The National Fire Association (NFPA) has developed a color-coded number system called NFPA 704. The system uses a color-coded diamond with four quadrants in which numbers are used to signal the degree of health hazard (blue), flammability hazard (red), and instability hazard (yellow).
Flammability, Health, and Instability
Brief descriptions of the rating numbers for the flammability, health, and instability sections are given in the table below. Numbers in these sections can range from zero to four, with zero meaning essentially no hazard and four indicating an extreme hazard.
These numbers will appear on your caution sign if your inventory contains amounts in the quantities that require a fire permit. You may have chemicals in your lab, shop or maker space, but show zeros on your NFPA diamond if the quantities do not meet or exceed the permit quantities.
The bottom diamond segment is white and it shows specific hazard codes that identify special problems or require special fire-fighting techniques. These specific hazards include OX (oxidizers), COR (corrosive materials) and W (use no water to fight fires).
The numeric ratings on the colored sections of the diamond on your caution sign are based on the hazard category and the quantity of material as recorded in your MyChem inventory.
For more information refer to the Table of NFPA codes and Permit Quantities.