GHS classification explained for US and UK chemical teams
GHS classification assigns chemical hazard classes and categories. Learn what Category 1 to 4 means and which rules apply in the US, GB and NI.
GHS classification assigns a chemical substance or mixture to hazard classes and, where applicable, categories using defined criteria. A class identifies the hazard type. A category distinguishes severity within that class, sometimes by strength of evidence rather than potency. OSHA's Hazard Communication Standard defines both terms.
GHS means Globally Harmonized System of Classification and Labelling of Chemicals. It's an international framework that national or regional law puts into force. It is not a worldwide chemical approval. The destination market's law sets the criteria. HSE explains this for CLP.
Classification describes intrinsic hazardous properties. It doesn't replace a risk assessment for a particular use or exposure, as HSE's classification guidance explains.
What is a GHS hazard class?
A hazard class describes the nature of a hazard, such as acute toxicity or flammability. Physical, health and environmental hazards are groups of GHS hazard classes, not three individual classes. This short hazard classification chart uses examples from HSE's guidance:
| Hazard group | Examples |
|---|---|
| Physical | Explosivity, flammability |
| Health | Acute toxicity, carcinogenicity, skin and eye irritation |
| Environmental | Aquatic hazards |
But the destination market still changes what you must classify. OSHA requires classification for physical and health hazards. Environmental classification isn't mandatory under its scheme, though other US environmental obligations can still apply. The standard also covers simple asphyxiants, combustible dust, and hazards not otherwise classified. The scope is in OSHA's definitions.
What do GHS categories 1 to 4 mean?
There is no universal four-category ladder. Where a class uses numbered categories, a lower number generally means a more severe hazard inside that class only, so Category 1 acute toxicity and Category 1 flammable liquid are not a shared danger score. Some categories reflect strength of evidence rather than potency. OSHA warns against comparing categories across different classes.
Two OSHA examples, not a UN or CLP chart:
- Acute toxicity uses Categories 1 to 4, based on route-specific acute toxicity estimates, including LD50 or LC50 information. OSHA hasn't adopted GHS acute toxicity Category 5. Category 4 doesn't mean safe. The criteria are in Appendix A.
- Flammable liquids also use Categories 1 to 4. The criteria are flash point and, where relevant, initial boiling point, not toxicity. They are in Appendix B.
Keep the class name attached. "Category 2" alone doesn't name the hazard. Not every class uses numbers: OSHA self-reactive chemicals use Types A to G, and gases under pressure use named groups, both listed in Appendix B.
How does GHS hazard classification work?
Start with the chemical and the market where it will be supplied. ECHA's classification guidance and HSE's workflow support this sequence:
- Identify the substance or mixture, its composition and the relevant physical form.
- Collect and evaluate reliable hazard information.
- Compare that evidence with the applicable criteria.
- If the law sets a binding classification, apply it to the covered hazards, then assess the remaining hazards.
- Determine the label information, and review the classification when relevant information changes.
A mixture needs its own classification. Don't copy an ingredient classification onto the finished product. Depending on the hazard and the evidence, use whole-mixture data, applicable bridging principles, or ingredient information and calculations. New testing isn't always required. ECHA's mixture guidance describes those routes. A mixture test doesn't always override ingredient evidence, and OSHA Appendix A has exceptions for carcinogenicity, mutagenicity and reproductive toxicity.
If an SDS already exists, read Section 2 for the classification and the related label information. Section 3 is composition. For a mixture, an ingredient classification there is not the mixture's classification. OSHA's SDS requirements keep those sections apart.
Which rules apply in the US and UK?
Use OSHA HazCom for US workplace hazard classification. Great Britain uses GB CLP and the GB mandatory classification and labelling list (GB MCL). Northern Ireland uses EU CLP for substances and mixtures placed on its market. Later EU changes don't automatically apply in GB. Qualifying Northern Ireland goods supplied directly to Great Britain have special provisions, so that supply isn't an automatic reclassification. HSE sets out the jurisdictional differences.
RegAffairs AI's cited answer on GHS classification and hazard categories follows this same class, category and jurisdiction split.
For EU and NI substance lookups, a harmonised classification in CLP Annex VI binds only the hazards that entry covers. Assess other relevant hazards separately. Absence from Annex VI doesn't mean the substance is non-hazardous, which ECHA's classification guidance states directly.