The U.S. Department of Transportation sorts dangerous goods into nine classes of hazardous materials, each defined by the primary risk the material presents in transport. The classes run from explosives at Class 1 to a miscellaneous category at Class 9, and every class triggers its own packaging, labeling, and handling rules. The framework comes from the UN Recommendations on the Transport of Dangerous Goods, Model Regulations, which governments use to harmonize rules across every mode of transport.1United Nations Economic Commission for Europe. Recommendations on the Transport of Dangerous Goods Model Regulations Volume I In the United States, the Pipeline and Hazardous Materials Safety Administration (PHMSA) enforces the rules under 49 CFR Parts 100–185.2Pipeline and Hazardous Materials Safety Administration. Hazardous Materials Regulations
Packing Groups
Most hazardous materials also receive a packing group indicating how dangerous they are within their class. Packing Group I means great danger, Packing Group II means medium danger, and Packing Group III means minor danger.3Federal Aviation Administration. Packaging Your Dangerous Goods The group determines how robust the packaging has to be. Not every class uses packing groups. Explosives (Class 1), gases (Class 2), radioactive materials (Class 7), and some Class 9 items follow their own packaging schemes instead.
Class 1: Explosives
Class 1 covers any substance or device that can detonate or deflagrate through a chemical reaction, releasing gas, heat, and pressure suddenly. Common examples include ammunition, fireworks, blasting caps, and TNT.
The class splits into six divisions based on how an explosion behaves:
- Division 1.1: materials with a mass explosion hazard, where virtually the entire quantity detonates at once.
- Division 1.2: materials that throw fragments but do not produce a mass explosion.
- Division 1.3: materials that create a fire hazard with minor blast or fragment effects, such as certain propellants.
- Division 1.4: materials with only a minor explosion risk, where any effects stay largely confined to the package.
- Division 1.5: very insensitive substances that carry a mass explosion hazard but are extremely unlikely to detonate under normal transport conditions.
- Division 1.6: extremely insensitive articles with no mass explosion hazard.
The division matters for transport. A Division 1.1 shipment triggers mandatory placarding and routing restrictions that do not apply to a Division 1.4 consumer fireworks shipment.
Class 2: Gases
Class 2 includes any material that is entirely gaseous at 20°C and standard atmospheric pressure, or is transported under pressure in a compressed, liquefied, or dissolved state. Everyday examples include propane, oxygen, helium, and aerosol cans. Three divisions separate gases by their primary hazard:
- Division 2.1, flammable gases: gases that ignite readily in air, like propane and acetylene.
- Division 2.2, non-flammable and non-toxic gases: gases transported under pressure that are neither flammable nor poisonous, like nitrogen, helium, and carbon dioxide.
- Division 2.3, toxic gases: gases poisonous or corrosive enough to pose a health hazard, such as chlorine and ammonia.
Class 3: Flammable Liquids
A flammable liquid is any liquid with a flash point no higher than 60°C (140°F).4eCFR. 49 CFR 173.120 – Class 3 Definitions The flash point is the lowest temperature at which a liquid gives off enough vapor to ignite when an ignition source is present. Below that threshold, the vapor concentration is too low to catch fire. The regulation also covers liquids with higher flash points that are intentionally heated and shipped above their flash point in bulk packaging.
Gasoline, acetone, many paints, and alcohols all fall into Class 3. Because these liquids produce ignitable vapors at relatively low temperatures, even a small spill in a warm environment creates an immediate fire risk. Class 3 has no formal subdivisions, but materials are assigned to Packing Groups I, II, or III based on flash point and boiling point.
Class 4: Flammable Solids, Spontaneously Combustible, and Dangerous When Wet
Class 4 gathers three related fire hazards, each in its own division:
- Division 4.1, flammable solids: solids that ignite easily through friction or brief contact with a flame and keep burning. Matches and nitrocellulose are typical.
- Division 4.2, spontaneously combustible: materials that can heat up and ignite on their own without an external spark. White phosphorus is the textbook case; expose it to air and it catches fire.
- Division 4.3, dangerous when wet: materials that react with water to produce flammable gas. Alkali metals like sodium and lithium, along with calcium carbide, belong here. A water-based fire suppression response to a Division 4.3 spill would make things dramatically worse.
