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Liquid oxygen is a non-flammable gas carried as a cryogenic (refrigerated) liquid, so it takes the green Division 2.2 placard, often marked OXYGEN. The trick is that oxygen is an oxidizer by nature but is not placarded as a yellow Class 5; in transport it rides as a green 2.2 gas. The extreme cold and strong fire-feeding are real added hazards behind the green placard.
Liquid oxygen is a 2.2 gas
Oxygen in transport is a gas hazard, and as a non-flammable gas it takes the green Division 2.2 placard. Liquid oxygen is simply oxygen cooled to a cryogenic liquid to ship it in bulk, so it is a refrigerated liquefied gas, still placarded green as a non-flammable gas, often marked OXYGEN or as a refrigerated liquid.
The Class 5 oxidizer trick
The classic trap is that oxygen is an oxidizer by nature, it feeds fire intensely, so people reach for the yellow Class 5 oxidizer placard. But compressed or liquid oxygen is not placarded as Class 5; it rides as a green 2.2 gas. The yellow Class 5.1 oxidizer placard is for oxidizing solids and liquids, not the oxygen itself.
Liquid oxygen at a glance
The cues to recognize:
| Item | Liquid oxygen |
|---|---|
| Hazard class | Division 2.2 (non-flammable gas) |
| Color | Green |
| Marking | Often OXYGEN / refrigerated liquid |
| Common wrong guess | Yellow Class 5 oxidizer |
| Added hazards | Extreme cold; feeds fire |
Liquid oxygen is green 2.2, not yellow 5. Confirm in your official manual.
The cold and fire-feeding hazards
Even though it is placarded green, liquid oxygen carries serious added hazards: it is extremely cold (a cryogenic burn and pressure risk) and it dramatically feeds fire, making other materials ignite and burn far more intensely. So green non-flammable gas is the placard, but the oxygen still makes fire much worse, which is why it is handled so carefully.
How to study and verify
Lock in liquid oxygen as a green Division 2.2 gas marked OXYGEN, not a yellow Class 5 oxidizer, and remember the cold and fire-feeding as real dangers behind the green placard. Drill it against the yellow 5.1 oxidizer (for oxidizing solids and liquids) so the trick cannot bait you. Verify the specifics in your official state CDL manual.
Frequently asked questions
- What placard does liquid oxygen use?
- The green Division 2.2 non-flammable gas placard, often marked OXYGEN or as a refrigerated liquid. It is an oxidizer by nature but not placarded as a yellow Class 5; it rides as a green 2.2 gas. Confirm in your official manual.
- Why isn't oxygen a Class 5 oxidizer placard?
- Because compressed or liquid oxygen is placarded as a gas (green 2.2), not as an oxidizing solid or liquid. The yellow Class 5.1 oxidizer placard is for oxidizers, not the oxygen cylinder or tank itself.
- Is liquid oxygen dangerous if it is non-flammable?
- Yes. It does not burn, but it strongly feeds fire (making other materials burn hotter), and it is extremely cold as a cryogenic liquid, adding cold-burn and pressure hazards behind the green placard.