The DOT hazard classification system can be used to identify acute hazards.
explosives
The number 4 on the NFPA 704 placard indicates the highest hazard in any of the three categories (health, fire, reactivity).
COLOR
The radioactive placard on a vehicle indicates that the vehicle carries some amount or regulated radioactive material.
color of label or placard
color of the label or placard
A placard identifying hazard class 6 describes the presence of a poisonous gas. For an image, see the related link, below.
The hazard class of the substance being carried
symbol, UN hazard class number, color, UN identification number
A poisonous gas placard typically features the hazard symbol for toxic chemicals, which is a skull and crossbones symbol. The placard may also include the UN number for the specific poisonous gas, as well as any additional information required by regulations. The placard is usually in a diamond shape and is orange with black symbols.
It is a DOT hazmat placard meaning "compressed oxygen."