NFPA 704 Instability (Yellow)
The yellow quadrant on the right of the NFPA 704 diamond is the instability rating. It runs from 0 (normally stable, even under fire conditions) to 4 (readily capable of detonation or explosive decomposition at normal temperatures and pressures) and tells responders how likely a material is to release energy violently on its own or when disturbed.
Where it sits: the yellow quadrant on the right of the diamond.
Sample diamond for illustration only. The yellow box is the one this page explains.
How to read it
Older versions of the diamond called this box reactivity. The current standard uses instability, which better reflects what it covers: self-reaction, decomposition, polymerization and the potential for explosion from shock, heat or pressure, as well as violent reaction with water. Reaction with water at a level that would change the rating is reflected in the number. A “W” in the white box flags water reactivity separately.
The yellow number is about the energy a material can release by itself. Peroxides, certain monomers that polymerize, nitro compounds and many explosives rate high. Most common solvents, acids and bases rate 0 or 1, and a few rate 2 because of the way they react with other materials.
Instability does not tell you what a material is incompatible with. A product rated 0 can still react dangerously if mixed with an incompatible chemical, such as bleach and ammonia. The incompatibility information is in Section 10 of the SDS and is one of the most useful parts of that section for storage planning.
What each rating means
| Rating | Description | Meaning |
|---|---|---|
| 4 | May detonate | Readily capable of detonation or explosive decomposition at normal temperatures and pressures. |
| 3 | Shock and heat may detonate | 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. |
| 2 | Violent chemical change | Normally unstable and readily undergoes violent chemical change at elevated temperatures and pressures, but does not detonate. May also react violently with water or form potentially explosive mixtures with water. |
| 1 | Unstable if heated | Normally stable, but can become unstable at elevated temperatures and pressures. |
| 0 | Stable | Normally stable, even under fire conditions, and not reactive with water. |
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
- Peroxide formers and polymerizable monomers can become more unstable as they age or lose inhibitor. A rating on an old label may understate the hazard of a container that has been kept too long.
- Compressed gas cylinders are not rated high just for being under pressure; the diamond is about chemical energy release, so check cylinder hazards separately.
- 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 describes chemical stability, hazardous reactions, conditions to avoid, incompatible materials and hazardous decomposition products. Section 2 shows any GHS explosive, self-reactive or organic peroxide classification. See SDS Section 10 and Section 16 for optional NFPA and HMIS ratings.
Related GHS pages
- GHS explosives
- GHS physical hazard classes (self-reactive, organic peroxides, water-reactive)
- Browse chemical SDS pages
Frequently asked questions
What does the yellow number on an NFPA diamond mean?
The yellow number is the instability rating from 0 to 4. It describes how readily a material can detonate, decompose violently or react violently with water, with 4 being the most unstable and 0 meaning stable even under fire conditions.
Is instability the same as reactivity on NFPA 704?
Yes in practice. Older editions and many older labels call the yellow box reactivity. Current editions call it instability.
Does a yellow 0 mean the chemical is safe to mix with anything?
No. A rating of 0 means the material is stable by itself, including in a fire. It can still react hazardously with other chemicals, so check the incompatible materials listed in Section 10 of the SDS.