Class 2 of the IMDG Code covers every substance that is gaseous at ambient conditions or is maintained in gaseous form during transport by compression, liquefaction, or refrigeration. The class spans hydrogen at cryogenic temperatures in dewar flasks on container deck, industrial oxygen cylinders in cargo holds, and chlorine in T50 tank containers on ro-ro vessels. Three divisions, a dozen containment categories, and two entirely separate code regimes (IMDG for packaged cargo, IGC for bulk carriage) govern that range. The IMDG segregation calculator, the IMDG EmS lookup, the IMDG tank container calculator, and the IMO IMDG general calculator are the primary computational tools for Class 2 cargo handling; the regulatory framework behind those tools is laid out below.
Regulatory authority and mandatory status
The IMDG Code 2022 Edition, incorporating Amendment 41-22, was adopted by IMO Maritime Safety Committee Resolution MSC.501(105) on 28 April 2022. It became mandatory under SOLAS Chapter VII Part A on 1 January 2024, with voluntary early application permitted from 1 January 2023. The code aligns with the 22nd revised edition of the UN Model Regulations on the Transport of Dangerous Goods.
SOLAS Chapter VII Part A (Regulations 1-6) is the instrument that gives the IMDG Code its legal force for packaged dangerous goods, including all Class 2 gases in non-bulk packagings. Regulation 2 requires that all packaged dangerous goods comply with the IMDG Code as a condition of carriage. The shipper, the packer, the carrier, and the master each carry specific duties under Regulation 4 (documentation), Regulation 5 (packing and stowage), and Regulation 6 (reporting of incidents).
Class 2 is assigned its own chapter in the classification structure: IMDG Code Chapter 2.2 covers gases, with sub-sections 2.2.2 (flammable gases, Division 2.1), 2.2.3 (non-flammable non-toxic gases, Division 2.2), and 2.2.4 (toxic gases, Division 2.3). The classification criteria derive from the UN Model Regulations Part 2 Chapter 2.2 and are harmonized across all transport modes including road (ADR), rail (RID), inland waterways (ADN), and air (ICAO TI).
What the class covers: the four physical states
IMDG Code 41-22 Chapter 2.2.1 defines a Class 2 substance as a pure gas, a gas mixture, or a mixture of one or more gases with one or more other substances, that at 50 degrees C has an absolute vapour pressure greater than 300 kPa (3 bar), or is completely gaseous at 20 degrees C at 101.3 kPa (1 atm). Four physical states exist within the class.
Compressed gases are entirely gaseous at minus 50 degrees C and are packaged under pressure for transport. Oxygen (UN 1072), nitrogen (UN 1066), argon (UN 1006), helium (UN 1046), and compressed natural gas (UN 1971 or UN 3537) are the most common. The entire vessel capacity in the cylinder is occupied by gas phase; pressure at ambient temperature is the only containment mechanism.
Liquefied gases are partially liquid at transport temperature; a liquid phase and vapour phase coexist in the cylinder or tank at the saturation pressure of the cargo. LPG (UN 1075: propane/butane blends), anhydrous ammonia (UN 1005), chlorine (UN 1017), and vinyl chloride (UN 1086) are typical. Transport temperature is ambient; the vessel must withstand saturation pressure at the maximum expected temperature during the voyage.
Refrigerated liquefied gases are maintained in liquid phase by active or passive refrigeration rather than by pressure alone. LNG (UN 1972), liquid nitrogen (UN 1977), liquid oxygen (UN 1073), and liquid argon (UN 1951) fall here. The insulated vessel (a vacuum-jacketed “dewar” or a T75 portable tank) holds the cargo at or near atmospheric pressure, relying on low temperature for the liquid phase. Boil-off is the primary operational hazard.
Dissolved gases are a special subcategory: acetylene (UN 1001) is dissolved in acetone within a porous filler mass inside a cylinder to prevent explosive decomposition that would occur in a free acetylene gas space. No other common commercial gas uses this state.
The class also includes articles classed as aerosols (UN 1950), which are non-refillable metal, glass, or plastic receptacles containing a pressurized propellant gas and a liquid or powder product, fitted with a release valve. Aerosols are not pure Class 2 entries in the sense that they carry a mixed classification based on their contents, but the IMDG Code’s aerosol provisions in Chapter 2.2.1 and packing instruction P207 treat them under the Class 2 umbrella.
Division 2.1: flammable gases
Classification criteria
IMDG Code 41-22 Chapter 2.2.2.1 specifies a two-part test for flammability. A gas is Division 2.1 if:
- Its lower flammable limit (LFL) is 13% or less by volume in air at 20 degrees C and 101.3 kPa, or
- It has a flammable range with air of at least 12 percentage points at those same conditions, regardless of where the LFL falls.
Either criterion alone is sufficient. The two-criterion structure captures gases with a very low LFL (easily ignited at low concentration, such as hydrogen with LFL 4%) and separately captures gases with a wide flammable range even if the LFL is above 13% (carbon monoxide has LFL 12.5% and an upper flammable limit of 74%, giving a 61.5 percentage-point range that satisfies criterion 2 and essentially satisfies criterion 1 simultaneously). Hydrogen cyanide is an edge case: LFL about 5.6%, range about 34 percentage points. It classifies Division 2.1 on both criteria but also meets the Division 2.3 LC50 threshold, making it a dual-hazard substance.
The flammability criteria match those in the UN Model Regulations Rev. 22 Chapter 2.2.2.1. They are tested under ISO 10156 (for determining flammability and oxidizing ability of gases and gas mixtures) or equivalent standard methods approved by the competent authority.
