Fire diamonds located on tanks and buildings indicate the level of chemical hazard located there. The four colors are blue, red, yellow, and white. The numbers superimposed over the colors rank the severity or danger, ranging from one to four, with four being the highest rating. The blue indicates potential health effects. A four in the blue means severe and immediate health effects, including death, and a one time exposure can cause lasting health problems. The red indicates explosiveness or readiness to ignite. A four in the red indicates an extremely high ability to ignite and combust. The Yellow concerns Reactivity, or the chemical's ability to react with other chemicals in the environment. The white indicates special precautions, usually used for oxy, or oxidizing agent or whether water reactive chemicals are present (indicated by a W with a line through it).
Fire Hazard - Red
4 Flash Point below 73 F (Boiling Point below 100 F)
3 Flash Point below 73 F (Boiling point at/above 100 f) and/or at/above 73 F
- not exceeding 100 F
2 Flash Point above 100 F, not exceeding 200 F
1 Flash Point above 200 F
0 Will not burn

Reactivity (Instability) - Yellow
4 May detonate
3 Shock and heat may detonate
2 Violent chemical change
1 Unstable if heated
0 Stable

4 Deadly
3 Extreme Danger
2 Hazardous
1 Slightly Hazardous
0 Normal Material

Specific Hazard - White
OX or OXY Oxidizer
W (with line through it) Use no water

Sometimes fire diamonds can be misleading about the chemical hazards actually at a facility. The numbers on the fire diamond reflect the highest number for the type of hazard of all of the different chemicals for which an MSDS is kept at the facility, and do not necessarily indicate that a single specific chemical has all those characteristics. A facility could have one chemical with a high number in the health section and a low number in reactivity, for example, and have a second chemical with a low number in the health section and a high number in the reactivity section, but the fire diamond would show the highest number in both the health (blue) and reactivity (yellow) section that these chemicals have. Seeing a diamond with a 4 in the blue, red, and yellow boxes does not mean that such a chemical is at the facility. The highest number any chemical achieves for any of the colored sections is what's placed in the fire diamond. The fire diamond could be alluding to one chemical or a hundred different chemicals. It is easy to see why something more specific than a fire diamond is necessary for emergency response.