The National Fire Protection Agency (NFPA), in section 704 of the National Fire Code, specifies a system for identifying the hazards associated with materials. Information contained on this and linked pages comes directly from the 1990 edition of NFPA 704. Although the system was developed primarily with the needs of fire protection agencies in mind, it is of value to anyone, including someone enrolled in a chemistry laboratory course, who needs to handle potentially hazardous material.
The hazard identification signal is a color-coded array of four numbers or letters arranged in a diamond shape. An example is shown below. You will see hazard diamonds like this on trucks, storage tanks, bottles of chemicals, and in various other places around campus and around town. The blue, red, and yellow fields (health, flammability, and reactivity) all use a numbering scale ranging from 0 to 4. A value of zero means that the material poses essentially no hazard; a rating of four indicates extreme danger. The fourth value (associated with white) tends to be more variable, both in meaning and in what letters or numbers are written there.