The nine classes and their divisions

Every dangerous good belongs to one of nine classes, numbered 1 to 9. The class describes the main way the product can cause harm in transport: by exploding, by being a gas under pressure, by burning, by reacting, by poisoning or infecting, by emitting radiation, by corroding, or in some other way. Several classes are split into divisions, written with a decimal point, such as 2.3 for toxic gases or 6.2 for infectious substances. The class number is printed at the bottom corner of every label and placard, so learning the nine classes is the first step in reading any shipment.

The class numbers do not rank danger. Class 9 is not safer than class 1 in every situation, and some class 9 products, such as large lithium batteries, have caused serious fires. Within several classes, the packing group (lesson 4) does rank danger from great (I) to minor (III).

Class 1: Explosives

Class 1 covers substances and articles that can explode, burn rapidly or produce a pyrotechnic effect, from blasting explosives and ammunition to fireworks, airbag inflators and flares. It has six divisions based on the type of hazard: 1.1 mass explosion hazard, 1.2 projection hazard, 1.3 fire hazard with minor blast or projection, 1.4 minor hazard confined mostly to the package, 1.5 very insensitive substances with a mass explosion hazard, and 1.6 extremely insensitive articles. Each explosive also has a compatibility group letter, from A to S, that controls which explosives can travel together. Division 1.4S, the lowest-hazard group, includes many small arms cartridges and some safety devices.

Explosives are also regulated under the Explosives Act, administered by Natural Resources Canada, which authorizes explosive products and licenses their manufacture and storage. Most of the exemptions for small quantities in the TDG Regulations do not apply to class 1, apart from a short list of low-hazard items.

Class 2: Gases

Class 2 covers gases that are compressed, liquefied, dissolved under pressure or refrigerated to very low temperatures, and aerosols. Its three divisions describe the hazard of the gas itself:

• Division 2.1, flammable gases, such as propane, butane, acetylene, hydrogen and natural gas.
• Division 2.2, non-flammable, non-toxic gases, such as nitrogen, carbon dioxide, argon, compressed air and refrigerated liquid nitrogen. Oxygen and other oxidizing gases are also in 2.2, with a subsidiary class 5.1.
• Division 2.3, toxic gases, such as chlorine, ammonia (anhydrous), hydrogen sulfide and sulfur dioxide. These are among the most dangerous products in transport because a release can spread a lethal cloud downwind.

Even a non-flammable, non-toxic gas can kill. Nitrogen and carbon dioxide displace oxygen in an enclosed space such as a van cargo area, and a cylinder whose valve is knocked off becomes a projectile. Refrigerated liquids cause cold burns and expand enormously as they warm. Lesson 12 deals with carrying cylinders safely.

Class 3: Flammable liquids

Class 3 covers liquids with a flash point of not more than 60 °C, the temperature at which they give off enough vapour to ignite. Gasoline, diesel fuel (whose flash point is usually just above 52 °C), ethanol, acetone, paint thinners, many adhesives and crude oil are all class 3. Liquids with a flash point above 60 °C are not regulated as class 3 in Canada unless they are transported hot, at or above their flash point. Flammable liquids are by far the largest group of dangerous goods moved by road and rail.

Class 4: Flammable solids and related substances

Class 4 has three divisions that describe different ways a solid can start a fire:

• Division 4.1, flammable solids, such as matches, sulfur, magnesium powder and some metal powders, plus self-reactive substances that can decompose violently with heat, and desensitized explosives that are kept wet to suppress their explosive properties.
• Division 4.2, substances liable to spontaneous combustion, such as white phosphorus, oily rags and some activated carbon, which can heat up and ignite in air without an ignition source.
• Division 4.3, substances that are dangerous when wet, such as sodium, calcium carbide and some metal hydrides, which give off flammable gas when they contact water. Responders must know about 4.3 goods because water, their usual tool, makes the fire worse.

Class 5: Oxidizers and organic peroxides

Division 5.1, oxidizing substances, are products that are not necessarily combustible themselves but give off oxygen and make other materials burn more fiercely. Examples include ammonium nitrate, calcium hypochlorite (pool chlorine), hydrogen peroxide solutions and sodium chlorate. Division 5.2, organic peroxides, combine fuel and oxidizer in one molecule and can decompose explosively when heated or contaminated. Some organic peroxides must be kept below a control temperature during transport, with a higher emergency temperature at which emergency action begins.

