Thymol

Thymol	Last updated: 30/04/2025
(Also known as: isopropyl-m-cresol; 2-isopropyl-5-methylphenol; thyme camphor; thymic acid; hydroxy cymene)

GENERAL INFORMATION

Description

Plant derived substancewith applications in animal health

Availability status

Current

Introduction & key dates

1842, first synthesised; 1964, first registered USA

Examples of veterinary uses

Used in beehives to control varroa mite and to prevent fermentation and the growth of mould in bee colonies. Also used in poultice preparations for its antiseptic properties and in animal nutrition for its antimicrobial action

Examples of species treated

Honeybees; Horses

Approval status

VMR 2013/2033 approval status (GB/UK)

Approved

EU Regulatory approval status

Approved

Chemical structure

Isomerism

Chemical formula

C₁₀H₁₄O

Canonical SMILES

CC1=CC(=C(C=C1)C(C)C)O

Isomeric SMILES

No data

International Chemical Identifier key (InChIKey)

MGSRCZKZVOBKFT-UHFFFAOYSA-N

International Chemical Identifier (InChI)

InChI=1S/C10H14O/c1-7(2)9-5-4-8(3)6-10(9)11/h4-7,11H,1-3H3

2D structure diagram/image available?

Yes

Cambridge Crystallographic Data Centre diagrams

Common Name	Relationship	Link
thymol	-

General status

Veterinary substance type

Antimicrobial, Antibacterial, Antiseptic, Miticide

Metabolite Type

Biocide, Biodrug

Substance groups

Plant-derived substance

Minimum active substance purity

>90%; 990 g kg⁻¹

Known relevant impurities

EU dossier - none declared

Substance origin

Natural

Mode of action

Contact, GABAergic activity and also repels vertebrate pests by a non-toxic mode of action but is toxic to micro-organisms

Molecular targets

[Adrenergic receptors (α1, α2, and β), Agonist]

CAS RN

89-83-8

EC number

201-944-8

CIPAC number

969

US EPA chemical code

080402

PubChem CID

Therapeutic Class

Antiparasitic products, insecticides & repellents: Ectoparasiticides

ATCvet Code

QP53AX22, QP53AX30

Controlled Drug?

Regulation 37/2010 MRL Classification

Allowed substance (Table 1)

Molecular mass

150.22

PIN (Preferred Identification Name)

IUPAC name

5-methyl-2-isopropyl-1-phenol

CAS name

phenol, 5-methyl-2-(1-methylethyl)-

Other status information

Recognised water pollutant; UK registered Commodity Substance

Relevant Environmental Water Quality Standards

Physical state

White crystalline solid with a herbal odour

Related substances & organisms

geraniol

eugenol

Formulations

Property

Product

Manufacturer

Authorisation Route

Legal Class (GB/UK)

Example products (past and present) using this active

Apiguard gel (25% thymol)

Vita (Europe) Ltd

GB National authorisation

Authorised veterinary medicine for general sale (AVM-GSL)

Apilife Var bee-hive strip for honey bees

Chemicals Laif Srl

GB National authorisation

Authorised veterinary medicine for general sale (AVM-GSL)

Kaolin Poultice for Horses

KL Pharmaceutical Ltd

UK National authorisation

Authorised veterinary medicine for general sale (AVM-GSL)

Thymovar Beehive Strips

Andermatt Biovet GmbH

GB National authorisation

Authorised veterinary medicine for general sale (AVM-GSL)

Formulation and application details

Formulated as gels and impregnated strips for bee hive use and as medicated poultices for bandaging infected areas. Poultices often contain multiple active substances and plant oils

ENVIRONMENTAL FATE

Property

Value

Source; quality score; and other information

Interpretation

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

596

Moderate

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

>250000

p-Xylene

>250000

Methanol

>250000

Ethyl acetate

200000

n-Heptane

Melting point (°C)

48.75

Boiling point (°C)

232.3

Degradation point (°C)

Flashpoint (°C)

105.5

(closed cup)

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

2.75 X 10⁰³

Calculated

Log P

3.44

High

Fat solubility of residues

Solubility

Data type

Density (g ml⁻¹)

0.97

Dissociation constant pKa) at 25 °C

10.77

Vapour pressure at 20 °C (mPa)

3400

Highly volatile. If applied directly to plants or soil, drift is a concern & mitigation is advisable

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

0.86

Moderately volatile

Volatilisation as max % of applied dose lost

From plant surface

From soil surface

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

No absorption >290nm

Surface tension (mN m⁻¹)

52.3

Refractive Index

1.5208

Environmental release

Degradation

Property

Value

Source; quality score; and other information

Interpretation

Soil degradation (days) (aerobic)

DT₅₀ (typical)

Non-persistent

DT₅₀ (lab at 20 °C)

0.7

Non-persistent

DT₅₀ (field)

