SFFMA Firefighter I (FF1) HazMat Operations/Awareness Practice Test

Hazard placards are showing the specific risk category of the material. The label "Dangerous when wet" is the official designation for materials that react with water to release hazards, such as flammable gases or heat. This is the class known as dangerous when wet (Class 4, Division 4.3). Examples include alkali metals like sodium and potassium, and calcium carbide. When these substances come into contact with water, they react vigorously, often producing flammable hydrogen gas or other dangerous byproducts.

This isn’t describing radioactive materials (that’s a different class with a radioactive symbol), toxic liquids (that would be labeled for toxicity/toxic substances), or corrosives (which have their own corrosive labeling). So the placard directly indicating danger when wet points to that water-reactive class.