2.3 Class 2: Gases
Key Takeaways
Class 2 covers pure gases, gas mixtures, and articles containing gases, categorized by their physical state (compressed, liquefied, refrigerated liquefied, dissolved, adsorbed, and aerosols).
Class 2 is structured into three primary hazard divisions: 2.1 Flammable gases, 2.2 Non-flammable non-toxic gases, and 2.3 Toxic gases.
Physical hazards unique to gases include massive pressure energy, extreme cold causing frostbite and brittle metal failure, rapid asphyxiation in confined spaces, and explosive BLEVE hazards.
A Boiling Liquid Expanding Vapor Explosion (BLEVE) occurs when external fire heats a pressure vessel containing liquefied gas, causing catastrophic structural rupture, blast shockwaves, and massive thermal fireballs.
Pressure receptacles (cylinders, bundles, tubes, drums, cryogenic receptacles) must have protective valve caps securely fitted, valves fully closed, and be secured in the orientation permitted for that receptacle and gas; upright carriage is not universal for every cylinder.
Class 2: Gases
Core Concept: Class 2 encompasses substances that have a vapor pressure greater than 300 kPa (3 bar) at 50°C or are completely gaseous at 20°C at standard atmospheric pressure (101.3 kPa). Because gases are transported under substantial pressure or in liquefied cryogenic states, their containment and physical behaviors present immediate, multi-faceted hazards.
1. Physical States of Gases in Transport
Unlike solids or liquids, gases change physical properties dramatically based on pressure and temperature. ADR 2.2.2.1.2 categorizes gases into distinct physical states:
- Compressed gas: A gas which, when packaged under pressure for carriage, remains entirely gaseous at -50°C (e.g., UN 1066 Nitrogen, UN 1002 Air, compressed).
- Liquefied gas: A gas which, when packaged under pressure, is partially liquid at temperatures above -50°C. This includes high-pressure liquefied gases (critical temperature between -50°C and +65°C, such as UN 1013 Carbon Dioxide) and low-pressure liquefied gases (critical temperature above +65°C, such as UN 1978 Propane and UN 1011 Butane).
- Refrigerated liquefied gas (Cryogenic gas): A gas which, when packaged for transport, is maintained in a liquid state at very low temperatures due to its refrigeration (e.g., UN 1977 Nitrogen, refrigerated liquid at -196°C, UN 1073 Oxygen, refrigerated liquid).
- Dissolved gas: A gas which, when packaged under pressure, is dissolved in a liquid phase solvent (e.g., UN 1001 Acetylene dissolved in acetone within a porous mass inside the cylinder).
- Adsorbed gas: A gas adsorbed onto a solid porous material inside a receptacle.
- Aerosols (UN 1950) and gas cartridges (UN 2037): Non-refillable receptacles containing gas under pressure used to expel liquids, pastes, or powders.
2. Hazard Divisions and Classification Codes
ADR Class 2 does not assign packing groups. Instead, gases are categorized into three hazard divisions, denoted by their primary hazard label diamonds:
| Division | Classification | Label Diamond | Characteristic Properties | Common Examples |
|---|---|---|---|---|
| Division 2.1 | Flammable gases | Red diamond with flame symbol | Ignitable in a mixture of 13% or less by volume with air, or having a flammable range of at least 12 percentage points. | UN 1978 Propane, UN 1077 Propylene, UN 1001 Acetylene, UN 1035 Ethane. |
| Division 2.2 | Non-flammable, non-toxic gases | Green diamond with gas cylinder symbol | Asphyxiant gases (diluting or replacing atmospheric oxygen) or oxidizing gases (providing oxygen more readily than air to stimulate fire). | UN 1066 Nitrogen, UN 1013 Carbon Dioxide, UN 1006 Argon, UN 1072 Oxygen (requires 5.1 oxidizer subsidiary label). |
| Division 2.3 | Toxic gases | White diamond with skull and crossbones | Known to be toxic or corrosive to humans, or having an LC50 <= 5,000 ppm. Highly dangerous by inhalation. | UN 1017 Chlorine, UN 1005 Ammonia, anhydrous, UN 1076 Phosgene, UN 1079 Sulfur dioxide. |
In transport documents and ADR classification codes, Class 2 substances combine a numerical physical state (including compressed, liquefied, refrigerated, dissolved and adsorbed groups) with a capital letter denoting the hazard group: A (Asphyxiant), O (Oxidizing), F (Flammable), T (Toxic), TF (Toxic, flammable), TC (Toxic, corrosive), TO (Toxic, oxidizing), TFC (Toxic, flammable, corrosive), or TOC (Toxic, oxidizing, corrosive).
3. Physical Hazards of Gases
Transporting Class 2 dangerous goods exposes drivers and emergency personnel to severe, rapid-onset hazards:
- High Pressure Energy: Industrial gas cylinders operate at pressures from 200 to over 300 bar. A sheared valve turns a 70 kg steel cylinder into an unguided kinetic rocket capable of smashing through brick walls.
