Two loads can sit in the same hazard class and still carry very different levels of risk. A Class 8 drain cleaner and a drum of fuming sulphuric acid are both corrosive, but no trainer would treat them as equals. ADR deals with this through packing groups: PG I for high danger, PG II for medium danger and PG III for low danger (2.1.1.3).
One point first. Much of the training material on this subject comes from the United States and is written against 49 CFR and the DOT Hazmat Table. The idea of packing groups is the same in both systems because both come from the UN Model Regulations. The references, tables and some category names are not the same. This post sets out what ADR 2025 says, as applied in Great Britain, and you should not read a US reference as if it covered a UK road load.
What sets the packing group in each class
Packing groups only apply to some classes. Where they do apply, each class uses its own test for how severe the hazard is:
| Class | What decides PG I, II or III |
|---|---|
| 3 Flammable liquids | Flash point and initial boiling point (2.2.3.1.3) |
| 4.1 Flammable solids | Burning-rate tests from the UN Manual of Tests and Criteria |
| 4.2 Spontaneously combustible | Pyrophoric substances are PG I. Self-heating substances are PG II or III, decided by test |
| 4.3 Dangerous when wet | How readily the substance reacts with water and how fast it gives off flammable gas |
| 5.1 Oxidisers | Solids: mean burning time. Liquids: rate of pressure rise or spontaneous ignition |
| 6.1 Toxic | Oral and dermal LD50, and inhalation LC50 |
| 8 Corrosive | How fast it destroys skin, or corrodes steel or aluminium |
| 9 Miscellaneous | No general criteria. The PG, where one exists, is shown entry by entry in the Dangerous Goods List |
Class 3 and Class 6.1 have the clearest thresholds:
| Class 3 | Flash point | Initial boiling point |
|---|---|---|
| PG I | — | ≤ 35 °C |
| PG II | < 23 °C | > 35 °C |
| PG III | ≥ 23 °C and ≤ 60 °C | > 35 °C |
| Class 6.1 | Oral LD50 (mg/kg) | Dermal LD50 (mg/kg) | Dust/mist LC50 (mg/l) |
|---|---|---|---|
| PG I | ≤ 5 | ≤ 50 | ≤ 0.2 |
| PG II | > 5 and ≤ 50 | > 50 and ≤ 200 | > 0.2 and ≤ 2 |
| PG III | > 50 and ≤ 300 | > 200 and ≤ 1,000 | > 2 and ≤ 4 |
US materials split poison-by-inhalation liquids into “Hazard Zones A and B”. ADR does not use those zones. Toxicity by vapour inhalation is assessed against the criteria in 2.2.61.1.9, and the result shows up in the packing group and in the entry’s own provisions.
Some classes have no packing groups at all. These are Class 1, Class 2, Class 5.2, Class 6.2, Class 7 and the self-reactive substances of 4.1. Each uses its own system, such as divisions and compatibility groups, classification codes or categories. The 13 ADR classes explained covers how those work.
One name, several packing groups
Many generic and n.o.s. entries appear more than once in the Dangerous Goods List of Chapter 3.2, once for each packing group. The packing group is in column (4) of Table A, and that row decides everything else:
| UN 1760 Corrosive liquid, n.o.s. | PG I | PG II | PG III |
|---|---|---|---|
| Limited quantity, column (7a) | 0 (not permitted) | 1 L | 5 L |
| Transport category, column (15) | 1 | 2 | 3 |
| 1.1.3.6 multiplier | 50 | 3 | 1 |
| 1.1.3.6 threshold | 20 | 333 | 1,000 |
A product name like “drain cleaner” does not tell you the packing group. The consignor has to classify the product against the Part 2 criteria. They then pick the matching row, and the transport document must state that packing group.
Here’s a correction to a common example. Concentrated sulphuric acid is often called a PG I corrosive. In ADR, UN 1830 Sulphuric acid with more than 51% acid is PG II. The PG I entries are the fuming acids, such as UN 1831. This is why you check the actual entry instead of going by reputation.
What the packing group changes for the driver
The driver does not assign the packing group, but it affects nearly every check you make:
| What it affects | ADR reference | Why it matters on the road |
|---|---|---|
| Transport document | 5.4.1.1.1 | The PG is part of the description, for example “UN 1760 CORROSIVE LIQUID, N.O.S., 8, PG II” |
| Packaging standard | 6.1.3 | X, Y or Z on the UN mark shows which packing groups the packaging is approved for |
| Limited quantity | 3.4 | The LQ limit per inner packaging changes with PG, and PG I often has none |
| Excepted quantity | 3.5 | The E-code in column (7b) changes with PG |
| Load threshold | 1.1.3.6 | The transport category, and so the points per unit, usually follows the PG |
| Marking and placarding | 5.3 | Exceeding 1.1.3.6 brings in orange plates and full crew requirements |
Read the packaging mark against the paperwork. A PG II substance in a drum marked only “Z” is in packaging approved for PG III only, and that is non-compliant. Don’t load it. Report it to the consignor or your DGSA. Reading a UN packaging mark explains the X, Y and Z codes in detail.
The PG also feeds your load-points sum. Two hundred litres at PG III in Transport Category 3 is 200 points. The same volume of a PG I entry in Transport Category 1 is 10,000 points, which is well over the 1,000-point threshold and needs a vehicle and driver fully equipped and certified under ADR. Never use a lower packing group to bring a load under the threshold. The packing group comes from classification, not from what suits the job.
How ADR Driver Pro Fits Into This
Packing groups come up throughout the core syllabus and in each hazard class, so they appear across all 13 modules of ADR Driver Pro. The 1820 revision questions, 110 of them free, test reading Table A rows, transport document entries and packaging marks, not just recalling definitions.
The packages route, one of 6 driver routes, focuses on the checks above. Among the 5 calculators, the web load-points calculator shows the 1.1.3.6 arithmetic step by step. The background is in what the load-points calculator actually does. The 8 quick reference sets and 6 field tools keep the PG criteria and transport-category multipliers on hand offline. The classification still has to come from the consignor’s transport information, and the final call rests with your carrier or DGSA.