Common Division 2.1 UN numbers
The Dangerous Goods List in IMDG Code Chapter 3.2 contains the individual entries. Key examples:
- UN 1001 Acetylene, dissolved (also Division 2.1 by property but handled as dissolved gas; packing instruction P200)
- UN 1011 Butane (LFL 1.8%, UFL 8.5%)
- UN 1049 Hydrogen, compressed (LFL 4.0%, UFL 75.0%)
- UN 1075 Petroleum gases, liquefied (LPG: propane/butane blends; LFL roughly 1.8-2.1%)
- UN 1079 This entry is sulphur dioxide and is Division 2.3; do not confuse with UN 1075
- UN 1086 Vinyl chloride, stabilized (LFL 3.6%, UFL 33.0%)
- UN 1971 Methane, compressed; or natural gas, compressed (LFL 5.0%, UFL 15.0%)
- UN 1972 Methane, refrigerated liquid; LNG (same flammability as UN 1971)
- UN 1978 Propane (LFL 2.1%, UFL 9.5%)
- UN 2034 Hydrogen and methane mixture, compressed
- UN 3374 Acetylene, solvent free (no added solvent; limited shipments under special authorization)
Stowage codes assigned in the Dangerous Goods List for Division 2.1 entries typically require stowage away from heat sources (stowage code SW2) and protection from direct sunlight. Most entries carry stowage category B (on deck or under deck, preferably on deck) or A (on deck or under deck). A few high-vapour-pressure entries such as UN 1009 (bromotrifluoromethane, refrigerant R-13B1) carry category D (on deck only, in a CTU).
Hazard mechanisms specific to Division 2.1
Flammable gases mix freely with air and find ignition sources. Vapour cloud explosions (VCEs) are possible when a large release generates a flammable cloud that subsequently finds a delayed ignition source. The overpressure from a VCE can cause structural damage at distances of hundreds of metres.
Boiling liquid expanding vapour explosions (BLEVEs) are a separate mechanism specific to pressurized liquefied gases (LPG, propylene, butylene). If the vessel shell is exposed to flame without adequate water-spray cooling, the metal weakens and fails before the pressure-relief valve can vent; the sudden depressurization vaporizes the entire liquid contents in milliseconds, producing a fireball. The IMDG Code’s EmS schedule F-D for flammable gas fires instructs crews not to fight a flammable gas fire if the source cannot be isolated, precisely because extinguishing the fire without isolating the gas produces an unburned vapour cloud that will re-ignite with greater violence.
The IMDG EmS lookup returns the specific fire and spillage schedules for any Division 2.1 UN entry.
Division 2.2: non-flammable non-toxic gases
Classification criteria
Division 2.2 is the residual class: a gas is 2.2 if it meets the general Class 2 pressure criterion but does not meet the Division 2.1 flammability criteria and does not meet the Division 2.3 toxicity criteria. IMDG Code Chapter 2.2.3.1 also requires that the gas exerts a gauge pressure of at least 200 kPa at 20 degrees C, is a liquefied gas, or is a refrigerated liquefied gas, as confirmation that the substance is genuinely a gas for transport purposes.
Division 2.2 covers four sub-groups in practice, though the code does not formally name them:
Asphyxiant gases displace breathable air in confined spaces without any toxic chemical action. Nitrogen (UN 1066), argon (UN 1006), helium (UN 1046), neon (UN 1065), krypton (UN 1056), and xenon (UN 2036) fall here. Carbon dioxide (UN 1013 compressed, UN 2187 refrigerated liquid) is also primarily asphyxiant but causes mild narcosis at concentrations above 3% and convulsions above 7%; it is still Division 2.2 because it does not meet the LC50 threshold for Division 2.3. Asphyxiants are the most common cause of gas-related confined-space fatalities on ships: a refrigerated hold filled with nitrogen (used to inhibit oxidation of perishable cargo) has killed port workers who entered without oxygen monitoring.
Oxidizing gases promote the combustion of other materials without being themselves flammable. Oxygen (UN 1072 compressed, UN 1073 refrigerated liquid) is the primary example. Nitrous oxide (UN 1070) and compressed air (UN 1002) also fall here. Oxidizing gases are Division 2.2 by classification but carry the oxygen symbol (division 5.1 subsidiary risk) in many DGL entries, and the segregation rules treat them as oxidizers for proximity-to-flammables purposes.
Refrigerant gases include hydrofluorocarbons and hydrochlorofluorocarbons that have no flammable range under normal conditions and are not acutely toxic. R-134a (1,1,1,2-tetrafluoroethane, UN 3159), R-22 (chlorodifluoromethane, UN 1018), and R-410A (difluoromethane/pentafluoroethane blend, UN 3340) are common examples. Note: some refrigerants have been reclassified in successive UN/IMDG amendments as newer data established flammable ranges; R-290 (propane) is Division 2.1, not 2.2.
Refrigerated liquefied gases in Division 2.2 include liquid nitrogen (UN 1977), liquid argon (UN 1951), and liquid oxygen (UN 1073). These are the same substances as compressed gas entries at the cryogenic state. They travel in vacuum-insulated cryogenic vessels, and the primary operational risk is the cold-burn hazard from skin contact with liquid at minus 183 degrees C (liquid oxygen) or minus 196 degrees C (liquid nitrogen).
Common Division 2.2 UN numbers
- UN 1002 Air, compressed (stowage A)
- UN 1006 Argon, compressed (stowage A)
- UN 1013 Carbon dioxide (stowage A)
- UN 1046 Helium, compressed (stowage A)
- UN 1066 Nitrogen, compressed (stowage A)
- UN 1070 Nitrous oxide (stowage A; subsidiary risk 5.1)
- UN 1072 Oxygen, compressed (stowage A; subsidiary risk 5.1)
- UN 1073 Oxygen, refrigerated liquid (stowage B; subsidiary risk 5.1)
- UN 1951 Argon, refrigerated liquid (stowage A)
- UN 1977 Nitrogen, refrigerated liquid (stowage A)
- UN 2187 Carbon dioxide, refrigerated liquid (stowage B)
Stowage categories for Division 2.2 are the lightest in Class 2, usually category A. The segregation requirements between Division 2.2 and other classes are minimal: no mandatory separation from Class 3 (flammable liquids), no mandatory separation from Class 4. The main segregation obligation is for oxidizing gases (oxygen, nitrous oxide), which must be kept away from flammable substances by the oxidizer segregation rules even though those rules come from the 2.2/5.1 combination rather than from 2.2 alone.
