Pesticide Toxicity Classification
How are pesticides classified by toxicity?
Pesticides are mostly classified by acute toxicity: the LD50 (the dose that kills half of a test-animal population) by the oral and dermal routes, and in some schemes inhalation, eye and skin irritation as well. The result is a hazard class, and each class has its own label signal word or colour band. The international reference is the WHO Recommended Classification of Pesticides by Hazard (2019 edition), which has five classes: Ia, Ib, II, III and U. National schemes such as US EPA Toxicity Categories I-IV, India's Insecticides Rules 1971, Mexico's NOM-232 and Brazil's framework use their own categories, and several are now anchored to GHS.
1. WHO Recommended Classification of Pesticides by Hazard (2019)
The criteria use LD50 values for the rat, in mg/kg body weight (WHO 2019, p.6 of the publication):
| Class | Description | Oral LD50 | Dermal LD50 |
|---|---|---|---|
| Ia | Extremely hazardous | < 5 | < 50 |
| Ib | Highly hazardous | 5–50 | 50–200 |
| II | Moderately hazardous | 50–2000 | 200–2000 |
| III | Slightly hazardous | Over 2000 | Over 2000 |
| U | Unlikely to present acute hazard | 5000 or higher |
How the WHO scheme is applied (same publication):
- What "hazard" means. It is the acute risk to health from single or multiple exposures over a relatively short period.
- Dermal value can override oral. Most classifications rest on the oral LD50. If the dermal LD50 would put a compound in a more restrictive class, the compound is always placed in that class.
- Adjustments. A compound can be moved to a higher class if it causes irreversible damage to vital organs, is highly volatile, is markedly cumulative, or is particularly hazardous or allergenic to humans. It can be moved down if shown to be less hazardous than its LD50 suggests.
- GHS alignment. Since the 2009 update, WHO uses the GHS Acute Toxicity Hazard Categories as its starting point, and each listed active ingredient also shows its GHS category. The old distinction between liquids and solids no longer applies. A few liquids with oral LD50 of 50–200 mg/kg were deliberately kept in Class Ib.
- The final class applies to the formulation. The tables classify technical active ingredients only. Where formulation data are lacking, the formulation's LD50 is estimated as (LD50 of the active ingredient × 100) ÷ % active ingredient. There is also a formula for mixtures.
- Fumigants are outside the scheme. Gaseous or volatile fumigants are listed separately (Table 8) and not classified.
- Labelling. Class Ia and Ib labels should bear a high-hazard symbol (usually a skull and crossbones) and a signal word such as POISON or TOXIC, plus poisoning symptoms and first aid.
- Highly Hazardous Pesticides. WHO class Ia/Ib is Criterion 1 of the FAO/WHO definition. Other criteria include GHS carcinogen, mutagen or reproductive toxicant Category 1A/1B, Stockholm Convention Annex A/B listing, Rotterdam Convention Annex III listing, and the Montreal Protocol.
The WHO tables list individual active ingredients by class. For example, tebupirimfos is Class Ia ("Extremely hazardous by skin contact", LD50 9.4 mg/kg) [1], and spiroxamine is Class II (dermal LD50 1068 mg/kg) [2].
2. United States: EPA Toxicity Categories (40 CFR 156.62)
40 CFR 156.62 sets four categories, with Category I the most toxic. A category is assigned for each of five acute exposure types, based on the product as sold or distributed (eCFR 40 CFR 156.62):
| Indicator | I | II | III | IV |
|---|---|---|---|---|
| Oral LD50 | Up to and including 50 mg/kg | >50 thru 500 mg/kg | >500 thru 5,000 mg/kg | >5,000 mg/kg |
| Dermal LD50 | Up to and including 200 mg/kg | >200 thru 2000 mg/kg | >2000 thru 20,000 mg/kg | >20,000 mg/kg |
| Inhalation LC50 | Up to and including 0.2 mg/liter | >0.2 thru 2 mg/liter | >2 thru 20 mg/liter | >20 mg/liter |
| 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 |
The regulation also says categories may be used for restricted-use classification and child-resistant packaging, not only labelling.
