What Are Hazards Not Otherwise Classified (HNOC)?

Under OSHA’s Hazard Communication Standard, hazards not otherwise classified (HNOC) is the catch-all category for chemicals that have a documented physical or health danger but don’t fit any of the standard hazard classes defined by the Globally Harmonized System. Manufacturers and importers have to evaluate every chemical they produce or bring into the country against the recognized GHS classes — flammability, acute toxicity, carcinogenicity, and the rest.¹ When the evidence shows a real danger but none of the predefined classes fits, the HNOC designation keeps the hazard from going uncommunicated. With compliance deadlines from OSHA’s 2024 rule update arriving in 2026, how this classification works matters for manufacturers, safety officers, and the people handling these chemicals on the floor.

What Counts as an HNOC

The regulation defines an HNOC as “an adverse physical or health effect identified through evaluation of scientific evidence during the classification process that does not meet the specified criteria for the physical and health hazard classes addressed in this section.”¹ If testing shows a chemical is dangerous but the danger doesn’t check the boxes of any recognized GHS class, it still gets flagged under this designation.

There is an important boundary. HNOC does not cover effects where a relevant hazard class exists but the chemical falls below the class’s cut-off value, and it does not cover effects that belong to a GHS category OSHA chose not to adopt. Acute toxicity Category 5 exists in the broader GHS framework but isn’t recognized under the U.S. standard, so a chemical that only qualifies for Category 5 isn’t technically an HNOC under the strict regulatory definition.¹ Many safety professionals still treat those borderline situations with extra caution on their data sheets, because the hazard to workers doesn’t vanish because a regulatory threshold wasn’t met.

How a Chemical Gets Classified This Way

Identifying an HNOC takes credible scientific evidence of an adverse physical or health effect — testing, peer-reviewed research, or clinical data showing a repeatable negative outcome from exposure.² OSHA uses a weight-of-evidence approach for health effects, so evaluators consider the totality of available data rather than any single study.

The process is performance-oriented. OSHA doesn’t prescribe a specific testing protocol, but companies have to demonstrate that their evaluation was thorough and scientifically defensible.³ Evaluators compare the documented effects against every existing GHS hazard class to confirm no standard classification fits. If the evidence is inconclusive, or if the effect only appears at exposure levels far beyond realistic workplace conditions, the HNOC designation may not apply.

Physical HNOCs tend to involve properties that create a risk of fire, explosion, or reactivity without meeting the criteria for classes like flammable liquids or oxidizers. Health HNOCs mean the substance causes a measurable biological change in humans or animals that doesn’t match any defined health hazard category.

Common Examples

A few situations where the hazard is real but doesn’t map to a standard GHS class:

Static-accumulating flammable liquids. Gasoline tends to build up static charge during transfer, creating ignition risks beyond its standard flammability classification.
Metal fume fever. Heating solder above its melting point can release fumes that cause flu-like symptoms, a response not captured by standard acute toxicity or inhalation hazard criteria.
Spontaneous combustion of soaked materials. Linseed oil can cause rags and other porous materials to self-ignite after drying, even if the chemical itself isn’t classified as spontaneously combustible.
Exothermic polymerization. Certain monomers like methylmethacrylate release significant heat during polymerization, which doesn’t fit the self-reactive substance criteria but still poses a real workplace hazard.

Without the HNOC category, a manufacturer could document a known danger, confirm it doesn’t fit any GHS class, and then have no duty to communicate it.

What the Safety Data Sheet Has to Say

Once a chemical carries an HNOC designation, the manufacturer has to describe the hazard in Section 2 (Hazard Identification) of the Safety Data Sheet. Appendix D to the Hazard Communication Standard requires it: the SDS must describe any hazards not otherwise classified identified during the classification process.⁴ OSHA has confirmed that HNOC information can be included alongside other hazard classifications under Section 2, subheading (a), rather than needing its own sub-header.⁵

The disclosure should explain what the adverse effect is and the conditions under which it’s likely to occur. That narrative fills the gap left by the absence of a standard hazard pictogram. A safety officer reading the SDS needs enough detail to select appropriate protective equipment and design handling procedures.

One common misconception is that because HNOCs don’t require standard GHS pictograms, no signal word needs to appear on the SDS either. OSHA has clarified that if a signal word is used for an HNOC on the SDS, it must be either “Danger” or “Warning,” and that the correct signal word for the HNOC must appear on the data sheet.⁵ Skipping the disclosure entirely is a fast route to a citation. Hazard communication was the second most cited OSHA standard in fiscal year 2024.⁶

Why the Shipped Label Usually Won’t Mention It

Labeling is where HNOCs diverge from standard hazard classes. OSHA does not require manufacturers to include HNOC information on the container label that ships to a customer’s workplace. The regulation explicitly states that “hazards not otherwise classified…do not have to be addressed on the container.”⁷ This keeps U.S. labeling consistent with international GHS practice, which doesn’t recognize the category for label purposes.

