2.1 DOT Hazard Classes 1–9

Hazardous materials technicians are tested on classification before they are tested on meters. NFPA 470 Chapter 11.2 (Analyze) and OSHA 29 CFR 1910.120(q)(6)(iii)(B) both require you to classify and identify known and unknown materials. This chapter is the communication-system half of that job: DOT hazard classes, placards, labels, GHS pictograms, UN/NA numbers, NFPA 704, safety data sheets, and shipping papers. Field survey instruments belong in later chapters. If you misread the diamond on the tank, every later decision — PPE, isolation, and product control — starts from the wrong hazard.

PHMSA's Hazardous Materials Regulations in 49 CFR Part 173 organize dangerous goods into nine classes. The class, and where it exists the division, is what the number in the lower corner of a DOT diamond is telling you. Learn official class names, typical placard color, symbol, and the divisions that appear on technician exams. Then immediately learn the trap: color is a clue, not an identification.

The nine classes at a glance

Keep this table in working memory. Exams and street work both start here.

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Class 1 — Explosives (orange, bursting ball)

The orange diamond with a bursting-ball symbol is Class 1. The division, not just the class number, tells you how the energy comes off:

• 1.1 — Mass explosion hazard (the entire load can detonate as a unit)
• 1.2 — Projection hazard without a mass explosion (fragments, not one blast of the whole quantity)
• 1.3 — Predominantly a fire hazard, with minor blast or minor projection, not a mass explosion
• 1.4 — No significant blast hazard (many consumer fireworks and some small-arms ammunition)
• 1.5 — Very insensitive explosives with a mass explosion hazard (blasting agents)
• 1.6 — Extremely insensitive articles without a mass explosion hazard

Compatibility groups (letters) further restrict what may be loaded together. For identification at the scene, read the division on the placard. Isolation for 1.1 is not the isolation for 1.4. Orange is not a generic warning color; orange with a bursting ball is explosives. Divisions 1.1, 1.2, and 1.3 are Table 1 materials under 49 CFR 172.504: any quantity on a transport vehicle generally requires explosives placards.

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Class 2 — Gases (2.1 / 2.2 / 2.3)

Class 2 is where color-alone thinking first fails, because three divisions share a class number and use three colors:

• 2.1 Flammable gas — red diamond, flame. Propane, propylene, and many flammable aerosols. A red diamond on a cargo tank is not automatically gasoline.
• 2.2 Non-flammable, non-toxic gas — green diamond, gas cylinder. Nitrogen, helium, carbon dioxide, and many compressed gases. Oxygen is a 2.2 material that may instead display the dedicated yellow Oxygen placard; that yellow oxygen diamond is still not Class 5.1.
• 2.3 Poison (toxic) gas — white diamond, skull and crossbones. Chlorine and other toxic gases. Many 2.3 entries also carry subsidiary hazards such as corrosive or oxidizer. Poison gas is a Table 1 placard material: any quantity generally requires the 2.3 placard.

The words poison and toxic are synonymous in the HMR. On the exam, 2.3 is a gas; 6.1 is a toxic liquid or solid. Do not swap them because both diamonds can be white with a skull.

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Class 3 — Flammable liquids, and the U.S. combustible distinction

The red FLAMMABLE diamond with a flame is the classic gasoline picture, and it is the right picture for UN1203 gasoline on an MC 306/DOT 406 cargo tank. Under 49 CFR 173.120, a flammable liquid has a flash point of not more than 60 °C (140 °F).

The United States also recognizes combustible liquids for domestic transport: liquids that do not meet another hazard class and have a flash point above 60 °C (140 °F) and below 93 °C (200 °F). A domestic exception also allows some liquids with flash points at or above 38 °C (100 °F) to be reclassed as combustible for highway and rail (not a free pass for vessel or aircraft). Combustible loads may show a red COMBUSTIBLE placard with a white bar rather than FLAMMABLE, and they often travel under NA numbers such as NA1993 rather than a UN flammable-liquid entry. Internationally there is no separate combustible class — it is a U.S. transport distinction. Combustible liquid is not Class 4.

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Class 4 — Flammable solids, spontaneous, dangerous when wet

Three divisions, three diamonds:

• 4.1 Flammable solids, self-reactive substances, and solid desensitized explosives — red and white vertical stripes with a flame. Matches, some metal powders, and wetted explosives that have been desensitized.
• 4.2 Spontaneously combustible (liable to spontaneous combustion) — white upper half, red lower half, flame. Pyrophoric materials and self-heating substances.
• 4.3 Dangerous when wet (emit flammable gas on contact with water) — blue diamond, flame. Sodium, calcium carbide, and similar water-reactive metals and hydrides. Division 4.3 is a Table 1 any-quantity placard hazard. Water on a 4.3 leak is not a default tactic.

