4.3 Hazard Identification Numbers (Kemler Codes) and UN Numbers

4.3 Hazard Identification Numbers (Kemler Codes) and UN Numbers

Quick Answer: Numbered orange plates provide instantaneous chemical hazard data for road tankers, bulk carriers, and containerized bulk units under ADR 5.3.2. A 15 mm horizontal dividing line separates the plate: the upper half displays the Hazard Identification Number (HIN / Kemler code, 2 or 3 figures, optionally preceded by 'X'), while the lower half displays the 4-digit UN substance identification number. Digits 2 through 9 define specific hazards, doubled digits denote intensified risks (e.g., 33, 66, 88), and the 'X' prefix warns of dangerous water reaction: water use requires expert approval.

Plate Anatomy and Layout (ADR 5.3.2.2.2)

Numbered orange plates provide emergency responders with immediate, actionable data regarding the nature of a chemical threat:

• Dividing Line: A standard 400 mm by 300 mm plate is divided across the center by a horizontal black line 15 mm thick.
• Upper Half — Hazard Identification Number (HIN): Commonly called the Kemler Code, this box contains two or three black digits (100 mm high with a 15 mm stroke width), optionally preceded by the letter X. It defines the type and severity of the chemical hazard.
• Lower Half — UN Number: Contains the 4-digit United Nations substance identification number (e.g., 1202 for gas oil/diesel, 1203 for petrol/gasoline, 1830 for sulfuric acid, 1075 for petroleum gases).

Fundamental Meaning of Kemler Digits

The figures generally indicate the following hazards. The code is not constructed by mechanically copying a class number; use the assigned HIN in Table A column (20), then its exact meaning in 5.3.2.3.2:

When the primary hazard of a substance has no secondary risks and does not warrant intensification, a zero is appended as the second digit (for example, 30 indicates a listed flammable/self-heating liquid or hot flammable-material case, 50 an oxidizing substance, 60 a toxic substance, and 80 a corrosive substance).

Special Syntax Rules: Digit Doubling, 'X' Prefix, and Violent Reactions

The Kemler code syntax follows three critical formatting rules tested heavily on ADR examinations:

1. Doubling of Digits: Intensification of Hazard

When a digit is repeated twice in succession, it signifies an intensification of that specific hazard:

• 33: Highly flammable liquid (flash point below 23°C, e.g., gasoline, acetone, ethanol).
• 66: Highly toxic substance (use the actual entry; a substance with additional hazards may require further figures).
• 88: Highly corrosive substance (use the actual entry and concentration; do not infer 88 merely because an acid is strong).
• 22: A special listed combination meaning refrigerated liquefied gas, asphyxiant (e.g., liquid nitrogen UN 1977, liquid argon UN 1951).
• 55: Strongly oxidising substance. 559: Strongly oxidising substance which can spontaneously lead to violent reaction. Do not omit the reaction hazard when the third figure is present.

2. The Prefix Letter 'X': Dangerous Reaction with Water

When the Hazard Identification Number is preceded by the capital letter X, the substance reacts dangerously with water:

• Meaning: The substance reacts dangerously with water. ADR permits water use only with expert approval.
• Driver action: Report the full HIN and UN number, keep away and do not attempt cargo firefighting or improvised cleanup. The prefix does not specify that ABC powder, carbon dioxide or sand will be compatible.
• Examples of meanings: X333 means a pyrophoric liquid reacting dangerously with water; X423 concerns a solid reacting dangerously with water and emitting flammable gases; X88 is highly corrosive and reacts dangerously with water. The actual substance’s assigned code must be read from Table A, not guessed from its name.

3. Digit '9' at the End: Spontaneous Violent Reaction

When the figure 9 appears as the final digit (e.g., 339, 669, 889), it alerts responders that the substance is prone to a spontaneous violent reaction. This includes sudden polymerization, self-accelerating thermal decomposition, or explosive decomposition under confinement when heated.

Selected HIN Meanings Worth Comparing

The complete list in 5.3.2.3.2 is authoritative. This table is a reading exercise, not a substitute catalogue of all goods and every concentration.

For familiar assignments, petrol UN 1203 uses 33 and methanol UN 1230 uses 336. Diesel UN 1202 uses 30 even though diesel may fall under the explicit flash-point exception to Class 3. A product’s concentration, state or assigned entry can change the code; avoid memorising an unverified commodity/HIN pair.

A document mismatch scenario

A driver is given a tank loading instruction for methanol, while a side plate still shows the previous product’s UN number. It is unsafe to “correct” only the top HIN by looking at a hazard label. Stop the release process and have the responsible operator reconcile the substance, compartment, transport document, required placards and both plate fields. Correct numbers must describe the actual contents or uncleaned residues. Two four-digit numbers that look similar can identify completely different hazards.

Rapid Exam Decoding Strategy for Drivers

To decode any Kemler code instantly under exam conditions, apply this systematic 5-step sequence:

1. Check for 'X': If an X precedes the numbers, water use requires expert approval; the driver must not improvise a response.
2. Identify Primary Risk (First Digit): Use the figures as a prompt to check the exact listed hazard meaning (e.g., 2 = Gas, 3 = Flammable liquid, 8 = Corrosive).
3. Evaluate Second Digit:
   • If identical to the first (e.g., 33, 66, 88): Note hazard intensification (highly flammable, highly toxic, highly corrosive).
   • If 0: The first figure adequately indicates the hazard; do not infer that the substance is harmless.
   • If different (e.g., 3 in 23, 6 in 336, 8 in 338): Note the secondary hazard (flammable, toxic, corrosive).
4. Evaluate Third Digit (if present): Note tertiary hazards, further intensification, or spontaneous violent reaction (9).
5. Read UN Number: Consult Table A or the transport document to match the 4-digit code in the lower half with the chemical substance name.

Source check: 17 October 2026 — ADR 2025, English text published by the Slovak Ministry of Transport, 5.3.2.3.