Signal words follow the category: "Danger = I", "Warning = II", "Caution = III or IV" [3]. "POISON" plus skull and crossbones is added if the product is Category I by the oral, inhalation or dermal route [4].
3. India: Insecticides Rules, 1971 (labelling rule)
Labels "shall contain" a diamond-shaped square covering at least one-sixteenth of the label face, split into two triangles. The upper triangle carries the symbol and signal word, the lower one the colour band:
- Category I (Extremely toxic): skull and crossbones and "POISON" in red.
- Category II (Highly toxic): "POISON" in red.
- Category III (Moderately toxic): "DANGER".
- Category IV (Slightly toxic): "CAUTION".
The colour table continues on the next page with "Moderately toxic 501-5000 | 2001-20000 | Bright blue" and "Slightly toxic | More than 5000 | More than 20000 | Bright green" [6]. Two parts of the table are missing from the text I could read: the Category I and II rows, and the column headings. The rule number is not visible on these pages either.
4. Mexico: NOM-232-SSA1-2009 (pesticide labelling)
- Acute-toxicity cut-offs (Anexo Normativo 1, Tabla 1):
- Ecotoxicological categories (on the label):
5. Brazil: Lei 14.785/2023 and Decreto 10.833/2021
- The label must show "a classificação toxicológica e ambiental do produto, de acordo com o GHS" [13].
- Under Decree Art. 10-E, toxicological classification and health-hazard communication on labels follow the GHS guidelines ("serão observadas as diretrizes do GHS") [14][15].
- The federal health body (ANVISA) sets the technical criteria and carries out the classification [14].
- I did not retrieve ANVISA's detailed category criteria.
Approach: I used the WHO 2019 classification (the official publication and its listed active ingredients), the eCFR text of 40 CFR 156.62 and EPA PR Notices, India's Insecticides Rules 1971, Mexico's NOM-232-SSA1-2009, and Brazil's Lei 14.785/2023 and Decreto 10.833/2021. I did not cover EU CLP classification of plant-protection products, ANVISA's detailed category rules, or other national schemes; I can look any of these up if you need them.
What LD50 puts a pesticide in WHO class Ib?
WHO Class Ib ("Highly hazardous") covers a rat oral LD50 of 5–50 mg/kg body weight or a rat dermal LD50 of 50–200 mg/kg body weight (WHO 2019, publication p.6). The table gives only these ranges. It does not say which class a value sitting exactly on a boundary (5, 50 or 200) falls into.
For comparison, the classes on either side are:
| Class | Oral LD50 (mg/kg) | Dermal LD50 (mg/kg) |
|---|---|---|
| Ia, extremely hazardous | < 5 | < 50 |
| Ib, highly hazardous | 5–50 | 50–200 |
| II, moderately hazardous | 50–2000 | 200–2000 |
The guidelines in the same publication add some rules that can change the result (publication pp.8-12):
- Dermal can override oral. The oral value is normally used. The dermal value is used instead if it puts the compound in a more hazardous class, or if it is much lower than the oral value even within the same class.
- Some liquids stay in Ib above 50 mg/kg oral. Under the pre-2009 scheme, some liquids with an oral LD50 of 50–200 mg/kg were Class Ib. Moving to the GHS cut-offs would have put them in GHS Category 3. WHO adjusted them so they "remain in Class Ib".
- Which LD50 is used. WHO takes the lowest value judged reliable, and the value for the more sensitive sex. Borderline cases are placed in the more or less hazardous class after looking at their toxicology and use experience.
- Formulations are classed separately. The tables classify technical-grade active ingredients. The final class is meant to apply to the formulation, based on data for that formulation where available.
- Ib makes a pesticide "highly hazardous". A formulation meeting the Class Ia or Ib criteria meets Criterion 1 of the FAO/WHO definition of Highly Hazardous Pesticides.
Some examples of Class Ib active ingredients, all also listed in WHO's Rotterdam Convention table, are azinphos-methyl [16], carbofuran [17] and fluoroacetamide [18].
