DOT Hazard Classes 1–9

The full 49 CFR §173 taxonomy — Explosives, Gases, Flammable Liquids, Flammable Solids, Oxidizers & Peroxides, Toxic & Infectious, Radioactive, Corrosive, and Miscellaneous. Each card links to a full reference with definition, hazards, public safety response, packing groups, and common UN numbers.

Class 1
Explosives
A substance or article, including a device, designed to function by explosion (extremely rapid release of gas and heat), or which, by chemical reaction within itself, is able to function in a similar manner even if not designed to function by explosion.
45 UN numbers · 49 CFR §173.50
Class 2.1
Flammable Gas
A gas that at 20 °C (68 °F) and standard atmospheric pressure (101.3 kPa) is ignitable when in a mixture of 13% or less by volume with air, OR has a flammable range with air of at least 12 percentage points regardless of the lower flammable limit.
161 UN numbers · 49 CFR §173.115(a)
Class 2.2
Non-flammable, Non-toxic Gas
A non-flammable, non-toxic gas that is asphyxiant (dilutes or replaces oxygen in the atmosphere) or oxidizing (may generally cause or contribute to the combustion of other material more than air does).
72 UN numbers · 49 CFR §173.115(b)
Class 2.3
Toxic Gas
A gas that is known to be so toxic or corrosive to humans as to pose a hazard to health during transportation, OR in the absence of adequate human toxicity data has an LC50 value of not more than 5000 mL/m³.
207 UN numbers · 49 CFR §173.115(c)
Class 3
Flammable and Combustible Liquids
A liquid having a flash point of not more than 60 °C (140 °F), or any material in a liquid phase with a flash point at or above 37.8 °C (100 °F) that is intentionally heated and offered for transportation or transported at or above its flash point in a bulk packaging (elevated-temperature flammable).
593 UN numbers · 49 CFR §173.120
Class 4.1
Flammable Solid
A material, other than an explosive, that under conditions normally incident to transportation is liable to cause fire through friction, absorption of moisture, spontaneous chemical change, or retained heat from manufacturing/processing; or one that can be readily ignited and burns vigorously.
167 UN numbers · 49 CFR §173.124(a)
Class 4.2
Spontaneously Combustible
A pyrophoric material (liquid or solid) that even in small quantities and without external ignition sources can ignite within 5 minutes of contact with air, OR a self-heating material that when in contact with air and without external energy supply is liable to self-heat.
85 UN numbers · 49 CFR §173.124(b)
Class 4.3
Dangerous When Wet
A material that, by contact with water, is liable to become spontaneously flammable or to give off flammable or toxic gas at a rate greater than 1 liter per kilogram of the material per hour.
118 UN numbers · 49 CFR §173.124(c)
Class 5.1
Oxidizer
A material that may, generally by yielding oxygen, cause or enhance the combustion of other materials.
183 UN numbers · 49 CFR §173.127
Class 5.2
Organic Peroxide
An organic compound containing oxygen in the bivalent -O-O- structure that may be considered a derivative of hydrogen peroxide, where one or both of the hydrogen atoms have been replaced by organic radicals.
25 UN numbers · 49 CFR §173.128
Class 6.1
Toxic (Poison)
A material, other than a gas, that is known to be so toxic to humans as to afford a hazard during transportation, or that in the absence of adequate data on human toxicity is presumed to be toxic to humans based on animal test criteria (oral, dermal, or inhalation LD50/LC50).
797 UN numbers · 49 CFR §173.132
Class 6.2
Infectious Substance
A material known or reasonably expected to contain a pathogen — a microorganism (including bacteria, viruses, rickettsiae, parasites, and fungi) or other agent (such as a prion) that can cause disease in humans or animals.
9 UN numbers · 49 CFR §173.134
Class 7
Radioactive
Any material containing radionuclides where both the activity concentration and the total activity in the consignment exceed the values specified in 49 CFR §173.436 (or determined per §173.433).
31 UN numbers · 49 CFR §173.401 - §173.478
Class 8
Corrosive
A liquid or solid that causes full-thickness destruction of human skin at the site of contact within a specified period of time, OR a liquid that has a severe corrosion rate on steel or aluminum surfaces exceeding 6.25 mm/year at a test temperature of 55 °C (130 °F).
74 UN numbers · 49 CFR §173.136
Class 9
Miscellaneous
A material which presents a hazard during transportation but which does not meet the definition of any other hazard class (Classes 1-8).
200 UN numbers · 49 CFR §173.140