UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

MEMORANDUM 

Date: May 14, 2015 

OFFICE OF 

CHEMICAL SAFETY AND 

POLLUTION PREVENTION 

SUBJECT: Thiabendazole: ID# l 5ID02 - Section 18 Emergency Exemption for the Use of 

Thiabendazole as a Seed Treatment on Succulent Peas in Bonneville and Latah 

Counties in Idaho. 

PC Code: 060 I 0 I 

Decision No.: 500376 

Petition No.: NA 

Risk Assessment Type: Single 

Chemica l Aggregate 

TXR No.: NA 

MRID No.: NA 

DP Barcode: 425853 

Registration No.: 151002 

Regulatory Action: Section 18 

Case No.: 2670 

CAS No.: 148-79-8 

40 CFR: 180.242 

FROM: Jessica Kidwell, Risk Assessor ~ ~ ~~ {'°"1\il, - '(~ t<-,.--

Bonnie Cropp-Kohlligian, es idu/Ch ~~i~t~ and Dietary Exposure Ass~~~ 0 

Abdallah Khasawinah, PhD., Toxicologist ~CC~ 

Kelly O'Rourke, Occupational/Residential Exposure Assessor --Y~ Wl.0 ''f!cJL'--

Risk Assessment Branch 4 

Health Effects Division (7509P) 

THROUGH: Elissa Reaves, PhD, Branch Chi 

Risk Assessment Branch 4 

Health Effects Division (7509P) 

TO: Keri Grinstead, Andrea Conrath, and Tawanda Maignan, RM09 

Fungicide Branch 

Registration Division (7505P) 

Page 1 of 25 
Thiabendazole/060I0 1 Human Health Risk Assessment 0425853 

1.0 Executive Summary .............. ............... .... .......................................................................... 3 

2.0 HED Recommendations ... .... ................... ................ .... .......... .............. ............. ....... .......... 4 

2.1 

2.2 

2.2.1 

2.2.2 

2.2.3 

2.2.4 

2.3 

Data Deficiencies ..................................... ........... .... .............. ... ... ........... .................... 4 

Tolerance Considerations ......................................................................................... 4 

Enforcement Analytical Method ........................................ ..................................... 4 

International Harmonization ................................................................................... 4 

Recommended Tolerances .. .... .... .... ........................... .......... ... .............................. .... 4 

Revisions to Petitioned-For Tolerances ................................................. ................. 5 

Label Recon1mendations .......................................................................................... 5 

3.0 

3.1 

3.2 

3.3 

Introduction ........................................................................ ................................................ 5 

Chemical Identity .... .... ... ............ ............................................ .... ... ............................ 5 

Physical/Chemical Characteristics ...................................... .... ... .......... ............. ...... 6 

Pesticide Use Pattern ............................................................ .............................. ...... 6 

3.4 Anticipated Exposure Pathways .............................................................................. 7 

3.5 Consideration of Environmental Justice ... ............................................. ................ 7 

4.0 Hazard Characterization and Dose-Response Assessment .............. ...... ........................ 8 

5.0 Dietary Exposure and Risk Assessment.. ....................................................................... 11 

5.1 Metabolite/Degradate Residue Profile .................................................................. 11 

5.1.1 Summary of Plant and Animal Metabolism Studies .............................. ............. 11 

5.1.2 Summary of Environmental Degr adation ............................................................ 11 

5.1.3 Comparison of Metabolic Pathways ... ........................................................ ........... 11 

5.1.4 Residues of Concern Summary and Rationale ..................................................... 12 

5.2 Food Residue Profile ............................................................................................... 13 

5.3 Water Residue Profile ............................................................................................. 14 

5.4 Dietary Risk Assessment .................... .......................... ...... .... ................................ 15 

5.4.1 Description of Residue Data Used in Dietary Assessment .................................. 15 

5.4.2 Percent Crop Treated Used in Dietary Assessment ............................................. 16 

5.4.3 Acute Dietary Risk Assessment ........................ ..................... ................................ 16 

5.4.4 Chronic Dietary Risk Assessment .... .... ............................. .................................... 17 

5.4.5 Cancer Dietary Risk Assessment.. ......................................................................... 17 

5.4.6 Summary Table ......................................................................... ............. ................. 17 

6.0 Residential (Non-Occupational) Exposure/Risk Characterization ............................. 17 

7.0 Aggregate Exposure/Risk Characterization ................................... ............................... 18 

8.0 Cumulative Exposure/Risk Characterization ............................................................... 18 

9.0 Occupational Exposure/Risk Characterization ............................................................ 18 

9.1 Short- and Intermediate-Term Handler Risk ...................................................... 18 

9.2 Short-/Intermediate Post-Application Risk ..... ..................................................... 21 

10.0 References ............................................................................................... .... .. .... ... ....... ...... 2 1 

Appendix A. International Residue Limits ............................................................ ................... 23 

Appendix B. Physical/Chemical Properties ............................................................................. 24 

Appendix C. Studies Reviewed for Ethical Conduct ........ ...................................................... 25 

Page 2of25 
Thiabendazole/060101 Human Health Risk Assessment D425853

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1.0 Executive Summary

The Idaho State Department of Agriculture has requested a Section 18 emergency exemption 

(ID#15ID02) for the use of thiabendazole, formulated as the product Mertect 340-F (EPA Reg 

No. 199-889), as a seed treatment on succulent shelled peas (English Peas) to control Fusarium 

and Ascochyta Blight in Bonneville and Latah Counties. The request entails application of 1.02 

fluid ounces (0.03 lb a.i.) of Mertect 340-F per 100 pounds of seed. A maximum of 239 gallons 

of Mertect 340-F (980 lbs a.i.) may be applied in Bonneville and Latah Counties of Idaho 

between April 1 through June 15, 2015.

The Health Effects Division (HED) has examined the Section 18 emergency exemption request 

materials and available data and has determined that the proposed use of thiabendazole on 

succulent shelled peas (Pisum spp., including English pea, garden pea and green pea) is 

adequately supported by available data and that a time-limited tolerance based on the proposed 

use pattern (i.e., 0.03 lbs a.i. per 100 pounds of seed) may be established. 

