NFPA 704 is a standard maintained by the U.S. National Fire Protection Association. It defines the “fire diamond” used by emergency personnel to quickly and easily identify the risks posed by nearby hazardous materials. This is necessary to help determine what, if any, specialty equipment should be used, procedures followed, or precautions taken during the first moments of an emergency response.

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

Blue: Health

• 0 – Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material, for example peanut oil.
• 1 – Exposure would cause irritation but only minor residual injury, for example turpentine.
• 2 – Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury, for example chloroform.
• 3 – Short exposure could cause serious temporary or residual injury, for example chlorine gas.
• 4 – Very short exposure could cause death or major residual injury, for example hydrogen cyanide.

Red: Flammability

• 0 – Will not burn, for example water.
• 1 – Must be pre-heated before ignition can occur, for example canola oil. Flash point over 93°C (200°F).
• 2 – Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur, for example diesel fuel. Flash point between 38°C (100°F) and 93°C (200°F).
• 3 – Liquids and solids that can be ignited under almost all ambient temperature conditions, for example gasoline. Flash point below 38°C (100°F) but above 23°C (73°F).
• 4 – Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily, for example propane. Flash point below 23°C (73°F).

Yellow: Reactivity

• 0 – Normally stable, even under fire exposure conditions, and is not reactive with water, for example liquid nitrogen.
• 1 – Normally stable, but can become unstable at elevated temperatures and pressures, for example phosphorus.
• 2 – Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water, for example calcium.
• 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, for example fluorine.
• 4 – Readily capable of detonation or explosive decomposition at normal temperatures and pressures, for example nitroglycerin.

White: Special

The white special notice area can contain several symbols:

• W – reacts with water in an unusual or dangerous manner, for example cesium.
• OX – oxidizer.
• COR – corrosive, strong acid or base. ACID and ALK are sometimes used to be more specific.
• BIO – biohazardous.
• The radioactive trefoil – is radioactive.
• CRYO – cryogenic.

Note: Only W and OX are officially part of the NFPA 704 standard, but other self-explanatory symbols are occasionally used in an unofficial manner.