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Class 5 — Oxidizers (5.1) and organic peroxides (5.2)

• 5.1 Oxidizing substances — yellow diamond, flame over circle. These materials yield oxygen or otherwise intensify combustion. The oxidizer is not the same thing as the fuel; the fuel may be whatever the oxidizer finds, including your turnout gear or a Class 3 pool next to it.
• 5.2 Organic peroxides — yellow and red diamond with a flame. Organic peroxides oxidize and are often thermally sensitive or self-reactive. Certain temperature-controlled organic peroxides are treated as particularly serious for placarding. Do not interchange the 5.1 and 5.2 diamonds on an exam item.

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Class 6 — Toxic (6.1) and infectious (6.2)

• 6.1 Toxic (poison) substances — white diamond, skull. Liquids and solids poisonous by inhalation, ingestion, or skin absorption. Packing group and whether the material is a poison-inhalation-hazard (PIH/TIH) change isolation and PPE. Poison-inhalation PG I is a Table 1 placard material.
• 6.2 Infectious substances — white diamond, biohazard symbol, not a skull. Category A infectious substances can cause permanent disability or life-threatening or fatal disease in otherwise healthy humans or animals; Category B is a lower-risk biological substance. Identify 6.2 by the infectious-substance label and proper shipping name, not by a 6.1 skull.

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Class 7 — Radioactive (white/yellow, trefoil)

White or yellow diamonds with the trefoil symbol. Identification is not finished at it is radioactive. Packages are labeled Radioactive White-I, Yellow-II, or Yellow-III based on radiation level at the surface and the transport index (TI). TI is the maximum radiation level in mrem/h at 1 meter from the package surface for the common non-exclusive-use case. White-I is the lowest category (TI = 0). Yellow-II and Yellow-III indicate higher TI and higher surface dose; Yellow-III is also a Table 1 placarding trigger. The number in the lower corner is 7. It is not a UN number and not an NFPA 704 rating.

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Class 8 — Corrosive (black and white, test tubes on hand and metal)

White upper half, black lower half, test tubes dripping onto a hand and a metal bar. The symbol teaches both tissue damage and metal corrosion. Acids and bases both travel as Class 8. A smaller-diameter circular cargo tank with exterior stiffening rings (MC 312 / DOT 412) plus a Class 8 placard is the corrosive-tanker picture — not an MC 306/DOT 406 gasoline oval.

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Class 9 — Miscellaneous

Black-and-white vertical stripes in the top half, class number 9. This remainder class includes lithium batteries, dry ice, asbestos, PCBs, environmentally hazardous substances, and other materials that pose a transport hazard not captured in Classes 1–8. Class 9 is still a regulated class. It is not a synonym for non-hazardous.

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Limited quantity and ORM-D (current DOT status)

After December 31, 2020, shippers may not use the ORM-D (Other Regulated Materials — Domestic) Consumer Commodity marking. Limited-quantity packages must use the limited-quantity marking (square-on-point with black top and bottom corners) under 49 CFR 173.156 and related sections. Older textbooks and older DOT charts still show ORM-D. Current PHMSA practice does not. Treat ORM-D as historical unless an item is explicitly about the phase-out.

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Primary class and subsidiary hazards

When a material meets more than one class, 49 CFR 173.2a assigns a primary (precedence) class. That primary class drives the main placard. Subsidiary hazards require additional labels, and in specified cases additional placards, so a flammable liquid that is also toxic is not finished when you have read only the red diamond. Each diamond still shows its class number in the lower corner. Identification means reading every diamond, then confirming with the four-digit ID number and the shipping paper.

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Exam trap: color is not identification

Red appears on 2.1 flammable gas, Class 3 flammable liquid, the lower half of 4.2, part of 5.2, and the combustible placard. White appears on 2.3, 6.1, 6.2, and Class 7 White-I. Yellow appears on 5.1, part of 5.2, Oxygen, and Class 7 Yellow-II/III. Orange is Class 1. Blue is 4.3. Green is 2.2. Always read the class or division number in the lower corner and the four-digit ID number.

Scene: MC 306/DOT 406 gasoline versus corrosive cargo tank

You arrive on a two-vehicle highway incident. Unit A is an oval aluminum cargo tank (MC 306 / DOT 406) with a red Class 3 placard and ID 1203 — gasoline, flammable liquid, atmospheric-pressure petroleum tank. Unit B is a smaller-diameter circular tank with stiffening rings (MC 312 / DOT 412), black-and-white Class 8 placard, and a corrosive UN/NA number. Treating both as fuel tankers because they are shiny and on the highway is how technicians take a corrosive burn or pick an incompatible water tactic. Tank specification is a recognition clue. The class diamond and ID number are the identification. Do not fight a Class 8 acid leak with the same foam plan you use on UN1203.

Before you leave this section, you should be able to name all nine classes, match color-and-symbol to class, recite 1.1–1.6, 2.1–2.3, 4.1–4.3, 5.1–5.2, and 6.1–6.2, explain the U.S. combustible-liquid distinction, and refuse to identify a material by color alone.