Which type of materials are designated as hazard Class 7 in the DOT system?

• A. Explosives
• B. Gases
• C. Radioactive substances
• D. Flammable liquids

Answer: (C)

Radioactive materials are placed in Class 7 because their danger comes from ionizing radiation rather than flammability or chemical reactivity. The DOT assigns class numbers to group hazards by the dominant risk during transport, and Class 7 specifically covers radioactive substances and related materials, which require special packaging, labeling, and handling controls to limit exposure. This is different from explosives (which are in Class 1), gases (Class 2), or flammable liquids (Class 3), whose primary hazards are rapid energy release, pressurized contents, or flammability, respectively. So radioactive substances are the ones designated Class 7.