Means of containment and UN specification marks

The best protection against a dangerous goods release is a container that does not leak. The TDG Regulations call any container used for dangerous goods a means of containment: a can, bottle, drum, box, bag, cylinder, intermediate bulk container, highway tank, portable tank or rail tank car. Part 5 of the regulations, together with safety standards incorporated by reference, sets out which means of containment can be used for which dangerous goods and how they must be designed, tested, marked, filled and closed.

The general rule

Whatever the container, the same general requirement applies: it must be designed, constructed, filled, closed, secured and maintained so that under normal conditions of transport, including handling, there is no accidental release of the dangerous goods that could endanger public safety. That sentence appears again and again in the regulations, including in the conditions of the small-quantity exemptions. It means more than choosing a strong container. The container must be compatible with the product (some plastics soften with solvents, and some metals corrode with acids), the closure must be tight and secured, there must be enough empty space (ullage) to allow liquids to expand with heat, and the container must be in good condition, without dents, rust or cracks that weaken it.

Normal conditions of transport include vibration, stacking, temperature changes from below freezing to the inside of a sunlit trailer, and the drops and bumps of loading and unloading. For air transport they also include pressure changes: a closure that is tight at ground level can leak at altitude unless the packaging is designed for the pressure difference.

Types of packaging

Small packagings fall into two broad types. A single packaging, such as a steel drum or plastic jerrican, holds the product directly. A combination packaging consists of inner packagings, such as glass bottles or metal cans, placed inside an outer packaging, such as a fibreboard box, often with cushioning and absorbent material. Combination packagings are common for laboratory chemicals, paints and consumer products. A composite packaging, such as a plastic receptacle in a steel drum, behaves as a single unit.

Larger containers come in several forms. Intermediate bulk containers (IBCs), often called totes, hold up to 3,000 litres and are common for liquids such as cleaning chemicals. Bulk containers include highway cargo tanks, portable tanks, tank cars, bulk bags and rail hopper cars. Cylinders and tubes hold gases under pressure. Each type has its own design standard and inspection schedule.

Performance-tested packagings

Most small packagings used for dangerous goods must be UN standardized packagings. That means the design has been tested to show it can survive drops, stacking, leakage tests and, for liquids, internal pressure, at a performance level matched to the packing group. Packing group I products need the toughest tests, marked X. Packing group II needs Y, and packing group III needs Z. A packaging tested to X may be used for products of any packing group; one tested to Y may be used for packing groups II and III; one tested to Z may be used only for packing group III.

Manufacturers of UN standardized packagings in Canada must be registered with Transport Canada and must test their designs at set intervals. The standard CAN/CGSB-43.150, incorporated into the regulations in 2026, sets out how small packagings and intermediate bulk containers are selected and used for different classes.

Reading the UN specification mark

A UN standardized packaging carries a permanent mark, usually embossed or printed, that tells you exactly what it was tested for. The mark begins with the UN packaging symbol, a small u over n in a circle, or the letters UN, followed by a series of codes separated by slashes.

The first code identifies the type of packaging. A number gives the kind (1 drum, 3 jerrican, 4 box, 5 bag, 6 composite). A letter gives the material (A steel, B aluminum, C natural wood, D plywood, G fibreboard, H plastic, L textile, M paper, N other metal). A second number, where used, gives the category, such as 1 for a closed head and 2 for an open (removable) head. So 1A1 is a closed-head steel drum, 1H2 an open-head plastic drum, 3H1 a closed-head plastic jerrican and 4G a fibreboard box.

Next comes the performance level letter (X, Y or Z). For packagings meant for solids or inner packagings, the letter is followed by the maximum gross mass in kilograms, and then the letter S. For packagings meant for liquids, the letter is followed by the maximum specific gravity (relative density) of liquid the design was tested for, and then the hydraulic test pressure in kilopascals. The mark ends with the last two digits of the year of manufacture, the country that authorized the mark (CA for Canada, USA for the United States) and a code identifying the manufacturer or the testing agency. Plastic drums and jerricans also show the month of manufacture, because plastic degrades with age and is allowed only a limited service life for dangerous goods.

