GHS / HazCom term · Glossary
Gas Under Pressure
Verified vs OSHA sources · 2026-10-05
A GHS/HazCom physical hazard class (Appendix B.5 of 29 CFR 1910.1200) for gases contained in a receptacle at a pressure of 200 kPa (gauge) or more, or that are liquefied or refrigerated liquefied. It is divided into four groups by the physical state of the gas in the container: compressed gas, liquefied gas, refrigerated liquefied gas, and dissolved gas. The hazard is primarily physical — the stored energy of the pressurized (or cryogenic) contents can cause an explosion, projectile, or cold burn. Gases under pressure carry the GHS04 (gas cylinder) pictogram.
Also known as: gases under pressure, compressed gas, GHS gas cylinder class
Gas Under Pressure at a glance
- Defined in 29 CFR 1910.1200 Appendix B.5 — contained at ≥ 200 kPa gauge, or liquefied/refrigerated liquefied.
- Compressed gas: entirely gaseous when packaged at −50°C (critical temperature ≤ −50°C).
- Liquefied gas: partly liquid when packaged above −50°C — split into high-pressure (crit. temp −50 to +65°C) and low-pressure (crit. temp > +65°C).
- Refrigerated liquefied gas: made partly liquid by its low (cryogenic) temperature.
- Dissolved gas: dissolved under pressure in a liquid-phase solvent (e.g., acetylene in acetone).
- Pictogram: GHS04 (gas cylinder); signal word 'Warning.' Refrigerated liquefied gases add a cold-burn hazard statement.
- This is a PHYSICAL hazard from pressure/temperature — it is separate from any flammable or toxic property the gas may also have.
In plain English
A gas stored under pressure in a cylinder or as a cold liquid. Even if the gas itself weren't flammable or toxic, the pressure alone is dangerous — a damaged cylinder can rocket across a room or burst, and very cold (cryogenic) gases can cause frostbite-like burns. GHS sorts them into four kinds by how the gas is held: compressed, liquefied, refrigerated-liquefied, or dissolved. They all carry the gas-cylinder symbol.
What the source says, in summary
Gases under pressure are gases which are contained in a receptacle at a pressure of 200 kPa (gauge) or more, or which are liquefied or liquefied and refrigerated. They comprise compressed gases, liquefied gases, dissolved gases and refrigerated liquefied gases.
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.5Read on eCFR / OSHA.gov
- 29 CFR 1910.1200Read on eCFR / OSHA.gov
Example
A cylinder of nitrogen is classified as a compressed gas (nitrogen is entirely gaseous at −50°C), while a propane cylinder is a liquefied gas and a dewar of liquid nitrogen is a refrigerated liquefied gas. All three carry the GHS04 gas-cylinder pictogram for the pressure hazard. The liquid-nitrogen dewar additionally warns of cryogenic cold burns, and because nitrogen displaces oxygen, it is also a simple asphyxiant. A single gas can therefore carry multiple, independent classifications — pressure (GHS04), plus flammability or asphyxiation as applicable.
Why it matters
The gas-under-pressure class captures a hazard that has nothing to do with the gas's chemistry: the stored mechanical energy of the pressurized container. A knocked-over cylinder with a sheared valve becomes a high-velocity projectile, and cryogenic liquefied gases cause severe cold burns and can expand explosively. Recognizing GHS04 as a physical-pressure warning — independent of, and additional to, any flammable or toxic hazard — drives proper cylinder securing, storage, and handling.
Gas under pressure vs. chemicals under pressure vs. aerosols
Three related pressurized-container classes are easy to confuse. GAS UNDER PRESSURE (B.5) is a gas held at pressure (or liquefied/refrigerated) in a cylinder — the classic compressed-gas cylinder. AEROSOLS (B.3) are non-refillable spray cans with a propellant that dispense contents as a spray, foam, or powder. CHEMICALS UNDER PRESSURE (B.3.2) — new in HCS 2024 — are liquids or solids (not just gases) pressurized with a gas in a container other than an aerosol dispenser. The GHS04 gas-cylinder pictogram appears on gases under pressure and chemicals under pressure, while aerosols are classified by their flammability. Choosing the right class depends on the physical form of the contents and the type of container.
GHS Gas Under Pressure — Four Groups (App B.5)
| Group | Definition |
|---|---|
| Compressed gas | Entirely gaseous when packaged at −50°C (critical temperature ≤ −50°C) |
| Liquefied gas | Partly liquid above −50°C (high-pressure: crit. temp −50 to +65°C; low-pressure: > +65°C) |
| Refrigerated liquefied gas | Made partly liquid by its low (cryogenic) temperature |
| Dissolved gas | Dissolved under pressure in a liquid-phase solvent (e.g., acetylene in acetone) |
Gas Under Pressure: frequently asked questions
- What are the four groups of gases under pressure?
- Compressed gas (entirely gaseous when packaged, critical temperature ≤ −50°C, e.g., nitrogen or oxygen); liquefied gas (partly liquid in the container above −50°C, subdivided into high-pressure and low-pressure by critical temperature, e.g., propane); refrigerated liquefied gas (kept partly liquid by cryogenic temperature, e.g., liquid nitrogen); and dissolved gas (dissolved under pressure in a solvent, e.g., acetylene dissolved in acetone). All four carry the GHS04 gas-cylinder pictogram.
- Why is a gas under pressure hazardous even if it isn't flammable or toxic?
- Because of the stored energy in the pressurized container. A cylinder holds gas at high pressure, so physical damage — especially a broken valve — can turn it into a high-speed projectile or cause it to burst. Refrigerated liquefied (cryogenic) gases add the hazard of severe cold burns and rapid volume expansion. These are physical hazards from pressure and temperature alone, which is why an inert gas like nitrogen still carries the GHS04 pictogram.
- What is the difference between a gas under pressure and a chemical under pressure?
- A gas under pressure (App B.5) is a gas contained at pressure — or liquefied/refrigerated — in a receptacle such as a compressed-gas cylinder. A chemical under pressure (App B.3.2, new in HCS 2024) is a liquid or solid pressurized with a gas inside a container that is NOT an aerosol dispenser. Both use the GHS04 gas-cylinder pictogram, but the gas-under-pressure class covers gases themselves, while chemicals under pressure covers pressurized liquids/solids.
Related terms
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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.5 (eCFR / OSHA.gov)
- 29 CFR 1910.1200 (eCFR / OSHA.gov)
- OSHA — 1910.1200 Appendix B.5 (Gases Under Pressure)
- OSHA — Hazard Communication (Physical Hazards)
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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