The Fire Diamond categorises hazardous substances according to flammability, instability, and danger to human health. One material ranks the maximum on all three scales.
I am sure that you’ve seen this symbol out in the wild. The Fire Diamond, technically known as the NFPA 704, is a label used to alert fire fighters to the presence of dangerous materials. You would want to know, for example, if a chemical is present that explodes when it comes into contact with water… preferably before you hose it down with water. The four quadrants of the symbol indicate the degree of danger according to specific measures.
The top red section describes flammability: how does this substance burn? The scale runs from 0 (doesn’t burn under almost all conditions, e.g. sand or concrete) to 4 (vaporises in air and burns at below room temperature).
The blue section describes its effect on human health, from 0 (no effect) to 4 (touching it for a moment can cause severe burns or even kill you). This is where the extremely poisonous materials go.
The yellow section is about the instability of the substance: at 0, it doesn’t really change much even if it’s in a fire. Yellow 4 materials are highly explosive – think TNT or nitroglycerin.
The bottom quadrant is reserved for additional special warnings, if it reacts badly with water for example, or includes a gas that can suffocate you. Sometimes it also includes warnings like radioactivity or biological danger.
So, when I first learnt about this, my mind immediately went to one question: is there any material that is ranked at the maximum on every scale? Propane is absurdly flammable (Red 4) but touching it won’t kill you (Blue 2). Breathing in hydrogen cyanide will kill you (Blue 4) but it doesn’t easily explode at room temperature (Yellow 2). Well, I found a post on the chemistry Stack Exchange that suggests a likely candidate: tert-butyl hydroperoxide.
This charming little peroxide is used to make polyurethane, amongst other things. In its undiluted form it ranks at a 4 for flammability, a 4 for instability, and a 4 for its effect on health (don’t touch it!). Tert-butyl hydroperoxide is so bad that it’s actually illegal in the United States to transport it in undiluted form. Fortunately it mixes with water without blowing up, so it is always transported as a solution – and that is much less flammable.