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 also need an approval from PHMSA before they can be shipped, which is why each explosive shipment carries an approval number (an EX number). The Bureau of Alcohol, Tobacco, Firearms and Explosives separately regulates manufacture, sale and storage. Most of the exceptions for small quantities in the HMR do not apply to class 1.

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 compressed gases, such as nitrogen, carbon dioxide, argon, compressed air and refrigerated liquid nitrogen. Oxygen is also in 2.2, labeled with an oxidizer label.
• Division 2.3, gases poisonous by inhalation (called poison gases in the labels), such as chlorine, hydrogen sulfide and sulfur dioxide. Anhydrous ammonia is classed 2.2 for domestic US transport, with an inhalation hazard marking, but 2.3 internationally. 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 (140 °F), the temperature at which they give off enough vapor to ignite. Gasoline, diesel fuel, ethanol, acetone, paint thinners, many adhesives and crude oil are all class 3. The HMR add a domestic category called combustible liquids: liquids with a flash point above 60 °C and below 93 °C (200 °F). Combustible liquids in non-bulk packaging are largely unregulated, but in bulk packaging, such as a tank truck, they carry placards and shipping papers. A flammable liquid with a flash point of 38 °C (100 °F) or higher may also be reclassed as combustible for domestic ground transport. Flammable liquids are by far the largest group of hazmat moved by highway 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, oxidizers, 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, poisonous materials (labeled toxic or poison), 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. Some 6.1 liquids are also poisonous by inhalation and need an inhalation hazard marking. 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. Regulated medical waste is also class 6.2. Laboratory and medical shippers need specific training for division 6.2.

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 hexafluoride. Radioactive packages carry one of three labels, Radioactive White-I, Yellow-II and Yellow-III, depending on the radiation level at the surface and at one meter (about 3.3 feet), which is expressed as the transport index. The Nuclear Regulatory Commission's rules in 10 CFR Part 71 add packaging requirements for many shipments.

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 materials that present a hazard in transport but do not meet the criteria of any other class. It includes lithium batteries, environmentally hazardous substances, hazardous substances with a reportable quantity that are not otherwise regulated, hazardous wastes that meet no other class, dry ice (UN1845), elevated temperature materials such as molten sulfur and hot asphalt, asbestos, genetically modified organisms, life-saving appliances and vehicles and engines. Lithium batteries now generate a large share of incidents across all modes because damaged or poorly packed cells can overheat and catch fire.

The old ORM-D (other regulated material, domestic) marking for consumer commodities was phased out at the end of 2020. Those products now ship as limited quantities, covered in lesson 10.

Primary and subsidiary hazards

Many products have more than one hazard. Methanol is flammable and poisonous. Hydrofluoric acid is corrosive and poisonous. A material gets one primary hazard class and one or more subsidiary hazards. The primary class is chosen using the precedence of hazard table in 49 CFR 173.2a; for many combinations, the class with the more severe packing group wins. Subsidiary hazards are shown in parentheses after the primary class on the shipping paper, for example 3 (6.1), and usually require an additional label.

# 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.