For years a large share of aerosol products in US commerce carried the gas-cylinder pictogram (GHS04) on the label, stacked on top of the flame. The reasoning seemed sound: the can is pressurized, "gases under pressure" is a hazard class keyed to pressure, so the product is a gas under pressure too. OSHA's own 2015 letter of interpretation reinforced the instinct, stating that a flammable aerosol meeting the Table B.5.1 criteria would be classified as both a flammable aerosol and a gas under pressure, with both sets of label elements required. The HazCom 2024 rule and a follow-on clarification to Table B.5.1 reverse that practice. Aerosols and chemicals under pressure are no longer to be classified as gases under pressure, and for substances the deadline to conform labels and SDSs (May 19, 2026 after the four-month extension in the January 15, 2026 rule) has passed.
The result is a remediation task across existing product lines and a classification decision that authors now have to get right going forward: a pressurized product is an aerosol, a chemical under pressure, or a gas under pressure, and with narrow exceptions it is exactly one of the three. The confusion is understandable because all three definitions share the same 200 kPa pressure anchor. The differentiators are what is inside the receptacle and what kind of receptacle it is, not the pressure itself. This post works through each class precisely, shows where the boundaries fall, explains why OSHA drew them where it did, and flags the pictogram trap that will catch anyone doing a blanket cleanup.
01 — DefinitionsWhy the three classes look alike on paper
Each definition turns on 200 kPa gauge at 20°C, which is exactly why practitioners conflate them.
A gas under pressure (Appendix B.5, GHS Chapter 2.5) is a gas contained in a receptacle at 200 kPa (29 psi) gauge or more at 20°C, or which is liquefied or liquefied and refrigerated. The defining fact is that the contents are a gas.
An aerosol (Appendix B.3.1, GHS Chapter 2.3) is any non-refillable receptacle containing a gas compressed, liquefied, or dissolved under pressure, with or without a liquid, paste, or powder, fitted with a release device allowing the contents to be ejected as particles suspended in a gas, or as a foam, paste, powder, liquid, or gas. The defining facts are the dispenser type (non-refillable, with a release device) and the dispersal mechanism.
A chemical under pressure (Appendix B.3.2, GHS Chapter 2.3, imported from GHS Revision 8) is a liquid or solid substance or mixture pressurized with a gas at 200 kPa gauge or more at 20°C, in a receptacle other than an aerosol dispenser, and which is not classified as a gas under pressure. The defining facts are that the bulk of the product is a liquid or solid (the gas is only the propellant), and the receptacle is not an aerosol dispenser.
The pressure is common to all three and tells you nothing about which one you are looking at. Always resolve the contents-and-receptacle question first.
Read those together and the logic is a decision sequence, not three overlapping buckets. Is the substance being classified a gas? Then it is a gas under pressure. Is it a liquid or solid dispersed from a non-refillable dispenser with a release valve? Then it is an aerosol. Is it a liquid or solid pressurized with a gas in some other kind of receptacle? Then it is a chemical under pressure.
02 — GasesGases under pressure: the four groups
Gases under pressure are classified by physical state when packaged into one of four groups. The split is driven by critical temperature — the temperature above which a pure gas cannot be liquefied no matter how much it is compressed.
| Group | Defining condition |
|---|---|
| Compressed gas | Entirely gaseous at −50°C when packaged under pressure; critical temperature at or below −50°C |
| Liquefied gas | Partially liquid at temperatures above −50°C. High-pressure liquefied: critical temperature above −50°C and at or below +65°C. Low-pressure liquefied: critical temperature above +65°C |
| Refrigerated liquefied gas | Made partially liquid because of its low temperature |
| Dissolved gas | Dissolved in a liquid-phase solvent when packaged |
All four groups carry the same label allocation under Appendix C: the gas-cylinder pictogram (GHS04) and the signal word "Warning." The hazard statement is H280 (Contains gas under pressure; may explode if heated) for compressed, liquefied, and dissolved gases, and H281 (Contains refrigerated gas; may cause cryogenic burns or injury) for refrigerated liquefied gases. There is no Danger tier for this class; the most severe gases under pressure still signal "Warning."