The most recent human health risk assessment (D416471, 8/26/2014) identified no risks of 

concern. The addition of succulent shelled peas (Pisum spp., including English pea, garden pea 

and green pea), treated according to the proposed Section 18 emergency exemption use rate, did 

not change the findings of this most recent risk assessment for drinking water, dietary (food and 

water), residential and aggregate exposures; hence, updated assessments for these exposures 

were not needed. However, a new occupational exposure and risk assessment was needed for this 

Section 18 emergency exemption request. 

Based on the proposed use pattern, short-/intermediate-term durations of exposure are expected 

for occupational handlers. The estimates of exposure to pesticide seed treatment handlers 

(treatment and planting) are based upon ExpoSAC Policies 14, 15, and 15.1 and a memorandum 

from the Biological and Economic Analysis Division (BEAD), “Acres Planted Per Day and 

Seeding Rates of Crops Grown in the United States” (J. Becker, March 2011). Short-/

intermediate-term exposure and risk for seed treatment resulted in total (combined dermal and 

inhalation) Margins of Exposure (MOEs) ranging from 3,200 to 35,000 for primary handlers 

(seed treaters) and an MOE of 6,800 for secondary handlers (seed planters). Note that the shortand intermediate-term Points of Departure (PODs) are the same, and the intermediate-term 

exposure inputs are less conservative than the short-term exposure inputs for seed treatment; 

therefore, short-term exposure risk estimates are protective of intermediate-term exposure risk 

estimates. These risk estimates are not of concern. A quantitative post-application assessment is 

not required for exposure to treated seeds that have already been planted.
Thiabendazole/060101 Human Health Risk Assessment D425853

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2.0 HED Recommendations

HED concludes that a time-limited tolerance for residues of thiabendazole and its regulated 

metabolite, benzimidazole, can be established for pea, succulent shelled for the proposed seed 

treatment use of thiabendazole on succulent shelled peas (Pisum spp., including English pea, 

garden pea and green pea). The specific tolerance recommendations are discussed in 2.2, and 

label modifications are discussed in section 2.3.

2.1 Data Deficiencies

None for this action.

2.2 Tolerance Considerations

2.2.1 Enforcement Analytical Method

HED Memo: D410807, 2/25/14, S. Hummel

Acceptable enforcement analytical methods are available for thiabendazole and benzimidazole in 

plant commodities. Four spectrophotofluorometric methods for the determination of 

thiabendazole are published in the Pesticide Analytical Manual (PAM) Vol. II, and a high 

performance liquid chromatography (HPLC) method with fluorescence detection (FLD) for the 

determination of benzimidazole (free and conjugated) is identified in the U.S. EPA Index of 

Residue Analytical Methods under thiabendazole as Study No. 93020. The validated limits of 

detection (LODs) for the thiabendazole methods range from 0.05-0.1 ppm, and the validated 

limit of quantitation (LOQ) for the benzimidazole method is 0.1 ppm. 

Thiabendazole is completely recovered using US Food and Drug Administration (FDA)

multiresidue methods (MRMs) 302 and 404, but is not recovered using MRM 303. 

Thiabendazole residues are routinely generated by the Pesticide Data Program (PDP) of the US 

Department of Agriculture (USDA) using MRMs with very low LODs. While MRM data 

submitted to EPA suggest that benzimidazole and 5-hydroxy-thiabendazole are not recovered by 

the FDA MRMs, 5-hydroxy-thiabendazole residues are also routinely generated by PDP.

2.2.2 International Harmonization

There is no issue with International Harmonization (See Appendix A). Neither CODEX nor 

Canada has established a maximum residue limit (MRL) for thiabendazole on the requested 

commodity. We note that the U.S. tolerance definition is not harmonized with Canada or 

CODEX. 

2.2.3 Recommended Tolerances

The current permanent tolerance definition for residues of thiabendazole in/on plant commodities 

is for the combined residues of the fungicide thiabendazole (2-(4-thiazolyl)benzimidazole) and 

its metabolite benzimidazole (free and conjugated) and the current tolerance expression is in 

accordance with the HED Interim Guidance on Tolerance Expressions (S. Knizner, 5/27/09).
Thiabendazole/060101 Human Health Risk Assessment D425853

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Table 2.2.3.1 summarizes the recommended time-limited tolerance level and commodity 

definition required to support the proposed new Section 18 emergency exemption use on pea, 

succulent shelled. This tolerance should be added to 40 CFR §180.242(b), using the current 

permanent tolerance definition and expression. Time-limited tolerances in/on pea vines and hay 

will not be established for this Section 18 emergency exemption, given (1) that the proposed use 

is limited and prohibits the feeding of pea vines and hay grown from treated seed to livestock 

and (2) that the available soybean forage and hay data indicate that residues in/on these livestock 

feed commodities are not likely to be detectable. However, a Section 3 registration for use on 

peas, if that use includes field peas, would most likely require the establishment of tolerances 

in/on pea vines and hay and a restriction would not be appropriate.

Table 2.2.3.1 Tolerance Summary for Thiabendazole. 

Commodity

Proposed

Tolerance

(ppm)

Recommended 

Tolerance

(ppm)

Comments; Correct Commodity Definition

Pea, succulent shelled Not Applicable 0.021 Pea, succulent shelled

1 Tolerance set at LOQ for both, thiabendazole and benzimidazole

2.2.4 Revisions to Petitioned-For Tolerances

No tolerances were proposed by the petitioner. 

2.3 Label Recommendations

The risk assessment supports the proposed thiabendazole seed treatment labels requiring only 

baseline level of clothing (i.e., long-sleeve shirt, long pants, shoes/socks) without use of 

chemical-resistant gloves. However, additional personal protective equipment (PPE) may be 

needed based on the acute toxicity of the end-use product. HED recommends that the 

Registration Division (RD) ensure that the appropriate PPE (e.g., chemical-resistant gloves) be 

included on the final label if necessary.

