NFPA Safety Diamond

The four divisions are typically color-coded, with blue indicating level of health hazard, red indicating flammability, yellow (chemical) reactivity, and white containing special codes for unique hazards. Each of health, flammability and reactivity is rated on a scale from 0 (no hazard; normal substance) to 4 (severe risk).

Health (Blue) Flammability (Red)
4 Very short exposure could cause death or major residual injury (e.g., hydrogen cyanide, phosphine) 4 Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily (e.g., propane). Flash point below 23°C (73°F)
3 Short exposure could cause serious temporary or moderate residual injury (e.g., chlorine gas) 3 Liquids and solids that can be ignited under almost all ambient temperature conditions (e.g., gasoline). Liquids having a Flash point below 23°C(73°F) and having a Boiling point at or above 38°C (100°F) or having a Flash point between 23°C (73°F) and 38°C (100°F)
2 Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury (e.g., ethyl ether) 2 Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur (e.g., diesel fuel). Flash point between 38°C (100°F) and 93°C (200°F)
1 Exposure would cause irritation with only minor residual injury (e.g., acetone) 1 Must be heated before ignition can occur (e.g., soybean oil). Flash point over 93°C (200°F)
0 Poses no health hazard, no precautions necessary. (e.g., lanolin) 0 Will not burn (e.g., water)
Instability/Reactivity (Yellow) Special (White)
4 Readily capable of detonation or explosive decomposition at normal temperatures and pressures (e.g., nitroglycerine, RDX) The white "special notice" area can contain several symbols. The following symbols are defined by the NFPA 704 standard.
3 Capable of detonation or explosive decomposition but requires a strong initiating source, must be heated under confinement before initiation, reacts explosively with water, or will detonate if severely shocked (e.g. ammonium nitrate) W Reacts with water in an unusual or dangerous manner (e.g., caesium, sodium, sulfuric acid)
2 Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water (e.g., phosphorus, potassium, sodium) OXY Oxidizer (e.g., potassium perchlorate, ammonium nitrate, hydrogen peroxide)
1 Normally stable, but can become unstable at elevated temperatures and pressures (e.g. propene) SA Simple asphyxiant gas (includes nitrogen, helium, neon, argon, krypton, and xenon)
0 Normally stable, even under fire exposure conditions, and is not reactive with water (e.g. helium)