03 — AerosolsAerosols: three categories, and no gas-cylinder pictogram
The 2024 rule renamed the class from "Flammable Aerosols" to "Aerosols" and added Category 3 for non-flammable products, matching GHS Revision 7. Aerosols are classified on flammable-component content (by mass) and chemical heat of combustion (ΔHc), with flammability tests breaking ties.
| Category | Criteria | Label elements |
|---|---|---|
| 1 | Contains ≥ 85% flammable components and ΔHc ≥ 30 kJ/g; or meets Category 1 by the ignition-distance, enclosed-space, or foam flammability test | Flame (GHS02), Danger, H222 (Extremely flammable aerosol) and H229 (Pressurized container: may burst if heated) |
| 2 | Contains > 1% flammable components or ΔHc ≥ 20 kJ/g, does not meet Category 1, and meets Category 2 by the applicable test | Flame (GHS02), Warning, H223 (Flammable aerosol) and H229 |
| 3 | Contains ≤ 1% flammable components and ΔHc < 20 kJ/g; or contains > 1% flammable components or ΔHc ≥ 20 kJ/g but fails the Category 1 and Category 2 tests | No pictogram, Warning, H229 |
The absence of GHS04 from aerosol labels is deliberate and applies to every aerosol category. An aerosol communicates its pressure hazard through H229, not the gas-cylinder pictogram. The pictogram never belonged there; it appeared because authors were separately running the product through the gases-under-pressure criteria.
The default-to-Category-1 rule still applies: an aerosol that has not been evaluated against the flammability classification procedures is classified as Category 1. Flammable components for this purpose are those classified as flammable liquids, flammable gases, or flammable solids, and they exclude pyrophoric, self-heating, and water-reactive substances.
It is also worth noting that an aerosol does not additionally fall within the flammable gases, flammable liquids, or flammable solids classes even when its propellant or solvent would qualify on its own. You classify the finished aerosol as an aerosol. You do not also stamp the product with a standalone flammable-gas classification because the propellant is, say, propane.
04 — ChemUPChemicals under pressure: the class that keeps the cylinder
Chemicals under pressure are the Revision 8 addition, and they are the reason a blanket "strip the cylinder from anything pressurized" cleanup is wrong. The class covers liquids and solids (including pastes and gels) that are pressurized with a propellant gas in a receptacle that is not an aerosol dispenser — things like pressurized adhesives, sealants, foams, and certain lubricants packaged in larger non-aerosol pressure vessels. Like aerosols, they are sorted into three categories on flammable-component content and heat of combustion, with Category 1 the most flammable and Category 3 non-flammable.
The label allocation is where they diverge from aerosols. Category 1 and Category 2 chemicals under pressure carry both the flame pictogram (GHS02) and the gas-cylinder pictogram (GHS04), with Danger and Warning respectively. Category 3, being non-flammable, drops the flame but retains the gas-cylinder pictogram with "Warning." All categories carry a hazard statement ending in "may burst if heated."
So the cylinder pictogram appears on a chemicals-under-pressure label, but it is allocated by the chemicals-under-pressure class itself under Appendix C. It is not the product being separately classified as a gas under pressure. That distinction is invisible on the finished label, where the same GHS04 image appears, but it matters enormously for how you classify and for what you do during cleanup.
The mutual exclusivity is explicit in the standard. Appendix B Note 2 states that chemicals under pressure do not additionally fall within the scope of flammable gases (B.2), aerosols (B.3.1), gases under pressure (B.5), flammable liquids (B.6), or flammable solids (B.7), though depending on their contents they may fall within other hazard classes such as the health hazards. The GHS source text is equally direct: a substance or mixture is classified as either an aerosol or a chemical under pressure, never both.
05 — ChangeWhat actually changed, and the logic behind it
The substantive change is the removal of the dual classification, codified in the updated Table B.5.1, which now makes clear that aerosols and chemicals under pressure are not classified as gases under pressure. The practical consequence is that the gas-cylinder pictogram comes off aerosol labels and SDSs, because it was only ever there as a byproduct of the now-prohibited dual classification. The pressure hazard for aerosols is preserved through H229.
The regulatory logic is about failure mechanisms and signal integrity. When OSHA aligned with GHS Revision 3 in 2012, many aerosols ended up classified as gases under pressure because of how the design criteria mapped onto DOT and transport classifications. OSHA later concluded that this did not represent the full spectrum of aerosol hazards, because an aerosol dispenser fails differently from a high-pressure industrial gas cylinder. Putting the same gas-cylinder pictogram on a small disposable can and on a compressed-gas cylinder equates two very different physical hazards.
Over-classification has a real cost: when a pictogram appears on products that do not present the hazard it is meant to flag, it stops carrying information, and workers learn to ignore it.
