CDL Hazmat classes and definitions study guide
The Hazardous Materials Regulations (49 CFR Parts 171 to 180) use defined terms throughout, and many duties depend on them: whether a packaging is bulk or non-bulk, whether a tank is a cargo tank, or whether a person is an offeror, a carrier or a hazmat employee. This guide sets out what the rule text states in the 173.2 index of hazard classes and divisions, which points to the section that defines each class, and in the 171.8 definitions most relevant to drivers and motor carriers. The detailed classification criteria in the sections 173.2 lists are not covered. A hazardous materials (H) endorsement also requires a TSA security threat assessment and a State knowledge test, which this guide does not cover. An amendment to 171.8 was published at 91 FR 49310 on Aug. 4, 2026; this guide follows the eCFR text as of Nov 4, 2026, so check the current text. Your State may have its own requirements, so check them too. Every point links to the regulation and quotes its exact words.
Hazard classes and divisions
The hazard class of a hazardous material is shown by either its class or division number or its class name.
“The hazard class of a hazardous material is indicated by either the class or division number or the class name.”
49 CFR 173.2, Hazardous material classes and index to hazard class definitions
The table in 173.2 lists class numbers, division numbers, class or division names and the sections that contain the definitions for classifying hazardous materials, including forbidden materials.
“The following table lists class numbers, division numbers, class or division names and those sections of this subchapter which contain definitions for classifying hazardous materials, including forbidden materials.”
49 CFR 173.2, Hazardous material classes and index to hazard class definitions
A hazard class is the category of hazard assigned to a hazardous material under the definitional criteria of part 173 and the 172.101 table. A material may meet the criteria for more than one hazard class but is assigned to only one.
“Hazard class means the category of hazard assigned to a hazardous material under the definitional criteria of part 173 of this subchapter and the provisions of the § 172.101 table. A material may meet the defining criteria for more than one hazard class but is assigned to only one hazard class.”
A division is a subdivision of a hazard class.
“Division means a subdivision of a hazard class.”
The primary hazard is the hazard class of a material as assigned in the 172.101 table.
“Primary hazard means the hazard class of a material as assigned in the § 172.101 table.”
A subsidiary hazard is a hazard of a material other than the primary hazard.
“Subsidiary hazard means a hazard of a material other than the primary hazard.”
A packing group groups hazardous materials by degree of danger: Packing Group I indicates great danger, Packing Group II medium danger and Packing Group III minor danger.
“Packing group means a grouping according to the degree of danger presented by hazardous materials. Packing Group I indicates great danger; Packing Group II, medium danger; Packing Group III, minor danger.”
Where each class is defined
In the 173.2 table, forbidden materials have no class number, and their definitions are in 173.21.
“None FORBIDDEN MATERIALS 173.21”
49 CFR 173.2, Hazardous material classes and index to hazard class definitions
For the definition of an explosive, 171.8 points to 173.50.
“Explosive. See § 173.50 of this subchapter.”
In the 173.2 table, Class 3 is named flammable and combustible liquid, with its definitions in 173.120.
“3 FLAMMABLE AND COMBUSTIBLE LIQUID 173.120”
49 CFR 173.2, Hazardous material classes and index to hazard class definitions
For the definition of an oxidizer, 171.8 points to 173.127.
“Oxidizer. See § 173.127 of this subchapter.”
For the definition of an organic peroxide, 171.8 points to 173.128.
“Organic peroxide. See § 173.128 of this subchapter.”
For the definition of an infectious substance (etiologic agent), 171.8 points to 173.134.
“Infectious substance (etiologic agent). See § 173.134 of this subchapter.”
For definitions relating to radioactive materials, 171.8 points to 173.403.
“Radioactive materials. See § 173.403 of this subchapter for definitions relating to radioactive materials.”
For the definition of a miscellaneous hazardous material, 171.8 points to 173.140.
“Miscellaneous hazardous material. See § 173.140 of this subchapter.”
