Last Updated: 2026-10-12
Answer Capsule: NFPA 704 authorizes exactly three symbols in the white quadrant of the fire diamond. W with a horizontal line through it means the material reacts dangerously with water. OX means oxidizer, a material that feeds a fire oxygen. SA means simple asphyxiant gas, limited to seven gases such as nitrogen. COR, ACID, ALK and BIO are not NFPA 704 symbols.
Your new sulfuric acid placard came with a W that has a line through it. The last one had COR in the same spot. Both signs are on the wall, the fire marshal walks through next week, and you are not sure which one is right. This is the most common question Print Pro AZ gets about the white quadrant, and the answer surprises people.
The white square is the only part of the NFPA 704 symbol set that does not use a number. It holds one of three symbols, or nothing. Yet older signs, supplier labels and internet charts have filled it with everything from ACID to a radiation trefoil.
This post covers the three legal NFPA 704 symbols and the ones that are not legal. It also covers what a firefighter does differently for each one. Brent Hanke has replaced a lot of COR placards, and this is what he tells the customer.
What Are the Three NFPA 704 Symbols Allowed in the White Quadrant?
The three NFPA 704 symbols allowed in the white quadrant are W with a horizontal line through it (water reactive), OX (oxidizer), and SA (simple asphyxiant). The 2022 edition of NFPA 704 authorizes only these three. Anything else inside the diamond is not part of the standard.
| Symbol | Meaning | Common examples |
|---|---|---|
| W with a horizontal line through it | Reacts dangerously or unusually with water | Concentrated sulfuric acid, sodium metal, calcium carbide |
| OX | Oxidizer; can supply oxygen to a fire without outside air | Hydrogen peroxide, chlorine, nitric acid, calcium hypochlorite |
| SA | Simple asphyxiant gas; displaces oxygen | Nitrogen, helium, neon, argon, krypton, xenon, liquefied carbon dioxide |
A blank white square is correct for most materials. Gasoline (1-3-0), acetone (1-3-0), hydrochloric acid (3-0-0) and sodium hydroxide (3-0-1) all leave it empty. Do not fill the square just because it looks unfinished. The rest of the diamond is explained in the NFPA 704 diamond explained guide.
What Does the W With a Line Through It Mean?
The W with a horizontal line through it means the material reacts dangerously or unusually with water. Firefighters read it as "do not use water," which matters because water is the default tool on every engine.
Concentrated sulfuric acid is the everyday example. It rates 3-0-2 with the W symbol. It reacts exothermically with water, generating enough heat to boil and spatter, which is why the rule in every lab is to add acid to water and never the reverse. Sodium metal and calcium carbide carry the symbol too, and both give off flammable gas on contact with water.
What a firefighter does differently: the crew holds the hose line back from the material itself. They protect exposures instead of attacking the product, and they look for a dry chemical or other non-water agent. A missing W on a room that stores concentrated sulfuric acid is not a paperwork problem. It sends a crew in with the wrong tool.
Dilute sulfuric acid rates lower and may not carry the symbol. Check the SDS for the concentration you actually store.
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What Does the OX Symbol Mean?
The OX symbol means oxidizer: the material can supply oxygen to a fire without outside air. A fire fed by an oxidizer burns hotter and faster, and the usual method of smothering it, cutting off the air, does not work.
Examples from the field guide: chlorine gas (4-0-0-OX), calcium hypochlorite pool shock (3-0-2-OX), compressed oxygen (0-0-0-OX) and liquid oxygen (3-0-0-OX). Hydrogen peroxide at 30 to 35% is 3-0-1-OX on most sheets, but suppliers differ, so use the SDS for your product and concentration. The same goes for nitric acid and potassium permanganate. Sodium hypochlorite bleach carries OX on some sheets and not others.
What a firefighter does differently: the crew expects a faster-growing fire and treats stored oxidizers as fuel for everything around them. They keep oxidizers and flammables separate in their plan, the same way you should keep them separate on the shelf. Pool and water treatment operators see OX more than anyone, because chlorine and hypochlorite are the whole job.
Callout: Compressed oxygen is 0-0-0-OX. The zeros say the gas itself is not toxic, flammable or unstable. The OX says it will turn any small fire in the room into a large one. Both halves matter.
What Does the SA Symbol Mean?
The SA symbol means simple asphyxiant: a gas that displaces the oxygen in a closed space. NFPA 704 limits SA to seven gases: nitrogen, helium, neon, argon, krypton, xenon and liquefied carbon dioxide. It is not used for any other material.
