Standards Guide
NFPA 704 Fire Diamond Explained
The NFPA 704 "fire diamond" is a rapid-recognition placard for emergency responders. In four colored quadrants it answers the three questions a fire crew asks before approaching a building or tank — how toxic, how flammable, how unstable — plus any special hazard that changes tactics, such as reacting violently with water. It is defined by NFPA 704, currently in its 2022 edition, and it is not a substitute for GHS container labels.
The Four Quadrants
| Quadrant | Color | Hazard Dimension |
|---|---|---|
| Left | Blue | Health — what exposure does to a person |
| Top | Red | Flammability — how easily the material ignites |
| Right | Yellow | Instability — how violently it can react or decompose |
| Bottom | White | Special hazards — symbols that change response tactics |
What the Numbers 0–4 Mean
Each colored quadrant carries a single digit from 0 (minimal hazard) to 4 (severe hazard). The ratings are assigned from measurable properties — toxicity data for health, flash point and boiling point for flammability, and reactive chemistry for instability — normally taken from the safety data sheet or from published rating sources.
| Rating | Health (blue) | Flammability (red) | Instability (yellow) |
|---|---|---|---|
| 4 | Can be lethal from very short exposure | Flammable gas or extremely volatile liquid; ignites readily at ambient conditions | Capable of detonation at normal temperatures and pressures |
| 3 | Serious or permanent injury possible from short exposure | Ignitable at almost all ambient temperatures (flash point below 23 °C / 73 °F) | Capable of detonation with a strong initiating source or if heated under confinement |
| 2 | Intense or continued exposure can cause incapacitation or residual injury | Must be moderately heated before ignition (flash point roughly 38–93 °C / 100–200 °F) | Violent chemical change possible at elevated temperature and pressure |
| 1 | Exposure causes irritation and minor residual injury | Must be preheated before ignition (flash point above 93 °C / 200 °F) | Normally stable but can become unstable when heated |
| 0 | No hazard beyond ordinary combustible material | Will not burn under typical fire conditions | Normally stable, even under fire exposure |
The White Quadrant: Special Symbols
NFPA 704 defines three symbols for the white quadrant. W with a horizontal strike-through means the material reacts dangerously with water, telling crews to avoid or carefully manage water application — sodium and sulfuric acid are classic examples. OX marks an oxidizer that intensifies any fire by supplying oxygen. SA marks a simple asphyxiant gas (nitrogen, argon, helium, neon, krypton, xenon) that can displace air in enclosed spaces. Other letters sometimes seen in the field, such as ACID, ALK, COR, or the radiation trefoil, are legacy or local additions rather than part of the current standard.
Worked Examples
| Material | Health | Flammability | Instability | Special |
|---|---|---|---|---|
| Gasoline | 1 | 3 | 0 | — |
| Acetone | 1 | 3 | 0 | — |
| Anhydrous ammonia | 3 | 1 | 0 | — |
| Concentrated sulfuric acid | 3 | 0 | 2 | W̶ (water reactive) |
| Hydrogen | 0 | 4 | 0 | — |
See the gasoline fire diamond reference page for the placard artwork and technical data. Ratings for a specific product should always be taken from that product's SDS or the supplier, because concentration and formulation change the numbers.
Where the Diamond Is Required
NFPA 704 placards are a fire-code requirement, not an OSHA labeling requirement. US fire codes — the International Fire Code and NFPA 1 as adopted by state and local jurisdictions — require the placard on buildings, storage areas, and tanks where hazardous materials are present above permit or threshold quantities. The authority having jurisdiction (usually the local fire marshal) decides placement details, but the normal pattern is exterior placards at building entrances the fire service would use, on outdoor tanks, and at the entrance to interior rooms where the materials are stored.
When one placard covers many chemicals, the standard allows a composite rating that shows the highest number in each category among the materials present — which is why a warehouse placard often looks more severe than any single product inside.
NFPA 704 vs GHS Labels
The two systems answer different questions and legally coexist. The GHS label on a container tells the worker who handles that product about its hazards; the numbers run 1–4 with 1 as the most severe category in most classes. The NFPA diamond on the building tells an arriving responder about aggregate risk; its numbers run 0–4 with 4 as the most severe. The inverted numbering is the single most common source of confusion in audits, so facilities should train the difference explicitly rather than assume the systems agree.