Not covered here
The classification criteria themselves (flash points, toxicity values, test methods and so on) are in the sections listed and are not covered here. The two sections word the Class 5 cross-references differently: the 171.8 entry reads "Class 5. See § 173.128 of this subchapter." while the 173.2 table lists 5.1 OXIDIZER at 173.127 and 5.2 ORGANIC PEROXIDE at 173.128; this guide quotes each text as written and does not resolve the difference. We only write what an official source states, so study these parts with your certifying program's materials.
What counts as a hazardous material
A hazardous material is a substance or material that the Secretary of Transportation has determined is capable of posing an unreasonable risk to health, safety, and property when transported in commerce, and has designated as hazardous under 49 U.S.C. 5103.
“Hazardous material means a substance or material that the Secretary of Transportation has determined is capable of posing an unreasonable risk to health, safety, and property when transported in commerce, and has designated as hazardous under section 5103 of Federal hazardous materials transportation law (49 U.S.C. 5103).”
The term hazardous material includes hazardous substances, hazardous wastes, marine pollutants, elevated temperature materials, materials designated as hazardous in the Hazardous Materials Table, and materials that meet the defining criteria for hazard classes and divisions in part 173.
“The term includes hazardous substances, hazardous wastes, marine pollutants, elevated temperature materials, materials designated as hazardous in the Hazardous Materials Table (see 49 CFR 172.101), and materials that meet the defining criteria for hazard classes and divisions in part 173 of this subchapter.”
A hazardous substance is a material, including its mixtures and solutions, that is listed in appendix A to 172.101 and is in a quantity, in one package, that equals or exceeds the reportable quantity (RQ) listed there; paragraph (3) of the definition adds a further condition when it is in a mixture or solution.
“Hazardous substance for the purposes of this subchapter, means a material, including its mixtures and solutions, that— (1) Is listed in the appendix A to § 172.101 of this subchapter; (2) Is in a quantity, in one package, which equals or exceeds the reportable quantity (RQ) listed in the appendix A to § 172.101 of this subchapter; and (3) When in a mixture or solution—”
The term hazardous substance does not include petroleum, including crude oil or any fraction of it, that is not otherwise specifically listed or designated as a hazardous substance in appendix A to 172.101. It also does not include natural gas, natural gas liquids, liquefied natural gas, or synthetic gas usable for fuel (or mixtures of natural gas and such synthetic gas).
“The term does not include petroleum, including crude oil or any fraction thereof which is not otherwise specifically listed or designated as a hazardous substance in appendix A to § 172.101 of this subchapter, and the term does not include natural gas, natural gas liquids, liquefied natural gas, or synthetic gas usable for fuel (or mixtures of natural gas and such synthetic gas).”
A hazardous waste, for the purposes of this chapter, is any material that is subject to the U.S. EPA Hazardous Waste Manifest Requirements in 40 CFR part 262.
“Hazardous waste, for the purposes of this chapter, means any material that is subject to the Hazardous Waste Manifest Requirements of the U.S. Environmental Protection Agency specified in 40 CFR part 262.”
A marine pollutant is a material listed in appendix B to 172.101 that, when in a solution or mixture of one or more marine pollutants, is packaged in a concentration that equals or exceeds one of two thresholds: 10 percent by weight of the solution or mixture for materials listed in the appendix, or 1 percent by weight of the solution or mixture for materials identified there as severe marine pollutants.
“Marine pollutant, means a material which is listed in appendix B to § 172.101 of this subchapter (also see § 171.4) and, when in a solution or mixture of one or more marine pollutants, is packaged in a concentration which equals or exceeds: (1) Ten percent by weight of the solution or mixture for materials listed in the appendix; or (2) One percent by weight of the solution or mixture for materials that are identified as severe marine pollutants in the appendix.”
An elevated temperature material is a material that, when offered for transportation or transported in a bulk packaging, meets one of the following: it is a liquid at or above 100 °C (212 °F); it is a liquid with a flash point at or above 38 °C (100 °F) that is intentionally heated and offered or transported at or above its flash point; or it is a solid at or above 240 °C (464 °F).
“Elevated temperature material means a material which, when offered for transportation or transported in a bulk packaging: (1) Is in a liquid phase and at a temperature at or above 100 °C (212 °F); (2) Is in a liquid phase with a flash point at or above 38 °C (100 °F) that is intentionally heated and offered for transportation or transported at or above its flash point; or (3) Is in a solid phase and at a temperature at or above 240 °C (464 °F).”