Division 2.3: toxic gases
Classification criteria
IMDG Code 41-22 Chapter 2.2.4.1 sets the primary criterion as an LC50 of 5,000 ml/m3 (5,000 ppm by volume) or less for acute inhalation toxicity in a one-hour exposure (four-hour in some test protocols with adjustment). The LC50 is the concentration that kills 50% of a test animal population at that exposure. Gases below this threshold are so dangerous by inhalation that any significant atmospheric release constitutes a life-safety emergency.
A secondary criterion allows classification of a gas as Division 2.3 on human experience alone, even without an LC50 value, if the competent authority determines it is known to be toxic or corrosive to humans to such a degree as to present a hazard to health during transport.
The toxic-by-inhalation (TIH) zone system further subdivides Division 2.3 for emergency response planning. The zones do not appear in the IMDG Code itself (they are primarily a North American Transport Canada/US DOT emergency-planning tool), but the underlying LC50 ranges are consistent with the IMDG Code’s hazard communication system:
| TIH Zone | LC50 range (ppm, 1-hour inhalation) | Representative gases |
|---|---|---|
| A | 200 or less | Phosgene (UN 1076) ~5 ppm, arsine (UN 2188) ~20 ppm, hydrogen cyanide (UN 1051) ~135 ppm |
| B | 201 to 1,000 | Chlorine (UN 1017) ~293 ppm, ethylene oxide (UN 1040) ~800 ppm |
| C | 1,001 to 3,000 | Hydrogen sulphide (UN 1053) ~713 ppm, sulphur dioxide (UN 1079) ~2,520 ppm |
| D | 3,001 to 5,000 | Ammonia (UN 1005) ~4,740 ppm, methyl bromide (UN 1062) ~4,000 ppm |
The Zone A boundary at 200 ppm LC50 is extremely severe: a chlorine cylinder leak that releases 1 kg of gas into a 100,000 cubic metre cargo hold would produce about 15 ppm, already above the immediately dangerous to life and health (IDLH) threshold of 10 ppm established by NIOSH. At Zone A concentrations, even brief exposure in a non-respirator atmosphere is lethal.
Dual-hazard Division 2.3 substances
Several Division 2.3 gases are also Division 2.1 flammable. The IMDG Code assigns the primary class based on the predominant hazard during transport. Hydrogen sulphide (UN 1053) is toxic (LC50 ~713 ppm) and flammable (LFL 4.3%). Its primary class is 2.3, with 2.1 as subsidiary hazard. Hydrogen cyanide (UN 1051) is toxic (LC50 ~135 ppm) and flammable (LFL ~5.6%). Ethylene oxide (UN 1040) is toxic (LC50 ~800 ppm), flammable (LFL 3.6%), and a Group I carcinogen. The compound classification means the shipment triggers both the toxic-gas stowage restrictions and the flammable-gas ignition-source controls.
Common Division 2.3 UN numbers
- UN 1005 Ammonia, anhydrous (also 8 corrosive; stowage D, segregation SG35, SG46, SGG18)
- UN 1008 Boron trifluoride (also 8 corrosive; stowage D)
- UN 1017 Chlorine (stowage D; special provision SW2, SG36, SG69)
- UN 1023 Coal gas, compressed (also 2.1; stowage D)
- UN 1040 Ethylene oxide (also 2.1 and 6.1; stowage D)
- UN 1051 Hydrogen cyanide, stabilized, with less than 3% water (also 2.1 and 6.1; stowage E)
- UN 1052 Hydrogen fluoride, anhydrous (also 8 corrosive; stowage D)
- UN 1053 Hydrogen sulphide (also 2.1; stowage D)
- UN 1062 Methyl bromide (stowage D; restricted under Montreal Protocol)
- UN 1067 Dinitrogen tetroxide (also 8 and 5.1; stowage D)
- UN 1076 Phosgene (stowage D; often port-restricted)
- UN 1079 Sulphur dioxide (also 8 corrosive; stowage D)
- UN 2188 Arsine (stowage D)
- UN 2199 Phosphine (also 2.1; stowage D)
- UN 2810 Toxic liquids, organic, n.o.s. (some liquefied gases fall here)
The stowage category D (on deck only, in a closed CTU) that most Division 2.3 entries carry means these cannot be loaded into enclosed cargo holds where a leak could accumulate. Category E (on deck only, away from living quarters) applies to the most hazardous entries.
Division comparison table
| Parameter | Division 2.1 Flammable | Division 2.2 Non-flammable Non-toxic | Division 2.3 Toxic |
|---|---|---|---|
| Primary hazard | Fire and explosion | Physical (pressure, asphyxiation, cold) | Acute inhalation toxicity |
| Classification criterion | LFL ≤ 13%, or flammable range ≥ 12 pp | Neither 2.1 nor 2.3 | LC50 ≤ 5,000 ml/m3 |
| Typical stowage category | A or B | A | D or E |
| Typical EmS fire code | F-D | F-C | F-C or F-E |
| Typical EmS spillage code | S-Q | S-V | S-U |
| Packing instruction | P200 (cylinders), T50 tanks | P200, T50, T75 | P200, T50 |
| MEGC permitted | Yes | Yes | Yes, with individual isolation valves |
| Subsidiary risk labels common | None (some also 2.3 or 6.1) | 5.1 for oxidizers | 2.1 if also flammable; 8 if corrosive |
| Representative UN entries | 1049 H2, 1075 LPG, 1972 LNG | 1072 O2, 1066 N2, 1977 LN2 | 1017 Cl2, 1005 NH3, 1053 H2S |
Containment: pressure receptacles under packing instruction P200
Scope of P200
Packing instruction P200 in IMDG Code Chapter 4.1 is the governing document for gases in all non-tank pressure receptacles: cylinders (seamless and welded), tubes (large-diameter high-pressure seamless cylinders), pressure drums (large-capacity welded vessels up to 3,000 litres), bundles of cylinders (manifolded clusters, up to 3,000 litres total), closed cryogenic receptacles (for refrigerated liquefied gases in dewars), and metal hydride storage systems (for hydrogen in solid-state form).