Companies can voluntarily add supplemental information about HNOC hazards to the label. If they do, the supplemental text cannot obscure or contradict the required GHS label elements; signal words, pictograms, and hazard statements must stay prominent.¹ The GHS exclamation mark pictogram may be used for HNOCs, but only if the words “Hazard Not Otherwise Classified” or the letters “HNOC” appear directly below it, and only if the exclamation mark isn’t already required for another hazard class on the same label.

The practical result is that a worker handling a shipped container usually won’t see an HNOC warning just by reading the label. They have to go to the Safety Data Sheet. That is one reason SDS access and training matter so much for these substances.

Employer Duties: Inventory and Training

Manufacturers classify the chemical, but employers carry the compliance burden at the workplace. The Hazard Communication Standard defines “hazardous chemical” to include any chemical classified as an HNOC, so these substances have to appear on the employer’s written hazard communication program and chemical inventory.⁸ You can’t leave them off the list because they lack a standard pictogram.

Training is specific. Employers have to provide effective information and training on all hazardous chemicals in the work area, and the standard explicitly calls out “hazards not otherwise classified” as required content.⁸ Training has to happen at initial assignment and whenever a new chemical hazard enters the work area. It should cover what the HNOC hazard is, the protective measures to take, emergency procedures, and how to find and read the Safety Data Sheet.

This is where employers often slip. They train workers on the chemicals with vivid pictograms and overlook the HNOC entries buried in Section 2 of the SDS. An inspector won’t care that the hazard was hard to spot. The obligation is the same whether the chemical carries a skull-and-crossbones or a text-only HNOC description.

Deadlines Under the 2024 Rule Update

OSHA finalized a significant update to the Hazard Communication Standard in May 2024, aligning the U.S. system more closely with GHS Revision 7.⁹ The core HNOC definition didn’t change, and the evidentiary standard for classification is the same. The update did clarify that nuisance particulates are excluded from the standard’s scope only if they don’t pose any physical hazard, health hazard, or HNOC, closing a loophole some manufacturers had used.⁷

The deadlines phase in:

May 19, 2026. Manufacturers, importers, and distributors must update Safety Data Sheets and labels for substances.
November 20, 2026. Employers must update workplace labels, written hazard communication programs, and employee training for substances to reflect new classifications and any newly identified HNOCs.⁸
November 19, 2027. Manufacturers, importers, and distributors must update SDS and labels for mixtures.
May 19, 2028. Employers must update programs, labels, and training for mixtures.⁸

If you handle chemical substances, the November 2026 deadline means you should already be reviewing incoming SDS updates from suppliers and flagging any new HNOC entries that will drive updated training and inventory records.

Penalties for Getting It Wrong

Effective January 15, 2025, a serious OSHA violation carries a maximum penalty of $16,550 per violation, while willful or repeated violations can reach $165,514 per violation. Failure-to-abate violations accrue at $16,550 per day beyond the abatement deadline.¹⁰

Hazard communication citations commonly involve incomplete SDS documentation, missing chemicals on workplace inventories, and inadequate employee training.⁶ An HNOC left off the chemical inventory, or a training program that only covers GHS-classified hazards, is exactly the gap inspectors are trained to find.

If You Ship Across the U.S.-Canada Border

OSHA uses a single HNOC category covering both physical and health hazards. Canada’s Hazardous Products Regulations split the concept into two distinct classes: Physical Hazards Not Otherwise Classified (PHNOC) and Health Hazards Not Otherwise Classified (HHNOC).¹¹

Labeling obligations differ too. In the U.S., HNOC label elements are voluntary. In Canada, label elements for PHNOC and HHNOC are required. Both countries permit the exclamation mark pictogram, but the U.S. requires the words “Hazard Not Otherwise Classified” or the letters “HNOC” to appear below it, while Canada doesn’t require the acronym (though it allows it). OSHA and Health Canada issued joint guidance to help manufacturers producing labels that have to satisfy both systems.¹¹

1
eCFR. 29 CFR 1910.1200 – Hazard Communication
2
Occupational Safety and Health Administration. OSHA’s Amended Hazard Communication Standard (HCS 2012) About Hazards Not Otherwise Classified
3
Occupational Safety and Health Administration. Hazard Communication – Guidance for Hazard Determination for Compliance with the OSHA Hazard Communication Standard
4
Occupational Safety and Health Administration. Appendix D to 1910.1200 – Safety Data Sheets (Mandatory)
5
Occupational Safety and Health Administration. Clarification on Labeling and SDS Requirements Under HCS 2012
6
Occupational Safety and Health Administration. Top 10 Most Frequently Cited Standards
7
Federal Register. Hazard Communication Standard
8
Occupational Safety and Health Administration. 1910.1200 – Hazard Communication
9
Occupational Safety and Health Administration. Hazard Communication Standard Final Rule
10
Occupational Safety and Health Administration. OSHA Penalties
11
Occupational Safety and Health Administration. Joint OSHA/Health Canada Guidance on Labeling Pictogram for Hazards Not Otherwise Classified (HNOC), Physical Hazards Not Otherwise Classified (PHNOC) and Health Hazards Not Otherwise Classified (HHNOC)