safety or strike-anywhere matches ). Hazardous Materials Class 4.2 : Spontaneously Combustible Solids 4.2 Spontaneously Combustible : Solid substances that ignite spontaneously (aluminium alkyls , white phosphorus ). Hazardous Materials Class 4.3 : Dangerous when Wet 4.3 Dangerous when Wet : Solid substances that emit a flammable gas when wet or react