P200 cross-references the cylinder design standards accepted by the IMDG Code: UN pressure receptacles approved under Chapter 6.2, or pressure receptacles meeting the standards of a UN competent authority (DOT specifications, ISO 9809, EN 1964, and equivalents). The IMDG Code 41-22 has expanded Chapter 6.2 to include requirements for refillable-cylinder type approval in section 6.2.1.4.4, a change from Amendment 40-20 that tightened the inspection body requirements.
Periodic inspection under P200 and Chapter 6.2
Pressure receptacles used for Class 2 transport are subject to periodic inspection and testing under IMDG Code Chapter 6.2. The inspection regime under P200 requires:
Standard industrial gas cylinders (oxygen, nitrogen, argon, hydrogen in seamless steel cylinders): the P200 table column “Periodic inspection intervals” typically specifies 10-year intervals with a 5-year intermediate inspection for welded cylinders and certain low-pressure entries. The 10-year standard applies to non-corrosive, non-toxic gases in steel cylinders. IMDG Code Amendment 41-22 added a provision that for gases where the periodic inspection interval is 10 years or more, the month need not be marked on the test date stamp; only the year is required.
Cylinders for corrosive or toxic gases (chlorine, hydrogen fluoride, hydrogen sulphide, phosgene): P200 assigns shorter intervals, typically 5 years, because corrosive cargo attacks the cylinder material and valve seats between inspections. The actual interval for each substance appears in the P200 table column for that UN number.
Closed cryogenic receptacles (liquid nitrogen, LNG portable cryogenic vessels): periodic inspection every 5 years with intermediate inspection after 2.5 years, under IMDG Code section 6.2.3.
Acetylene cylinders: 10-year periodic inspection under P200 with special requirements for porous mass integrity and solvent quantity checks.
Non-refillable receptacles (aerosol cans, gas cartridges under P003 and P207) are exempt from periodic inspection because they are manufactured for single use and are not intended for refilling or retesting.
Chapter 6.2 of the IMDG Code 41-22 also updated the requirements for inspection bodies: bodies must now be recognized by a competent authority, and their recognition is linked to a defined inspection regime that covers both new cylinder approval (section 6.2.1.4.3) and periodic inspection (section 6.2.1.6.1). Amendment 41-22 made those requirements more explicit than they were in Amendment 40-20.
Filling limits under P200
P200 tabulates the maximum permissible filling pressure and filling ratio for each gas. Two measures control filling:
Maximum filling pressure applies to compressed gases: the highest gauge pressure the cylinder may reach at 15 degrees C during filling. For most industrial gases this is 200 bar for standard cylinders and up to 300 bar for high-pressure cylinders. The actual test pressure is 1.5 times the maximum filling pressure.
Maximum filling ratio (or filling density) applies to liquefied gases: the maximum mass of gas per unit volume of the cylinder in kg/litre. For propane the filling ratio is 0.43 kg/litre; for chlorine it is 1.25 kg/litre. The ratio is set so that the liquid phase at the maximum anticipated transport temperature does not exceed 95% of the cylinder volume, leaving a 5% vapour space to prevent hydraulic pressure rupture.
Containment: portable tanks (T50 and T75)
T50: non-refrigerated liquefied gases
Portable tank instruction T50, in IMDG Code Chapter 4.2.5.2.5, applies to non-refrigerated liquefied gases, which are the Division 2.1 and Division 2.3 gases that are liquid at ambient temperature because they are above their boiling point but below their critical temperature at the transport pressure. LPG, anhydrous ammonia, chlorine, vinyl chloride, and most refrigerant gases travel as T50 cargo.
T50 specifies the portable tank as a vacuum-tested pressure vessel with minimum test pressure (which varies by entry in the T50 table, but ranges from 1.5 bar to 22 bar or more for highly hazardous entries), maximum allowable working pressure, and opening restrictions. For many toxic or flammable entries, T50 prohibits bottom openings (discharge must be from the top via a dip pipe) to reduce the risk of a catastrophic release during a bottom-fitting failure.
The T50 table column “Maximum degree of filling” sets the maximum liquid fill to 95% of the tank’s water capacity by default, adjusted downward for substances with high thermal expansion coefficients. For anhydrous ammonia (UN 1005), the filling density in T50 tanks is typically 0.53 kg/litre. For chlorine (UN 1017) the filling density is 1.25 kg/litre, and the T50 entry for chlorine carries special provision TP19 which limits the maximum allowable working pressure and specifies materials compatible with chlorine (the tank shell must be carbon steel, and all valve materials must be chlorine-compatible alloys; rubber and most elastomers corrode in chlorine service).
Periodic inspection of T50 portable tanks follows IMDG Code Chapter 6.7 rather than Chapter 6.2. Chapter 6.7 requires initial inspection and test before first use, with periodic inspection every 5 years, and an intermediate inspection of the portable tank every 2.5 years. The intermediate inspection is an external examination rather than a full pressure test. Amendment 41-22 clarified that if an intermediate inspection is missed, the owner must conduct a full 5-year inspection rather than resuming at the intermediate-inspection interval.