- Asphyxiation Hazard: Division 2.2 gases (such as nitrogen, argon, and carbon dioxide) have no odor or color. If a cylinder leaks into an enclosed truck cab or box van, oxygen can fall to a dangerous level without a reliable sensory warning. There is no guaranteed time to escape or safe number of breaths. Furthermore, gases heavier than air (e.g., propane, carbon dioxide, butane) pool in low-lying ground, culverts, inspection pits, and storm drains.
- Extreme Cold and Cryogenic Burns: Rapid depressurization of liquefied gas causes flash vaporization, absorbing heat and plunging metal fittings to sub-zero temperatures. Contact can cause serious cold injury (cryogenic frostbite). Cryogenic liquids expand greatly when vaporised: a small volume of cryogenic liquid can produce a much larger volume of gas on warming, displacing oxygen in a confined space.
The BLEVE Hazard (Boiling Liquid Expanding Vapor Explosion)
The single most catastrophic event in dangerous goods road transport is a BLEVE.
- Mechanism: A transport tanker or cylinder containing liquefied gas (such as LPG or propane) is engulfed in an external fire (often originating from dragging vehicle brakes or blown tires). The liquid inside boils rapidly, raising internal pressure and triggering safety relief valves. However, as the liquid level drops, the upper steel shell of the tank is exposed directly to intense flames without liquid behind it to conduct heat away. The steel softens, loses structural tensile strength, and catastrophically ruptures.
- Consequences: The superheated pressurized liquid instantaneously vaporizes in a fraction of a second, causing a gigantic blast overpressure wave, launching tank fragments thousands of meters, and igniting a massive fireball (up to several hundred meters in diameter) radiating lethal heat.
- Driver Countermeasures: If a fire impinges directly upon the vapor space of a gas tank and cannot be brought under control immediately, drivers must never approach. Evacuate the surrounding area immediately, retreat well clear, warn others without approaching and tell responders about the pressure-vessel fire. ADR does not prescribe a universal 500–800-metre driver perimeter; do not approach to establish one.
4. Pressure Receptacles, Handling, and Load Securing
ADR 6.2 and 7.5 enforce strict physical safety rules for handling gas packaging:
- Types of receptacles: Individual cylinders, bundles of cylinders (manifolds connected together in a rigid frame), tubes, pressure drums, and closed cryogenic receptacles.
- Valve Protection: Cylinders must have their protective valve caps, shrouds, or collars securely attached during transit. Protection may be provided by the valve design, a permanent guard or another authorised arrangement; not every receptacle needs a removable cap.
- Valve Closure: Drivers must verify that applicable isolation valves and closures are secured as designed, without obstructing pressure-relief or cryogenic vent systems before commencing transport.
- Orientation and Securing: Use the cylinder’s authorised orientation and secure it against falling or rolling. Receptacles designed for upright use, including many LPG cylinders, must remain upright so their pressure relief arrangement functions as designed. In open or curtainsider vehicles, cylinders must be secured using lashing straps (EN 12195-1) against headboards or heavy-duty transport cradles to resist vehicle braking forces (0.8g forward, 0.5g lateral and rearward).
- Ventilation: Apply the ventilation provisions assigned to the gas, including CV36 where specified. Opening a door is not proof that an atmosphere is safe. Do not enter a suspected oxygen-deficient compartment; controlled ventilation and assessment belong to trained authorised personnel.
Source check: 17 October 2026 — ADR 2025, English text published by the Slovak Ministry of Transport, 2.2.2, P200 and CV9/CV10/CV36.
What dangerous phenomenon occurs when a transport tank containing liquefied gas is engulfed in an external fire, causing the dry upper metal shell to soften and catastrophically rupture?
Adiabatic catalytic decomposition
Vapor cloud flashback
Boiling Liquid Expanding Vapor Explosion (BLEVE)
Cryogenic sublimation collapse
Why are Division 2.2 non-flammable, non-toxic gases such as Nitrogen (UN 1066) particularly lethal when leaking into enclosed driver cabs or poorly ventilated box trailers?
They spontaneously ignite upon contact with vehicle upholstery
They rapidly dissolve standard automotive safety glass
They generate intense ionizing radiation that penetrates steel
They are completely odorless and displace atmospheric oxygen, causing rapid unconsciousness and asphyxiation
What is the primary physical reason that cylinders containing liquefied flammable gases like propane (UN 1978) should be stowed and transported in an upright vertical position?
To ensure that pressure relief safety valves remain in communication with the vapor phase rather than liquid
To prevent the cylinder steel wall from experiencing atmospheric friction
To comply with minimum axle load distribution requirements under ADR Chapter 9
To ensure the cylinder serial markings remain visible from the rear doors
Which Class 2 gas division requires a white hazard diamond displaying the skull and crossbones symbol?
Division 2.1 Flammable gases
Division 2.3 Toxic gases
Division 2.2 Asphyxiant gases
Division 2.2 Oxidizing gases
Sections you finish are checked off in the contents.