Chemicals Under Pressure: OSHA's New HCS 2024 Hazard Class (Appendix B.3.2) Explained

Chemicals Under Pressure is a new physical hazard class introduced by HCS 2024 and codified at Appendix B.3.2 of 29 CFR 1910.1200. It covers liquids or solids that are pressurized with a gas — at a gauge pressure of 200 kPa (about 29 psi) or more at 20 °C — inside a container that is not an aerosol dispenser. Before this update, the hazard posed by such pressurized non-aerosol products had no dedicated home in OSHA's classification scheme. HCS 2024 closes that gap, aligning primarily with GHS Revision 7, which created the class. This is a finalized, real OSHA hazard class — not a proposal — and employers must train on it before the deadline.

Why OSHA Added Chemicals Under Pressure

Prior to HCS 2024, OSHA's physical-hazard appendix (Appendix B) had a structural gap: a chemical pressurized with a propellant gas, but not packaged as an aerosol, did not fit cleanly into any class. It was not an aerosol (those go in a throwaway dispenser), and it was not itself a gas under pressure (the contents are liquid or solid). The stored pressure is a genuine physical hazard — a ruptured or over-heated container can fail violently — yet under the old scheme that hazard was often under-communicated or shoehorned into an ill-fitting category.

Appendix B.3.2 fixes that. It ensures the pressurization hazard is classified on its own terms, appears on the shipped-container label (29 CFR 1910.1200(f)(1)), and is documented on the safety data sheet. Construction employers inherit the identical obligation through 29 CFR 1926.59, which adopts the Hazard Communication Standard for construction work.

What Falls Under Appendix B.3.2 (the criteria)

The class applies to a liquid or solid chemical that is pressurized with a gas at a defined threshold inside a container that is not an aerosol dispenser. The defining features:

• Physical state of the contents: a liquid or a solid (not a gas — a gas in a cylinder is a Gas Under Pressure).
• Pressurization: the product is pressurized by a gas (a propellant or a dissolved/compressed gas) at a gauge pressure of 200 kPa (≈ 29 psi) or more at 20 °C.
• Packaging: the container is not a non-refillable aerosol can (those fall under Aerosols, Appendix B.3) — for example a refillable pressurized vessel or cylinder dispensing a liquid/solid product.
• Layered classification: the pressure-driven hazard is classified in addition to any flammability, toxicity, corrosivity, or other hazard the chemical also presents.

In short: content + pressure + container all have to line up. Change the container to a throwaway aerosol can and it becomes an Aerosol; change the contents to a gas and it becomes a Gas Under Pressure.

Chemicals Under Pressure vs Aerosols vs Gases Under Pressure

These three pressurized-product classes are the most commonly confused part of HCS 2024. The distinction is mostly about packaging and the physical state of the contents — not the brand of chemical inside.

The practical decision test:

1. Is it a gas in a cylinder? → Gas Under Pressure (B.5).
2. Is it sprayed from a throwaway aerosol can? → Aerosol (B.3).
3. Is it a liquid or solid, pressurized by a gas, in a non-aerosol container at ≥ 200 kPa? → Chemical Under Pressure (B.3.2).

Worked Example: Classifying a Pressurized Product

Suppose a supplier ships a refillable cylinder containing a flammable liquid solvent that is pressurized with nitrogen to 300 kPa at 20 °C so it dispenses on demand. Walk it through:

1. Contents? A liquid (the solvent), not a gas → rules out Gas Under Pressure (B.5).
2. Container? A refillable cylinder, not a non-refillable aerosol can → rules out Aerosols (B.3).
3. Pressurized by a gas at ≥ 200 kPa @ 20 °C? Yes — nitrogen at 300 kPa → meets Appendix B.3.2.
4. Other hazards? The solvent is also a flammable liquid (Appendix B.6.x criteria) → that classification stands in addition to B.3.2.

Result: the product is classified as a Chemical Under Pressure and a Flammable Liquid. Its SDS Section 2 and its shipped label carry the hazard and precautionary statements for both, plus every applicable pictogram (for example, the flame and the gas-cylinder pictograms, layered per the SDS classification).

Compliance Dates That Apply to This Class

Chemicals Under Pressure follows the same staggered HCS 2024 calendar as every other class. A January 15, 2026 final rule (FR Doc. 2026-00653) extended each original deadline by four months; these are the post-extension dates now in force under 29 CFR 1910.1200(j):

During the interim, a party may comply with the 2012 HCS, the 2024 HCS, or both. If a product newly classifies as a Chemical Under Pressure, the supplier must update its SDS and shipped label by the applicable manufacturer date, and the employer must update workplace labeling and training by the applicable employer date. Note there is no separate distributor date — manufacturers, importers, and distributors share the (j)(2)(i)/(j)(3)(i) dates.