DT₉₀ (lab at 20 °C)

2.2

Non-persistent

DT₉₀ (field)

Note

EU 2023 dossier Lab studies DT₅₀ (normalised) range 0.6-0.8 days, DT₉₀ range 1.8-2.6 days, Soils=4;

Manure DT₅₀ (days)

Aqueous photolysis DT₅₀ (days) at pH 7

Value

Note

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

Value

Note

Water-sediment DT₅₀ (days)

Water phase only DT₅₀ (days)

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)

Slightly mobile

K_oc (mL g⁻¹)

2188

Notes and range

Data caluated using predictive software

Freundlich

K_f (mL g⁻¹)

3.06

Moderately mobile

K_foc (mL g⁻¹)

119.14

¹/_n

0.906

Notes and range

EU 2023dossier K_f range 1.021-5.361 mL g⁻¹, K_foc range 92.8-171.0 mL g⁻¹, ¹/_n range 0.830-0.958, Soils=4

pH sensitivity

Fate indices

Property

Value

Source; quality score; and other information

Interpretation

GUS leaching potential index

-0.30

Calculated

Low leachability

Bio-concentration factor

BCF (l kg⁻¹)

CT₅₀ (days)

Known metabolites

None

ECOTOXICOLOGY

Terrestrial ecotoxicology

Property

Value

Source; quality score; and other information

Interpretation

Mammals - Acute oral LD₅₀ (mg kg⁻¹)

980

Rat

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)

> 640

Colinus virginianus

Moderate

Birds - Short term dietary (LC₅₀/LD₅₀)

> 5866 mg kg bw⁻¹ day⁻¹

Colinus virginianus as product (66 g l⁻¹)

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

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

> 500

Eisenia foetida as product (66g l⁻¹) corr

Moderate

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

1.65

Eisenia andrei corr

Moderate

Soil micro-organisms

Nitrogen mineralisation: No significant adverse effect
Carbon mineralisation: No significant adverse effect

Dose 54.5 mg product/kg soil

Collembola

Acute LC₅₀ (mg kg⁻¹)

Chronic NOEC (mg kg⁻¹)

Non-target plants

Honeybees (Apis spp.)

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

> 200

Apis mellifera as product (66 g l⁻¹ thymol)

Low

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

> 224.6

Apis mellifera as product (66 g l⁻¹ thymol)

Low

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

Chronic LDD₅₀ 10d

123.5

Apis mellifera as product (66 g l⁻¹ thymol)

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) as Mortality LR₅₀ g ha⁻¹

> 12420

Aphidius rhopalosiphi as product (66 g l⁻¹ thymol)

Beneficial insects (Predatory mites) as Mortality LR₅₀ g ha⁻¹

> 12420

Typhlodromus pyri as product (66 g l⁻¹ thymol)

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)

3.0

Oncorhynchus mykiss

Moderate

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)

4.9

Daphnia magna

Moderate

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)

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, growth) - Acute 7 day EC₅₀, biomass (mg l⁻¹)

Aquatic plants (rooted, growth) - Acute 14 day EC₅₀, biomass (mg l⁻¹)

Algae - Acute 72 hour EC₅₀, growth (mg l⁻¹)

11.7

Pseudokirchneriella subcapitata Growth

Low

Algae - Chronic 96 hour NOEC, growth (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)

Low (class I)

Mammals - Acute oral LD₅₀ (mg kg⁻¹)

980

Rat

Moderate

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

2000

Rat

Mammals - Inhalation LC₅₀ (mg l⁻¹)

> 5.0

Rat

Other Mammal toxicity endpoints

Subcutaneous LD₅₀ = 243 mg kg⁻¹

Mouse

Intravenous LD₅₀ > 100 mg kg⁻¹

Mouse

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

0.07

Rat SF=600

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

0.4

Rat SF=100

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

0.4

Rat SF =100

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

0.4

Rat SF =100

Dermal penetration studies (%)

25-70

Default

Dangerous Substances Directive 76/464

Exposure Routes

Public

Acceptable for proposed uses

Occupational

PPE/PPC recommended

Mammalian dose elimination route and rate

Extensively and rapidly absorbed, excreted via urine (>50% in 24hr)

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

Eye irritant

Phototoxicant

No data found

General human health issues

May cause gastrointestinal problems, central hyperactivity and occasionally convulsions and coma
Possible renal, kidney and liver toxicant
May cause CNS or muscular contractions

Handling issues

Property

Value and interpretation

General

Corrosive
Not explosive or oxidising

CLP classification 2013

Health: H302, H314, H317, H318, H336

WHO Classification

Not listed (Not listed)

UN Number

Waste disposal & packaging

Shelf-life, storage, stability and reactivity

TRANSLATIONS

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Name

English

thymol

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Record last updated:	30/04/2025
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