2.1 The Nine Hazard Classes

Why Hazard Classes Matter

The Hazardous Materials Regulations (HMR) — issued by the U.S. Department of Transportation (DOT) and found in Title 49 of the Code of Federal Regulations (49 CFR) — divide every regulated material into one of nine hazard classes. The class number is not just a label. It tells you the placard color and symbol, which materials it can and cannot ride with, and what an emergency responder should do if it spills.

On the CDL HazMat knowledge test you will be asked to match a material to its class number, recognize the divisions inside Class 1 and Class 2, and explain what a Packing Group means. Learn the table below cold — several test questions come straight from it.

The Nine Hazard Classes

Classes That Have Divisions

Three classes are split further, and the divisions are common test material.

Class 1 — Explosives (Divisions 1.1 to 1.6)

• 1.1 — Mass explosion hazard (the whole load can detonate at once); dynamite.
• 1.2 — Projection hazard but not a mass explosion; some ammunition.
• 1.3 — Fire hazard with minor blast or projection; display fireworks, propellant.
• 1.4 — Minor explosion hazard, effects mostly stay inside the package; small-arms ammunition.
• 1.5 — Very insensitive substances with a mass explosion hazard; blasting agents.
• 1.6 — Extremely insensitive articles with no mass explosion hazard.

Divisions 1.1 and 1.2 are the most strictly regulated and trigger extra rules such as a written route plan.

Class 2 — Gases (Divisions 2.1, 2.2, 2.3)

• 2.1 — Flammable gas: propane, hydrogen, acetylene.
• 2.2 — Non-flammable, non-poisonous gas: oxygen, nitrogen, carbon dioxide.
• 2.3 — Poison (toxic) gas: chlorine, anhydrous ammonia. A 2.3 gas is also a Poison Inhalation Hazard (PIH) material.

Class 4 — Flammable Solids (Divisions 4.1, 4.2, 4.3)

• 4.1 — Flammable solid: matches, lithium metal.
• 4.2 — Spontaneously combustible: ignites on contact with air.
• 4.3 — Dangerous when wet: gives off a flammable gas when it touches water; carries a flame symbol on a blue placard.

Class 5 — Oxidizers and Organic Peroxides

• 5.1 — Oxidizer: supplies oxygen and makes other materials burn faster or more violently; ammonium nitrate.
• 5.2 — Organic peroxide: thermally unstable and can self-accelerate into a fire or explosion when exposed to heat, friction, or contamination.

Class 6 — Poisons and Infectious Substances

• 6.1 — Toxic (poisonous): harmful or fatal if swallowed, inhaled, or absorbed; pesticides, cyanide. Some 6.1 liquids are also Poison Inhalation Hazard materials.
• 6.2 — Infectious substances: contain pathogens, such as medical and biomedical waste.

Packing Groups: How Dangerous Is It?

Most materials in Classes 3, 4, 5, 6, and 8 are assigned a Packing Group (PG). The Packing Group rates the degree of danger within a class and helps decide how strong the packaging must be.

Not every class uses Packing Groups. Class 1 (explosives), Class 2 (gases), and Class 7 (radioactive) do not have Packing Groups — their danger is described by their division or by other measures instead.

Identification Numbers: UN and NA

Every material also has a four-digit identification number that pins down the exact substance. Gasoline, for example, is UN1203.

• UN stands for United Nations — an internationally recognized number used worldwide.
• NA stands for North America — used for materials recognized in U.S. and Canadian transport but not the international UN system.

This number appears on shipping papers, on package markings, and often on the placard or an orange panel. It is the number an emergency responder looks up in the Emergency Response Guidebook (ERG) to find the right response. Do not confuse the four-digit identification number with the hazard class number (1–9) or with a chemical's CAS number — the test will offer those as wrong answers.