A flammable liquid is any liquid with a flashpoint below a defined regulatory threshold, making it capable of producing vapors that can form an ignitable mixture with air at or near ambient temperatures. Because the vapors rather than the liquid itself burn, the flashpoint — not the liquid’s temperature — determines whether a hazardous vapor atmosphere exists at a given location.
The exact threshold varies by authority.
- OSHA (29 CFR 1910.106) defines a flammable liquid as one with a flashpoint below 100°F (37.8°C).
- The GHS/HazCom 2012 system (OSHA 29 CFR 1910.1200) classifies liquids with flashpoints up to 60°C (140°F) as flammable, divided into three subcategories.
- NFPA 30 uses 100°F (37.8°C) as the dividing line between flammable and combustible liquids.
- DOT (49 CFR 173.120) sets the transport threshold at 60°C (140°F).
- OSHA’s PSM has their own unique definition
Engineers must identify which regulatory framework governs their specific application. It can be annoying and cause confusing.
Key Points
- Flashpoint is the primary classification parameter — a lower flashpoint means the liquid produces ignitable vapors at lower ambient temperatures and across a wider range of operating conditions.
- The flammable/combustible boundary differs across standards: OSHA and NFPA 30 use 100°F (37.8°C), while GHS and DOT use 60°C (140°F) — the same liquid can be “flammable” under one framework and “combustible” under another.
- Flammable liquids stored or processed above their flashpoint are treated as continuous or intermittent vapor sources in hazardous area classification under IEC 60079-10-1, driving zone designation and equipment selection.
- Common process industry examples include gasoline (flashpoint ≈ −43°C), methanol (11°C), acetone (−18°C), and toluene (4°C).
See Also: flammable gas, flashpoint, hazardous areas, NFPA hazard ratings
A methanol storage tank with a flashpoint of 11°C produces ignitable vapors on any day warmer than 11°C. In a typical temperate climate, this means the area around the tank qualifies as a hazardous area for most of the year. Under IEC 60079-10-1, the tank vent and bund area would be designated Zone 1 or Zone 2 depending on ventilation conditions and release frequency, requiring explosion-protected electrical equipment throughout.
See Also: Flammable Gas, Flashpoint, Hazardous Areas, NFPA Hazard Ratings
Cited Sources
- OSHA 29 CFR 1910.106 — Flammable Liquids
- OSHA 29 CFR 1910.1200, Appendix B — GHS Hazard Communication, Physical Hazard Criteria
- NFPA 30, Flammable and Combustible Liquids Code
- DOT 49 CFR 173.120 — Flammable Liquids (Division 3)