GHS Classification Meaning
What does GHS classification mean?
GHS classification means sorting a chemical (or mixture) into standardised hazard classes and categories using the criteria of the Globally Harmonized System of Classification and Labelling of Chemicals. That classification then fixes what the label says and what goes on the safety data sheet. OSHA describes the GHS as "a common and coherent approach to classifying chemicals and communicating hazard information on labels and safety data sheets" (OSHA Hazard Communication).
How it works, as OSHA describes it for its GHS-aligned Hazard Communication Standard (29 CFR 1910.1200) (OSHA):
- Classification: there are set criteria for health hazards, physical hazards and mixtures. Chemical manufacturers and importers "are required to evaluate the hazards of the chemicals they produce or import."
- Labels: each hazard class and category has its own signal word, pictogram and hazard statement, and the label must carry all of them. Precautionary statements "must also be provided."
- Safety data sheets: these follow a fixed 16-section format.
- Training: employers are required to train workers on the label elements and the SDS format.
What a classification looks like in practice. Here is benzene (CAS 71-43-2) in Safe Work Australia's HCIS register of GHS classifications [1]:
| Element | Entry |
|---|---|
| Hazard classes and categories | Health: Carc. 1A; Muta. 1B; STOT RE 1; Asp. Haz. 1; Skin Irrit. 2; Eye Irrit. 2. Physical: Flam. Liq. 2 |
| Hazard statements | H350 "May cause cancer", H340 "May cause genetic defects", H372, H304, H315, H319, H225 "Highly flammable liquid and vapour" |
| Pictograms | GHS02, GHS07, GHS08 |
| Signal word | Danger |
The class name (for example "Carc.") gives the type of hazard. The number or letter after it (1A, 1B, 2) gives the category, which shows how severe the hazard is or how strong the evidence is. Each class and category combination brings a fixed set of label elements with it.
One point that matters in practice: the GHS is a UN system that each jurisdiction puts into its own law. The US does this through the OSHA Hazard Communication Standard, and the benzene example above comes from Australia's own GHS register. For a specific market, the classification that counts is the one under that jurisdiction's law (in the EU, that would be CLP), not the UN text itself.
Approach: I used OSHA's Hazard Communication page and Safe Work Australia's HCIS register of GHS classifications. I couldn't open UNECE's GHS page, which publishes the UN GHS text (the "Purple Book"), so nothing here is quoted from it directly.
If useful, I can pull the GHS classification for a specific substance in the EU (CLP Annex VI), Japan (NITE), Korea or Australia and compare them.
What do the hazard categories mean?
A hazard category is a severity tier inside a hazard class. CLP defines it as "the division of criteria within each hazard class, specifying hazard severity", while the hazard class is "the nature of the physical, health or environmental hazard" [2]. The general rule is that Category 1 is the most severe, and higher numbers are less severe. What actually separates one category from the next depends on the class. In some classes it is a measured value; in others it is how strong the evidence is.
Approach: I took the definitions and criteria from the current consolidated EU CLP Regulation (EC) No 1272/2008, Article 2 and Annex I. CLP is the EU's implementation of the GHS, so the cut-offs below are CLP's and may differ in other jurisdictions.
1. Categories set by measured values (potency or physical properties)
Acute toxicity has four categories. The cut-offs are acute toxicity estimates (ATE), which are LD50/LC50-type values. A lower ATE means a more toxic substance and a lower category number [3][4]:
| Route | Cat. 1 | Cat. 2 | Cat. 3 | Cat. 4 |
|---|---|---|---|---|
| Oral (mg/kg bw) | ≤ 5 | > 5 to ≤ 50 | > 50 to ≤ 300 | > 300 to ≤ 2 000 |
| Dermal (mg/kg bw) | ≤ 50 | > 50 to ≤ 200 | > 200 to ≤ 1 000 | > 1 000 to ≤ 2 000 |
| Gases (ppmV) | ≤ 100 | > 100 to ≤ 500 | > 500 to ≤ 2 500 | > 2 500 to ≤ 20 000 |
| Vapours (mg/l) | ≤ 0.5 | > 0.5 to ≤ 2.0 | > 2.0 to ≤ 10.0 | > 10.0 to ≤ 20.0 |
| Dusts and mists (mg/l) | ≤ 0.05 | > 0.05 to ≤ 0.5 | > 0.5 to ≤ 1.0 | > 1.0 to ≤ 5.0 |
The inhalation ranges are based on 4-hour exposures [4].
