Classification of Dangerous Goods
The nine classes
Dangerous goods are grouped into nine classes: 1 explosives, 2 gases, 3 flammable liquids, 4 flammable solids (including spontaneously combustible and dangerous-when-wet substances), 5 oxidizers and organic peroxides, 6 toxic and infectious substances, 7 radioactive material, 8 corrosives, and 9 miscellaneous (including lithium batteries and environmentally hazardous substances). Each class is divided into divisions and packing groups.
Packing groups and precedence
Packing groups I, II and III reflect the degree of danger (great, medium, minor). When a substance meets criteria for more than one class, the "precedence of hazard" rules determine the primary class. Questions love the case of a substance that is both flammable and toxic: the primary class is not necessarily the one with the lowest number.
Lithium batteries
Lithium batteries are a frequent exam topic. They fall under Class 9 (UN 3480/3481 for lithium-ion cells and batteries, UN 3090/3091 for lithium metal). The state of charge, watt-hour rating and packaging requirements differ between cells, batteries and equipment. A common trap is assuming a battery installed in equipment is treated the same as a standalone battery.
Worked example
A liquid has a flash point of 40 degrees Celsius and is toxic on inhalation. Flash point at or below 60 degrees Celsius makes it a flammable liquid (Class 3). Toxicity may make it Class 6.1 as a subsidiary risk. The primary class is determined by the precedence rules and the packing group by the most severe criterion. The answer: Class 3, with a subsidiary Class 6.1 risk, and the packing group assigned from the most restrictive test result.
Rule of thumb
Classify by the substance's properties, not by its name or its common use. Always check for subsidiary risks and assign the packing group from the most severe criterion.