The 9 DOT Hazard Classes, Explained
Every hazardous material shipped in the U.S. falls into one of nine DOT hazard classes. Here's what each class covers, with the two subdivided classes broken down further.
The Department of Transportation sorts every regulated hazardous material into one of nine hazard classes, numbered 1 through 9, based on the type of danger it poses — not how dangerous it is overall. Classes 1, 2, 4, 5, and 6 are further split into divisions (like 2.1 Flammable Gas vs. 2.3 Toxic Gas) for more precision.
Why classes instead of one danger scale
It would be simpler to just rank materials from "mild" to "extreme," but that's not what the hazard class system does. Instead, it groups materials by the kind of hazard they pose, because a flammable liquid and a corrosive both need serious precautions, just completely different ones. The class tells you what kind of incident to plan for; the packing group (a separate column) tells you how severe it is within that class.
Class 1 — Explosives
Split into six divisions (1.1 through 1.6) based on the type of explosion hazard — mass detonation, projection hazard, fire hazard, and so on down to 1.6, materials that are extremely insensitive and pose almost no mass-explosion risk.
Class 2 — Gases
Three divisions: 2.1 Flammable gas (propane, butane), 2.2 Non-flammable, non-toxic gas (compressed air, nitrogen), and 2.3 Toxic gas (chlorine, ammonia in certain forms). Ammonia's exact division depends on concentration and form.
Class 3 — Flammable liquids
Not divided further. Covers liquids with a flash point at or below 60°C (140°F) — gasoline, acetone, most paint thinners and solvents fall here.
Class 4 — Flammable solids
Three divisions: 4.1 Flammable solid (matches, some metal powders), 4.2 Spontaneously combustible (materials that can ignite without an external spark, like certain oily rags or white phosphorus), and 4.3 Dangerous when wet (materials that react with water to release flammable gas, like sodium).
Class 5 — Oxidizers and organic peroxides
5.1 Oxidizer (materials that can intensify a fire by yielding oxygen, like ammonium nitrate) and 5.2 Organic peroxide (a chemically unstable class that's both an oxidizer and often flammable).
Class 6 — Toxic and infectious substances
6.1 Toxic (poisonous) materials, and 6.2 Infectious substances — this division covers regulated medical waste, diagnostic specimens, and biological agents.
Class 7 — Radioactive material
Not divided into sub-numbers, but internally categorized by activity level and packaging type (Type A, Type B, etc.) which determines the packaging and labeling requirements.
Class 8 — Corrosives
Materials that cause visible destruction of skin tissue or corrode steel/aluminum — this covers everything from battery acid to many industrial cleaning compounds.
Class 9 — Miscellaneous
The catch-all for materials that present a hazard during transport but don't fit classes 1–8 — this includes things like lithium batteries, dry ice, and environmentally hazardous substances (materials dangerous to aquatic life, for example).
A material can only have one primary class
Even though many chemicals are dangerous in more than one way (both flammable and toxic, say), the Hazmat Table assigns one primary hazard class per entry, with any additional hazards noted as a "subsidiary hazard" in the label codes column. The primary class is generally whichever hazard is considered most significant for transport purposes, which isn't always the most intuitive choice — always check the actual table entry rather than assuming.