Approach: I searched the WHO Recommended Classification of Pesticides by Hazard (2019) tables of classified active ingredients. That source does not hold the criteria table itself, so I took the cut-offs and rules from the official WHO 2019 publication.
How does that compare with EPA toxicity categories?
By oral and dermal LD50, EPA Toxicity Category I takes in everything WHO puts in Class Ia and Class Ib. Category I runs up to and including 50 mg/kg oral and 200 mg/kg dermal. Those are the same upper limits as WHO Class Ib (5–50 mg/kg oral, 50–200 mg/kg dermal). The two schemes split apart below Class Ib: EPA has no separate "extremely hazardous" tier like WHO Class Ia, and the lower bands are set at different points.
Oral LD50 (rat, mg/kg)
| WHO 2019 class | WHO range | EPA category (40 CFR 156.62) | EPA range |
|---|---|---|---|
| Ia, extremely hazardous | < 5 | I | Up to and including 50 |
| Ib, highly hazardous | 5–50 | I | Up to and including 50 |
| II, moderately hazardous | 50–2000 | II, then part of III | >50 thru 500; >500 thru 5,000 |
| III, slightly hazardous | Over 2000 | III | >500 thru 5,000 |
| U, unlikely to present acute hazard | 5000 or higher | IV | >5,000 |
Dermal LD50 (mg/kg)
| WHO class | WHO range | EPA category | EPA range |
|---|---|---|---|
| Ia | < 50 | I | Up to and including 200 |
| Ib | 50–200 | I | Up to and including 200 |
| II | 200–2000 | II | >200 thru 2000 |
| III | Over 2000 | III / IV | >2000 thru 20,000; >20,000 |
The EPA values are from 40 CFR 156.62 on the eCFR (current to 22 October 2026, no changes since 3 January 2017). The WHO values are from the WHO 2019 publication cited above.
Differences that matter in practice
- Exact boundaries. EPA states its edges: 50 mg/kg oral and 200 mg/kg dermal are in Category I ("up to and including"). As noted earlier, the WHO table does not say which class takes a value exactly at 5, 50 or 200.
- WHO Class II is much wider. It spans EPA Category II and the lower half of Category III by the oral route. So an active at an oral LD50 of 1,000 mg/kg is WHO Class II but EPA Category III ("Caution").
- Liquids WHO keeps in Ib. WHO keeps certain liquids with an oral LD50 of 50–200 mg/kg in Class Ib. EPA has no such adjustment: by its oral cut-offs, that range is Category II.
- What gets classified. EPA assigns the category to "the pesticide product as sold or distributed". Statements based on the product as diluted for use are permitted in certain cases. The WHO tables classify the technical active ingredient, and the final class is meant to apply to the formulation.
- Number of end-points. EPA assigns a category for each of five acute exposures: oral, dermal, inhalation LC50 (Category I up to and including 0.2 mg/liter), eye irritation and skin irritation. WHO classes on oral and dermal LD50 only, with the dermal value overriding the oral where it is more restrictive.
- What the class is used for. EPA says the categories may also be used for restricted-use classification and child-resistant packaging. WHO Ia/Ib is Criterion 1 of the FAO/WHO Highly Hazardous Pesticides definition.
Label consequences
- A product with a WHO Ia or Ib active is likely to end up EPA Category I if its formulation is toxic enough. The signal words are "Danger = I", "Warning = II" and "Caution = III or IV" [3].
- "POISON" with skull and crossbones is added if the product is Toxicity Category I by the oral, inhalation or dermal route [4].
- First-aid statements for products in Toxicity Category I for oral, dermal or inhalation toxicity "must appear on the front panel of the label" [19].
- On data: for granular products, EPA "will not accept calculations that bridge down" from Category I or II source products [20]. WHO, by contrast, allows the formulation LD50 to be estimated from the active-ingredient LD50.
Approach: I searched EPA's Pesticide Registration Notices for the category and signal-word rules. I took the cut-off table from the current eCFR text of 40 CFR 156.62, because the notices I searched do not reproduce it.