Placards are diamond shaped, 4-point signs placed on trucks carrying hazardous materials. There are more than two dozen placards used to represent dangerous goods, and you can determine what a truck is carrying by the specific details on the sign. 

A HAZMAT placard has six main parts. Not every placard includes all six:

• Hazard class number 
• UN/NA number
• Compatibility letters 
• Color
• Words
• Graphics

Class numbers 

Numbers 1-9 represent the different hazardous classes and their divisions (class numbers are located at the bottom of the sign and division numbers are in the middle):  

• Class 1 — Explosives 
  • 1.1: Products with the potential to create a mass explosion 
  • 1.2: Products with the potential to create a projectile hazard  
  • 1.3: Products with the potential to create a fire or minor blast 
  • 1.4: Products with no significant risk of creating a blast 
  • 1.5: Products considered very insensitive that are used as blasting agents 
  • 1.6: Products considered extremely insensitive with no risk to create a mass explosion  
• Class 2 — Gases 
  • 2.1: Flammable gases 
  • 2.2: Nonflammable gases 
  • 2.3: Toxic gases 
• Class 3 — Flammable and combustible liquids
• Class 4 — Flammable materials 
  • 4.1: Flammable solids 
  • 4.2: Spontaneously combustible 
  • 4.3: Dangerous when wet 
• Class 5 — Oxidizer and organic peroxide 
  • 5.1: Oxidizing substances 
  • 5.2: Organic peroxides 
• Class 6 — Poisons
  • 6.1: Toxic substances  
  • 6.2: Infectious substances 
• Class 7 — Radioactive  
• Class 8 — Corrosive 
• Class 9 — Miscellaneous 

UN/NA numbers 

Four-digit numbers ranging from 0004-3534 are called United Nations (UN) numbers. They help identify hazardous international cargo traveling in the United States. Goods that aren’t classified or regulated by the United Nations receive North American (NA) numbers. These four-digit numbers range from 8000-9279 and are assigned by the DOT. All UN and NA placards come with an identifier that helps shippers determine the cargo’s class, division and compatibility group.   

Compatibility letters 

On some signs, you may see the letters A-S. These compatibility letters help shippers and carriers know which explosives can be loaded together onto a trailer.  

Colors, words and graphics 

One of the easiest ways to identify hazmat placards, other than the class numbers, is by the color (along with the words and graphics on each sign):  

• Orange – Orange represents explosive materials which can include products like dynamite, fireworks and ammunition. These signs typically have the words explosives or blasting agents on them and a graphic indicating something blowing up. They’ll also have the number 1 to indicate the class. 
• Red – Red is for flammable goods like gasoline, rubbing alcohol, paint and acetone, which can fall into Classes 2 or 3. These placards feature a flame image and usually have the words flammable, gasoline, combustible or fuel oil. 
• Green – If the truck has a green sign, it’s transporting nonflammable substances like compressed and liquefied gases. You’ll see the word nonflammable gas, an image of a gas canister and the number 2.  
• Yellow – Yellow indicates oxidizers — substances, that when mixed with oxygen, are likely to combust (Classes 2 or 5). Common oxidizers include ammonium nitrate, potassium nitrate, halogens and nitric acid. These signs have oxygen written on them and a graphic of an “O” with flames.  
• White – White indicates poisonous and biohazardous substances like dyes, acids, aerosols and medical waste. For toxic materials, the sign will be labeled poison, PG III (PG = packing group) or inhalation hazard with a skull-and-crossbones image. For biohazards, the placard will say infectious substance and have a biohazard symbol (three circles overlapping one center circle). These types of materials can fall into Classes 2 or 6.   
• Blue – Blue represents goods that are dangerous when wet — meaning when these materials meet water, they can become flammable. Examples include sodium, calcium and potassium. You’ll see dangerous when wet, an image of a flame and the number 4 on these placards. 
• Red and white – If you see a sign that’s half red and half white with spontaneously combustible written on it, a flame graphic and the number 4, that means there are substances present that may ignite when exposed to air. This can include things like aluminum and lithium alkyls or white phosphorous. 
• Red and white stripes  – Signs that have red and white vertical stripes with the number 4 represent flammable solids such as matches and magnesium. These placards are labeled flammable solid and have a fire graphic. 
• Red and yellow – Red and yellow indicate organic peroxides which have the potential to ignite or explode (these fall under the division 5.2). Common examples are methyl ethyl ketone peroxide and benzoyl peroxide. These signs say organic peroxide and will either have the graphic of “O” with flames or just a normal fire graphic. 
• Yellow and white – Yellow and white represent radioactive substances that are often found in medical equipment. You’ll see the word radioactive, the radiation symbol of three blades surrounding a small circle, and the number 7. 
• White and black – Half white and half black signifies corrosive materials that can irritate and harm the skin. Examples include batteries, hydrochloric acid, sulfuric acid and sodium hydroxide. These signs say corrosive, show substances spilling onto hands, and have the number 8. 
• White with black stripes – A white sign with black vertical stripes at the top and the number 9 at the bottom signals miscellaneous dangerous goods. This includes environmentally hazardous substances that don’t fall into a specific class like asbestos and dry ice.