Every UN/DOT hazard class and subdivision, with the placard you'll see in the field, the materials each class covers, and common UN numbers you can look up instantly.
The United Nations system organizes hazardous materials into nine hazard classes, each describing the primary danger a substance presents during transport. Several classes are further divided into divisions — Class 2, for example, splits into flammable gas (2.1), non-flammable non-toxic gas (2.2), and toxic gas (2.3). The framework is used worldwide and is incorporated into U.S. law through 49 CFR Part 173, into air transport through the IATA Dangerous Goods Regulations, and into ocean shipments through the IMDG Code.
Every regulated material is assigned a four-digit UN number that ties it to a specific hazard class, an optional packing group, and an Emergency Response Guide number used by first responders. A single material can carry both a primary class and one or more subsidiary hazards — both must appear on placards and shipping papers.
The chart below covers each class with its placard, what it includes, and a handful of common UN-numbered examples. Click any UN number to see full details — packing group, ERG guide, proper shipping name, and shipping notes.
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Class 1
Explosives
Orange placard · 6 divisions · No packing groups
Materials capable of producing a sudden release of energy through chemical reaction — blast, projection, or heat. Covers commercial explosives, propellants, pyrotechnics, ammunition, and detonating devices. Class 1 uses compatibility groups (letters A through S) in addition to divisions, controlling which explosives can be stored or transported together.
Division
Description
1.1
Mass explosion hazard — the entire load can detonate at once.
1.2
Projection hazard but not mass explosion (e.g. some ammunition).
1.3
Fire hazard with minor blast or projection (e.g. propellants).
1.4
Minor explosion hazard, effects largely contained within package.
1.5
Very insensitive substances with mass explosion potential.
1.6
Extremely insensitive articles with no mass explosion potential.
Red / Green / White · 3 divisions · No packing groups
Materials transported at gauge pressure above 200 kPa or as a refrigerated liquid. Classification depends on whether the gas burns, suffocates by displacement, or actively poisons.
Division
Description & placard color
2.1
Flammable Gas (red) — ignites in air at ≤13% concentration or has a flammable range of ≥12%.
2.2
Non-Flammable, Non-Toxic Gas (green) — asphyxiants, oxidizing gases, compressed air.
2.3
Toxic Gas (white) — LC50 ≤5000 mL/m³. Acutely dangerous; small leaks can be fatal.
Liquids, liquid mixtures, or solids in solution that give off a flammable vapor at or below 60 °C (140 °F) closed-cup flash point. The most-shipped hazard class in the world — fuels, solvents, paints, adhesives, alcohols. Packing group is determined by flash point and initial boiling point.
Packing Group
Criteria
PG I
Initial boiling point ≤35 °C — extreme volatility.
Red/white striped · Red/white split · Blue · 3 divisions · PG I, II, III
Solid materials that ignite easily, ignite spontaneously, or release flammable gas on contact with water. Three very different hazards live under one class number — read the division, not just the "4."
Materials that release oxygen or otherwise support combustion of other materials. They don't necessarily burn themselves — they make other things burn faster, hotter, and harder to extinguish. Organic peroxides (5.2) are particularly temperature-sensitive and many require shipment under controlled temperature.
Division
Description
5.1
Oxidizer — yields oxygen, intensifies combustion of other materials. Yellow placard.
5.2
Organic Peroxide — thermally unstable, may decompose exothermically. Red over yellow placard.
Materials that cause death, serious injury, or harm to human health if swallowed, inhaled, or absorbed through skin (6.1), or that contain pathogens capable of infecting humans or animals (6.2).
Division
Description
6.1
Toxic Substance — uses Packing Groups I/II/III. White placard with skull and crossbones.
6.2
Infectious Substance — pathogens (Category A or B). No packing groups; separate package design requirements (e.g. P620, P650).
Yellow/white placard · 3 categories · No packing groups
Any material containing radionuclides with both activity concentration and total activity exceeding specified thresholds. Class 7 uses label categories (I-White, II-Yellow, III-Yellow) based on the surface and 1-meter radiation level, plus a Transport Index on labels II and III. Fissile materials carry an additional FISSILE label.
White over black placard · No subdivisions · PG I, II, III
Materials that cause full-thickness destruction of intact skin tissue or significant corrosion to steel or aluminum on contact. Includes strong acids and bases — sulfuric, hydrochloric, nitric, sodium hydroxide, ammonium hydroxide solutions. Many Class 8 materials also carry subsidiary hazards (Class 6.1 toxic is common).
Packing Group
Criteria (skin destruction time)
PG I
≤3 minutes exposure causes full-thickness destruction within 60 min.
PG II
3 min – 1 hour exposure causes full-thickness destruction within 14 days.
PG III
1–4 hour exposure causes destruction, or steel/aluminum corrosion >6.25 mm/year.
Black/white striped placard · No subdivisions · PG II, III where applicable
The catch-all class. Substances that present a hazard during transport but don't fit cleanly into Classes 1–8. Lithium batteries, dry ice, environmentally hazardous substances, elevated-temperature materials, and genetically modified organisms all live here.