T75: refrigerated liquefied gases
Portable tank instruction T75, in IMDG Code Chapter 4.2.5.2.6, applies to refrigerated liquefied gases: cargo that is liquid because it is below its boiling point, maintained by insulation rather than by pressure. LNG (UN 1972), liquid oxygen (UN 1073), liquid nitrogen (UN 1977), liquid argon (UN 1951), and liquid helium (UN 1963) travel in T75 tanks.
The defining design feature of a T75 tank is the holding time: the time calculated before the pressure of the refrigerated liquefied gas in the tank reaches the start-to-discharge pressure of the pressure-relief device, starting from the maximum allowable working pressure and the maximum filling density at the intended transport temperature. The holding time must exceed the expected voyage duration plus a safety margin. For LNG in a well-insulated T75 tank with a 24-hour boil-off rate of about 0.3% of liquid volume, a 20-day voyage requires a minimum holding time of roughly 30 days to provide adequate margin.
T75 tanks are vacuum-insulated: a vacuum annular space between the inner and outer shells, typically filled with multi-layer insulation (MLI), reduces heat ingress to milliwatts per square metre. Test pressure for T75 tanks is generally low (4 to 10 bar) because the cargo is carried near atmospheric pressure. Pressure-relief devices vent boil-off gas to atmosphere or to a recovery system. The outer shell must be designed to withstand the maximum vacuum that could develop if the inner shell fails.
Multiple-element gas containers (MEGCs)
MEGCs are clusters of cylinders, tubes, or pressure receptacles mounted in a frame as a single transport unit, sharing a manifold and common pressure-relief device. They are covered in IMDG Code Chapter 4.2.4 and Chapter 6.7.4. MEGCs are common for high-volume industrial gas shipments: a 20-foot frame holding twelve high-pressure tubes of 450 litres each gives a total capacity of 5,400 litres, far more than a standard bundle of cylinders.
For Division 2.3 toxic gases, IMDG Code Chapter 4.2.4.2 requires that each element of a MEGC carrying a toxic gas be fitted with an individual isolation valve so that a leaking element can be isolated from the rest of the cluster without venting the entire manifold. General-service MEGCs used for inert or flammable gases may share a single manifold with no element-level isolation.
MEGCs are subject to the same periodic inspection regime as portable tanks under Chapter 6.7: initial inspection, 5-year periodic inspection, and 2.5-year intermediate inspection.
Aerosols under UN 1950 and the P207/LP02 scheme
Aerosols present a special case in Class 2 regulation. An aerosol is an article, not a substance, and its classification depends on the flammability and toxicity of both the propellant and the product. IMDG Code Chapter 2.2.1 (aerosols) defines the classification procedure.
Aerosol classification hierarchy:
- If the aerosol contains any substance classified as toxic (Division 6.1, Packing Group I or II) or corrosive (Class 8, Packing Group I): classified as Division 2.3 or 8 as appropriate, not under UN 1950.
- If flammable components (liquids with flash point at or below 93 degrees C, flammable gases, or flammable solids) comprise 85% or more by mass of the contents: classified as extremely flammable (aerosol category 1, UN 1950 with label 2.1).
- If flammable components comprise 1% to 85% by mass, or heat of combustion is at least 20 kJ/g: flammable aerosol (category 2, UN 1950 with label 2.1).
- If flammable components are below 1% and heat of combustion is below 20 kJ/g: non-flammable (category 3, UN 1950 with label 2.2).
Packing instruction P207 applies to aerosols in IMDG Code Chapter 4.1. P207 limits receptacle volume to 1,000 ml (1 litre). The complementary instruction LP02 covers large aerosols above 1,000 ml up to 1,000 ml containers… actually P207 covers up to 1,000 ml and LP02 covers aerosols exceeding 1,000 ml capacity up to maximum 5 litres. Both instructions require the aerosol to pass the aerosol flame projection test and the aerosol density test defined in the UN Manual of Tests and Criteria.
The limited-quantity provisions for UN 1950 aerosols are generous: the IMDG Code limited-quantity (LQ) exemption allows up to 500 ml per aerosol and up to 30 kg gross mass per package without the full DGL entry requirements, significantly reducing documentation and marking requirements for small retail aerosol shipments. The IMDG limited quantity calculator evaluates whether a specific consignment qualifies.
Stowage requirements for Class 2
Stowage categories in the Dangerous Goods List
The IMDG Code Chapter 7.1 Stowage category system assigns each DGL entry to one of five stowage categories (A through E):
| Category | Permission |
|---|---|
| A | On deck or under deck |
| B | On deck or under deck, but on deck preferred |
| C | On deck only |
| D | On deck only, in a closed cargo transport unit (CTU) |
| E | On deck only, in a closed CTU, away from living quarters and ignition sources |
Division 2.1 compressed gases (oxygen cylinders, helium) typically carry category A. Division 2.1 liquefied gases (LPG, propylene) and refrigerated liquefied gases typically carry category B. Division 2.3 toxic gases nearly universally carry category D or E: on-deck only in a closed CTU. This prevents leak accumulation in enclosed cargo spaces, where concentrations can reach lethal levels before detection.
Special stowage provisions from the SW codes
Beyond the category, each DGL entry may carry stowage codes (SW codes) that add to the category. The relevant SW codes for Class 2:
SW1 (recommended to be stowed under deck on passenger ships): limits passenger ship carriage of many flammable gas entries to under-deck stowage where the fire-detection and suppression systems are more effective.
SW2 (protected from heat): required for many Division 2.1 liquefied gas entries. The vapour pressure of an LPG tank rises about 0.4 bar per degree C at ambient temperature; a 30-degree C rise from 20 to 50 degrees C increases the tank pressure by about 12 bar, potentially exceeding the working pressure of smaller cylinders. Protection from heat means: not adjacent to heated structures (funnel casings, engine room bulkheads), not in direct sunlight without shade, not in a position that concentrates solar radiation.