3.0 Introduction

3.1 Chemical Identity

The chemical structure and nomenclature of thiabendazole and its regulated metabolites are 

presented in Table 3.1. 

Table 3.1. Test Compound Nomenclature.

Compound

Common name Thiabendazole

Company experimental name None

IUPAC name 2-(thiazol-4-yl)benzimidazole

CAS name 2-(4-thiazolyl)-1H-benzimidazole

N

H

N

S

N
Thiabendazole/060101 Human Health Risk Assessment D425853

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Table 3.1. Test Compound Nomenclature.

CAS registry number 148-79-8

End-use product (EP) 4.1 lb ai/gal FlC formulation (Mertect 34F; EPA Reg. No 100-889)

Compound

Common name Benzimidazole

Chemical name 1H-benzimidazole

CAS registry number 51-17-2

Compound

Common name 5-hydroxy-thiabendazole

Chemical name 2-(1,3-thiazol-4-yl)-1H-benzimidazol-5-ol) 

3.2 Physical/Chemical Characteristics

Technical thiabendazole is a colorless crystalline solid with a melting point of 304-305 C, bulk 

density of 25-30 cc (tapped), octanol/water partition coefficient (Kow) of 240-285 at pH7, and has 

a low vapor pressure of 4 X 10-9 mm Hg at 25°C. Thiabendazole is sparingly soluble in water at 

0.028-0.030 mg/mL at 25°C, and is soluble in several organic solvents. The physicochemical 

properties of the technical grade of thiabendazole are presented in Appendix B. 

3.3 Pesticide Use Pattern

The petitioner has submitted a draft Section 18 emergency exemption label (not dated) for 

Mertect® 340-F Fungicide (EPA Reg. No. 100-889) which includes proposed regionally 

restricted use directions for succulent shelled cultivars of pea (Pisum spp.) including English pea, 

garden pea and green pea. The proposed use directions are summarized in Table 3.3.1. See 

Section 2.3 for recommended modifications to the proposed label.

N

H

N

N

H

N

S

HO N
Thiabendazole/060101 Human Health Risk Assessment D425853

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Table 3.3.1 Summary of Directions for Use of Thiabendazole.

Applic. Timing, 

Type, and Equip.

Formulation

[EPA Reg. No.] Applic. Rate 

Max. 

No. 

Applic

. per 

Season

PHI Use Directions and Limitations

Pea, succulent shelled(Pisum spp.) including English pea, garden pea and green pea.

Seed treatment,

Seed treatment 

application 

equipment

4.1 lb ai/gal F

[100-889]

1.02 fl oz /100 lb seed

32.68 g ai/100 kg seed

0.033lb ai/100 lb seed

Note: mg ai/seed not 

provided on label

1 N/A Restricted to use in Bonneville and 

Latah Counties in the State of Idaho.

For ready mix or slurry treaters, mix 

in adequate water to provide 

complete coverage. The minimum 

slurry volume is 4.0 fl oz/100 lb seed 

and more diluent may be required for 

completed coverage which is 

essential.

Do not feed vines or hay grown 

from treated seed to livestock

3.4 Anticipated Exposure Pathways

The Registration Division has requested an assessment of human health risk to support the 

proposed new seed treatment use of thiabendazole on pea, succulent shelled. Humans may be 

exposed to thiabendazole in food and drinking water. The proposed seed treatment uses of 

thiabendazole are not expected to result in residential (non-occupational) exposures. However, 

residential exposure is possible via antimicrobial uses in products such as paints and sponges. 

Occupational exposure is expected for primary handlers (seed treaters) and secondary handlers 

(seed planters). Post application exposure is not expected for workers.

Risk assessments have been previously prepared for the existing uses of thiabendazole. This risk 

assessment considers all of the aforementioned exposure pathways based on the proposed new 

uses of thiabendazole, but also considers the existing uses as well. 

3.5 Consideration of Environmental Justice

Potential areas of environmental justice concerns, to the extent possible, were considered in this 

human health risk assessment, in accordance with U.S. Executive Order 12898, "Federal Actions 

to Address Environmental Justice in Minority Populations and Low-Income Populations," 

(http://www.eh.doe.gov/oepa/guidance/justice/eo12898.pdf. As a part of every pesticide risk 

assessment, OPP considers a large variety of consumer subgroups according to well-established 

procedures. In line with OPP policy, HED estimates risks to population subgroups from 

pesticide exposures that are based on patterns of that subgroup’s food and water consumption, 

and activities in and around the home that involve pesticide use in a residential setting. 

Extensive data on food consumption patterns are compiled by the USDA under the Continuing 

Survey of Food Intake by Individuals (CSFII) and are used in pesticide risk assessments for all 

registered food uses of a pesticide. These data are analyzed and categorized by subgroups based 

on age, season of the year, ethnic group, and region of the country. Additionally, OPP is able to 

assess dietary exposure to smaller, specialized subgroups and exposure assessments are 
Thiabendazole/060101 Human Health Risk Assessment D425853

Page 8 of 25

performed when conditions or circumstances warrant. Whenever appropriate, non-dietary 

exposures based on home use of pesticide products and associated risks for adult applicators and 

for toddlers, youths, and adults entering or playing on treated areas post application are 

evaluated. Further considerations are currently in development as OPP has committed resources 

and expertise to the development of specialized software and models that consider exposure to 

bystanders and farm workers as well as lifestyle and traditional dietary patterns among specific 

subgroups.

4.0 Hazard Characterization and Dose-Response Assessment

The hazard characterization and dose response assessment were last updated in the 8/26/14 risk 

assessment (D416471). No updates were needed for this assessment. The toxicological endpoint 

selection table is presented below. Refer to the previous assessment (D416471) for any detailed 

information. 