Flammable liquids have three categories, set by flash point and initial boiling point [5]:
- Cat. 1: flash point < 23 °C and initial boiling point ≤ 35 °C. Label: H224 "Extremely flammable liquid and vapour", signal word Danger.
- Cat. 2: flash point < 23 °C and initial boiling point > 35 °C. Label: H225 "Highly flammable…", Danger.
- Cat. 3: flash point ≥ 23 °C and ≤ 60 °C. Label: H226 "Flammable liquid and vapour", Warning.
2. Categories set by strength of evidence (the CMR classes)
For carcinogenicity, the category reflects how convincing the evidence is, not how potent the substance is [6][7]:
- Category 1A: "known to have carcinogenic potential for humans", based largely on human evidence.
- Category 1B: "presumed to have carcinogenic potential for humans", based largely on animal evidence.
- Category 2: "suspected human carcinogens". There is evidence from human and/or animal studies, but it is "not sufficiently convincing" for 1A or 1B.
CLP states plainly that this classification "does not provide information on the level of the human cancer risk which the use of the substance or mixture may represent" [6]. Germ cell mutagenicity uses the same 1A/1B/2 structure. For example, Category 1 carries H340 "May cause genetic defects" with Danger, and Category 2 carries H341 "Suspected of causing genetic defects" with Warning [6].
3. Sub-categories
Some classes split Category 1 further. Skin corrosion, for instance, has Sub-categories 1A, 1B and 1C. All of them, and Category 1 without a sub-category, carry H314 "Causes severe skin burns and eye damage" with Danger. Skin irritation Category 2 carries H315 with Warning [8].
4. How the category shows up on the label
The signal word follows the category. "Danger" is for "the more severe hazard categories" and "Warning" for "the less severe hazard categories" [2]. Each category also has its own hazard statement, which describes the hazard "including, where appropriate, the degree of hazard" [2].
Reading the benzene example from earlier: "Carc. 1A" means carcinogenicity with the strongest evidence category (human evidence). "Flam. Liq. 2" means a flash point below 23 °C with a boiling point above 35 °C.
Classes are also sometimes split by route of exposure or type of effect. CLP calls this "differentiation" [9]. Acute toxicity, for example, is assessed separately for the oral, dermal and inhalation routes [3].
How do US and UK requirements differ?
The US and Great Britain both build on the UN GHS, but they apply it for different purposes. The US rule is OSHA's Hazard Communication Standard (HCS, 29 CFR 1910.1200), a workplace standard covering physical and health hazards only. GB's rule is GB CLP (the retained Regulation (EC) No 1272/2008), which applies whenever a chemical is placed on the market. It also covers environmental hazards and has a legally binding list of classifications set by HSE. The day-to-day differences are in who sets the classification, which hazard classes count, and what labels and safety data sheets (SDSs) must contain.
Approach: For GB I used the GB CLP text, the GB mandatory classification and labelling list (GB MCL list), and UK REACH Article 31 and Annex II. For the US I used OSHA's HCS 2024 inspection directive (CPL 02-02-079, effective 19 May 2026), which explains each paragraph of the standard; I did not open the 29 CFR 1910.1200 text itself. "UK" here means Great Britain; I did not check Northern Ireland.