SW22 (keep as dry as possible): applies to some hygroscopic gas entries where moisture absorption into the propellant system could cause hazards.
The IMDG segregation calculator computes both the category-based and SW-code stowage requirements for any pair of Class 2 entries and any other dangerous goods class.
Ventilation requirements
IMDG Code Chapter 7.1.7 requires that cargo spaces containing gases from the Dangerous Goods List (other than on-deck stowage) have mechanical ventilation capable of a minimum number of air changes per hour. Division 2.2 inert gases in closed cargo holds require standard ventilation (at least 6 air changes per hour). Division 2.1 flammable gases require ventilation capable of preventing accumulation above 25% of the LFL. Division 2.3 toxic gases must be on deck (category D/E) rather than in a ventilated hold, precisely because no credible mechanical ventilation rate prevents lethal concentrations from a significant tank or cylinder failure.
Segregation requirements for Class 2
The segregation table in Chapter 7.2
IMDG Code Chapter 7.2 establishes the segregation requirements between classes. The table uses four separation levels:
- Away from (1 unit distance in the same cargo space)
- Separated from (in a different compartment or different container on deck)
- Separated by a complete compartment or hold from (a structural ship division between the two)
- Separated longitudinally by an intervening complete compartment or hold from (the most stringent: a full hold-length separation)
Key Class 2 segregation requirements:
Division 2.1 from Class 1 (Explosives): segregation levels vary by compatibility group, but flammable gases from Class 1 typically require “separated from” or stricter.
Division 2.1 from Class 5.1 (Oxidizers): “separated from.” Flammable gas next to an oxidizer creates a fire-on-contact risk.
Division 2.2 (Oxidizing gases, i.e., oxygen) from Class 2.1: “separated from.” Oxygen accelerates flammable gas fires.
Division 2.3 from Class 3 (Flammable Liquids): “separated by a complete compartment or hold from” in most interpretations, because a toxic gas leak near flammable liquid vapour combines two independently dangerous atmospheres.
Division 2.3 from Class 8 (Corrosives): “separated from” minimum. Corrosives attack the valve seats and cylinder bodies of Division 2.3 cylinders, increasing leak probability.
Division 2.3 from foodstuffs: “separated by a complete compartment or hold from,” because a toxic gas leak can contaminate edible cargo with lethal residues.
Substance-specific segregation codes (SG codes) in the DGL may impose stricter requirements for individual UN numbers. For UN 1005 (ammonia, anhydrous), code SG35 prohibits stowage near ammonium nitrate; SG46 requires segregation from chlorine (UN 1017) because ammonia plus chlorine produces nitrogen trichloride, an explosive and toxic compound. SG codes take precedence over the general class-level segregation table.
The IMDG segregation calculator applies both the class-level table and the SG codes simultaneously for any pair of UN numbers.
Emergency response: EmS and MFAG
EmS fire schedules for Class 2
The Emergency Response Procedures for Ships Carrying Dangerous Goods (EmS Guide), published as part of the IMDG Code Supplement, assigns each Class 2 entry two codes: a fire schedule (F-x) and a spillage schedule (S-x). The codes are tabulated in the EmS Guide rather than in the Code itself.
F-C (general fire involving non-flammable gases): apply to cargo fires near Division 2.2 entries. Cool containers with water, control the fire in surrounding materials, allow the gas to vent through pressure-relief devices. Do not attempt to stop pressure-relief venting: the alternative is cylinder rupture.
F-D (fire involving flammable gases): the critical instruction here is to not extinguish a flammable gas fire unless the source can be simultaneously isolated. A burning flammable gas fire is self-limiting because the combustion zone contains the gas. Extinguishing it creates an unburned vapour cloud that will re-ignite with potentially explosive force. F-D instructs: isolate the source if safe, cool surrounding structures with water spray to prevent heat-induced BLEVE, fight any secondary fire in the surrounding materials, do not attempt to extinguish the primary gas flame.
F-E (fire involving gases producing toxic combustion products): applies to Division 2.3 entries that produce toxic combustion products other than carbon dioxide. Phosgene (UN 1076) is the extreme case: burning phosgene produces carbon dioxide, but phosgene’s hydrolysis products in the presence of moisture are hydrochloric acid and carbon dioxide. Chlorine combustion produces hydrogen chloride and phosgene. F-E instructs full SCBA for all personnel within the smoke plume.
EmS spillage schedules for Class 2
S-Q (spill of flammable liquefied gas from tank): applies to spillage of Division 2.1 liquefied gases from portable tanks. Evacuate the area, ventilate, eliminate ignition sources, allow the gas to evaporate from a safe distance.
S-U (spillage of toxic gas): the most demanding spillage schedule in the EmS system. Muster all crew on the windward side immediately. Don self-contained breathing apparatus (SCBA) before approaching the affected area. Attempt source isolation only with full PPE, and only with command authorization and a designated backup team in SCBA. If the leak cannot be controlled, consider ship abandonment. S-U applies to all Division 2.3 entries.
S-V (spillage of refrigerated liquefied gases): applies to cryogenic spills. Avoid all contact with the liquid (cold burns are instantaneous and severe). Do not direct water jets at the liquid pool (water causes violent evaporation and a larger vapour cloud). Allow the liquid to evaporate from a safe upwind position. The vapour cloud from liquid nitrogen is visible (condensed atmospheric moisture) but the gas itself is colourless and will flow into low areas; personnel in basements or engine rooms must evacuate.
The IMDG EmS lookup returns the specific F-x and S-x codes for any Class 2 UN entry.