Table 4.1 Summary of Toxicological Doses and Endpoints for Thiabendazole for Use in Dietary and NonOccupational Human Health Risk Assessments

Exposure/ 

Scenario

Point of 

Departure

Uncertainty/FQ

PA Safety 

Factors

RfD, PAD, 

Level of 

Concern for 

Risk 

Assessment

Study and Toxicological Effects

Acute Dietary

(general 

population 

including 

females 13 –

49 years and 

children)

NOAEL =

50 mg/kg

UFA = 10X

UFH = 10X

FQPA UFDB = 

10X

aRfD =

0.05mg/kg/day

aPAD =

0.05 mg/kg

Acute neurotoxicity Study (MRID 48996310)

LOAEL = 200 mg/kg based decreases in the 

FOB (reduced body temperature in males 

p<0.05, and reduced rearing in females ↓54%, 

p<0.01), reduced locomotor activity in males 

and females (↓37-46%, p<0.01), at time of peak 

effect (approximately 3 hours post-dose).

Reduced body weight gain (p<0.01) and food 

consumption (↓44%, p<0.01) occurred on day 

1.

Chronic 

Dietary

NOAEL =

10 mg/kg/day

UFA = 3X

UFH = 10X

FQPA UFDB = 

10X

cRfD =

0.033 

mg/kg/day

cPAD =

0.033 

mg/kg/day

2-Year chronic carcinogenicity in the Rat 

(MRID 43593201)

Chronic LOAEL = 30 mg/kg/day based on 

decreased body weight gains and liver 

hypertrophy. Thiabendazole induced thyroid 

adenomas in male rats at dosages of ≥30 

mg/kg/day.

Supported by subchronic toxicity rat study 

(MRID 42942802)

Subchronic LOAEL = 40 mg/kg/ based on 

reduced body weight and body weight gains 

((↓11%) and histopathological changes in the 

bone marrow (erythroid hyperplasia), liver 

(centrilobular hypertrophy), thyroid (follicular 

cell hypertrophy) and spleen (pigmented).
Thiabendazole/060101 Human Health Risk Assessment D425853

Page 9 of 25

Table 4.1 Summary of Toxicological Doses and Endpoints for Thiabendazole for Use in Dietary and NonOccupational Human Health Risk Assessments

Exposure/ 

Scenario

Point of 

Departure

Uncertainty/FQ

PA Safety 

Factors

RfD, PAD, 

Level of 

Concern for 

Risk 

Assessment

Study and Toxicological Effects

Incidental 

Oral (Short –

term (1-30 

days) and 

intermediateterm (1-6 

months)

NOAEL= 10

mg/kg/day

UFA= 3x

UFH=10x

FQPA UFDB = 

10x

Residential 

LOC for MOE 

= 300

Subchronic oral toxicity study – rat

MRID 42942802 

LOAEL = 40 mg/kg/day based on reduced 

body weight gains and histopathological 

changes in the bone marrow, liver and thyroid

Dermal 

Short-Term 

(1-30 day s) 

and 

Intermediateterm (1-6 

months) 

DAF = 0.5%

NOAEL= 10

mg/kg/day

DAF = 0.5%

UFA= 3x

UFH=10

FQPA UFDB = 

10x

Residential 

LOC for MOE 

= 300 Subchronic oral toxicity study – rat

MRID 42942802 

LOAEL = 40 mg/kg/day based on reduced 

body weight gains and histopathological 

changes in the bone marrow, liver and thyroid

Inhalation 

Short-Term 

(1-30 days) 

and 

Intermediateterm (1-6 

months) 

NOAEL= 10

mg/kg/day

UFA= 3x

UFH=10x

FQPA UFDB = 

10x

Residential 

LOC for MOE 

= 300

Subchronic oral toxicity study – rat

MRID 42942802 

LOAEL = 40 mg/kg/day based on reduced 

body weight gains and histopathological 

changes in the bone marrow, liver and thyroid

Cancer (oral, 

dermal, 

inhalation)

Likely to be carcinogenic at doses high enough to cause a disturbance of the thyroid hormonal balance 

but not likely to be carcinogenic at doses lower that those which could cause a disturbance of this 

hormonal balance (TXR 0050554). Quantification of cancer risk is not required.

Point of Departure (POD) = A data point or an estimated point that is derived from observed dose-response data and 

used to mark the beginning of extrapolation to determine risk associated with lower environmentally relevant human 

exposures. NOAEL = no observed adverse effect level. LOAEL = lowest observed adverse effect level. UF = 

uncertainty factor. UFA = extrapolation from animal to human (interspecies). UFH = potential variation in 

sensitivity among members of the human population (intraspecies). UFDB = to account for the absence of key data 

(i.e., lack of a critical study). FQPA SF = FQPA Safety Factor. PAD = population adjusted dose (a = acute, c = 

chronic). RfD = reference dose. MOE = margin of exposure. LOC = level of concern. N/A = not applicable. DAF 

= dermal absorption factor (TXR 0055373)
Thiabendazole/060101 Human Health Risk Assessment D425853

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Table 4..2 Summary of Toxicological Doses and Endpoints for Thiabendazole for Use in Occupational Human 

Health Risk Assessments

Exposure/ 

Scenario

Point of 

Departure

Uncertainty 

Factors

Level of Concern 

for Risk 

Assessment

Study and Toxicological Effects

Dermal ShortTerm (1-30 

day s) and 

Intermediateterm (1-6 

months) 

DAF = 0.5%

NOAEL= 10

mg/kg/day

DAF = 0.5%

UFA= 3x

UFH=10

UFDB = 10x

Occupational l 

LOC for MOE = 

300

Subchronic oral toxicity study – rat

MRID 42942802 

LOAEL = 40 mg/kg/day based on reduced 

body weight gains and histopathological 

changes in the bone marrow, liver and 

thyroid

Inhalation 

Short-Term (1-

30 days) and 

Intermediateterm (1-6 

months) 

NOAEL= 10

mg/kg/day

UFA= 3x

UFH=10x

UFDB = 10x

Occupational 

LOC for MOE = 

300

Subchronic oral toxicity study – rat

MRID 42942802 

LOAEL = 40 mg/kg/day based on reduced 

body weight gains and histopathological 

changes in the bone marrow, liver and 

thyroid

Cancer (oral, 

dermal, 

inhalation)

Likely to be carcinogenic at doses high enough to cause a disturbance of the thyroid hormonal 

balance but not likely to be carcinogenic at doses lower that those which could cause a 

disturbance of this hormonal balance (TXR 0050554). Quantification of cancer risk is not 

required.

