The Nine DOT Hazardous Materials Classes
The Department of Transportation classifies hazardous materials into nine classes based on their primary hazard. These classes are internationally standardized through the United Nations' Globally Harmonized System (GHS) and appear on placards, labels, and shipping documentation. Understanding the classes is foundational to hazmat transport theory — the hazmat endorsement (H endorsement) requires detailed knowledge, but even drivers without the endorsement encounter placarded vehicles daily and must understand basic response.
Class 1: Explosives — divided into six divisions based on explosion type and risk. Division 1.1 is the most dangerous (mass explosion hazard); Division 1.4 is the least dangerous (minor explosion hazard). Common examples: ammunition (Div 1.3), fireworks (Div 1.3 or 1.4), detonators (Div 1.1).
Class 2: Gases — including flammable gases (2.1: propane, hydrogen), non-flammable compressed gases (2.2: nitrogen, helium), and poisonous/toxic gases (2.3: chlorine, ammonia, hydrogen cyanide). Poisonous gas poses the highest hazard: inhalation can be lethal in minutes.
Class 3: Flammable and Combustible Liquids — materials with flash points below 60°C (140°F). Common examples: gasoline, diesel fuel (combustible), alcohol, paint.
Class 4: Flammable Solids — divided into three divisions: 4.1 (flammable solids: matches, sulfur), 4.2 (spontaneously combustible materials: white phosphorus), 4.3 (dangerous when wet: sodium, calcium carbide).
Class 5: Oxidizers and Organic Peroxides — 5.1 are oxidizers that can accelerate combustion of other materials (ammonium nitrate, potassium permanganate); 5.2 are organic peroxides (highly unstable, heat and shock sensitive).
Class 6: Toxic/Poisonous and Infectious Materials — 6.1 are toxic by ingestion, inhalation, or skin absorption (pesticides, cyanides); 6.2 are infectious substances (medical waste, diagnostic specimens) subject to additional DOT-CDC regulations.
Class 7: Radioactive Materials — classified by radioactivity level into White I, Yellow II, Yellow III, and Fissile categories. Transport index (TI) on the label indicates radiation dose rate at one meter.
Class 8: Corrosives — materials that cause visible destruction of human skin or steel. Battery acid (sulfuric acid), sodium hydroxide, hydrochloric acid.
Class 9: Miscellaneous Hazardous Materials — hazardous materials not classified in 1–8, including dry ice (CO2), lithium batteries, magnetized materials, and elevated temperature materials.