. This proactive approach to regulatory adherence can enhance a company’s standing and serve as a competitive advantage. Conversely, incidents of non-compliance can severely damage a company’s brand, leading to a loss of business, erosion of public confidence, and long-term reputational harm. Therefore, compliance is not just a cost center but a critical aspect of risk management and business continuity.
Proactive Compliance
Establishing a robust compliance framework involves continuous efforts: conducting thorough audits, creating comprehensive checklists, implementing rigorous quality assurance measures, and regularly training employees. Utilizing proven, compliant products like the ai650 specimen bag is a key part of this proactive strategy, ensuring that the packaging itself meets the highest regulatory standards from design through post-market surveillance. This systematic approach protects against costly disruptions and reinforces an organization’s commitment to safety and excellence.