Flammable Liquid Categories Decoded: Flash Point and Boiling Point Thresholds (Cat 1-4)

Under GHS and OSHA's Hazard Communication Standard, flammable liquids are classified into four categories by two physical properties: flash point and initial boiling point. Category 1 is the most hazardous; Category 4 is the least. A liquid is "flammable" only if its flash point is at or below 93°C (199.4°F) — above that, it falls outside this hazard class. Getting the category right is not academic: it locks in the signal word, pictogram, and hazard statement on every label, so classification is exactly where compliant labeling begins.

What makes a liquid "flammable" under GHS?

A flammable liquid is a liquid (or a liquid containing solids in solution or suspension) with a flash point of not more than 93°C (199.4°F), per OSHA's classification criteria in 29 CFR 1910.1200, Appendix B.6. The flash point is the lowest temperature at which the liquid gives off enough vapor to form an ignitable mixture with air near its surface. A spark or flame at that temperature flashes across the surface — hence the name.

Two properties drive the category assignment:

• Flash point — the primary driver. A lower flash point means the liquid ignites at lower (often ambient or below-ambient) temperatures, which is more dangerous.
• Initial boiling point — used as a tiebreaker to separate the single most volatile group (Category 1) from everything else with the same low flash point.

It is the vapor, not the liquid itself, that burns. That is why a low flash point matters so much on a jobsite: a Category 1 or 2 solvent can release an ignitable vapor cloud at temperatures you would never think of as "hot."

The four flammable liquid categories (exact thresholds)

OSHA's criteria are written in both Celsius and Fahrenheit. The table below gives both so you can match an SDS regardless of which unit it reports. Thresholds are from 29 CFR 1910.1200, Appendix B.6.1 (mirrored in 29 CFR 1910.106(a) for the underlying definitions).

The single most-misread distinction is Category 1 vs Category 2: both have a flash point below 23°C (73.4°F). What separates them is the initial boiling point. If the boiling point is at or below 35°C (95°F), it is Category 1 — the most volatile, most readily ignitable group. If the boiling point is above 35°C, it drops to Category 2. Boiling point is irrelevant for Categories 3 and 4; those are decided by flash point alone.

Label elements by category

Once you have the category, the required label elements follow directly under 29 CFR 1910.1200, Appendix C (label provisions) and Appendix B.6 (hazard statements):

Three things to internalize:

• Category 4 carries no pictogram. It is the one flammable-liquid category with no flame symbol — only the word "Warning" and the "Combustible liquid" statement. A label with a flame on a true Category 4 product is over-classifying it.
• The hazard statements are standardized. You cannot substitute custom wording like "very flammable" — Appendix B.6 fixes the exact phrasing. (OSHA accepts the British spelling "vapor" because it adopts the harmonized GHS text.)
• Signal word steps down at Category 3. Cat 1 and 2 say "Danger"; Cat 3 and 4 say "Warning." Only one signal word ever appears on a label — the most severe one across all of the product's hazards.

Worked examples (classify these four liquids)

Walk through the second one. Acetone-type solvent has a flash point of 10°C, which is below 23°C — so it is Category 1 or Category 2. Now check the boiling point: 56°C is above 35°C, so it cannot be Category 1. It is Category 2 → flame pictogram, signal word "Danger," "Highly flammable liquid and vapor." If that same flash point had paired with a boiling point of, say, 34°C, it would jump up to Category 1 ("Extremely flammable").

Always classify from test data or SDS Section 9 (Physical and Chemical Properties), and confirm it agrees with the GHS classification stated in SDS Section 2. If Section 2 and Section 9 disagree, the SDS has a problem — go back to the manufacturer before you print a label.

Classification (HazCom) is NOT storage (1926.152 / 1910.106)

This is the trap that trips up most contractors, so it gets its own section. The flash-point categories above govern labeling under HazCom — what goes on the label and the SDS. They do not tell you how much of a flammable liquid you can keep on site or how to segregate it. HazCom (29 CFR 1910.1200, adopted for construction by 29 CFR 1926.59) is labels, SDSs, and training only — it imposes no storage quantities and no segregation rules.

Flammable- and combustible-liquid storage lives in a different standard:

A second wrinkle is terminology. GHS Category 4 uses the words "combustible liquid," but the older 1910.106 storage scheme has its own "combustible liquid" classes (Class II and Class IIIA, by flash point) that pre-date GHS and use different break points. So "combustible liquid" can mean two different things depending on whether someone is talking about the GHS label or the 1910.106 storage class. When you compare numbers across documents, always confirm which framework is being used.

