Pesticide toxicity classification under WHO, GHS and EPA rules
Pesticide toxicity classification uses rat LD50: WHO classes Ia to U, GHS Categories 1 to 5, EU CLP Categories 1 to 4 and US EPA Categories I to IV.
Pesticide toxicity classification
Pesticide toxicity classification mainly uses acute oral and dermal toxicity, measured as rat LD50 in mg/kg body weight. LD50 is the dose lethal to half the test animals. The WHO scheme uses classes Ia, Ib, II, III and U. The US EPA uses Toxicity Categories I to IV, while EU CLP uses acute toxicity Categories 1 to 4.
The schemes use different cut-offs and classify different things: an active ingredient, a formulation or the product as sold. Don't transfer a class number from one scheme to another (WHO classification, EPA categories, CLP).
WHO pesticide classification and LD50 cut-offs
The current edition is the 2019 WHO classification, with corrigenda in August 2020 and June 2021. WHO's pesticides page still links that edition. The criteria table gives these rat LD50 values in mg/kg body weight:
| WHO class | Name | Oral LD50 | Dermal LD50 |
|---|---|---|---|
| Ia | Extremely hazardous | Under 5 | Under 50 |
| Ib | Highly hazardous | 5 to 50 | 50 to 200 |
| II | Moderately hazardous | 50 to 2,000 | 200 to 2,000 |
| III | Slightly hazardous | Over 2,000 | Over 2,000 |
| U | Unlikely to present acute hazard | 5,000 or higher* |
Class U has one figure for the class, not separate oral and dermal limits.
The printed ranges overlap, including at an oral LD50 of 50 mg/kg, and Class III overlaps Class U on paper. Table 5 is a separate list, with an assumed cut point of 5,000 mg/kg or more, and fumigants in Table 8 are not classified. Use the listed class, not the range on its own (WHO, criteria and table notes).
Since the 2009 update, WHO starts from the GHS acute toxicity categories and then adjusts some entries. Some liquids with an oral LD50 of 50 to 200 mg/kg were kept in Class Ib, so the move to GHS would not lower their class (WHO, column 7 notes).
How WHO assigns a class
If the dermal LD50 would put a compound in a stricter class than the oral LD50, WHO uses the stricter class. Tebupirimfos is Class Ia for skin contact (LD50 9.4 mg/kg). Dermal values are marked "D" (WHO, Table 1 and column 8 notes).
WHO uses the lowest reliable LD50. It can raise the class for irreversible organ damage, high volatility, marked accumulation, or a particular human hazard or allergenicity, and lower it where the evidence shows less hazard. Each listed active also shows its GHS acute toxicity category (WHO, classification criteria and table notes).
Classifying a pesticide formulation
The tables classify technical active ingredients. The final class is the formulation's. Without test data, the single-active estimate is:
Formulation LD50 = (active ingredient LD50 × 100) / percentage active ingredient
A 10% formulation of an active with an oral LD50 of 40 mg/kg estimates at 400 mg/kg: Class Ib for the active, Class II on that oral figure. For mixtures, test the actual mixture, then the most hazardous constituent at the total active concentration, then an additive formula (WHO, formulation guidance).
Check the corrigenda before you rely on a row. Zeta-cypermethrin moved from Table 2 (Class Ib) to Table 3 (Class II) in June 2021.
GHS and CLP acute toxicity categories
UN GHS has five acute toxicity categories. These oral and dermal limits are from GHS Revision 10, Table 3.1.2. Revision 11, published in 2025, is the latest edition.
| GHS category | Oral (mg/kg) | Dermal (mg/kg) |
|---|---|---|
| 1 | 5 or less | 50 or less |
| 2 | Over 5 to 50 | Over 50 to 200 |
| 3 | Over 50 to 300 | Over 200 to 1,000 |
| 4 | Over 300 to 2,000 | Over 1,000 to 2,000 |
| 5 | Over 2,000 to 5,000 | Over 2,000 to 5,000 |
The top of each band is included. EU CLP keeps Categories 1 to 4 on those same oral and dermal edges, and has no Category 5. An oral or dermal LD50 over 2,000 mg/kg is not an acute toxicity category under CLP. EU plant protection products are classified and labelled under CLP, with further labelling rules in Regulation (EU) No 547/2011.