Limited quantity, when specified as such in a section that applies to a particular material, means the maximum amount of a hazardous material for which there is a specific labeling or packaging exception.
“Limited quantity, when specified as such in a section applicable to a particular material, means the maximum amount of a hazardous material for which there is a specific labeling or packaging exception.”
A material of trade is a hazardous material, other than a hazardous waste, carried on a motor vehicle for one of these purposes: protecting the health and safety of the operator or passengers; supporting the operation or maintenance of a motor vehicle; or, by a private motor carrier, in direct support of a principal business that is other than transportation by motor vehicle.
“Material of trade means a hazardous material, other than a hazardous waste, that is carried on a motor vehicle— (1) For the purpose of protecting the health and safety of the motor vehicle operator or passengers; (2) For the purpose of supporting the operation or maintenance of a motor vehicle (including its auxiliary equipment); or (3) By a private motor carrier (including vehicles operated by a rail carrier) in direct support of a principal business that is other than transportation by motor vehicle.”
Not covered here
The RQ concentration table for mixtures and solutions and the lists in appendices A and B to 172.101 are not covered. We only write what an official source states, so study these parts with your certifying program's materials.
Bulk and non-bulk packaging
A bulk packaging is a packaging, other than a vessel or a barge, including a transport vehicle or freight container, in which hazardous materials are loaded with no intermediate form of containment.
“Bulk packaging means a packaging, other than a vessel or a barge, including a transport vehicle or freight container, in which hazardous materials are loaded with no intermediate form of containment.”
A Large Packaging in which hazardous materials are loaded with an intermediate form of containment, such as articles or inner packagings, is also a bulk packaging.
“A Large Packaging in which hazardous materials are loaded with an intermediate form of containment, such as one or more articles or inner packagings, is also a bulk packaging.”
A bulk packaging also has one of the following: a maximum capacity greater than 450 L (119 gallons) as a receptacle for a liquid; a maximum net mass greater than 400 kg (882 pounds) and a maximum capacity greater than 450 L (119 gallons) as a receptacle for a solid; or a water capacity greater than 454 kg (1000 pounds) as a receptacle for a gas.
“Additionally, a bulk packaging has: (1) A maximum capacity greater than 450 L (119 gallons) as a receptacle for a liquid; (2) A maximum net mass greater than 400 kg (882 pounds) and a maximum capacity greater than 450 L (119 gallons) as a receptacle for a solid; or (3) A water capacity greater than 454 kg (1000 pounds) as a receptacle for a gas as defined in § 173.115 of this subchapter.”
A non-bulk packaging has one of the following: a maximum capacity of 450 L (119 gallons) or less as a receptacle for a liquid; a maximum net mass of 400 kg (882 pounds) or less and a maximum capacity of 450 L (119 gallons) or less as a receptacle for a solid; or a water capacity of 454 kg (1000 pounds) or less as a receptacle for a gas. A fourth option, for certain bags and boxes, is in the next point.
“Non-bulk packaging means a packaging which has: (1) A maximum capacity of 450 L (119 gallons) or less as a receptacle for a liquid; (2) A maximum net mass of 400 kg (882 pounds) or less and a maximum capacity of 450 L (119 gallons) or less as a receptacle for a solid; (3) A water capacity of 454 kg (1000 pounds) or less as a receptacle for a gas as defined in § 173.115 of this subchapter; or”
Regardless of the definition of bulk packaging, a bag or a box that conforms to the specification packaging requirements in subpart L of part 178, with a maximum net mass of 400 kg (882 pounds) or less, is also a non-bulk packaging.
“(4) Regardless of the definition of bulk packaging, a maximum net mass of 400 kg (882 pounds) or less for a bag or a box conforming to the applicable requirements for specification packagings, including the maximum net mass limitations, provided in subpart L of part 178 of this subchapter.”
A packaging is a receptacle and any other components or materials needed for the receptacle to perform its containment function in conformance with the minimum packing requirements of the HMR.
“Packaging means a receptacle and any other components or materials necessary for the receptacle to perform its containment function in conformance with the minimum packing requirements of this subchapter.”
