Fully regulated hazardous materials must be shipped in UN specification packaging, which means containers built from specific approved materials, assembled in approved configurations, and tested to meet performance standards set by the UN and enforced in the U.S. through 49 CFR (the Hazardous Materials Regulations administered by PHMSA). There is no single “approved material.” Instead, the regulations define a menu of approved material types, container styles, and performance levels that must be matched to the specific hazard class and packing group of the item you’re shipping.
Approved Outer Packaging Materials
The UN system recognizes a defined set of materials for constructing outer packaging. Each is identified by a letter code that appears in the UN specification marking stamped on every compliant container:
- A: Steel
- B: Aluminum
- C: Natural wood
- D: Plywood
- F: Reconstituted wood
- G: Fiberboard
- H: Plastic
- L: Textile
- M: Paper (multi-wall)
- N: Metal other than steel or aluminum
- P: Glass, porcelain, or stoneware
These materials can be formed into different container types: drums (code 1), jerricans (code 3), boxes (code 4), bags (code 5), and composite packaging (code 6). Not every material works with every container type. You won’t find a glass drum, for instance, but you will find glass receptacles nested inside protective steel, aluminum, fiberboard, or plywood outer containers as part of a composite packaging system.
How UN Packaging Codes Work
Every approved container carries a UN specification mark that tells you exactly what it is and what it’s rated for. The code follows a consistent structure. The first number identifies the container type (1 for drum, 4 for box, etc.), the letter identifies the material, and a final digit indicates the closure style where applicable. A steel drum with a non-removable head is coded 1A1. The same drum with a removable head is 1A2. A fiberboard box is 4G.
After the packaging code, the mark includes a packing group rating (X, Y, or Z), a maximum gross mass or liquid density, and the year of manufacture. X corresponds to Packing Group I (the highest danger level), Y to Packing Group II, and Z to Packing Group III. A mark reading “4G/Y25/S” tells you the container is a fiberboard box rated for Packing Group II solids with a maximum gross mass of 25 kg. If the packaging is approved for liquids, the mark will show liquid density and pressure capability instead of “S.”
If a container is approved for both solids and liquids, it will carry two separate UN marks.
Composite and Combination Packaging
Many fully regulated items require a layered approach: an inner receptacle held inside a protective outer container. These composite packagings use two material codes in sequence, the first for the inner receptacle and the second for the outer. Common configurations include a plastic receptacle inside a protective steel drum, a plastic receptacle inside a fiberboard box, or glass receptacles inside wooden, plywood, or fiberboard outer packaging.
For glass, porcelain, or stoneware inner receptacles, the regulations require that they be cylindrical or pear-shaped, made from defect-free material, and firmly secured within the outer packaging. Plastic inner receptacles must fit snugly inside the outer container, which cannot have projections that could abrade the plastic.
Packing Groups and Performance Levels
The packaging you choose must match the packing group assigned to the hazardous material. Packing Group I covers the most dangerous substances and demands the strongest packaging (marked X). Packing Group II is moderate danger (marked Y), and Packing Group III is the lowest regulated level (marked Z). An X-rated package can be used for all three packing groups. A Y-rated package can be used for Packing Groups II and III. A Z-rated package is only acceptable for Packing Group III.
All UN specification packaging must pass performance tests before it can be used. These include drop testing (the filled container is dropped from a specified height onto a rigid, flat surface), stacking tests, and for liquid-rated packaging, leakproofness and hydrostatic pressure testing. Packaging intended for air transport must also pass internal pressure testing. These aren’t optional certifications. Every marked container must be capable of passing all applicable tests set out in the Hazardous Materials Regulations.
Chemical Compatibility for Plastic Packaging
When a fully regulated liquid goes into plastic packaging, the container must be tested for chemical compatibility and permeation. The standard procedure fills actual containers with the hazardous material and stores them under controlled conditions. Three test methods exist, ranging from 180 days at room temperature to 14 days at 60°C (140°F). At least three sample containers must be tested for each combination of hazardous material and container design.
After storage, testers check for deformation, swelling, cracking, corrosion, embrittlement, leakage, or any defect that could cause premature failure. They also measure weight loss to determine how much material permeated through the plastic walls. For poisons, the permeation rate cannot exceed 0.5% of the original weight over the test period. For all other hazardous materials, the limit is 2.0%. The container must then survive a drop test while filled with water. Any of these failures disqualifies the packaging.
Lithium Batteries as a Common Example
Fully regulated lithium batteries (both lithium-ion under UN3480 and lithium-metal under UN3090) illustrate how the system works in practice. The outer packaging must be UN specification packaging rated to at least Packing Group II performance standards. PHMSA’s lithium battery guide lists the specific authorized outer container types:
- Boxes: Metal (4A, 4B, 4N), wooden (4C1, 4C2, 4D, 4F), fiberboard (4G), or solid plastic (4H1, 4H2)
- Drums: Metal (1A2, 1B2, 1N2), plywood (1D), fiber (1G), or plastic (1H2)
- Jerricans: Metal (3A2, 3B2) or plastic (3H2)
Inside, each cell or battery must sit in a non-metallic inner packaging that completely encloses it and prevents contact with any electrically conductive material. Terminals must be protected against short circuits, and the cells must be packed to prevent shifting. FAA testing has shown that standard fiberboard packaging can ignite during lithium battery thermal runaway, which is why inner packaging separation and proper performance ratings are critical, not optional.
Special Packaging Designations
You may encounter additional letter qualifiers stamped after the packaging code. A “V” after the code (for example, 4GV) indicates “special packaging” that meets an alternative set of requirements under the UN Model Regulations. “W” means the container was manufactured to a different specification but has been deemed equivalent. “T” designates salvage packaging, used to overpack damaged or leaking containers. “U” identifies special packaging for infectious substances.
These qualifiers change what the package is authorized for, so reading the full UN specification mark matters. A 4GV box has different allowances than a standard 4G box, even though both are fiberboard.
Matching Material to Your Shipment
The approved material for your specific item depends on its hazard class, packing group, physical state (solid or liquid), and mode of transport. The Hazardous Materials Table in 49 CFR 172.101 assigns each regulated substance a proper shipping name, hazard class, and packing group. From there, a column in the table directs you to the specific packaging section in Part 173, which lists the authorized inner, intermediate, and outer packaging combinations for that substance.
Steel and aluminum drums are common for corrosive or flammable liquids. Fiberboard boxes (4G) are among the most widely used outer containers for solids and for combination packaging where inner containers hold the liquid. Plastic containers work for many chemical classes but require compatibility testing. Glass inner receptacles are used for smaller quantities of highly reactive materials that would attack metal or plastic. The material choice is never arbitrary: it flows from the specific entry in the Hazardous Materials Table through to the packaging instructions for that substance.