For the storage side — incompatible segregation, cabinet limits, and dispensing — plan from SDS Section 7 (Handling and Storage) and SDS Section 10 (Stability and Reactivity / incompatible materials), and see OSHA Chemical Storage Requirements: Complete Guide.

Where this fits in HazCom 2024

OSHA's classification rules for flammable liquids sit in 29 CFR 1910.1200, Appendix B. The HCS 2024 update aligns the standard primarily with GHS Revision 7 and adds new physical-hazard classes alongside the existing ones, including:

• Aerosols, Categories 1–3 (Appendix B.3) — including a new non-flammable Category 3
• Chemicals Under Pressure (Appendix B.3.2)
• Desensitized Explosives (Appendix B.17)

These are separate hazard classes from flammable liquids, but they frequently appear on the same product — for example, an aerosol can whose contents are also a flammable liquid will be classified for both hazard classes, and the label must reflect each.

The phased compliance dates under HCS 2024 (post-extension, per 29 CFR 1910.1200(j)) are:

For the full picture, see the Complete OSHA HazCom 2026 Guide and the Hazard Communication Standard 1910.1200 overview.

Common mistakes and myths

• Forgetting the boiling-point test for Category 1. A low flash point alone does not make a liquid Category 1. You must also check the initial boiling point (≤ 35°C). Skipping it routinely over-classifies Category 2 liquids as Category 1.
• Putting a flame pictogram on a Category 4 liquid. Category 4 uses no pictogram — only "Warning" and "Combustible liquid." Adding a flame is wrong.
• Confusing GHS categories with NFPA 704 ratings. The red diamond on a label (GHS, red/black, for product labels) is not the multicolored NFPA 704 building placard (for emergency responders). NFPA's 0–4 flammability number runs the opposite direction from GHS categories and uses different break points.
• Confusing "combustible liquid" (GHS Cat 4) with the 1910.106 storage classes. Same words, different frameworks and numbers. See the section above.
• Assuming HazCom dictates storage. It does not. Storage is 1926.152 / 1910.106.
• Inventing your own hazard statement. The wording is fixed by Appendix B.6. Use it verbatim.
• Trusting the supplier's artwork over the SDS. Classify from SDS Sections 2 and 9; verify the label matches.

Why getting it wrong costs money

Mislabeling a flammable liquid — wrong signal word, missing flame pictogram, or no label at all — is a labeling violation under 29 CFR 1910.1200(f), and Hazard Communication ranked second on OSHA's FY2025 list of most-cited standards. A serious citation can run up to $16,550 (the 2026 maximum, unchanged from 2025 — there was no inflation increase), and willful or repeated violations reach up to $165,514. Construction employers are on the hook through 29 CFR 1926.59, which adopts 1910.1200 by reference. See The $16,550 Mistake: HazCom Violations in 2026.

Note: This is general guidance, not legal advice. OSHA-approved State Plans may have their own HazCom requirements that must be at least as effective as the federal standard — verify your state's rules.

Classification checklist

• Find the flash point (test data or SDS Section 9).
• Confirm it is ≤ 93°C (199.4°F) — otherwise it is not a flammable liquid under this class.
• If flash point < 23°C (73.4°F), get the initial boiling point to split Cat 1 (BP ≤ 35°C) from Cat 2 (BP > 35°C).
• Assign the category from the threshold table.
• Apply the correct pictogram (none for Cat 4), signal word, and standardized hazard statement.
• Confirm the label and SDS Section 2 agree on classification.
• Remember storage is a separate standard (1926.152 / 1910.106), planned from SDS Sections 7 and 10.

Classify and label flammable liquids correctly

Turn a flammable-liquid classification into a compliant label — correct flame pictogram, signal word, and hazard statement — with the GHS Label Generator. For in-house bottles, see the Secondary Container Label Maker. Related reading: The 8 OSHA Pictograms vs the 9 GHS Pictograms · GHS Label Requirements Complete Guide · OSHA Chemical Storage Requirements.

Sources & verification: 29 CFR 1910.1200 Appendix B.6 (flammable-liquid classification) and Appendix C (label elements); 29 CFR 1910.106(a) (definitions); storage via 29 CFR 1926.152 (construction) / 1910.106 (general industry); construction HazCom adoption via 29 CFR 1926.59; penalty maximums per 29 CFR 1903.15(d) (2026); HCS 2024 deadlines per 29 CFR 1910.1200(j). Facts verified against the HazComFast regulatory source of truth (ghsClassification, hcsDeadlines, oshaPenalties, standardsCitations), last verified 2026-06-26. Not legal advice.