Point of Departure (POD) = A data point or an estimated point that is derived from observed dose-response data and 

used to mark the beginning of extrapolation to determine risk associated with lower environmentally relevant human 

exposures. NOAEL = no observed adverse effect level. LOAEL = lowest observed adverse effect level. UF = 

uncertainty factor. UFA = extrapolation from animal to human (interspecies). UFH = potential variation in 

sensitivity among members of the human population (intraspecies). UFL = use of a LOAEL to extrapolate a 

NOAEL. UFS = use of a short-term study for long-term risk assessment. UFDB = to account for the absence of key 

data (i.e., lack of a critical study). MOE = margin of exposure. LOC = level of concern. N/A = not applicable. DAF 

= dermal absorption factor (TXR 0055373)
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5.0 Dietary Exposure and Risk Assessment 

5.1 Metabolite/Degradate Residue Profile

5.1.1 Summary of Plant and Animal Metabolism Studies

The nature of the residue in plants is understood based on acceptable plant metabolism studies 

conducted in wheat, soybean, and sugar beet. In the three plant metabolism studies submitted, the 

parent compound and benzimidazole (free and conjugated) were the major residues. In the 

confined rotational crop study, thiabendazole (TBZ), benzimidazole (BNZ), and 5-hydroxy 

thiabendazole (5-OH-TBZ) were identified. The residues of concern in primary and rotational 

crops for purposes of tolerance enforcement and risk assessment are thiabendazole and its 

metabolite benzimidazole (free and conjugated). The nature of the residue in livestock is 

understood based on acceptable hen and goat metabolism studies. The major metabolite in goat 

milk and tissues is the 5-OH-TBZ compound (and its conjugates); BNZ and TBZ are also 

present. TBZ degrades predominantly to 5-OH-TBZ and its conjugates in hens. Varying amounts 

of BNZ and unchanged TBZ are also present in eggs and hen tissues. The residues of concern in 

livestock commodities are thiabendazole, 5-hydroxy-thiabendazole (free and conjugated), and 

benzimidazole. 

5.1.2 Summary of Environmental Degradation

Environmental fate studies show that thiabendazole is persistent and immobile. Thiabendazole is 

stable to hydrolysis and soil photolysis and degrades slowly in aerobic (half-life ~ 700 days) and 

anaerobic soil environments (>200 days). Aquatic photolysis, with a half-life of 1.2 days in clear 

water, appears to be the only significant degradation pathway. Under anaerobic aquatic 

conditions thiabendazole showed no evidence of abiotic or biotic degradation but moved rapidly 

from water phase to the sediment phase. Thiabendazole binds tightly to soil due to its high 

soil/water partitioning coefficients (Kocs > 1,000 mL/goc), thus limiting the amount available for 

leaching into ground water and for solution phase runoff into surface water. With the high 

sorption capacity, thiabendazole would be transported on eroded sediment. Sorption was weakly 

correlated to organic carbon. Terrestrial field dissipation studies showed thiabendazole to be 

extremely persistent with no observable leaching. Vapor pressure and Henry’s Constant indicate 

that thiabendazole has a low potential to volatilize from soil and water. 

5.1.3 Comparison of Metabolic Pathways

In the plant metabolism studies submitted, the parent compound and benzimidazole (free and 

conjugated) were the major residues when foliar application was used. Although 5-hydroxy 

thiabendazole is a major metabolite in rats and livestock, it is minor in plants. In the confined 

rotational crop study, thiabendazole, benzimidazole, and 5-hydroxy thiabendazole were 

identified. The major metabolite in livestock is the 5-OH-TBZ compound (and its conjugates); 

BNZ and TBZ are also present. 
Thiabendazole/060101 Human Health Risk Assessment D425853

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5.1.4 Residues of Concern Summary and Rationale

Residues of Concern: The qualitative nature of the residue in plants is adequately understood 

based on soybean, sugar beet and wheat metabolism studies reflecting foliar treatments. The 

HED Metabolism Assessment Review Committee (MARC) determined that residues of concern 

in plants for regulation and dietary risk assessment include thiabendazole and its metabolite 

benzimidazole (free and conjugated). No residues of 5-hydroxythiabendazole were identified in 

these plant metabolism studies. [Note: The 5-hydroxythiabendazole metabolite was identified 

as a minor residue in rotational crops.] Post-harvest treatments are expected to result in the 

highest potential residues of thiabendazole per se in/on plant commodities and potential residues 

of benzimidazole (free and conjugated) are unlikely to contribute significantly to the total 

thiabendazole residues of concern for these uses. Based on re-evaluation of the available foliar 

metabolism data, the reviewer has determined that maximum total thiabendazole residues of 

concern in soybean (foliage, immature plant, straw and seed), sugar beet roots, sugar beet tops 

and wheat (foliage, forage, straw and grain) may be estimated by multiplying the thiabendazole 

residue level by 1.5x, 1.4x, 1.8x and 1.8x, respectively. 

The qualitative nature of the residue in animals is adequately understood based on acceptable 

ruminant and poultry metabolism studies. The HED Metabolism Committee determined that 

residues of concern in livestock for regulation and dietary risk assessment include thiabendazole, 

5-hydroxythiabendazole (free and conjugated), and benzimidazole.

