Classification of Dangerous Goods, Hazard Properties and Classes
The nine classes
ADR classifies dangerous goods into nine classes: 1 explosives, 2 gases, 3 flammable liquids, 4.1 flammable solids, 4.2 spontaneously combustible, 4.3 water-reactive, 5.1 oxidising, 5.2 organic peroxides, 6.1 toxic, 6.2 infectious, 7 radioactive, 8 corrosive, 9 miscellaneous. Subclasses matter: a substance in 4.2 behaves very differently from 4.1. Candidates must know the class number, the hazard, and the typical packing group logic.
Packing groups and hazard precedence
Packing groups I (high danger), II (medium) and III (low) apply mainly to classes 3, 4, 5, 6.1, 8 and 9. Where a substance has several hazards, ADR uses a precedence table to assign the primary class and subsidiary risks. Exam traps: assuming the most obvious property is always the primary class; forgetting that subsidiary risk labels must also appear; and confusing UN numbers (four digits identifying the substance) with class numbers.
Reading the Dangerous Goods List
The Dangerous Goods List in Part 3 gives, for each UN number, the name, class, packing group, labels, special provisions, packing instructions and tank codes. A candidate should be able to read a row and answer: what class, what label, what packing group, what special provision applies.
Worked rule of thumb
For a substance with several hazards, find the UN number, read the class column, then read the subsidiary risk column. The class column is the primary hazard; the subsidiary column adds labels and sometimes extra requirements. Never infer class from the name alone.