Lithium batteries are the highest-volume Class 9 shipment. UN3480 (loose lithium-ion), UN3481 (lithium-ion in or with equipment), UN3090 (lithium metal), and UN3091 (lithium metal in or with equipment) each have distinct packaging and quantity rules under 49 CFR §173.185 and the IATA Lithium Battery Guidance Document. Misclassification is one of the most-fined HazMat violations.
Most hazard classes use Packing Groups to indicate the degree of danger within the class. The packing group determines the strength and specification of the packaging required, and in many cases the quantity limits per mode of transport.
PG I
Great danger
Most hazardous. Requires the highest packaging performance standards. Example: UN2814 Infectious Substance, UN1051 Hydrogen Cyanide.
Lowest hazard tier in the regulated range. Reduced packaging strength acceptable. Example: UN1170 Ethanol Solution <70%.
Classes that don't use packing groups: Class 1 (Explosives) uses divisions plus compatibility groups. Class 2 (Gases) uses divisions only. Class 6.2 (Infectious) uses Category A/B designations. Class 7 (Radioactive) uses label categories and Transport Index.
Multiple risks
Subsidiary hazards: when one class isn't enough
A material can present more than one type of danger. The primary class is the most significant hazard; any additional dangers are subsidiary hazards. Both must be shown on placards and shipping papers, both must be considered for segregation, and both can affect packing group assignment.
Example:UN1098 Allyl Alcohol is primarily Class 6.1 (Toxic, PG I) but is also flammable — it carries a subsidiary Class 3 hazard. A truck carrying it must placard both 6.1 and 3, and shipping papers must list both class numbers.
Subsidiary hazards are a common compliance trap. Inspectors specifically look for them, and shippers regularly get cited for missing the second placard or omitting the subsidiary class from the shipping paper.
Regulatory Context
Where these classes come from
The 9-class system originates with the UN Recommendations on the Transport of Dangerous Goods (the "Orange Book"), published by the UN Economic Commission for Europe and updated on a two-year cycle. National and modal regulations adopt the framework with local additions:
49 CFR — Ground transport in the United States
Title 49 of the Code of Federal Regulations (Parts 171–180) is administered by the Pipeline and Hazardous Materials Safety Administration (PHMSA). Class definitions are in Part 173, Subparts D and E. The Hazardous Materials Table (49 CFR §172.101) assigns proper shipping names, classes, packing groups, and ID numbers.
IATA DGR — Air transport
The IATA Dangerous Goods Regulations are updated annually and incorporate the ICAO Technical Instructions, which carry treaty force under the Chicago Convention. Air shipments enforce stricter quantity limits and packaging than ground for nearly every class — particularly for Class 2, Class 3, and Class 9 (lithium batteries).
IMDG Code — Marine transport
The International Maritime Dangerous Goods Code is mandatory under the SOLAS Convention for vessels carrying dangerous goods at sea. It adds marine-specific provisions: stowage and segregation requirements, marine pollutant designations, and limitations for passenger vessels.
FAQ
Common questions
How many DOT hazard classes are there?
Nine, plus subdivisions. The full list runs Class 1 (Explosives) through Class 9 (Miscellaneous), with Classes 2, 4, 5, and 6 each split into divisions. All 9 are defined in 49 CFR §173 and harmonized with the UN Recommendations on the Transport of Dangerous Goods.
What color is each hazard class placard?
Class 1 orange · Class 2.1 red · Class 2.2 green · Class 2.3 white · Class 3 red · Class 4.1 red/white stripes · Class 4.2 red-over-white · Class 4.3 blue · Class 5.1 yellow · Class 5.2 red-over-yellow · Class 6.1 white with skull · Class 6.2 white · Class 7 yellow-over-white · Class 8 white-over-black · Class 9 black/white stripes over white.
What is a subsidiary hazard?
A secondary danger a material presents beyond its primary classification. For example, UN1098 Allyl Alcohol is primarily Class 6.1 toxic but also flammable (subsidiary Class 3). Both must appear on placards and shipping documents.
Do all hazard classes use packing groups?
No. Classes 3, 4, 5, 6.1, 8, and some entries in Class 9 use Packing Groups I (high danger), II (medium), and III (low). Class 1, Class 2, Class 6.2, and Class 7 use different categorization systems — compatibility groups, divisions, package types, label categories.
Where are the hazard classes legally defined?
In the United States, in 49 CFR Part 173 (Subparts D and E), administered by PHMSA. Internationally, in the UN Recommendations on the Transport of Dangerous Goods, the IATA DGR for air, and the IMDG Code for sea.
UNLookup is a reference utility. The chart above summarizes how the 9-class system works but is not a substitute for current editions of 49 CFR, the IATA DGR, the IMDG Code, or the Emergency Response Guidebook. Always verify against the official source applicable to your mode of transport before tendering a shipment. Regulations change; this page is updated periodically and may not reflect the most recent amendments.