Except as provided in subpart K of part 178, an overpack is an enclosure used by a single consignor to protect a package, make it easier to handle, or consolidate two or more packages. A transport vehicle, freight container, or aircraft unit load device is not an overpack.
“Overpack, except as provided in subpart K of part 178 of this subchapter, means an enclosure that is used by a single consignor to provide protection or convenience in handling of a package or to consolidate two or more packages. Overpack does not include a transport vehicle, freight container, or aircraft unit load device.”
Examples of overpacks are one or more packages placed or stacked onto a load board such as a pallet and secured by strapping, shrink wrapping, stretch wrapping, or other suitable means, or placed in a protective outer packaging such as a box or crate.
“Examples of overpacks are one or more packages: (1) Placed or stacked onto a load board such as a pallet and secured by strapping, shrink wrapping, stretch wrapping, or other suitable means; or (2) Placed in a protective outer packaging such as a box or crate.”
Vehicles, containers and tanks
A transport vehicle is a cargo-carrying vehicle such as an automobile, van, tractor, truck, semitrailer, tank car or rail car used to transport cargo by any mode. Each cargo-carrying body, such as each trailer, is a separate transport vehicle.
“Transport vehicle means a cargo-carrying vehicle such as an automobile, van, tractor, truck, semitrailer, tank car or rail car used for the transportation of cargo by any mode. Each cargo-carrying body (trailer, rail car, etc.) is a separate transport vehicle.”
A motor vehicle includes a vehicle, machine, tractor, trailer, or semitrailer, or any combination of them, propelled or drawn by mechanical power and used upon the highways to transport passengers or property. It does not include a vehicle, locomotive, or car operated exclusively on rails, or a trolley bus powered from a fixed overhead wire that furnishes local passenger transportation similar to street-railway service.
“Motor vehicle includes a vehicle, machine, tractor, trailer, or semitrailer, or any combination thereof, propelled or drawn by mechanical power and used upon the highways in the transportation of passengers or property. It does not include a vehicle, locomotive, or car operated exclusively on a rail or rails, or a trolley bus operated by electric power derived from a fixed overhead wire, furnishing local passenger transportation similar to street-railway service.”
A freight container is a reusable container with a volume of 64 cubic feet or more, designed to be lifted with its contents intact and intended mainly to contain packages (in unit form) during transportation.
“Freight container means a reusable container having a volume of 64 cubic feet or more, designed and constructed to permit being lifted with its contents intact and intended primarily for containment of packages (in unit form) during transportation.”
A cargo tank is a bulk packaging that meets all three conditions: it is a tank intended primarily for carrying liquids or gases; it is permanently attached to or forms part of a motor vehicle, or is not permanently attached but, because of its size, construction or attachment, is loaded or unloaded without being removed from the motor vehicle; and it is not made under a specification for cylinders, intermediate bulk containers, multi-unit tank car tanks, portable tanks, or tank cars.
“Cargo tank means a bulk packaging that: (1) Is a tank intended primarily for the carriage of liquids or gases and includes appurtenances, reinforcements, fittings, and closures (for the definition of a tank, see 49 CFR 178.320, 178.337-1, or 178.338-1, as applicable); (2) Is permanently attached to or forms a part of a motor vehicle, or is not permanently attached to a motor vehicle but which, by reason of its size, construction or attachment to a motor vehicle is loaded or unloaded without being removed from the motor vehicle; and (3) Is not fabricated under a specification for cylinders, intermediate bulk containers, multi-unit tank car tanks, portable tanks, or tank cars.”
A cargo tank motor vehicle is a motor vehicle with one or more cargo tanks permanently attached to or forming an integral part of it.
“Cargo tank motor vehicle means a motor vehicle with one or more cargo tanks permanently attached to or forming an integral part of the motor vehicle.”
A portable tank is a bulk packaging (except a cylinder with a water capacity of 1000 pounds or less) designed primarily to be loaded onto, or on, or temporarily attached to a transport vehicle or ship, with skids, mountings, or accessories for handling by mechanical means. It does not include a cargo tank, tank car, multi-unit tank car tank, or trailer carrying 3AX, 3AAX, or 3T cylinders.
