Pyrethrins (pyrethrin I)

Pyrethrins (pyrethrin I)	Last updated: 28/08/2025
(Also known as: pyrethum)

GENERAL INFORMATION

Description

A non-persistent insecticide extracted from Pyrethrum used to control a variety of insect parasites

Examples of veterinary uses

Used as a topical treatment for fleas and ticks as the pyrethrins mixture

Examples of species treated

Dogs; Cats; Horses

Approval status

VMR 2013/2033 approval status (GB/UK)

Not approved as pyrethrins I; Approved as pyrethrins (mix)

EU Regulatory approval status

Not approved as pyrethrins I; Approved as pyrethrins (mix)

Chemical structure

Isomerism

Pyrethrins exhibit stereoisomerism, specifically cis-trans isomerism and chirality due to their complex molecular structures. Each pyrethrin molecule consists of an ester linkage between chrysanthemic acid or pyrethric acid and an alcohol component like pyrethrolone or cinerolone. The cyclopropane ring in chrysanthemic acid introduces two chiral centres, allowing for multiple stereoisomers. These include cis- and trans-isomers, based on the relative positions of substituents around the cyclopropane ring, and enantiomers, which are non-superimposable mirror images. There are six major pyrethrins: pyrethrin I and II, cinerin I and II, and jasmolin I and II. Each pair differs by the alcohol moiety and the acid component, and each exists in multiple stereoisomeric forms. The cis-isomers are generally more biologically active than their trans counterparts.

Chemical formula

C₂₁H₂₈O₃

Canonical SMILES

CC1=C(C(=O)CC1OC(=O)C2C(C2(C)C)C=C(C)C)CC=CC=C

Isomeric SMILES

CC1=C(C(=O)C[C@@H]1OC(=O)[C@@H]2[C@H](C2(C)C)C=C(C)C)C/C=C\C=C

International Chemical Identifier key (InChIKey)

ROVGZAWFACYCSP-WIURDZCKSA-N

International Chemical Identifier (InChI)

InChI=1S/C21H28O3/c1-7-8-9-10-15-14(4)18(12-17(15)22)24-20(23)19-16(11-13(2)3)21(19,5)6/h7-9,11,16,18-19H,1,10,12H2,2-6H3/b9-8+/t16-,18+,19+/m1/s1

2D structure diagram/image available?

Yes

General status

Veterinary substance type

Insecticide, Antiparasitic

Substance groups

Plant-derived substance

Minimum active substance purity

Known relevant impurities

Substance origin

Natural

Mode of action

Contact acting neurotoxins with repellent properties. Sodium channel modulator.

Molecular targets

CAS RN

121-21-1

Alternative/old CAS RN

8003-34-7

EC number

204-455-8

CIPAC number

US EPA chemical code

069001

PubChem CID

Therapeutic Class

Antiparasitic Products, Insecticides and Repellents: Ectoparasiticides, Insecticides and Repellents

ATCvet Code

QP53AC51

Controlled Drug?

Regulation 37/2010 MRL Classification

Allowed substance (Table 1: All food producing species)

Molecular mass

328.4

PIN (Preferred Identification Name)

IUPAC name

(Z)-(S)-2-methyl-4-oxo-3-(penta-2,4-dienyl)cyclopent-2-enyl(1R)-trans-2,2-dimethyl-3-(2-methylprop-1-enyl)cyclopropanecarboxylate

CAS name

(1R-(1α(S(Z),3β)))-2-methyl-4-oxo-3-(2,4-pantadienyl)cyclopenten-1-yl 2,2-dimethyl-3-(2-methyl-1-propenyl)cyclopropanecarboxylate

Forever chemical

Other status information

Relevant Environmental Water Quality Standards

Physical state

Viscous amber coloured liquid

Related substances & organisms

pyrethrins (cinerin I)

pyrethrins (jasmolin I)

pyrethrins (pyrethrin II)

pyrethrins (cinerin II)

pyrethrins (jasmolin II)

pyrethrins (mix)

Commercial

Property

Product

Manufacturer

Authorisation Route

Legal Class (GB/UK)

Availability status

Current

Introduction & key dates

circa 1930, introduced

Example products (past and present) using this active

Formulation and application details

ENVIRONMENTAL FATE

Property

Value

Source; quality score; and other information

Interpretation

Solubility - In water at 20 °C (mg l⁻¹)

0.96

Low

Solubility - In organic solvents at 20 °C (mg l⁻¹)

250000

n-Heptane

250000

p-Xylene

250000

Methane

250000

Acetone

Melting point (°C)

142

Boiling point (°C)

Degradation point (°C)

200

Flashpoint (°C)

Octanol-water partition coefficient at pH 7, 20 °C

3.89 X 10⁰⁵

Calculated

Log P

5.59

High

Fat solubility of residues

Solubility

Data type

Density (g ml⁻¹)

