The Complete Guide to UN Numbers and Hazard Classes

Every dangerous good in international commerce has a four-digit UN number, a proper shipping name, a hazard class, and (usually) a packing group. Together they uniquely identify the substance for regulators, carriers and emergency responders. Understanding how the system fits together is the foundation of every other compliance decision you will make.

Where UN numbers come from

UN numbers are assigned by the UN Committee of Experts on the Transport of Dangerous Goods, which publishes the UN Recommendations on the Transport of Dangerous Goods — Model Regulations (the "Orange Book") every two years. Every modal regulation in the world derives from this single source: IATA DGR for air, IMDG Code for sea, ADR for European road, RID for European rail, and 49 CFR for US domestic transport.

The four-digit numbers run from UN0004 (ammonium picrate) to roughly UN3552 in the current edition. New numbers are added each biennium — sodium-ion batteries received UN3551 / UN3552 only recently.

The 9 hazard classes

1. Class 1 — Explosives. Subdivisions 1.1 through 1.6 by hazard type, plus a compatibility group letter (A–S). Includes fireworks, ammunition, blasting caps, airbag inflators (UN0503).
2. Class 2 — Gases. Three divisions: 2.1 flammable (UN1950 aerosols, UN1075 LPG), 2.2 non-flammable / non-toxic (UN1066 nitrogen, UN1006 argon), 2.3 toxic (UN1017 chlorine).
3. Class 3 — Flammable liquids. Petrol (UN1203), ethanol (UN1170), paint (UN1263), perfumes. Packing group I, II or III by flash point and initial boiling point.
4. Class 4 — Flammable solids. 4.1 flammable solids (UN1944 matches), 4.2 spontaneously combustible (UN1362 activated carbon), 4.3 dangerous when wet (UN1428 sodium).
5. Class 5 — Oxidizers and organic peroxides. 5.1 oxidizers (UN1942 ammonium nitrate), 5.2 organic peroxides (UN3101).
6. Class 6 — Toxic and infectious substances. 6.1 toxic (UN1654 nicotine, UN3077 environmentally hazardous), 6.2 infectious (UN2814 Category A, UN3373 Category B biological).
7. Class 7 — Radioactive material. Excepted packages through Type C, with transport indices and criticality safety indices.
8. Class 8 — Corrosives. UN1789 hydrochloric acid, UN1830 sulphuric acid, UN2794 wet batteries.
9. Class 9 — Miscellaneous. The catch-all. Lithium batteries (UN3480/3481/3090/3091), dry ice (UN1845), magnetised material (UN2807), environmentally hazardous substances (UN3082).

Packing groups

Within most classes, the substance is assigned a Packing Group that reflects its degree of danger:

• PG I — high danger
• PG II — medium danger
• PG III — low danger

Class 1, 2, 5.2, 6.2 and 7 do not use packing groups; they have their own subdivision systems instead. Lithium batteries are formally PG II for documentation purposes even though Class 9 substances are otherwise unassigned.

How to use the Dangerous Goods List

The DGL (List of Dangerous Goods) is the master table. It is sorted alphabetically by proper shipping name and also exists in numeric order. Each row gives you:

• UN number
• Proper shipping name
• Class / Division
• Subsidiary risk(s)
• Packing group
• ERG code
• Special provisions (numeric codes)
• Excepted quantity code
• Limited quantity allowance
• Passenger / cargo aircraft packing instruction and quantity
• Cargo-only packing instruction and quantity

The right answer to "how do I ship X" almost always begins by reading a single row of the DGL carefully. The most common error is grabbing the wrong UN number for a substance that has multiple entries depending on concentration, packing group or subsidiary risk.

Subsidiary risks

A substance can have a primary class and one or more subsidiary risks. Hydrogen peroxide solutions, for example, are Class 5.1 (oxidiser) with a subsidiary risk of 8 (corrosive). Both labels go on the package; segregation rules consider all risks.

Special provisions

Special provisions (SP) are numeric codes appended to a DGL entry that modify the general rules for that substance. Examples:

• SP 188 — small lithium battery exception
• SP A1 — forbidden on passenger aircraft
• SP 274 — technical name required after PSN
• SP 230 — cells must comply with UN 38.3 testing

Special provisions are easy to overlook because they hide in a single column of the DGL, but they routinely change the answer to "is this shippable and how".

Putting it together

The compliance flow for any substance:

1. Identify the substance / mixture / article.
2. Find its UN number and proper shipping name in the DGL.
3. Read the class, subsidiary risks and packing group.
4. Check every special provision listed in that row.
5. Apply the correct packing instruction for the mode.
6. Mark, label, document.

The whole rest of the regulation is detail on top of these six steps. For modal specifics keep reading: IMDG quick reference, 49 CFR checklist, and ADR for beginners.