The U.S. Department of Transportation (DOT) requires hazardous materials, or hazmat, to be labeled with uniform, standardized warning signs. These color-coded, diamond-shaped hazmat placards (they're squares on end; with the words upright, the corners of the square point vertically and horizontally) must be affixed to all shipments of hazardous materials. The signs warn handlers and others that there is dangerous material present. In the event of an emergency situation, the signs also notify first responders of exactly what hazardous material they are dealing with. Hazmat placards indicate the type of material with both numbers and words. Some signs also have pictures, such as the toxic materials placard, which features a skull and crossbones.

Examine the shape of the placard. All hazmat signs are diamond-shaped.

Check the number at the bottom point of the placard. All hazmat signs have a single-digit number to indicate what class of hazardous material it is. There are nine classes. For example, a “1” indicates that the material is explosive, while a “7” indicates radioactivity. A “3” means that it is a flammable liquid, a “4” indicates a flammable solid and a “6” indicates toxic or poisonous material.

Read the lettering on the placard if you are unsure of what a number may indicate. For example, a class 1 placard will typically have “Explosives” or “Blasting Agents” printed on it. Flammable material will usually say “Flammable,” “Combustible” or “Fuel Oil.” A sign indicating radioactivity will simply say “Radioactive,” while a class 6 placard will say “Poison,” “Toxic” or “Inhalation Hazard.”

Look for a four-digit number. This is separate from the one-digit number indicating class of material, and it may not be evident on all placards. In some cases, this number may be located on an orange panel or a white square next to the hazmat placard. This four-digit number indicates the specific type of hazardous material.

Compare the four-digit number with the numbers found in the U.S. Department of Transportation Emergency Response Guidebook. The guidebook indicates exactly which materials correspond with which numbers. For example, number 1091 indicates that the material is acetone oils.