Class 5: Oxidizers and Organic Peroxides
Class 5 materials are not necessarily flammable on their own, but they supply the oxygen that makes other materials burn faster or more intensely.
- Division 5.1, oxidizing substances: materials that yield oxygen and intensify fires in surrounding combustibles. Ammonium nitrate fertilizers and hydrogen peroxide are common examples. The fertilizer connection is why storage regulations for these substances are especially strict.
- Division 5.2, organic peroxides: thermally unstable compounds that can decompose explosively or burn rapidly. They often require temperature-controlled transport because heat accelerates decomposition.
Class 6: Toxic and Infectious Substances
Class 6 addresses materials harmful to human health through two divisions:
- Division 6.1, toxic substances: materials capable of causing death or serious injury if swallowed, inhaled, or absorbed through the skin. Pesticides and cyanide compounds are among the more familiar entries. Toxicity ratings are based on lethal concentration and lethal dose data.
- Division 6.2, infectious substances: materials known or reasonably expected to contain pathogens that cause disease in humans or animals. This includes clinical waste, diagnostic specimens, and cultures of disease-causing organisms. Division 6.2 has its own dedicated packaging standards, often triple-containment systems, because the hazard is biological rather than chemical.
Class 7: Radioactive Material
Radioactive materials contain atoms that emit ionizing radiation as they decay. Medical isotopes used in cancer treatment, depleted uranium products, and certain industrial gauges all fall into this class. Prolonged or intense exposure can damage DNA and cause serious illness, which is why shielding requirements dominate Class 7 packaging rules.
Class 7 has no formal subdivisions. The regulations instead use a transport index and activity levels to determine labeling and vehicle placarding. A low-activity shipment of medical isotopes gets a different label category than a high-activity industrial source, even though both are Class 7.
Class 8: Corrosives
Corrosive materials destroy living tissue on contact and can eat through metals like steel and aluminum. Strong acids such as hydrochloric acid and sulfuric acid belong here, along with strong bases like sodium hydroxide (lye). The defining test under DOT rules is whether the substance causes full-thickness destruction of skin.
Corrosives span the pH scale. Highly acidic substances at very low pH and highly alkaline substances at very high pH both qualify. Because corrosives attack packaging material itself, container compatibility is a bigger concern here than in most other classes. A corrosive shipped in the wrong container can breach its own packaging from the inside.
Class 9: Miscellaneous Dangerous Goods
Class 9 is the catch-all for anything that poses a transport hazard but does not fit neatly into Classes 1 through 8. Lithium batteries are probably the most commonly shipped Class 9 item today. Other examples include dry ice (solid carbon dioxide, which displaces oxygen in enclosed spaces), asbestos, airbag modules, and environmentally hazardous substances.
Lithium-ion batteries present fire and thermal runaway risks, and beginning January 1, 2026, international air transport rules require certain lithium-ion battery shipments to be offered at a state of charge no higher than 30% of rated capacity. The restriction applies to batteries packed with equipment that exceed 2.7 watt-hours per cell, among other categories. Domestic ground shipment rules are less restrictive, but lithium battery packages still require the Class 9 lithium battery handling mark.
Limited Quantity Exception
Small shipments of certain hazardous materials across multiple classes can qualify for reduced regulatory requirements under the limited quantity provisions. A qualifying package gets marked with a black-bordered, square-on-point diamond, white in the center and black on the top and bottom portions, measuring at least 100 mm per side; packages too small for that can use a 50 mm version.5eCFR. 49 CFR 172.315 – Limited Quantities Packages displaying this mark are exempt from certain other marking requirements, though hazardous substances and hazardous waste still need their full markings. For cargo transport units being loaded onto vessels, the limited quantity mark must appear on each side and each end at a minimum size of 250 mm per side.
Penalties for Getting Classification Wrong
Misclassifying a hazardous material, or shipping it without the right labels and paperwork, carries steep financial penalties. As of December 30, 2024, the maximum civil penalty is $272,926 per violation per day the violation continues, up to $2,729,245 for a related series of violations.6Pipeline and Hazardous Materials Safety Administration. Civil Penalty Summary The amounts are adjusted periodically for inflation. Criminal penalties, including imprisonment, apply when violations are willful or involve knowing misrepresentation on shipping documents. Most enforcement actions do not hit the statutory maximum, but the per-day-per-violation structure means a single overlooked problem that persists for weeks can compound into a six-figure fine.