0.85

Dissociation constant pKa) at 25 °C

Not applicable

No dissociation

Vapour pressure at 20 °C (mPa)

3.13 X 10^-02

Low volatility

Henry's law constant at 25 °C (Pa m³ mol⁻¹)

2.96 X 10^-01

Moderately volatile

Volatilisation as max % of applied dose lost

From plant surface

From soil surface

Maximum UV-vis absorption L mol⁻¹ cm⁻¹

Surface tension (mN m⁻¹)

52.8

as pyrethrins

Refractive Index

Environmental release

Degradation

Property

Value

Source; quality score; and other information

Interpretation

Soil degradation (days) (aerobic)

DT₅₀ (typical)

3.3

Non-persistent

DT₅₀ (lab at 20 °C)

3.3

Non-persistent

DT₅₀ (field)

Non-persistent

DT₉₀ (lab at 20 °C)

12.92

Non-persistent

DT₉₀ (field)

Note

EU 2021 dossier lab studies DT₅₀ (normalised) range 2.5-4.69 days, DT₉₀ (measured) range 6.25-22.1 days, Soils=5

Manure DT₅₀ (days)

Aqueous photolysis DT₅₀ (days) at pH 7

Value

0.5

Fast

Note

Rapid

Aqueous hydrolysis DT₅₀ (days) at 20 °C and pH 7

Value

528

Very persistent

Note

Water-sediment DT₅₀ (days)

3.7

Fast

Water phase only DT₅₀ (days)

1.0

Moderately fast

Sediment phase only DT₅₀ (days)

Air degradation

As this parameter is not normally measured directly, a surrogate measure is used: ‘Photochemical oxidative DT₅₀’. Where data is available, this can be found in the Fate Indices section below.

Decay in stored produce DT₅₀

Soil adsorption and mobility

Property

Value

Source; quality score; and other information

Interpretation

Linear

K_d (mL g⁻¹)

Non-mobile

K_oc (mL g⁻¹)

100000

Notes and range

Literature data

Freundlich

K_f (mL g⁻¹)

298

Non-mobile

K_foc (mL g⁻¹)

24809

¹/_n

0.952

Notes and range

EU dossier data for pyrethrins K_f range 198-430 mL g⁻¹, kfoc range 12472-74175 mL g⁻¹, ¹/_n range 0.907-1.10, Soils=4

pH sensitivity

Fate indices

Property

Value

Source; quality score; and other information

Interpretation

GUS leaching potential index

-0.43

Calculated

Low leachability

Bio-concentration factor

BCF (l kg⁻¹)

471

Whole fish

Threshold for concern

CT₅₀ (days)

Not available

Known soil metabolites

Metabolite

Major/Minor fraction

$Click here for further information on Major/Minor fraction$

Estimated maximum occurrence fraction

Notes

chrysanthemic acid

Minor fraction

0.05

Known groundwater metabolites

None

Other known metabolites

Metabolite name and reference

Aliases

Formation medium / Rate

Estimated maximum occurrence fraction

hydroxy-chrysanthenic acid

Plant

dihydroxy-chrysanthenic acid

Plant

dicarboxylic-chrysanthenic acid

Plant; Humans (Urine)

aldehyde-chrysanthenic acid

Plant

ECOTOXICOLOGY

Terrestrial ecotoxicology

Property

Value

Source; quality score; and other information

Interpretation

Mammals - Acute oral LD₅₀ (mg kg⁻¹)

700

Rat as pyrethrins

Moderate

Mammals - Short term dietary NOEL

(mg kg⁻¹)

(ppm diet)

Mammals - Chronic 21d NOAEL (mg kg⁻¹ bw d⁻¹)

Birds - Acute LD₅₀ (mg kg⁻¹)

> 51151

Anas platyrhynchos as pyrethrins

Low

Birds - Short term dietary (LC₅₀/LD₅₀)

Birds - Chronic 21d NOEL (mg kg⁻¹ bw d⁻¹)

Earthworms - Acute 14 day LC₅₀ (mg kg⁻¹)

23.7

Eisenia foetida corr

Moderate

Earthworms - Chronic NOEC, reproduction (mg kg⁻¹)

0.25

Eisenia foetida corr

Moderate

Soil micro-organisms

Collembola

Acute LC₅₀ (mg kg⁻¹)

Chronic NOEC (mg kg⁻¹)

Non-target plants

Vegetative vigour ER₅₀ (g ha⁻¹)

Seedling emergence ER₅₀ (g ha⁻¹)

Honeybees (Apis spp.)

Contact acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

0.013

High

Oral acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Unknown mode acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Chronic

Notes

Bumblebees (Bombus spp.)

Contact acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Oral acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Mason bees (Osmia spp.)

