The dangerous-when-wet placard is a gift to anyone studying placards, because it is the only blue diamond in the entire system. If you see blue, you are done: it is division 4.3. The hazard behind it is unusual too, which is exactly why it gets its own placard.
This is study guidance, not regulatory advice. The binding definition is in 49 CFR 173.124 and your official state CDL manual.
What the hazard is
Division 4.3 materials are dangerous specifically because of water. On contact with water or even moist air, they react and give off flammable gas, and sometimes toxic gas, which can ignite. That is the opposite of how most people picture fighting a fire, so the placard exists to warn responders never to use water on these materials.
| Element | Dangerous-when-wet placard |
|---|---|
| Color | Blue |
| Symbol | Flame |
| Class number | 4, at the bottom point |
| Division | 4.3 |
| Table | Table 1 (placard in any amount) |
Common examples
The materials in 4.3 are mostly reactive metals and compounds. Examples include sodium, potassium, lithium metal, calcium carbide, and certain magnesium and aluminum powders. These are exactly the materials you do not want to meet water unexpectedly.
Why blue is your anchor
Most placard study is about separating look-alikes, but blue is the rare case where the color is the complete answer. Use it. When you train placards, the unique ones like blue (4.3), the black-and-white stripes of Class 9, and the yellow-over-white of Class 7 free up your attention for the genuinely confusing pairs in most confused hazmat placards. The role of color across the system is in hazmat placard colors explained.
When it must be placarded
Division 4.3 is a Table 1 material, so it must be placarded in any amount, with no weight threshold. That puts it in the same urgency tier as toxic gas and the most severe explosives, as explained in when hazmat placards are required under 49 CFR 172.504. For response procedures, the PHMSA Emergency Response Guidebook and the FMCSA hazardous materials regulations are the references.
Frequently asked questions
What does the dangerous-when-wet placard mean?
It marks division 4.3 materials, which react with water to release flammable or toxic gas. The placard warns handlers and responders to keep these materials away from water.
What color is the dangerous-when-wet placard?
Blue. It is the only blue placard in the system, so a blue diamond is a near-certain sign of a division 4.3 dangerous-when-wet material.
What are examples of dangerous-when-wet materials?
Reactive metals and compounds such as sodium, potassium, lithium metal, and calcium carbide. They give off flammable gas on contact with water.
What is the best way to remember the dangerous-when-wet placard?
Lock in that blue equals dangerous when wet, since blue is unique, then drill it with a recognition app such as CDL Placards so it is automatic. Your state CDL manual remains the authority on the rules.