MFAG for Class 2 casualties
The Medical First Aid Guide (MFAG), also published in the IMDG Code Supplement, provides onboard treatment protocols for exposures to each class. For Class 2 toxic gas exposures, the MFAG directs crews to remove the casualty to fresh air immediately, apply supplemental oxygen, and contact a medical professional by radio. For specific substances, MFAG tables address: chlorine (treat as pulmonary irritant; no specific antidote; supportive care, bronchodilators for bronchospasm), ammonia (pulmonary irritant; irrigation of affected mucous membranes; supplemental oxygen), hydrogen sulphide (rapid-onset paralysis of the respiratory centre at concentrations above 500 ppm; immediate rescue mandatory before the rescuer succumbs; supplemental oxygen and in severe cases sodium nitrite/thiosulphate antidote under medical supervision). For cryogenic injuries, the MFAG instructs standard cold-burn treatment: remove contaminated clothing, irrigate with tepid (not hot) water, do not rub frozen tissue.
The IGC Code boundary: packaged versus bulk gas carriage
SOLAS Chapter VII structure
SOLAS Chapter VII divides gas carriage into two regimes. Part A (Regulations 1-6) addresses carriage of dangerous goods in packaged form and gives the IMDG Code its mandatory force. Part C (Regulations 13-17) addresses ships carrying liquefied gases in bulk and gives the IGC Code its mandatory force. The two parts are mutually exclusive for any given parcel of cargo.
A container of LPG cylinders on a containership is Part A cargo (IMDG Code applies). The same LPG carried in the dedicated cargo tanks of an LPG carrier is Part C cargo (IGC Code applies). No LPG enters the IMDG Code regime and the IGC Code regime simultaneously for the same parcel.
IGC Code scope
The IGC Code, in its 2016 edition (consolidated), governs the construction, equipment, and operation of ships carrying liquefied natural gas, LPG, ethylene, ammonia, propylene oxide, and approximately 150 other liquefied gas products in bulk cargo containment systems. The code became mandatory under SOLAS from 1 July 1986 for ships built after that date.
IGC-classed ships carry dedicated cargo tanks designed for the specific cargo: pressure vessels for fully pressurized carriage (LPG at ambient temperature, 17 to 18 bar for propane), semi-refrigerated tanks for partial refrigeration, and atmospheric/low-pressure insulated tanks for fully refrigerated carriage (LNG at minus 163 degrees C, near atmospheric pressure). These tanks are integrated into the ship’s structure in a way that has no parallel in the IMDG Code’s portable-tank regime.
A vessel simultaneously carrying Class 2 packaged goods in deck containers (IMDG Code) and LNG cargo in IGC-classed tanks must comply with both codes. The IMDG Code stowage and segregation rules apply to the deck containers, including segregation from the IGC tank manifolds and vent masts.
LNG-fuelled ships and the IGF Code
Ships using LNG as fuel (gas-fuelled vessels) are regulated by the IGF Code (International Code of Safety for Ships Using Gases or Other Low-flashpoint Fuels), adopted by MSC.391(95) on 11 June 2015 and mandatory under SOLAS Chapter II-1 Part G from 1 January 2017. The IGF Code governs the LNG fuel tanks and fuel-gas supply systems on these ships. IMDG Code carriage of packaged Class 2 cargo on the same LNG-fuelled vessel is independent of the IGF Code, but the IMDG Code segregation rules apply to any packaged Class 2 cargo and the LNG fuel tanks.
Notable examples: LPG, LNG, CO2, chlorine, and ammonia
LPG (UN 1075)
LPG (petroleum gases, liquefied) is the most common Division 2.1 entry by volume in container and ro-ro shipping. Propane (UN 1978) and butane (UN 1011) are the pure-component equivalents; UN 1075 covers blends. Shipping in T50 portable tanks at filling densities of 0.43 kg/litre (propane) to 0.53 kg/litre (butane) and working pressures of 18 to 22 bar at 50 degrees C. Stowage category B; EmS F-D, S-Q. The IMDG tank container calculator returns the T-code and maximum filling density for UN 1075.
The BLEVE risk from LPG is well-documented. The 1971 Feyzin refinery disaster in France killed 18 people when a sphere of propane caught fire; the BLEVE destroyed a second sphere and 10 people sheltering nearby. The maritime analogy is a T50 tank container on a vessel fire; IMDG Code stowage codes require that LPG tanks on deck have cooling water available and that the ship carries adequate firefighting equipment for the cargo quantity.
LNG (UN 1972)
LNG as packaged cargo under IMDG Code is rare: the overwhelming majority of LNG carriage is bulk under IGC Code. When LNG does move as packaged cargo, it is in T75 cryogenic tanks at minus 162 degrees C, near atmospheric pressure, stowage category B. EmS F-D (flammable gas fire schedule), S-V (refrigerated liquefied gas spill). The holding time calculation for a T75 LNG tank assumes a boil-off rate of 0.3% per day; a 20-day voyage requires a holding time of at least 25 days to avoid pressure-relief venting during transport.
Carbon dioxide (UN 1013, UN 2187)
Carbon dioxide is Division 2.2, asphyxiant. Compressed CO2 (UN 1013) is shipped in small cylinders for food and beverage use and in large high-pressure cylinders for industrial processes. Refrigerated liquid CO2 (UN 2187) is shipped in insulated tanks for dry-ice production and food freezing. The critical operational hazard with CO2 is confined-space asphyxiation: CO2 is heavier than air (density 1.53 kg/m3 versus 1.22 kg/m3 for air) and settles in bilges, tank pockets, and enclosed holds. IMDG Code stowage category A for UN 1013; stowage category B for UN 2187 because the refrigerated liquid requires more careful handling.
Chlorine (UN 1017)
Chlorine is Division 2.3, TIH Zone B (LC50 ~293 ppm). It is also a strong oxidizer (subsidiary risk 5.1) and corrosive (subsidiary risk 8). Stowage category D, EmS F-C, S-U. Chlorine travels in T50 tanks at filling density 1.25 kg/litre. Special provision TP19 limits the maximum allowable working pressure of T50 tanks used for chlorine to 4 bar at 65 degrees C, consistent with its saturation pressure at that temperature.