Contact acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Oral acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Other bee species (1)

Acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg insect⁻¹)

Mode of exposure

Other bee species (2)

Acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg insect⁻¹)

Mode of exposure

Beneficial insects (Ladybirds)

Beneficial insects (Lacewings)

Beneficial insects (Parasitic wasps)

Harmful

Aphidius rhopalosiphi

Beneficial insects (Predatory mites)

Harmless

Typhlodromus pyri

Beneficial insects (Ground beetles)

Aquatic ecotoxicology

Property

Value

Source; quality score; and other information

Interpretation

Temperate Freshwater Fish - Acute 96 hour LC₅₀ (mg l⁻¹)

0.005

Oncorhynchus mykiss as pyrethrins

High

Temperate Freshwater Fish - Chronic 21 day NOEC (mg l⁻¹)

Tropical Freshwater Fish - Acute 96 hour LC₅₀ (mg l⁻¹)

Temperate Freshwater Aquatic invertebrates - Acute 48 hour EC₅₀ (mg l⁻¹)

0.061

Daphnia magna

High

Temperate Freshwater Aquatic invertebrates - Chronic 21 day NOEC (mg l⁻¹)

Tropical Freshwater Aquatic invertebrates - Acute 48 hour EC₅₀ (mg l⁻¹)

Aquatic crustaceans - Acute 96 hour LC₅₀ (mg l⁻¹)

0.0014

Americamysis bahia as pyrethrins

High

Sediment dwelling organisms - Acute 96 hour LC₅₀ (mg l⁻¹)

Sediment dwelling organisms - Chronic 28 day NOEC, static, water (mg l⁻¹)

Sediment dwelling organisms - Chronic 28 day NOEC, sediment (mg kg⁻¹)

Aquatic Plants (free-floating, fonds growth, fresh) - 7 day (mg l⁻¹)

Aquatic plants (rooted, growth rate, fresh) - 14 day (mg l⁻¹)

Algae - Acute (growth rate, fresh; mg l⁻¹) (EC₅₀)

320

Unknown species

Low

Algae - Chronic (growth rate, fresh; mg l⁻¹)

Mesocosm study data

NOEAEC mg l⁻¹

Marine bivalves

HUMAN HEALTH AND PROTECTION

General

Property

Value

Source; quality score; and other information

Interpretation

Threshold of Toxicological Concern (Cramer Class)

Intermediate (class II)

Mammals - Acute oral LD₅₀ (mg kg⁻¹)

700

Rat as pyrethrins

Moderate

Mammals - Dermal LD₅₀ (mg kg⁻¹ body weight)

2000

Rat as Pyrethrins

Mammals - Inhalation LC₅₀ (mg l⁻¹)

3.4

Rat 4 hr as Pyrethrins

Other Mammal toxicity endpoints

Intraperitoneal LD₅₀ = 200 mg kg⁻¹

Rat as Pyrethrins

ADI - Acceptable Daily Intake (mg kg⁻¹ bw day⁻¹)

0.04

Rat SF=100

ARfD - Acute Reference Dose (mg kg⁻¹ bw day⁻¹)

0.2

Rat SF=100

AAOEL - Acute Acceptable Operator Exposure Level (mg kg⁻¹ bw day⁻¹)

AOEL - Acceptable Operator Exposure Level - Systemic (mg kg⁻¹ bw day⁻¹)

None allocated

Dermal penetration studies (%)

default

Dangerous Substances Directive 76/464

Exposure Routes

Public

Occupational

Mammalian dose elimination route and rate

Almost completely excreted within 72hrs

Health issues

Specific human health issues

Carcinogen

Genotoxic

Endocrine disruptor

; ; ; ;

Reproduction / development effects

Acetyl cholinesterase inhibitor

Neurotoxicant

No data found

Respiratory tract irritant

Skin irritant

Skin sensitiser

No data found

Eye irritant

Phototoxicant

No data found

General human health issues

May cause dermatitis
May cause gastointestinal problems
Possible thyroid and liver toxicant

Handling issues

Property

Value and interpretation

General

Not explosive or oxidising
IMDG Transport Hazard Class 9
Not expected to auto-ignite; Not highly flammable

CLP classification 2013

Health: H332, H312, H302
Environment: H400, H410

WHO Classification

II (Moderately hazardous)

UN Number

UN3082

Waste disposal & packaging

Packaging Group III (minor danger)

Shelf-life, storage, stability and reactivity

TRANSLATIONS

Language

Name

English

pyrethrins (pyrethrin I)

French

pyrethrines

German

Pyrethrine

Danish

pyrethrin

Italian

piretrine

Spanish

piretrina

Greek

Polish

pyretryny

Swedish

pyretriner

Hungarian

Dutch

pyrethrinen

Norwegian

Record last updated:	28/08/2025
Contact:	aeru@herts.ac.uk
Please cite as:	Lewis, K.A., Tzilivakis, J., Warner, D. and Green, A. (2016) An international database for pesticide risk assessments and management. Human and Ecological Risk Assessment: An International Journal, 22(4), 1050-1064. DOI: 10.1080/10807039.2015.1133242