NFPA 704
The red diamond tells how flammable the chemical compound is: how easily it catches fire. The yellow diamond tells about reactivity: how quickly the compound reacts with other materials. (For example, some chemicals, like ammonium nitrate, explode when they touch water; this is an example of reactivity.) The blue diamond tells how dangerous the chemical is to a person's health. Each of these three diamonds - red, yellow, and blue - are given a score between 0 and 4. A score of 0 means there is no danger. A score of 4 means there is the worst possible danger.