| Topic | US (OSHA HCS) | GB (GB CLP / UK REACH) |
|---|---|---|
| Who classifies | Manufacturers and importers use Appendices A and B of the standard. They "must consider all available data", and inspectors cite them under (d)(1)-(d)(3) if they get it wrong [10][11] | Manufacturers, importers and downstream users "shall classify" before placing on the market. If a substance has a GB MCL list entry, it "shall be classified in accordance with that entry" for the hazards that entry covers. The supplier classifies only for hazards the entry doesn't cover [12] |
| Binding classification list | The directive describes classification only as the supplier's own evaluation | Yes. HSE (defined as "the Agency") runs the list. Changes go to the Secretary of State to decide. Proposals already made in jurisdictions that adopted the GHS "in a similar way", including the EU, can be fast-tracked [13][14]. Example: the entry for benzene (Index 601-020-00-8) is Flam. Liq. 2, Carc. 1A, Muta. 1B, STOT RE 1, Asp. Tox. 1, Eye Irrit. 2, Skin Irrit. 2, with pictograms GHS02/GHS08/GHS07 and signal word Danger [15] |
| Hazard scope | "Hazard class" means the nature of the physical or health hazards. A "hazardous chemical" also includes simple asphyxiants, combustible dust and hazards not otherwise classified (HNOC) [16] | "Hazard class" means the nature of the physical, health or environmental hazard [17]. That brings in aquatic acute/chronic hazards (H400, H410-H413) [18] and hazardous to the ozone layer (H420) [19] |
| Environmental info on labels | Optional. Suppliers "can also choose" to list environmental effects and use the environmental pictogram [20] | Mandatory once the substance or mixture is classified, e.g. Aquatic Acute 1 means Warning and H400 [18] |
| Extra hazards | HNOCs (for example, polymerisation that may release heat) and reaction products from reasonably anticipated uses go in SDS Section 2 but "need not appear on the label" [16][21] | Supplemental hazard statements from Annex II and from the GB MCL list "shall be included" in the label's supplemental information section [22] |
| Label content | Product identifier, the signal word ("Danger" or "Warning" only, one per label), and other (f)(1) elements in English. "OSHA approved/compliant" wording is prohibited [20] | Supplier name, address and phone number; nominal quantity (for packages made available to the general public); product identifiers; pictograms; signal word; hazard and precautionary statements; supplemental information. The label "shall be written in English" [23]. Statements like "non-toxic" are banned [22] |
| Small containers | Containers of 3 ml or less need no label if the supplier can show any label interferes with normal use, but must carry at least the product identifier; full label on the immediate outer package [24] | Not covered in the GB provisions I read |
| SDS legal basis and scope | HCS paragraph (g) and Appendix D: 16 sections. Section 1 needs a US address and US phone number [25] | UK REACH Article 31 and Annex II. An SDS is required for hazardous substances and mixtures, PBT/vPvB substances and Candidate List substances. For unclassified mixtures, one is required on request when they contain certain substances at ≥ 1 % or ≥ 0.1 % by weight, or a substance with a GB workplace exposure limit. It "shall be supplied in English" [26] |
| SDS environmental sections | The headings of Sections 12-15 must be present, but OSHA does not require content in them because "this information is not within OSHA's jurisdiction" [27] | The SDS must enable "protection of the environment" as well as worker protection, and "shall not contain blank subsections" [28] |
| Trade secrets / confidentiality | The chemical identity can be withheld. A withheld concentration must be given as one of the prescribed ranges in (i)(1)(iv). Concentrations below 0.1 % cannot be withheld if the ingredient contributes to a health hazard classification [29] | The provisions I read cover confidentiality only for information in HSE opinions [30] |
US transition dates under HCS 2024 [31]:
- Substance labels and SDSs were due by 19 May 2026.
- Employers must update workplace labels, programmes and training for substances by 20 November 2026.
- Mixture labels and SDSs are due by 19 November 2027.
- Employer updates for mixtures are due by 19 May 2028.
- Until those dates, suppliers may comply with HCS 2012, HCS 2024, or both.
The practical effect is that one label or SDS rarely works in both markets. A GB label must show environmental classifications and follow any GB MCL entry. A US SDS needs a US contact and HNOC entries, and US labels must not claim OSHA approval.