NFPA 704 Special Hazards (White)
The white quadrant at the bottom of the NFPA 704 diamond holds special hazard symbols instead of a number. The three symbols in the standard are OX for an oxidizer, W with a bar through it for a material that reacts with water in an unusual or dangerous way, and SA for a simple asphyxiant gas.
Where it sits: the white quadrant on the bottom of the diamond.
Sample diamond for illustration only. The white box is the one this page explains.
How to read it
The white box is the only part of the diamond that is not rated from 0 to 4. It is either blank or carries one of these symbols, and it exists to warn responders of a hazard that changes how they would fight a fire or approach a spill. Spraying water on a water-reactive material, for example, can make things worse, and an oxidizer can make a fire burn far more intensely.
OX marks a material that can give up oxygen or otherwise promote combustion of other materials. Typical examples are nitrates, perchlorates and hypochlorites. W (drawn as a W with a horizontal bar through it) marks a material that reacts with water in an unusual or dangerous manner; common examples are sodium metal and some metal hydrides and reactive halides. SA marks simple asphyxiant gases: nitrogen, helium, neon, argon, krypton and xenon. Published summaries of the standard note that it is also used for some carbon dioxide installations, such as liquefied CO2 vapor withdrawal systems and areas with large amounts of dry ice.
Other markings appear in the wild. Some facilities and older labels add COR for corrosive, ACID, ALK, a radiation trefoil, or CRYO. These are not part of the core set in the standard, and meanings vary between users. If you see one, read it as a local convention and confirm the actual hazard from the SDS. Oxygen, for instance, is not marked SA; it is an oxidizer.
Special hazard symbols
| Symbol | Hazard | Meaning | Notes |
|---|---|---|---|
| OX | Oxidizer | Oxidizer. Can give up oxygen or otherwise promote combustion of other materials. | Examples: many nitrates, chlorates, perchlorates, hypochlorites and peroxides. Keep away from fuels and organic material. |
| W (with a bar) | Reacts with water | Water-reactive. Reacts with water in an unusual or dangerous manner. | Examples: alkali metals such as sodium and potassium, and some metal hydrides. Do not use water on a fire involving the material. |
| SA | Simple asphyxiant | Simple asphyxiant gas: nitrogen, helium, neon, argon, krypton and xenon (also used for some carbon dioxide installations). | The listed gases are not toxic but displace oxygen in enclosed spaces. Carbon dioxide also has toxic effects of its own at high concentrations. |
Paraphrased summary of the NFPA 704 criteria. Check the current edition of NFPA 704 for the authoritative text; ratings for specific products come from the supplier or the facility.
Points to remember
- A blank white box means none of these special hazards apply, not that the material is free of other hazards.
- A corrosive material is not marked in the white box under the core standard. Corrosivity is reflected in the blue health number and in GHS Category 1 skin corrosion.
- NFPA 704 numbers run the opposite way to GHS categories. See NFPA 704 versus GHS.
Where to find the information on an SDS
Section 10 lists water reactivity and incompatibilities such as oxidizers and combustibles. Section 2 shows GHS classifications such as oxidizing solid or liquid, and Section 5 explains which extinguishing media to avoid. See SDS Section 10 and Section 16 for optional NFPA and HMIS ratings.
Related GHS pages
- GHS oxidizing liquids and solids
- GHS oxidizing gases
- GHS gases under pressure
- Browse chemical SDS pages
Frequently asked questions
What does OX mean on an NFPA diamond?
OX means the material is an oxidizer. It can give up oxygen or otherwise help other materials burn, so it should be kept away from fuels and organic materials.
What does a W with a line through it mean on a hazard diamond?
It means the material reacts with water in an unusual or dangerous way, so water should not be used on a fire or spill involving it unless the SDS or emergency guidance says otherwise.
What does SA mean on an NFPA diamond?
SA stands for simple asphyxiant. It is used for nitrogen, helium, neon, argon, krypton and xenon, which are not toxic but can displace oxygen in a confined area, and in some carbon dioxide installations.