How OSHA classifies a flammable liquid
A flammable liquid's GHS category is set by two numbers from Section 9 of its Safety Data Sheet: the closed-cup flash point (the lowest temperature at which the liquid gives off enough vapor to ignite) and the initial boiling point. OSHA's rule lives in 29 CFR 1910.1200 App B.6.1, aligned with 29 CFR 1910.106(a). The decision is exact, and two boundaries trip people up:
- Category 1 vs Category 2 is split by the boiling point, not the flash point. Both require a flash point below 73.4°F (23°C); a boiling point at or below 95°F (35°C) is Category 1, above 95°F is Category 2. The 95°F cutoff is inclusive (95°F = Cat 1).
- The Category 3/4 line is at 140°F (60°C), inclusive of Category 3. A flash point of exactly 140°F is Category 3 — Category 4 starts strictly above 140°F. Never count 140°F in both categories.
GHS flammable-liquid categories (2026)
Classification criteria: 29 CFR 1910.1200 App B.6.2, table B.6.1. Label elements: App C, table C.4.19. Categories 1, 2 and 3 carry the Flame pictogram (GHS02); Category 4 carries no pictogram. Verified 2026-10-05.
| Category | Criteria (closed-cup flash point + boiling point) | Signal word | Hazard statement |
|---|---|---|---|
| Category 1 | FP < 73.4°F (23°C) and BP ≤ 95°F (35°C) | Danger | H224 Extremely flammable liquid and vapor |
| Category 2 | FP < 73.4°F (23°C) and BP > 95°F (35°C) | Danger | H225 Highly flammable liquid and vapor |
| Category 3 | FP 73.4°F (23°C) to 140°F (60°C) | Warning | H226 Flammable liquid and vapor |
| Category 4 | FP > 140°F (60°C) to 199.4°F (93°C) | Warning | H227 Combustible liquid |
A liquid with a closed-cup flash point above 199.4°F (93°C) is not a GHS flammable liquid — though it may still be regulated as a combustible liquid for storage.
Worked examples
Acetone (CAS 67-64-1). SDS Section 9 lists a flash point of −4°F (−20°C) and an initial boiling point of 133°F (56°C). Flash point is below 73.4°F, so it is Category 1 or 2; the boiling point of 133°F is above 95°F, so it is Category 2 — signal word Danger, hazard statement H225 "Highly flammable liquid and vapor," Flame pictogram.
Diesel fuel #2. A typical SDS lists a flash point of about 126°F (52°C). That is at or above 73.4°F and at or below 140°F, so the boiling point is irrelevant: it is Category 3 — signal word Warning, hazard statement H226 "Flammable liquid and vapor," Flame pictogram. (A diesel with a flash point above 140°F would instead be Category 4, "Combustible liquid," signal word Warning and no pictogram.)
What to do with the category
- Label correctly. The category sets the signal word, the H-statement, and the Flame pictogram on shipped labels 29 CFR 1910.1200(f)(1) and workplace/secondary labels 1910.1200(f)(6).
- Size your storage. The GHS category drives hazard communication; allowable container sizes and cabinet/room quantities come from the storage standard — 29 CFR 1926.152 (construction); 29 CFR 1910.106 (general industry). Classify first, then size the cabinet.
- Train your crew. HazCom training 29 CFR 1910.1200(h) must cover the hazards of the chemicals in the work area, including flammable-liquid handling and ignition control.
Frequently asked questions
What are the OSHA GHS flammable liquid categories?
Per 29 CFR 1910.1200 Appendix B.6.1 (aligned with 1910.106(a)): Category 1 — flash point < 73.4°F (23°C) AND initial boiling point ≤ 95°F (35°C). Category 2 — flash point < 73.4°F AND boiling point > 95°F. Category 3 — flash point ≥ 73.4°F and ≤ 140°F (60°C). Category 4 — flash point > 140°F and ≤ 199.4°F (93°C). A liquid with a flash point above 199.4°F is not a GHS flammable liquid.
Is a liquid with a 73°F flash point and 90°F boiling point Category 1 or 2?
Category 1. Category 1 vs Category 2 is decided by the BOILING point, not the flash point. Both categories require a flash point below 73.4°F (23°C). A liquid is Category 1 when its flash point is below 73.4°F AND its initial boiling point is at or below 95°F (35°C). If the boiling point is above 95°F, it is Category 2. So a liquid with a flash point of 73°F and a boiling point of 90°F (≤ 95°F) is Category 1.
Which flammable liquid categories use the signal word Danger versus Warning?
Categories 1 and 2 use the signal word DANGER. Categories 3 and 4 use WARNING. The Flame pictogram (GHS02) applies to Categories 1, 2 and 3 — Category 4 carries no pictogram at all. The hazard statements are H224 (Cat 1, extremely flammable), H225 (Cat 2, highly flammable), H226 (Cat 3, flammable), and H227 (Cat 4, combustible). Source: 29 CFR 1910.1200 Appendix C, table C.4.19.
Where is the boundary between Category 3 and Category 4?
At exactly 140°F (60°C). Category 3 is flash point ≥ 73.4°F and ≤ 140°F — so 140°F is Category 3 (inclusive). Category 4 begins above 140°F (flash point > 140°F and ≤ 199.4°F). 140°F belongs to Category 3 only; it must not be counted in both categories.
What is the difference between a flammable liquid and a combustible liquid?
Under HCS 2024, all four GHS categories are formally 'flammable liquids,' but Category 4 (flash point > 140°F to 199.4°F) carries the H227 statement 'Combustible liquid' and the signal word Warning. Historically OSHA 1910.106 split liquids into Class I 'flammable' (flash point < 100°F) and Class II/III 'combustible' (flash point ≥ 100°F). Use the GHS category for labels and SDS; the flammable-vs-combustible split still drives storage-quantity limits under 1926.152 / 1910.106.
Does the GHS category dictate how I store the flammable liquid?
No — HazCom (1910.1200, adopted for construction by 1926.59) governs labels, SDS, and training only. It does not impose chemical storage segregation. Flammable and combustible liquid storage quantities and cabinets are governed by 29 CFR 1926.152 for construction (which supersedes 1910.106 for field applications) and 29 CFR 1910.106 for general industry. The GHS category tells you the hazard; the storage standard tells you the allowable container and cabinet quantities.
Which flash point does OSHA use — closed cup or open cup?
Closed-cup. 1910.1200 Appendix B.6 and 1910.106(a) define classification by the closed-cup flash point determined by an accepted test method (e.g., ASTM D56, D93, or D3278). Open-cup values run higher and are not used for GHS classification. Always read the flash point from SDS Section 9, which reports the closed-cup value.
Sources & verification
- https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1200
- https://www.ecfr.gov/current/title-29/section-1910.1200
- https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1200AppC
- https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1200AppB
- https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.106
- https://www.federalregister.gov/documents/2024/05/20/2024-08568/hazard-communication-standard
Category boundaries verified 2026-10-05. This classifier is an estimate for planning only — it is not legal advice. Always classify from the manufacturer's SDS and verify against the applicable storage standard. State-Plan states may adopt equivalent or stricter requirements.
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