In the U.S., hazardous materials carried in bulk are required to be marked before shipment by road, rail, or air. The authority that regulates this marking is the Department of Transportation, or DOT. The rules for placarding are contained in CFR Title 49, sections 171 and 172.

Don't Panic! You don't need to run out to Office Depot to get a bunch of crazy stickers for your household chemicals--"Each person offering a motor carrier a hazardous material for transportation by highway shall provide to the motor carrier the required placards for the material being offered prior to or at the same time the material is offered for transportation, unless the carrier's motor vehicle is already placarded for the material as required by this subpart." So you do have to have placards, but you can get them from the same place you get your hazardous materials.

But which materials need to be placarded? I'm so glad you asked. 49 CFR 172 (subpart F) defines the following classes of hazmat placards and the regulations that apply:

  • Class 1: Explosives - placard must be orange with black lettering, bearing the appropriate division number (1,2,3...6) and the word "EXPLOSIVES".
    • Division 1.1 Explosives with a mass explosion hazard
    • Division 1.2 Explosives with a projection hazard
    • Division 1.3 Explosives with predominantly a fire hazard
    • Division 1.4 Explosives with no significant blast hazard
    • Division 1.5 Very insensitive explosives - carries the legend "BLASTING AGENTS" instead of "EXPLOSIVES".
    • Division 1.6 Extremely insensitive explosive articles
  • Class 2: Gases
  • Class 3: Flammable liquids.
    • Division 3.1 Flashpoint below -18oC (0oF) - red background with white lettering, a flame logo, and the word "FLAMMABLE" or "GASOLINE" where applicable.
    • Division 3.2 Flashpoint -18oC and above, but less than 23oC (73oF) - red background with white lettering, legend "COMBUSTIBLE" or "FUEL OIL" where applicable.
    • Division 3.3 Flashpoint 23oC and up to 61oC (141oF)
  • Class 4: Flammable solids; spontaneously combustible materials; and materials that are dangerous when wet
    • Division 4.1 Flammable solids - white with seven vertical red stripes, flame logo, legend "FLAMMABLE SOLIDS"
    • Division 4.2 Spontaneously combustible materials - upper half white, lower half red, with black lettering, flame logo, legend "SPONTANEOUSLY COMBUSTIBLE"
    • Division 4.3 Materials that are dangerous when wet - blue with white lettering, flame logo, legend "DANGEROUS WHEN WET".
  • Class 5: Oxidizers and organic peroxides - yellow with black lettering, legend "OXIDIZER" or "ORGANIC PEROXIDE", logo is a circle with a flame.
  • Class 6: Poisons and etiologic materials - white with black text, white skull and crossbones, legend "POISON" or "TOXIC"
    • Division 6.1 Poisonous materials
    • Division 6.2 Etiologic (infectious) materials
  • Class 7: Radioactive materials - yellow upper half, white lower half, black lettering, with radiation logo, legend "RADIOACTIVE".
    • Any material, or combination of materials, that spontaneously gives off ionizing radiation. It has a specific activity greater than 0.002 microcuries per gram.
  • Class 8: Corrosives - black with white lettering, upper corner in white with spilled test tube & dissolving hand logo drawn in black.
    • A material, liquid or solid, that causes visible destruction or irreversible alteration to human skin or a liquid that has a severe corrosion rate on steel or aluminum.
  • Class 9: Miscellaneous - white with seven vertical black stripes coverin the upper half. Number "9" is underlined and centered at the bottom.
    • A material which presents a hazard during transport, but which is not included in any other hazard class (such as a hazardous substance or a hazardous waste).
  • "Dangerous": white placard with upper and lower third in red, with black lettering "DANGEROUS" through the white center. Used when 2 or more from different categories are present in the same shipment (e.g. if 1.x and 2.x are together, but not necessary if 1.x and 1.y are together).

Also:

Placards must be square, and measure at least 273mm on each edge, with a solid line inner border at least 12.7mm from the edge. For other than Class 7 or the DANGEROUS placard, text indicating a hazard (for example, "FLAMMABLE") is not required. Text may be omitted from the OXYGEN placard only if the specific identification number is displayed on the placard.

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