Table 5.1.4 Summary of Metabolites and Degradates to be included in the Risk 

Assessment and Tolerance Expression

Matrix Residues included in Risk 

Assessment

Residues included in 

Tolerance Expression

Plants

Primary Crop

thiabendazole and 

benzimidazole (free and 

conjugated)

thiabendazole and 

benzimidazole (free and 

conjugated)

Rotational Crop

thiabendazole and 

benzimidazole (free and 

conjugated)

thiabendazole and 

benzimidazole (free and 

conjugated)

Livestock

Ruminant

thiabendazole, 5-hydroxythiabendazole (free and 

conjugated), and 

benzimidazole

thiabendazole, 5-hydroxythiabendazole (free and 

conjugated), and 

benzimidazole

Poultry

thiabendazole, 5-hydroxythiabendazole (free and 

conjugated), and 

benzimidazole

thiabendazole, 5-hydroxythiabendazole (free and 

conjugated), and 

benzimidazole

Drinking Water thiabendazole Not Applicable
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Page 13 of 25

5.2 Food Residue Profile

HED Memo: D410807/D410833/D313507, 2/25314, S. Hummel

To support the Section 18 emergency exemption request for use of thiabendazole as a seed 

treatment on succulent shelled peas (Pisum spp., including English pea, garden pea and green 

pea), the Idaho application included two new field trial data reports but not the full studies. The 

preliminary report of pea/bean field trial data from trials conducted by Syngenta Crop Protection, 

LLC (hereafter referred to as Syngenta) at an exaggerated rate, support previously submitted 

soybean field trial data which are deemed adequate to support the proposed use and these data 

are discussed briefly below. The preliminary report of fresh peas from Sleepy Eye, MN is 

consistent with the data above but provides too little detail to be of use and will not be discussed 

further. 

Syngenta reportedly conducted thirty-five field trials in the United States on peas and beans 

(succulent shelled, edible-podded and dried shelled varieties) using Mertect® 340-F, containing 

4.1 lb ai/gal thiabendazole, at a nominal seed treatment rate of 0.2 mg ai/seed. This rate is 

approximately 2x the proposed maximum use rate estimated by the reviewer at 0.11 mg ai/seed 

(use rate 0.033 lb ai/100 lb seed) and BEAD1estimated 1361-2800 garden pea seed/lb of seed). 

Samples were collected at normal harvest timing and stored frozen for up to 19 months prior to 

analysis. Samples were analyzed using the liquid chromatography with tandem mass 

spectrometry (LC-MS/MS) method GRM040.01A which has been previously deemed acceptable 

for data-collection; the limit of quantitation (LOQ) determined as the lowest level of method 

validation (LLMV) is 0.01 ppm for both thiabendazole and benzimidazole, for a combined LOQ 

of 0.02 ppm. Residues of thiabendazole and benzimidazole were reportedly less than the LOQ 

(<0.01 ppm, for each analyte) in/on succulent shelled peas/beans (6 pea trials and 6 bean trials), 

edible-podded peas/beans (3 peas trials and 6 bean trials) and dry shelled peas/beans (5 pea trials 

and 9 bean trials). These reported data are consistent with the previously submitted/reviewed 

soybean data discussed below.

Syngenta previously submitted soybean field trial data reflecting seed treatment use of 

thiabendazole which were reviewed by HED (D410807). Eighteen independent field trials were 

conducted in the United States during the 2010 growing season. Mertect® 340-F was applied to 

the seeds at a nominal rate calculated by the original reviewer of 0.06 mg ai/seed (based on use 

rate of 0.04 lb ai/100 lb seed and BEAD estimated 3,000 seeds/lb of seed). For each trial site, 

samples of treated seeds were planted within 3-30 days of treatment. Samples of soybean forage 

and hay were harvested from all trials 46-94 days after planting (DAP); seed was harvested 109-

153 DAP. Hay samples were dried to approximately 10-20% moisture content; drying intervals 

were not reported. Samples of soybean were stored frozen from collection to analysis for 221-

273 days (7.3-9.0 months) for forage, 222-274 days (7.3-9.0 months) for hay, and 181-224 days 

(6.0-7.4 months) for seed. Samples were analyzed for residues of thiabendazole and 

benzimidazole using the LC/MS/MS method GRM040.01A deemed adequate for data-collection. 

 

1 Becker, J. and Ratnayake, S. 3/24/11. No DP Barcode. Acres Planted per Day and Seeding Rates of Crops Grown 

in the United States.
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Residues of thiabendazole and benzimidazole were each below the LOQ (<0.01 ppm) in/on all 

samples of soybean seed, forage and hay.

Acceptable storage stability data are available indicating that residues of both analytes are stable 

during frozen storage for up to 22 months in/on soybean forage, hay, and seed (DP# 280701, 

12/9/10, T. Morton). These data are adequate to support the sample storage conditions and 

durations for the field trials discussed above.

5.3 Water Residue Profile

EFED Memo: D410805, 2/24/14, J. Wolf. (Drinking Water Assessment (DWA))

An updated drinking water assessment is not needed for this Section 18 emergency exemption 

request for use of thiabendazole on succulent shelled peas (Pisum spp., including English pea, 

garden pea and green pea). The most recent drinking water assessment (D410805) provided Tier 

1 estimated drinking water concentrations (EDWCs) resulting from the currently registered seed 

treatment use rate estimated for wheat at 0.20 lb ai/A which is higher than the estimated 

maximum proposed seed treatment use rate for succulent peas at 0.14 lb ai/A (calculated by the 

reviewer based on the proposed use rate of 0.033 lb ai/100 lb seed and the BEAD2estimated 

1361-2800 garden pea seed/lb of seed and BEAD estimated seeding rate for garden peas of 

560,057 seed/A). The findings of the most recent drinking water assessment (D410805) are 

reiterated below. 