EPA pesticide toxicity categories
40 CFR 156.62 assigns four Toxicity Categories. Category I is the highest. Each of five indicators is scored on the product as sold or distributed.
| Indicator | I | II | III | IV |
|---|---|---|---|---|
| Oral LD50 (mg/kg) | ≤50 | >50 to 500 | >500 to 5,000 | >5,000 |
| Dermal LD50 (mg/kg) | ≤200 | >200 to 2,000 | >2,000 to 20,000 | >20,000 |
| Inhalation LC50 (mg/L) | ≤0.2 | >0.2 to 2 | >2 to 20 | >20 |
| Eye irritation | Corrosive. Corneal opacity not reversible within 7 days | Corneal opacity reversible within 7 days. Irritation persisting for 7 days | No corneal opacity. Irritation reversible within 7 days | No irritation |
| Skin irritation | Corrosive | Severe irritation at 72 hours | Moderate irritation at 72 hours | Mild or slight irritation at 72 hours |
For Categories I to III, each stated LD50 or LC50 upper limit is included. Most hazard, precautionary and PPE statements follow that category. Some statements may use the diluted product. The signal word cannot follow that dilution, and it stays the same on every part of the label (40 CFR 156.62, 156.64).
WHO vs GHS vs EPA
The cut-offs don't match. WHO Class II spans GHS Categories 3 and 4 on both oral and dermal LD50. EPA oral Category II runs through 500 mg/kg and Category III through 5,000, so those bands cut across WHO Classes II and III. EPA Category I (oral up to and including 50 mg/kg, dermal up to and including 200) takes in WHO Classes Ia and Ib together. WHO Class III, over 2,000, lines up with GHS Category 5, which stops at 5,000. Class U is a separate list. EPA noted in August 2026 that its acute oral categories differ from GHS.
An oral LD50 of 1,000 mg/kg is WHO Class II, GHS and CLP Category 4, and EPA Category III. At 30 mg/kg it is WHO Class Ib and GHS/CLP Category 2 (Danger, Fatal if swallowed), and EPA Category I (DANGER, Poison, skull and crossbones). A liquid with an oral LD50 of 100 mg/kg that WHO kept in Class Ib is GHS and CLP Category 3, and EPA Category II (WHO, CLP, EPA signal-word rule).
Toxicity labels and colour bands
A label shows toxicity through the signal word, the symbol and any required colour band. Under CLP's label table, Categories 1 to 3 take the skull and crossbones and Danger. Category 4 takes the exclamation mark and Warning. The statements are Fatal if swallowed or fatal in contact with skin (Categories 1 and 2), Toxic if swallowed or toxic in contact with skin (Category 3), and Harmful if swallowed or harmful in contact with skin (Category 4).
WHO does not set a fixed symbol set. Class Ia and Ib labels should still show a high-hazard symbol, usually a skull and crossbones, a word such as POISON or TOXIC, and poisoning symptoms plus first aid (WHO classification). On a US label, the highest of the five indicators sets the signal word (40 CFR 156.64):
- Category I: DANGER. If that category comes from oral, dermal or inhalation toxicity, add Poison in red and the skull and crossbones. Eye or skin irritation alone does not.
- Category II: WARNING.
- Category III: CAUTION.
- Category IV on every indicator: no signal word is required. If one is used, it is CAUTION.
The FAO/WHO labelling guidelines set out colour bands that many countries require. Keep WHO bands with WHO classes, and GHS bands with GHS categories.
| Band | WHO-based scheme | GHS-based scheme |
|---|---|---|
| Red | Ia, Ib | 1, 2 |
| Yellow | II | 3 |
| Blue | III | 4, 5 |
| Green | U | Not classified |
WHO-based words run from Very toxic (Ia) to Caution (III), with no symbol for Class III and nothing for Class U. GHS-based bands use Danger for Categories 1 to 3 and Warning for 4 and 5. Category 5 is "May be harmful", with no symbol (FAO/WHO labelling guidelines).
Limits of the WHO classification
The WHO class covers acute hazard only. Chronic effects such as cancer sit in other GHS classes, though WHO can still adjust a class where the evidence supports it. A Class Ia or Ib formulation meets Criterion 1 of the FAO/WHO highly hazardous pesticide definition, one of eight criteria. The class is not a ban (WHO classification).
RegAffairs AI's cited answer sets out the WHO class cut-offs and the EPA category comparison for this question.