GHS / HazCom term · Glossary
Flammable Gas
Verified vs OSHA sources · 2026-10-05
A gas that has a flammable range with air at 20°C (68°F) and standard pressure. Under OSHA's HazCom 2024 update, flammable gases are classified as Category 1A, 1B, or 2. Category 1A is the most hazardous and now includes pyrophoric gases (those igniting spontaneously in air at or below 54°C) and chemically unstable gases (which can react explosively even without air). Examples include hydrogen, methane, propane, and acetylene.
Also known as: combustible gas
Flammable Gas at a glance
- Classified Category 1A, 1B, or 2 (HazCom 2024 update).
- Category 1A: ignitable at ≤13% by volume in air, OR flammable range ≥12 percentage points — and includes pyrophoric and chemically unstable gases.
- Category 1B: meets 1A but isn't pyrophoric/unstable, with a lower flammability limit > 6% or a burning velocity < 10 cm/s.
- Category 2: other gases that have a flammable range in air but don't meet Category 1.
In plain English
A gas that can catch fire when mixed with air — like propane, methane, hydrogen, or acetylene. OSHA sorts them into 1A (most dangerous, including gases that ignite on their own), 1B, and 2.
What the rule says
“Flammable gas means a gas having a flammable range with air at 20 °C (68 °F) and a standard pressure of 101.3 kPa (14.7 psi). A pyrophoric gas means a flammable gas that is liable to ignite spontaneously in air at a temperature of 54 °C (130 °F) or below.”
In context
A classification or labeling concept from the UN Globally Harmonized System, adopted into US law through OSHA's Hazard Communication Standard (29 CFR 1910.1200) and its appendices.
Where this is written in OSHA's rules
- 29 CFR 1910.1200 App B.2Read on eCFR / OSHA.gov
Example
Propane and methane have flammable ranges in air and carry the GHS02 flame pictogram. Silane — which bursts into flame the instant it meets air below 54°C — is a pyrophoric gas, placing it in Category 1A, the highest flammable-gas hazard tier.
Why it matters
A gas leak that reaches its flammable range needs only an ignition source to flash or explode, and pyrophoric gases skip the ignition source entirely. The HazCom 2024 restructuring into 1A/1B/2 — folding pyrophoric and chemically unstable gases into 1A — sharpened how these top-tier hazards must be labeled and communicated on the SDS.
Where pyrophoric gas actually sits
Pyrophoric gas is not a separate low category — it's part of Category 1A, the most hazardous flammable-gas tier, because it ignites spontaneously in air (≤54°C). The older shorthand that put pyrophoric gas in “Category 2” is incorrect under the current HazCom classification.
Flammable gas categories (HazCom 2024)
| Category | Criteria |
|---|---|
| 1A | ≤13% vol ignitable or range ≥12 pts; incl. pyrophoric (≤54°C) & chemically unstable gases |
| 1B | Meets 1A but not pyrophoric/unstable; LFL > 6% or burning velocity < 10 cm/s |
| 2 | Other gases with a flammable range in air |
Flammable Gas: frequently asked questions
- What is a flammable gas?
- A gas that has a flammable range with air at 20°C and standard pressure — such as hydrogen, methane, propane, or acetylene. OSHA's HazCom 2024 update classifies them as Category 1A, 1B, or 2.
- What is a pyrophoric gas, and what category is it?
- A flammable gas that ignites spontaneously in air at or below 54°C (130°F). It falls in Category 1A — the most hazardous flammable-gas tier — not a lower category.
- What's the difference between Category 1A and 1B flammable gas?
- Both meet the Category 1 flammability criteria, but 1B gases are not pyrophoric or chemically unstable and have a higher lower-flammability limit (> 6%) or a low burning velocity (< 10 cm/s), making them somewhat less hazardous than 1A.
Related terms
Free tools for this
Governing OSHA standards
More GHS terms
A GHS health-hazard class for the harmful effects that follow a single dose, several doses within 24 hours, or a 4-hour inhalation exposure. Chemicals are sorted into categories by route (oral, dermal, inhalation) using LD50/LC50 cut-offs — Category 1 is the most toxic. OSHA's HazCom standard uses Categories 1–4.
Aspiration HazardA GHS health-hazard class for chemicals that can be fatal if swallowed and then enter the airways — directly or by vomiting — causing chemical pneumonia or lung injury. Thin, low-viscosity hydrocarbons (gasoline, kerosene, lamp oil) are the classic example. It is Category 1, with the health-hazard pictogram (GHS08) and signal word “Danger.”
CarcinogenicityA GHS health-hazard class for chemicals that cause cancer or increase its incidence. GHS (and OSHA's HazCom standard) sorts carcinogens into Category 1A (known human carcinogen), 1B (presumed, from animal evidence), and Category 2 (suspected). Carcinogens carry the health-hazard pictogram (GHS08) and a hazard statement such as H350 (“May cause cancer”).
Chemicals Under PressureA physical-hazard class added by OSHA's 2024 HCS update (aligning with GHS Revision 7) for liquids or solids pressurized with a gas, held in pressure receptacles other than aerosol dispensers and not classified as gases under pressure. It is categorized by the flammability of its contents and always carries the gas-cylinder pictogram (GHS04) for the pressure hazard.
ClassificationThe process of evaluating the available scientific evidence for a chemical against the GHS criteria to decide whether it is hazardous and, if so, the hazard class and category of each effect. Classification is the first step in HazCom — its result determines every label element (pictogram, signal word, hazard and precautionary statements) and SDS content.
Corrosive to MetalsA GHS physical-hazard class for chemicals that, by chemical action, will materially damage or even destroy metals. It is a single category (Category 1), determined by corrosion-rate testing on steel and aluminum, and uses the corrosion pictogram (GHS05) with the hazard statement H290. It is a separate hazard from skin corrosion, even though both use GHS05.
Desensitized ExplosivesA physical-hazard class added by OSHA's 2024 HCS update (GHS Revision 7) for solid or liquid explosives that have been phlegmatized — wetted or diluted — to suppress their explosive properties for safe transport and storage. It has four categories based on the corrected burning rate, and uses the flame pictogram (GHS02), not the exploding-bomb.
Globally Harmonized System (GHS)An internationally agreed framework created by the United Nations (first adopted in 2003) that standardizes how chemical hazards are classified and communicated — using common hazard classes, pictograms, signal words (Danger / Warning), and coded hazard (H) and precautionary (P) statements on labels and safety data sheets. The US adopted GHS into law through OSHA's 2012 Hazard Communication Standard update.
Sources & verification
- 29 CFR 1910.1200 App B.2 (eCFR / OSHA.gov)
- OSHA — 1910.1200 Appendix B.2 (Flammable Gases)
- OSHA — Hazard Communication Standard 2024 Final Rule
Reviewed by HazComFast against eCFR, OSHA.gov, NIOSH, and the Federal Register. Last reviewed 2026-10-05. This glossary is general information, not legal advice; OSHA State-Plan states (e.g. California, Michigan) may adopt stricter requirements.
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