Port pre-notification requirements for chlorine are strict: most major ports require 96-hour advance notice of chlorine cargo, designated anchorages away from the port area for waiting periods, and pilotage and escort for harbour transit. Some ports refuse chlorine entirely. The Montreal Protocol does not restrict chlorine transport (it addresses chlorinated organic compounds, not elemental chlorine), but national regulations in several countries restrict chlorine imports.
Ammonia, anhydrous (UN 1005)
Ammonia is Division 2.3 (also 8 corrosive). LC50 ~4,740 ppm places it in TIH Zone D, the least toxic of the Division 2.3 zones, but ammonia at 1,000 ppm is still immediately dangerous to life (IDLH threshold). Stowage category D; EmS F-C (non-flammable gas fire schedule, because the flammable subsidiary risk of ammonia is marginal at LFL 15%), S-U (toxic gas spillage). Filling density in T50 tanks: 0.53 kg/litre. Special provisions SG35 and SG46 prohibit stowage near ammonium nitrate and chlorine respectively.
The flammability of ammonia is a real but secondary hazard: LFL 15%, UFL 28%, making it difficult to ignite without a strong ignition source and confined atmosphere. The toxic hazard dominates the emergency response calculus. The IGC ammonia calculator addresses ammonia carriage in IGC-classed bulk tankers; for packaged ammonia in T50 tanks, the IMDG IMDG tank container calculator applies.
Documentation requirements for Class 2 shipments
Dangerous Goods Declaration
IMDG Code Chapter 5.4 requires a Dangerous Goods Declaration (DGD) for every Class 2 shipment in packaged form. The DGD must contain:
- UN number (e.g., UN 1005)
- Proper shipping name (Ammonia, anhydrous)
- Class and division (2.3)
- Subsidiary risk(s) (8)
- Packing group (Class 2 entries have no packing group; column 4 of the DGL is blank)
- Net quantity and type of packaging or tank
- Container/vehicle identification
- Emergency contact name and telephone number (24-hour)
Class 2 entries do not carry a packing group: the IMDG Code assigns packing groups I, II, and III to Classes 3-6.2 and Class 8 based on degree of hazard within the class, but Class 2 uses the Division (2.1, 2.2, 2.3) as the hazard-level indicator rather than a packing group. This absence of a packing group is a common error in DGD preparation for gas shipments.
Container/Vehicle Packing Certificate
IMDG Code Chapter 5.4.2 requires a Container/Vehicle Packing Certificate (CPC) when Class 2 cargo is loaded into a container. The CPC confirms that the container was in a proper condition to receive the cargo, that packing was done in accordance with the IMDG Code, and that the container/vehicle was marked and placarded correctly.
Placarding and marking
Class 2 portable tanks and containers must display the appropriate placard: diamond-shaped, with the class number at the bottom and the division symbol (flame for 2.1, cylinder for 2.2, skull/crossbones for 2.3 or the subsidiary risk where applicable). The placard size for a portable tank or freight container is 250 mm x 250 mm. For Division 2.3 entries, an inhalation hazard mark (where applicable under the DGL entry) must also be displayed.
Limitations
The regulatory picture in this article reflects IMDG Code Amendment 41-22, mandatory from 1 January 2024 under SOLAS Chapter VII. Amendment 42-24 was adopted by Resolution MSC.556(108) in May 2024 and introduces further changes with a mandatory date of 1 January 2026; shipments after that date should be checked against 42-24.
Class 2 gas classifications are driven by measured physical properties (LFL, flammable range, LC50) that can vary with mixture composition, temperature, and pressure. The DGL entries in the IMDG Code specify classifications for pure substances and named mixtures; unnamed mixtures (gas blends, n.o.s. entries) require classification by the shipper using the test methods in the UN Manual of Tests and Criteria.
Port-state-specific restrictions for Division 2.3 entries (particularly chlorine, phosgene, and hydrogen cyanide) vary by jurisdiction and may be stricter than the IMDG Code minimum. The Montreal Protocol restricts the production and trade of methyl bromide (UN 1062) under its ozone-depleting-substance schedule; methyl bromide movements may require permits beyond what the IMDG Code mandates.
The T50 and T75 tank inspection regimes, and the Chapter 6.2 cylinder inspection requirements, create a time-sensitive compliance obligation: tanks and cylinders whose periodic inspection date has passed cannot be legally offered for transport. The IMDG tank container calculator identifies the T-code for a given UN entry but does not verify the inspection status of a specific tank; that verification is the shipper’s and packer’s responsibility.
This article covers packaged Class 2 cargo under IMDG Code only. Bulk gas carriage in IGC-classed ships and LNG fuel systems under the IGF Code are distinct regulatory regimes not covered here. The IGC Code wiki article covers bulk gas carriage.
See also
- IMDG Class 1: Explosives
- IMDG Class 3: Flammable Liquids
- IMDG Class 4: Flammable Solids
- IMDG Class 5: Oxidisers and Organic Peroxides
- IMDG Class 6: Toxic and Infectious Substances
- IMDG Class 8: Corrosive Substances
- IMDG Class 9: Miscellaneous Dangerous Goods
- IGC Code: Construction of Gas Carriers
- SOLAS Chapter VII: Carriage of Dangerous Goods
- IMSBC Code
- IMDG segregation calculator
- IMDG EmS lookup
- IMDG packing group calculator
- Container IMDG class lookup
- IMDG tank container calculator
- IMDG limited quantity calculator
- IMO IMDG general calculator
- Gas carrier tank pressure relief calculator
- IGC Type C tank MARVS check
- IGC ammonia calculator