How It Appears on Labels and SDSs

Because B.3.2 is a physical hazard class, a chemical under pressure carries the hazard statement and precautionary statements for pressurization, layered with whatever other classifications apply. On the safety data sheet, expect changes in:

• Section 2 — Hazard Identification: the new classification, signal word, and the hazard/precautionary statements for the pressurized contents.
• Section 9 — Physical and Chemical Properties and Section 10 — Stability and Reactivity: information tied to the stored pressure (conditions to avoid, e.g., heating that raises internal pressure).
• Shipped-container label (29 CFR 1910.1200(f)(1)) and your workplace/secondary-container label (29 CFR 1910.1200(f)(6)): updated signal word, statements, and pictograms.

The exact label elements, by category

Appendix C assigns these. Note the pattern: the gas cylinder appears on all three categories — the pressure hazard is the constant — and the flame is added only where the product is flammable.

The category itself is decided by flammable content and heat of combustion (Appendix B.3.2): Category 1 is ≥85% flammable components with a heat of combustion ≥20 kJ/g; Category 3 is ≤1% flammable components with a heat of combustion <20 kJ/g; Category 2 is everything in between.

Contrast this with an aerosol, because the two classes collide exactly here. An Aerosol Category 3 — also "the non-flammable one" — carries no pictogram at all under Appendix C, only "Warning" and H229, "Pressurized container: may burst if heated." Same intuition, opposite label. If you take one thing from this article: a non-flammable chemical under pressure shows the gas cylinder; a non-flammable aerosol shows nothing. Even the hazard statements differ — explode versus burst. See Aerosols Category 3 under HCS 2024.

Remember OSHA requires the eight GHS pictograms (Appendix C, Figure C.1); the environmental pictogram GHS09 remains voluntary and is not OSHA-required, because environmental hazards fall outside OSHA's jurisdiction. A pressurized container that is also flammable or toxic shows all applicable pictograms, not just one.

Common Mistakes and Myths

• Myth: "It's just a fancy name for aerosols." No — Aerosols (B.3) are a separate class with their own non-refillable-dispenser packaging test. B.3.2 is specifically for non-aerosol pressurized liquids/solids.
• Myth: "It's a gas, so it's a Gas Under Pressure." Only if the contents are a gas. B.3.2 contents are a liquid or solid pressurized by a gas.
• Myth: "Chemicals Under Pressure isn't real / OSHA didn't finalize it." It is finalized at Appendix B.3.2 in the HCS 2024 rule. Do not skip it in your hazard-class training.
• Mistake: classifying only the pressure hazard. The B.3.2 classification is additive — keep every other hazard (flammable, toxic, corrosive) on the label and SDS too.
• Mistake: assuming an environmental (GHS09) pictogram is now required. It is still voluntary under OSHA.

What Employers Should Do

• Ask suppliers whether any of your pressurized, non-aerosol products now classify under Appendix B.3.2
• Collect HCS 2024-compliant SDSs that reflect the new class (Sections 2, 9, 10)
• Verify shipped labels (29 CFR 1910.1200(f)(1)) include the new hazard information and pictograms
• Update workplace / secondary-container labels using a 29 CFR 1910.1200(f)(6) option
• Retrain employees on the new class as part of HCS 2024 training before November 20, 2026 (substances)
• Track which products are on the substance vs mixture deadline so nothing slips

What to Do Next

Map every pressurized product in your inventory to the right class and the right deadline. See the full schedule in our HCS 2024 compliance calendar, review the sibling new class in Desensitized Explosives (Appendix B.17), and understand the broader update in HCS 2024: what changed. When you are ready to relabel, build compliant shipped and secondary labels with the GHS Label Generator, and confirm you are using the right eight OSHA pictograms.

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*Verified August 5, 2026 against 29 CFR 1910.1200 Appendix B (B.3.2 category criteria, and the B.1–B.17 section list confirming Gases Under Pressure is B.5, not B.6) and Appendix C (the per-category label elements table added to this article), plus 1910.1200(j), the HCS 2024 final rule (89 FR 44144, May 20, 2024), and the deadline-extension rule (FR Doc. 2026-00653, Jan 15, 2026). ***