It is the IMO rulebook for dangerous goods on a ship: how to class them, pack them, mark them and stow them. Each substance gets a class, a UN number and packing rules that the carrier and the terminal have to follow.
Class 2
Gases
Read the divisions below as a planning note. The class name tells you the main hazard. Packing, labels and where the box may sit on the ship still come from the IMDG Code.
Gases, class 2
Division 2.1. Flammable gas
454 kg (1001 lbs) of a material that is a gas at 20°C (68°F) or below and at 101.3 kPa (14.7 psi). That is a material whose boiling point is 20°C (68°F) or lower at that pressure, and which:
1. Catches fire at 101.3 kPa (14.7 psi) when mixed with air at 13 percent or less by volume; or
2. Has a flammable range in air of at least 12 percent at 101.3 kPa (14.7 psi), whatever the lower limit is.
Division 2.2. Non-flammable, non-toxic gas
This division covers compressed gas, liquefied gas, pressurized cryogenic gas, gas dissolved under pressure, asphyxiant gas and oxidizing gas. A non-flammable, non-toxic compressed gas means a material, or a mixture, that:
1. Builds an absolute pressure of 280 kPa (40.6 psia) or more inside the packaging at 20°C (68°F), and
2. Does not meet the tests for division 2.1 or 2.3.
Division 2.3. Toxic gas
A gas that is poisonous if inhaled: it is a gas at 20°C or below and at 101.3 kPa (boiling point 20°C or lower at 14.7 psi), and it:
1. Is known to be toxic enough to threaten health while it is being carried, or
2. Lacks enough human data, so it is treated as toxic because animal tests give an LC50 of 5000 ml/m³ or less.
Questions
What a class does not settle
The class names the main hazard. Many goods also have a second one, such as a poisonous gas that can also burn. The papers and the placards show both, and the separation rules follow whichever hazard is stricter.
It belongs to the substance, or to a small group of similar substances, in the IMDG Dangerous Goods List. It is not one number for a whole class. Two boxes in the same class can have different numbers, packing groups and separation rules.