Nitrogen is the common case. Compressed nitrogen rates 0-0-0-SA. Liquid nitrogen rates 3-0-0-SA, because the cold liquid causes severe burns. Argon and helium are 0-0-0-SA. Carbon dioxide is 0-0-0-SA on most sheets, though some rate it 2-0-0-SA, so check your SDS.
What a firefighter does differently: an SA on the door tells the crew the air inside may not support life. That holds even with no fire, no smoke and no smell. They enter on breathing air and treat any unconscious person as an oxygen-deficient atmosphere case. For your facility, SA is the reason a nitrogen storage room or a CO2 tank room gets a placard even though every number is a zero.
Which Symbols Are Not Part of NFPA 704?
COR, ACID, ALK, BIO, POI and the radiation trefoil are not part of NFPA 704. They appear on older signs, on some SDS sheets, and on charts that mix NFPA 704 with other systems. The standard does not authorize them inside the diamond.
If your facility wants to keep them, NFPA 704 says to place them outside the diamond. A corrosive warning belongs on its own sign next to the placard, in the OSHA 29 CFR 1910.145 format, not in the white square. A strict fire marshal can call COR or ACID inside the diamond non-compliant, and some do.
Here is a call Print Pro AZ gets every month. A facility manager has a hydrochloric acid room with a placard that reads 3-0-0 and COR in the white square. The inspector flagged it.
The fix is a 3-0-0 diamond with a blank white square, plus a separate corrosive sign at the door. The corrosive hazard is still posted, just in the right place. Print Pro AZ prints both, and the corrosive sign is covered in the OSHA danger signs collection.
How Do You Put the Right Symbol on Your Placard?
You put the right symbol on your placard by checking two SDS sections for each material. Then you collect every symbol present in the room. Special hazards come from Section 5 (fire-fighting measures) and Section 9 (physical and chemical properties), and Section 16 often lists the NFPA rating outright.
Here is the check Print Pro AZ runs on each chemical inventory it receives:
- Does Section 5 warn against water? That is the W with a line.
- Does Section 5 or Section 9 call it an oxidizer? That is OX.
- Is it one of the seven SA gases? That is SA.
- Anything else? Leave the square blank and put the other warning on a separate sign.
A room with more than one symbol shows all of them. A room that stores concentrated sulfuric acid and calcium hypochlorite shows both W with a line and OX. Print Pro AZ prints NFPA 704 diamond signs with any symbol set, in vinyl for indoor doors and cabinets or .040 aluminum for outdoors. For the full symbol list and 50 chemical ratings on one sheet, grab the free NFPA 704 Diamond Field Guide with the download form on this page.
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Frequently Asked Questions
What does the W with a line through it mean on the NFPA diamond?
It means the material reacts dangerously or unusually with water, so responders should not apply water to it. Concentrated sulfuric acid, sodium metal and calcium carbide carry this symbol under NFPA 704.
What does OX mean on a hazard diamond?
OX means oxidizer. The material can supply oxygen to a fire without outside air, so the fire burns faster and cannot be smothered. Chlorine, calcium hypochlorite, nitric acid and hydrogen peroxide are common oxidizers.
What does SA mean on the NFPA 704 sign?
SA means simple asphyxiant, a gas that displaces oxygen in a closed space. NFPA 704 applies it only to nitrogen, helium, neon, argon, krypton, xenon and liquefied carbon dioxide. Compressed nitrogen rates 0-0-0-SA.
Is COR an NFPA 704 symbol?
No. COR, ACID, ALK, BIO, POI and the radiation trefoil are not part of NFPA 704. If you use them, the standard says to place them outside the diamond. A strict fire marshal can flag them inside the white square.
Conclusion
NFPA 704 allows exactly three symbols in the white quadrant. W with a line is water reactive, OX is oxidizer, and SA covers the seven simple asphyxiant gases. Each one changes what a firefighter does at your door, from holding back the hose to entering on breathing air. COR and its cousins belong on a separate sign, outside the diamond.
Getting the symbol right the first time saves you a failed fire inspection and a re-print. It also keeps a responder from reaching for water in a sulfuric acid room.
Shop NFPA 704 diamond signs with any symbol, or call Brent at Print Pro AZ with your SDS sheets: (602) 649-5305.
Sources
Brent Hanke | Print Pro AZ | (602) 649-5305 | b.hanke@printproaz.com
Brent Hanke is the founder of Print Pro AZ, supplying NEC-compliant labels to contractors across the country.