The drinking water estimates used in the dietary risk assessment were provided by the 

Environmental Fate and Effects Division (EFED; D410805). EFED conducted a Tier I drinking 

water assessment for thiabendazole in surface water using the FQPA Index Reservoir Screening 

Tool (FIRST) model and in ground water using the Pesticide Root Zone Model for GroundWater 

(PRZM-GW). The highest Estimated Drinking Water Concentrations (EDWCs) resulted from 

the currently registered use of thiabendazole on wheat as a seed treatment and not from the 

proposed new uses or requested soybean use rate increase. Surface water estimates were higher 

than ground water estimates. The highest EDWCs for peak and annual mean were 3.80 and 0.47 

μg/L (ppb), respectively. The peak concentration of 0.0038 ppm and annual mean of 0.00047 

ppm were used in the acute and chronic dietary exposure analyses, respectively. See Table 5.3 

below for details.

 

2 Becker, J. and Ratnayake, S. 3/24/11. No DP Barcode. Acres Planted per Day and Seeding Rates of Crops Grown 

in the United States.
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Table 5.3. Summary of Tier 1 Estimated Drinking Water Concentrations in Surface and Ground Water for 

the New Uses and Previously Assessed Wheat Use.

New Uses and Reassessment Thiabendazole

Peak Concentration Annual Average

Surface Water Crop (app rate lb a.i./A) Model μg/L

New Uses (0.15) FIRST 2.85 0.35

Wheat (0.20) FIRST 3.80 0.47

From RED Peak Concentration 56-day Average

Wheat (0.20) GENEEC 2.401 1.551

Groundwater Highest Daily 

Concentration

Average 

Concentration after 

Breakthrough

New Uses (0.15) PRZM-GW 0.46 -

Wheat (0.20) PRZM-GW 0.62 -

1

 The highest EDWCs in surface water for thiabendazole were previously prepared for the 2002 Reregistration 

Eligibility Decision (RED), using the Tier 1 GENEEC model (which has since been replaced by the FIRST model) 

for surface water and the SCI-GROW model (which has since been replaced with PRZM-GW) for groundwater. 

The peak concentrations are higher than previously determined for the RED primarily due to changes in the 

conceptual model (GENEEC vs. FIRST) (e.g., differences of field size and shape and the implementation of the 

Index Reservoir) and difference in fate input values for modeling. The chronic EDWCs are lower than before due 

the inclusion of the Index Reservoir (e.g., size and flow through the reservoir).

The drinking water models and their descriptions are available at the EPA internet site: 

http://www.epa.gov/oppefed1/models/water/. 

5.4 Dietary Risk Assessment

HED Memo: D416470, 3/10/14, B. Cropp-Kohlligian

An updated dietary risk assessment is not needed for this Section 18 emergency exemption 

request for use of thiabendazole on succulent shelled peas (Pisum spp., including English pea, 

garden pea and green pea). Based on the supporting residue chemistry data, the combined 

residues of thiabendazole and benzimidazole in/on succulent shelled peas (Pisum spp., including 

English pea, garden pea and green pea) is estimated at 0.01 ppm (i.e., 1⁄2 LOQ for each analyte) 

for the proposed Section 18 emergency exemption use. The previous drinking water estimates 

(D410805) are deemed adequate for the proposed use on succulent shelled peas. The addition of 

succulent shelled peas (Pisum spp., including English pea, garden pea and green pea) and the 

assumption of 100% of succulent shelled peas treated did not change the findings of the most 

recent dietary exposure and risk assessment (D416470) which are reiterated below. 

5.4.1 Description of Residue Data Used in Dietary Assessment

With regards to food, refined acute probabilistic and chronic dietary exposure assessments were 

conducted for thiabendazole using anticipated residue estimates for all existing uses except, 

cereal grains, citrus oil/peel, dried beans/peas and soybeans. Established/recommended 

tolerances were used for cereal grains (corn tolerance at the method limit of quantitation; 

estimated at 1⁄2 LOQ), citrus oil/peel, dried beans, soybeans and the proposed new seed treatment 

uses. Maximum residues based on field trial data were used for dried peas. Anticipated residue 

estimates for carrot, potato, sweet potato, cantaloupe, orange, apple, pear baby food, strawberry, 
Thiabendazole/060101 Human Health Risk Assessment D425853

Page 16 of 25

mushrooms, avocado, banana, mango, and papaya were based on USDA PDP monitoring data 

for thiabendazole and translated, as appropriate to yams, other citrus fruits, other pome fruits and 

plantains. Because USDA PDP monitoring data did not include benzimidazole (free and 

conjugated), an additional factor based on available metabolism data reflecting foliar use was 

applied to sweet potato/yam, avocado, cantaloupe, strawberry and mushrooms to account for this 

residue.

There is no reasonable expectation of finite residues of thiabendazole in poultry and eggs. 

Anticipated residue estimates in meat and milk considered exposure from thiabendazole-treated 

feedstuffs as well as from the use of thiabendazole as an animal drug, regulated by FDA. 

Anticipated residue estimates in tissues with the exceptions of beef meat and beef fat were based 

on feeding study data. Anticipated residue estimates in the meat and fat of beef were based on 

tolerances established under 21 CFR §556.730 for residues of thiabendazole at 0.1 ppm for 

negligible residues of thiabendazole in tissues. The anticipated residue estimate in milk was 

based on USDA PDP data for parent and available feeding study data for its metabolite, 5-

hydroxythiabendazole (free and conjugated). All estimates were supported by available ruminant 

metabolism data.

DEEM default processing factors were used for all commodities except potato 

chips/granules/flakes and wheat processed commodities which relied on empirical data 

demonstrating that residues do not concentration in these processed commodities. Also, the 

maximum theoretical concentration factor for citrus peel (3.3x) was applied to orange and lemon 

peel, since the use on citrus is post-harvest and most of the residues should be found in/on the 

peel.