Class 6: Toxic and infectious substances

Division 6.1, toxic substances, are liquids and solids that can kill or seriously harm people if swallowed, inhaled or absorbed through the skin, such as many pesticides, arsenic compounds, cyanides and phenol. Division 6.2, infectious substances, are materials known or reasonably expected to contain pathogens. Category A infectious substances (UN2814 affecting humans and UN2900 affecting only animals) can cause permanent disability or life-threatening disease; Category B substances, such as most diagnostic specimens, travel as UN3373 Biological Substance, Category B, with simpler packaging rules. Laboratory and medical shippers need specific training for class 6.2; the Human Pathogens and Toxins Act also applies to many of these materials.

Class 7: Radioactive materials

Class 7 covers materials that emit ionizing radiation above set activity limits, such as medical isotopes, industrial radiography sources, moisture density gauges used on construction sites and uranium ore concentrate. Radioactive packages carry one of three labels, Category I-White, II-Yellow and III-Yellow, depending on the radiation level at the surface and at one metre, which is expressed as the transport index. The Packaging and Transport of Nuclear Substances Regulations, 2015, administered by the Canadian Nuclear Safety Commission, add requirements to the TDG Regulations.

Class 8: Corrosives

Class 8 covers substances that destroy skin or corrode metal. A product is corrosive in transport if it causes full-thickness destruction of skin within set exposure times, or if it corrodes steel or aluminum faster than 6.25 mm (about 0.25 inch) a year at 55 °C. Sulfuric acid, hydrochloric acid, sodium hydroxide, battery fluid, many cleaners and some amines are class 8. Corrosives are common in industry and are frequent causes of package leaks.

Class 9: Miscellaneous products

Class 9 holds products that present a hazard in transport but do not meet the criteria of any other class. It includes lithium batteries, environmentally hazardous substances (products toxic to aquatic life), dry ice (UN1845), elevated temperature substances such as molten sulfur and hot asphalt above set temperatures, certain asbestos, genetically modified organisms, life-saving appliances such as inflatable rafts, and vehicles and engines powered by flammable fuel or batteries. Lithium batteries now generate a large share of incidents across all modes because damaged or poorly packed cells can overheat and catch fire.

The older consumer commodity designation ORM-D, still sometimes seen on old packaging, belongs to the US system and was phased out there at the end of 2020. In Canada, consumer products are handled through limited quantities and the small-quantity exemptions in lesson 10.

Primary and subsidiary hazards

Many products have more than one hazard. Methanol is flammable and toxic. Hydrofluoric acid is corrosive and toxic. A substance gets one primary class, the most important hazard in transport, and one or more subsidiary classes. The primary class is chosen using a precedence table in the regulations; for many combinations, the class with the more severe packing group wins. Subsidiary classes are shown in brackets after the primary class on the shipping document, for example 3 (6.1), and often require an additional label. Some subsidiary hazards also require a subsidiary placard on the vehicle.

# Simplified precedence between class 3 and class 8 for a flammable, corrosive liquid,
# following the pattern of the UN precedence table (illustrative; always check the regulations).
TABLE = {  # (class 3 PG, class 8 PG) -> primary class
    ("I", "I"): "3", ("I", "II"): "3", ("I", "III"): "3",
    ("II", "I"): "8", ("II", "II"): "3", ("II", "III"): "3",
    ("III", "I"): "8", ("III", "II"): "8", ("III", "III"): "3",
}
def primary(pg3, pg8):
    p = TABLE[(pg3, pg8)]
    sub = "8" if p == "3" else "3"
    pg = min(pg3, pg8, key=["I", "II", "III"].index)   # most severe packing group applies
    return f"Class {p} ({sub}), PG {pg}"

for case in [("II", "I"), ("II", "II"), ("III", "II"), ("I", "III")]:
    print(f"flammable PG {case[0]:<3} corrosive PG {case[1]:<3} -> {primary(*case)}")

flammable PG II  corrosive PG I   -> Class 8 (3), PG I
flammable PG II  corrosive PG II  -> Class 3 (8), PG II
flammable PG III corrosive PG II  -> Class 8 (3), PG II
flammable PG I   corrosive PG III -> Class 3 (8), PG I

The result always carries the most severe packing group of the hazards involved, even when the class with the less severe group is chosen as primary. The table is illustrative; real classification follows the exact table in the regulations and, where a product is listed by name, the class shown in the list overrides your own reasoning.