Dangerous When Wet Hazard Sign - Class 4 Safety Symbol Explained #2218581 (License: Personal Use)

This blue diamond-shaped warning sign features a black flame icon above the bold text “DANGEROUS WHEN WET” and the number “4” at the bottom, denoting UN Class 4.2 (spontaneously combustible) or Class 4.3 (dangerous when wet) materials. It alerts handlers to substances like alkali metals or metal hydrides that can ignite or release toxic gases upon contact with moisture.

Used on shipping containers, safety data sheets (SDS), chemical storage areas, and transport vehicles to warn personnel and emergency responders of water-reactive hazards; matches user intent for safety compliance, hazard identification, and risk mitigation training.