What Do the Numbered Placards on Trucks Mean?

Hazardous material carriers must indicate hazardous cargo with numbered placards.
Hazardous material carriers must indicate hazardous cargo with numbered placards. (Image: Joe Raedle/Getty Images News/Getty Images)

International and federal regulations require that trucks and rail cars carrying hazardous materials bear placards indicating hazmat transport and specifying the particular materials on board. When a transportation accident takes place involving any hazardous substance, emergency responders need to know what material is being transported so they can take appropriate steps to handle the accident, protect themselves and the public. Placard numbers denote the type of material aboard the truck or rail carrier.

Meaning of UN/NA Numbers

Rail cars and transportation trucks are required to carry numbered placards that warn of hazardous materials on board. These four-digit numbers, called the UN/NA numbers refer to specific chemicals, or groups of chemicals. These numbers are assigned by the United Nations (U.N.) or the United States Department of Transportation (US DOT).

Explosives and Compressed Gases

As you drive through your community or on the interstate, large trucks share the roads with you. Because of this, government agencies require the trucking companies to affix large placards that designate what chemicals their trucks are transporting. These chemicals include explosives or any device (detonators, primers, fuzes, powder cake or propelling charges) that is designed to function by exploding. If the device is not designed to explode, but instead functions through a chemical reaction such as an explosive chemical, the truck is required to carry this placard.

Compressed gases fall under the Class 2 hazardous materials US DOT law. They include flammable gases, non-flammable, non-poisonous gases, oxygen and any gas so toxic to humans it is regarded as a hazard to health during transportation, states 247Ritgs.com.


Flammables fall into the Class 3 category of hazardous materials and include flammable liquids, combustibles, gasoline and fuel oil.

A liquid that has a flash point of at least 35 F is required to be denoted by a hazardous materials placard. A combustible liquid with a flash point of at least 38 F is also required to be indicated with a placard.

Dry explosives, when wet with plasticizer, water or alcohol to suppress the explosive tendency, are required to be denoted by a Class 4 placard, says the Environmental Chemistry website.

Corrosives and Oxidizers

The Class 8 listing covers corrosive materials such as solids or liquids that produce full thickness skin destruction on contact. In addition, when transporting corrosives, the carrier must take every precaution to segregate incompatible corrosives from each other; many react violently when they come into contact with each other, states 247Rigs.com.

Radioactive and Poisons

Poisonous substances are Class 6 hazardous materials required to be placarded by the UN and US DOT. Any poison that does not fall under the Hazard Zone A or B is included in this category and is marked by a Class 6 “Poison” placard on a train or large truck, writes the Environmental Chemistry website.

A “Toxic” placard may be substituted for the “Poison” placard. A “Packing Group III” placard may also replace the “Poison” placard when the carrier is transporting Poison PG III materials.

Radioactive materials must be denoted by a bright yellow “Radioactive Class 7" placard. Some materials, in what is called “exclusive use,” are not required to carry this label. The “Radioactive” placard is required, says the Environmental Chemistry website. "Exclusive use" refers to "sole use" or "full load," in which, "all initial, intermediate, and final loading and unloading are carried out in accordance with the direction of the consignor or consignee," according to the USDOT Hazardous Materials Transportation placards website.

Miscellaneous and Other Markings

Any material with a noxious or anesthetic property is required to be placarded with the Class 9 “Miscellaneous” designation.

Marine pollutants fall under the “Other Related Markings” placard requirement, according to the 247Rigs website.

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