“Portable tank means a bulk packaging (except a cylinder having a water capacity of 1000 pounds or less) designed primarily to be loaded onto, or on, or temporarily attached to a transport vehicle or ship and equipped with skids, mountings, or accessories to facilitate handling of the tank by mechanical means. It does not include a cargo tank, tank car, multi-unit tank car tank, or trailer carrying 3AX, 3AAX, or 3T cylinders.”
Offerors, carriers and hazmat employees
A carrier is a person who transports passengers or property in commerce by rail car, aircraft, motor vehicle, or vessel.
“Carrier means a person who transports passengers or property in commerce by rail car, aircraft, motor vehicle, or vessel.”
An offeror (person who offers) is any person who does either or both of the following: performs, or is responsible for performing, any pre-transportation function required under the HMR; or tenders or makes the hazardous material available to a carrier for transportation in commerce.
“Person who offers or offeror means: (1) Any person who does either or both of the following: (i) Performs, or is responsible for performing, any pre-transportation function required under this subchapter for transportation of the hazardous material in commerce. (ii) Tenders or makes the hazardous material available to a carrier for transportation in commerce.”
A carrier is not an offeror when it performs a function required as a condition of accepting a hazardous material (such as reviewing shipping papers, examining packages, or preparing shipping documentation for its own use), or when it transfers a hazardous material to another carrier for continued transportation without performing a pre-transportation function.
“(2) A carrier is not an offeror when it performs a function required by this subchapter as a condition of acceptance of a hazardous material for transportation in commerce (e.g., reviewing shipping papers, examining packages to ensure that they are in conformance with this subchapter, or preparing shipping documentation for its own use) or when it transfers a hazardous material to another carrier for continued transportation in commerce without performing a pre-transportation function.”
A hazmat employee includes a person employed full time, part time, or temporarily by a hazmat employer who directly affects hazardous materials transportation safety, and a self-employed person (including an owner-operator of a motor vehicle) transporting hazardous materials in commerce who directly affects that safety.
“Hazmat employee means: (1) A person who is: (i) Employed on a full-time, part time, or temporary basis by a hazmat employer and who in the course of such full time, part time or temporary employment directly affects hazardous materials transportation safety; (ii) Self-employed (including an owner-operator of a motor vehicle, vessel, or aircraft) transporting hazardous materials in commerce who in the course of such self-employment directly affects hazardous materials transportation safety;”
The term hazmat employee includes an individual, employed by a hazmat employer or self-employed, who during the course of employment loads, unloads, or handles hazardous materials, prepares them for transportation, is responsible for their safe transport, or operates a vehicle used to transport them.
“(2) This term includes an individual, employed on a full time, part time, or temporary basis by a hazmat employer, or who is self-employed, who during the course of employment: (i) Loads, unloads, or handles hazardous materials; (ii) Designs, manufactures, fabricates, inspects, marks, maintains, reconditions, repairs, or tests a package, container or packaging component that is represented, marked, certified, or sold as qualified for use in transporting hazardous material in commerce. (iii) Prepares hazardous materials for transportation; (iv) Is responsible for safety of transporting hazardous materials; (v) Operates a vehicle used to transport hazardous materials.”
A hazmat employer includes a person who employs or uses at least one hazmat employee on a full-time, part time, or temporary basis and who transports hazardous materials in commerce or causes them to be transported in commerce.
“Hazmat employer means: (1) A person who employs or uses at least one hazmat employee on a full-time, part time, or temporary basis; and who: (i) Transports hazardous materials in commerce; (ii) Causes hazardous materials to be transported in commerce; or”
A self-employed person, including an owner-operator of a motor vehicle, transporting materials in commerce can also be a hazmat employer when the person transports hazardous materials in commerce or causes them to be transported.
“(2) A person who is self-employed (including an owner-operator of a motor vehicle, vessel, or aircraft) transporting materials in commerce; and who: (i) Transports hazardous materials in commerce; (ii) Causes hazardous materials to be transported in commerce; or”
A federal department, agency, or instrumentality, or an authority of a State, political subdivision of a State, or an Indian tribe, can be a hazmat employer when it transports hazardous materials in commerce or causes them to be transported.