# Decode a UN specification mark and check it against a product (simplified).
KIND = {"1": "drum", "3": "jerrican", "4": "box", "5": "bag", "6": "composite"}
MATERIAL = {"A": "steel", "B": "aluminum", "G": "paperboard", "H": "plastic", "C": "wood", "D": "plywood"}
LEVEL = {"X": {"I", "II", "III"}, "Y": {"II", "III"}, "Z": {"III"}}

def decode(mark):
    code, perf, third, year, country, maker = mark.split("/")[:6]
    kind = KIND[code[0]] + " of " + MATERIAL[code[1]]
    if len(code) > 2:
        kind += ", closed head" if code[2] == "1" else ", open head"
    groups = LEVEL[perf[0]]
    if third == "S":
        detail = f"solids or inner packagings, max gross mass {perf[1:]} kg"
    else:
        detail = f"liquids up to specific gravity {perf[1:]}, test pressure {third} kPa"
    return kind, "PG " + "/".join(sorted(groups, key=["I", "II", "III"].index)), detail, "20" + year, country

def suitable(mark, pg, sg=None):
    kind, groups, detail, year, country = decode(mark)
    ok = pg in groups.replace("PG ", "").split("/")
    if sg is not None and "specific gravity" in detail:
        ok = ok and sg <= float(mark.split("/")[1][1:])
    return ok

for mark in ["4G/Y25/S/26/CA/ABC", "1A1/X1.8/250/26/USA/M12", "3H1/Y1.2/100/25/CA/XYZ"]:
    print(mark, "->", decode(mark))
print("sulfuric acid, PG II, SG 1.84 in 1A1/X1.8:", suitable("1A1/X1.8/250/26/USA/M12", "II", 1.84))
print("solvent, PG II, SG 0.87 in 3H1/Y1.2:     ", suitable("3H1/Y1.2/100/25/CA/XYZ", "II", 0.87))
print("PG I solid in 4G/Y25:                     ", suitable("4G/Y25/S/26/CA/ABC", "I"))

4G/Y25/S/26/CA/ABC -> ('box of paperboard', 'PG II/III', 'solids or inner packagings, max gross mass 25 kg', '2026', 'CA')
1A1/X1.8/250/26/USA/M12 -> ('drum of steel, closed head', 'PG I/II/III', 'liquids up to specific gravity 1.8, test pressure 250 kPa', '2026', 'USA')
3H1/Y1.2/100/25/CA/XYZ -> ('jerrican of plastic, closed head', 'PG II/III', 'liquids up to specific gravity 1.2, test pressure 100 kPa', '2025', 'CA')
sulfuric acid, PG II, SG 1.84 in 1A1/X1.8: False
solvent, PG II, SG 0.87 in 3H1/Y1.2:      True
PG I solid in 4G/Y25:                      False

The check shows why the numbers matter. Concentrated sulfuric acid has a specific gravity of about 1.84, slightly above the 1.8 the drum was tested for, so that drum design is not suitable even though its performance letter allows packing group II. A packing group I solid cannot go in a box tested only to Y. These are common errors in real inspections.

Gas cylinders

Cylinders for gases must meet a specification and be requalified at set intervals, usually by a hydrostatic or ultrasonic test every five or ten years depending on the type, with the date stamped on the cylinder. In Canada cylinders must meet the TC specifications in CSA B339, or DOT or UN specifications where the regulations accept them, and selection and use are covered by CSA B340. Never transport a cylinder that is out of date for requalification, has a damaged valve or shows bulging, deep dents, fire damage or corrosion. Valves must be closed and, where the cylinder is designed for one, protected by a cap or guard.

Bulk containers

Highway tanks are built to TC specifications such as TC 406 for low-pressure flammable liquids like gasoline, TC 407 for chemicals and TC 331 for liquefied gases like propane, under standards such as CSA B620 and B621. They must be inspected and tested periodically by registered facilities, and the test dates must be marked on the tank. Rail tank cars carrying flammable liquids in trains must now be TC-117 cars or retrofitted equivalents, a requirement that resulted from the Lac-Mégantic disaster.

Handling and loading

Even the best container fails if it is mishandled. Packages must be loaded so they cannot shift, roll or fall during transport, using blocking, bracing, straps or load bars. Orientation arrows on combination packagings must point up, because the inner packagings may have been designed to stay upright. Packages must not be dropped, dragged or stacked higher than they were designed for, and leaking or damaged packages must not be transported. Lesson 7 covers segregation, which keeps incompatible goods apart, and lesson 12 covers securing cylinders in a vehicle.