The cleaner allocation — flame and H229 for aerosols, the cylinder reserved for actual gases under pressure and for chemicals under pressure where the bulk material sits under propellant pressure in a non-dispenser vessel — restores the cylinder pictogram's meaning.
06 — The trapThe trap: do not strip the cylinder from chemicals under pressure
The remediation task is to remove GHS04 from aerosol products that were dual-classified. The trap is doing it with a search-and-replace mindset that treats every pressurized container the same. If you sweep your library for products carrying both the flame and the cylinder and remove the cylinder wholesale, you will wrongly de-pictogram your chemicals under pressure, which legitimately carry GHS04 in Categories 1, 2, and 3.
Do not run a blanket "remove GHS04 from all pressurized products" cleanup. Aerosols lose the cylinder; chemicals under pressure keep it. The correct cleanup is a reclassification, not a relabeling — determine which class each product belongs to before touching any pictogram.
For each pressurized product, determine first which of the three classes it belongs to. If it is an aerosol, the cylinder comes off and H229 stays. If it is a chemical under pressure, the cylinder stays because the chemicals-under-pressure class allocates it. If it is genuinely a gas under pressure, nothing about the cylinder changes. A product previously labeled as both a flammable aerosol and a gas under pressure should end up as an aerosol with the flame and H229, and the cylinder gone.
07 — ErrorsCommon errors and gray areas
The most frequent mistake is treating "it's pressurized" as the classification trigger. Pressure at or above 200 kPa is necessary for all three classes but distinguishes none of them. Always resolve the contents-and-receptacle question first.
The second is the cleanup overreach described above — removing the cylinder from chemicals under pressure during an aerosol remediation pass.
The third comes from foreign and legacy SDSs. A supplier sheet authored before the alignment, or a sheet from a jurisdiction still on an older basis, may still show an aerosol dual-classified as a gas under pressure with GHS04. Under 1910.1200(d)(3)(ii) you may rely on an ingredient supplier's SDS, but not where you know or reasonably should know it misstates a classification. A dual-classified aerosol on an incoming sheet is now a known misstatement for US purposes, and carrying it through to your finished-product label is not defensible.
The fourth is the aerosol-versus-chemical-under-pressure boundary itself, which turns entirely on the receptacle. The deciding question is whether the package is an aerosol dispenser — meaning a non-refillable receptacle with a release device that ejects the contents as a spray, foam, paste, powder, or gas. A pressurized liquid or solid in any other kind of receptacle is a chemical under pressure. Two chemically identical formulations can land in different classes purely on packaging, which is correct under the standard because the hazard on container failure differs with the receptacle.
The fifth is forgetting the default-to-Category-1 rule for aerosols that have not been run through the flammability tests, and forgetting that flammable components for aerosol and chemical-under-pressure purposes exclude pyrophoric, self-heating, and water-reactive substances, which are not permitted in these products in the first place.
08 — Decision logicDecision rules
When classifying a pressurized product or cleaning up an existing label or SDS, work from these.
If the substance is a gas, it is a gas under pressure. If it is a liquid or solid dispensed from a non-refillable aerosol dispenser with a release valve, it is an aerosol. If it is a liquid or solid pressurized with a gas in any other receptacle, it is a chemical under pressure.
Appendix B Note 2 and the GHS text bar classifying a chemical under pressure or an aerosol as a gas under pressure. The dual classification of aerosols as gases under pressure is retired; conform substance labels and SDSs now, mixtures by November 19, 2027.
Aerosols carry the flame pictogram only (Categories 1 and 2) and no pictogram for Category 3. H229 carries the pressure communication. The cylinder pictogram was never correctly allocated to aerosols under the current standard.
It is allocated by that class under Appendix C across all three categories, with the flame added for Categories 1 and 2. Run your cleanup as a reclassification so this pictogram is not stripped by mistake.
Do not trust the cylinder pictogram's presence on an incoming SDS to mean the product is a gas under pressure. Verify the class from the contents and receptacle. Under 1910.1200(d)(3)(ii), a dual-classified aerosol on a supplier sheet is now a known misstatement for US purposes and carrying it through to a finished-product label is not defensible.
Written by the Chemply Regulatory Team. Citations refer to 29 CFR 1910.1200 and its appendices, the May 20, 2024 HazCom final rule (89 FR 44144), the January 15, 2026 deadline-extension rule (91 FR 1695), and the UN GHS through Revision 8.
This is general regulatory information, not legal advice, and it is not a substitute for professional regulatory judgment on a specific product.