“(3) A department, agency, or instrumentality of the United States Government, or an authority of a State, political subdivision of a State, or an Indian tribe; and who: (i) Transports hazardous materials in commerce; (ii) Causes hazardous materials to be transported in commerce; or”
Transportation and related functions
Transportation (or transport) means the movement of property and loading, unloading, or storage incidental to that movement.
“Transportation or transport means the movement of property and loading, unloading, or storage incidental to that movement.”
A pre-transportation function is a function specified in the HMR that is required to assure the safe transportation of a hazardous material in commerce. The list includes determining the hazard class, selecting and filling packagings, marking and labeling packages, preparing a shipping paper, and providing and maintaining emergency response information.
“Pre-transportation function means a function specified in the HMR that is required to assure the safe transportation of a hazardous material in commerce, including— (1) Determining the hazard class of a hazardous material. (2) Selecting a hazardous materials packaging. (3) Filling a hazardous materials packaging, including a bulk packaging. (4) Securing a closure on a filled or partially filled hazardous materials package or container or on a package or container containing a residue of a hazardous material. (5) Marking a package to indicate that it contains a hazardous material. (6) Labeling a package to indicate that it contains a hazardous material. (7) Preparing a shipping paper. (8) Providing and maintaining emergency response information.”
Pre-transportation functions also include loading, blocking, and bracing a hazmat package in a freight container or transport vehicle; segregating it from incompatible cargo; and selecting, providing, or affixing placards for a freight container or transport vehicle.
“(12) Loading, blocking, and bracing a hazardous materials package in a freight container or transport vehicle. (13) Segregating a hazardous materials package in a freight container or transport vehicle from incompatible cargo. (14) Selecting, providing, or affixing placards for a freight container or transport vehicle to indicate that it contains a hazardous material.”
Loading incidental to movement means loading of packaged or containerized hazardous material onto a transport vehicle, aircraft, or vessel by carrier personnel or in the presence of carrier personnel, for the purpose of transporting it.
“Loading incidental to movement means loading by carrier personnel or in the presence of carrier personnel of packaged or containerized hazardous material onto a transport vehicle, aircraft, or vessel for the purpose of transporting it”
For a bulk packaging, loading incidental to movement means filling the packaging with a hazardous material for the purpose of transporting it. Loading incidental to movement includes transloading.
“For a bulk packaging, loading incidental to movement means filling the packaging with a hazardous material for the purpose of transporting it. Loading incidental to movement includes transloading.”
Transloading is the transfer of a hazardous material by any person from one bulk packaging to another, from a bulk packaging to a non-bulk packaging, or from a non-bulk packaging to a bulk packaging, to continue its movement in commerce.
“Transloading means the transfer of a hazardous material by any person from one bulk packaging to another bulk packaging, from a bulk packaging to a non-bulk packaging, or from a non-bulk packaging to a bulk packaging for the purpose of continuing the movement of the hazardous material in commerce.”
Storage incidental to movement includes storage at the destination shown on a shipping document, including at a transloading facility, provided the shipping documentation identifies the shipment as a through-shipment and identifies the final destination or destinations.
“(1) Storage incidental to movement includes— (i) Storage at the destination shown on a shipping document, including storage at a transloading facility, provided the shipping documentation identifies the shipment as a through-shipment and identifies the final destination or destinations of the hazardous material; and”
Storage incidental to movement does not include storage of a hazardous material at its final destination as shown on a shipping document.
“(2) Storage incidental to movement does not include storage of a hazardous material at its final destination as shown on a shipping document.”
Sources
- eCFR: 49 CFR 173.2, Hazardous material classes and index to hazard class definitions (as of Nov 4, 2026)Checked November 6, 2026
- eCFR: 49 CFR 171.8, Definitions and abbreviations (as of Nov 4, 2026)Checked November 6, 2026
Other CDL study guides: License standards, Driver qualifications, Driving rules, Hours of service, Inspection and maintenance, Brakes, Cargo securement, Drug and alcohol testing, Hazmat shipping papers and emergency information, Hazmat marking, Hazmat placarding, Hazmat highway carriage, loading and segregation, Hazmat driving, parking and training