January 10, 2022 2 min read

Have you ever noticed the diamond-shaped signs on the back of truck trailers and wondered what they are and why they’re there? Well, those are called HAZMAT placards! They are required by the US Dept. of Transportation and provide information on what the truck is hauling and informs emergency responders of what substances are involved in case of an accident.

There are more than twenty-five different kinds of HAZMAT placards with nine hazard classes, some of those classes have a subclass used to identify more closely the substance being carried.

HAZMAT Classes

Class 1 — Explosives 
Class 1 HAZMAT placards are for explosives like TNT, ammunition, and fireworks. These are usually yellowish orange in color and has 6 subcategories:
1: Mass Explosion Hazard
2: Mass Projection Hazard
3: Fire/Minor Blast Hazard
4: Minor Explosion Hazard
5: Blasting Agent
6: Very Insensitive Material

Class 2 — Gases 
Class 2 is for items containing gases. Think along the lines of fire extinguishers, propane tanks, and gas cartridges. These placards come in different colors and have different meaning for each!
Green indicates non-flammable gas
Red indicates flammable gas.
White indicates toxic gas
White with a black skull indicates an inhalation hazard

Class 3 — Flammable and combustible liquids
Class 3 HAZMAT placards are for flammable gases like kerosene or gasoline. These are usually BRIGHT red and read "FLAMMABLE" or "COMBUSTIBLE".

Class 4 — Flammable materials 
Class 4 HAZMAT placards are for flammable solids, like matches or powdered metals. These come in 3 color schemes.
Red and White indicates flammable solids
White upper half, red lower half indicates spontaneously combustible solids
Blue indicates water-reactive solids.

Class 5 — Oxidizer and organic peroxide 
Class 5 HAZMAT placards are used for oxidizing substances and organic peroxides. They have two subcategories.
1: Oxidizer
2: Organic peroxide.

Class 6 — Poisons
Class 6 HAZMAT placards are for toxic or infectious substances like medical waste and strong poisons, such as cyanide. These have two designs: white to indicate toxicity and white with a skull to indicate inhalation hazard.

Class 7 — Radioactive 
Class 7 HAZMAT placards are for radioactive substances like radioactive ore or medical isotopes. They have a yellow top that shows the international symbol for radiation and a white bottom part.

Class 8 — Corrosive
Class 8 HAZMAT placards are used to indicate corrosive chemicals, like lye and certain paints. They have a white top with a detail of a chemical burning a human hand and a black bottom.

Class 9 — Miscellaneous 
Class 9 HAZMAT placards are for miscellaneous substances like dry ice. These are black and white striped on top with a white bottom.

For general guidance of US DOT Hazardous Materials, you can check out this link here!