What Is a Packing Group?
Packing groups (I, II, or III) tell you how severe a hazard is within its class, and directly determine how strong the packaging has to be. Here's how the three groups differ.
A packing group is a Roman numeral — I, II, or III — assigned to most hazardous materials to indicate the degree of danger within their hazard class. Packing Group I is the highest danger level and requires the toughest packaging; Packing Group III is the lowest and allows lighter packaging. Not every hazard class uses packing groups (Class 2 gases and Class 7 radioactive materials don't), but most of the flammable, corrosive, and toxic classes do.
What the roman numeral actually changes
The packing group doesn't change the hazard class — a Packing Group I flammable liquid and a Packing Group III flammable liquid are both still Class 3. What changes is the performance standard the packaging has to meet. Packaging is tested and rated (drop tests, stacking tests, pressure tests) to a specific packing group level, and you can't ship a Packing Group I material in packaging only rated for Packing Group III, even if it happens to physically fit.
The three groups
- Packing Group I — Great danger. The most severe materials within a class. For flammable liquids, this generally means a very low flash point combined with a low boiling point. Requires the most rigorous packaging.
- Packing Group II — Medium danger. The middle tier — most common industrial solvents and moderately hazardous corrosives tend to land here.
- Packing Group III — Minor danger. The least severe materials that still require regulation. Packaging requirements are the least strict of the three, though still far from casual.
How the group gets assigned
For most classes, the packing group is determined by lab criteria specific to that hazard type. A few examples:
- Flammable liquids (Class 3): based on flash point and boiling point — lower flash point and lower boiling point pushes a material toward Packing Group I.
- Corrosives (Class 8): based on how long it takes the material to cause full-thickness destruction of skin tissue in test conditions, and for materials that aren't tested on skin, how corrosive they are to steel or aluminum.
- Toxic substances (Class 6.1): based on LD50/LC50 toxicity data — the dose lethal to half of test subjects.
This means the packing group is a lab-determined value, not something a shipper estimates — for a listed entry in the Hazmat Table, the packing group is already fixed; you don't calculate it yourself unless you're dealing with an unlisted mixture that needs to be classified from scratch.
Why a single Hazmat Table entry sometimes lists multiple packing groups
Some table entries show something like "I, II, III" in the packing group column. That means the same proper shipping name can apply to material across a range of severities depending on concentration or specific formulation — a classic example is many acid solutions, where a higher concentration falls into a stricter packing group than a dilute version of the same acid. When that happens, you have to determine which packing group your specific product actually falls into using the criteria above, rather than picking whichever one is most convenient.
Classes that don't use packing groups
Explosives (Class 1), gases (Class 2), radioactive material (Class 7), and Class 9 miscellaneous materials generally don't use packing groups at all — these classes have their own separate systems for indicating severity (divisions, compatibility groups, or specific quantity-based rules instead).