5.4.2 Percent Crop Treated Used in Dietary Assessment

The available percent crop treated (%CT or PCT) data for the currently registered seed treatment 

uses of thiabendazole on soybeans and wheat (SLUA report 2/22/13) were used in the acute and 

chronic dietary assessments. The following maximum percent crop treated estimates were used 

in the acute dietary risk assessment for the following crops that are currently registered for 

thiabendazole: soybeans 2.5% and wheat 2.5%. The following average percent crop treated 

estimates were used in the chronic dietary risk assessment for the following crops that are 

currently registered for thiabendazole: soybeans 1% and wheat 1%. The available %CT data for 

the currently registered seed treatment use of thiabendazole on potatoes was not used since there 

is also a post-harvest use on potatoes. No other %CT data are available according to BEAD 

(Email dated 12/12/13 from S. Haddad (BEAD) to T. Morton (HED)) and, therefore, 100% crop 

treated was assumed for all other existing/proposed uses. 

5.4.3 Acute Dietary Risk Assessment

Estimated acute and chronic dietary exposures to thiabendazole from food and drinking water are 

below HED’s level of concern for all population subgroups. The combined acute dietary 

exposure from food and drinking water at the 99.9th percentile of exposure is estimated at 

0.012953 mg/kg/day for the general population, equivalent to 26% of the aPAD. The population 

subgroup with the highest estimated acute dietary exposure is children 1-2 years, with an 
Thiabendazole/060101 Human Health Risk Assessment D425853

Page 17 of 25

estimated exposure at the 99.9th percentile of 0.034322 mg/kg/day, equivalent to 69% of the 

aPAD. 

5.4.4 Chronic Dietary Risk Assessment

Chronic dietary exposure estimates for food and drinking water are well below HED’s level of 

concern (<100% CPAD) for all population subgroups. The dietary exposure is estimated at 

0.000379 mg/kg/day for the general population (1.1% of the cPAD) and 0.001562 mg/kg/day 

(4.7% of the cPAD) for Children 1-2 years old, the population subgroup with the highest 

estimated chronic dietary exposure. The results for all populations are summarized in Table 5 

below. 

5.4.5 Cancer Dietary Risk Assessment

Since thiabendazole is classified as “Likely to be carcinogenic at doses high enough to cause a 

disturbance of the thyroid hormonal balance but not likely to be carcinogenic at doses lower that 

those which could cause a disturbance of this hormonal balance,” a cancer dietary exposure 

assessment is not required. HED is currently regulating chronic dietary risk with a chronic RfD 

that reflects a dose level below dose levels at which thyroid hormone balance is impacted and 

consequently is also being protective of potential carcinogenic effects. Therefore, a cancer 

dietary assessment was not performed.

5.4.6 Summary Table

Table 5.4 6 Summary of Dietary (Food and Drinking Water) Exposure and Risk for Thiabendazole.

Population Subgroup

Acute Dietary

(99.9 Percentile) Chronic Dietary Cancer

Dietary 

Exposure 

(mg/kg/day)

% aPAD*

Dietary 

Exposure

(mg/kg/day)

% cPAD*

Dietary 

Exposure

(mg/kg/day)

Risk

General U.S. Population 0.012953 26 0.000379 1.1

All Infants (<1 year old) 0.021098 42 0.000612 1.9

N/A N/A

Children 1-2 years old* 0.034322 69 0.001562 4.7

Children 3-5 years old 0.028211 56 0.001081 3.3

Children 6-12 years old 0.017865 36 0.000597 1.8

Youth 13-19 years old 0.009042 18 0.000295 0.9

Adults 20-49 years old 0.007528 15 0.000267 0.8

Adults 50-99 years old 0.007786 16 0.000276 0.8

Females 13-49 years old 0.007789 16 0.000256 0.8

*The population subgroup(s) with the highest risk estimates.

6.0 Residential (Non-Occupational) Exposure/Risk Characterization

(AD memo of 6/23/10, T. Leighton, D379421)

The proposed seed treatment use of thiabendazole is not expected to result in residential (nonoccupational) exposures. However, residential exposure is possible via antimicrobial uses in 
Thiabendazole/060101 Human Health Risk Assessment D425853

Page 18 of 25

products such as paints and sponges. The Antimicrobials Division has previously conducted an 

assessment of these uses (D379421, T. Leighton, 6/23/10). No risks of concern were identified.

7.0 Aggregate Exposure/Risk Characterization

HED Memo: D416471, 8/26/14, J. Kidwell, et al.

An updated dietary exposure and risk assessment is not needed for this Section 18 emergency 

exemption request for use of thiabendazole on succulent shelled peas (Pisum spp., including 

English pea, garden pea and green pea). Hence, an updated aggregate exposure and risk 

assessment is not needed. The most recent aggregate exposure and risk assessment (D416471) 

did not result in any risks of concern. From the most recent aggregate assessment, the MOEs 

were 2000 for painters and 1400 (children 1-2 years) to 7300 (general population) for sponges 

and, therefore, were not of concern.

8.0 Cumulative Exposure/Risk Characterization

Unlike other pesticides for which EPA has followed a cumulative risk approach based on a 

common mechanism of toxicity, EPA has not made a common mechanism of toxicity finding as 

to thiabendazole and any other substances and thiabendazole does not appear to produce a toxic 

metabolite produced by other substances. For the purposes of this tolerance action, therefore, 

EPA has not assumed that thiabendazole has a common mechanism of toxicity with other 

substances. For information regarding EPA’s efforts to determine which chemicals have a 

common mechanism of toxicity and to evaluate the cumulative effects of such chemicals, see the 

policy statements released by EPA’s Office of Pesticide Programs concerning common 

mechanism determinations and procedures for cumulating effects from substances found to have 

a common mechanism on EPA’s website at http://www.epa.gov/pesticides/cumulative/.

9.0 Occupational Exposure/Risk Characterization

9.1 Short- and Intermediate-Term Handler Risk

HED uses the term handlers to describe those individuals who are involved in the pesticide 

application process. HED believes that there are distinct job functions or tasks related to 

applications and exposures can vary depending on the specifics of each task. Job requirements 

(amount of chemical used in each application), the kinds of equipment used, the target being 

treated, and the level of protective gear or controls used by a handler can cause exposure levels 

to differ in a manner specific to each application event. 

Based on the proposed seed treatment use, occupational handler exposure is expected. 

Occupational seed treatment handlers may experience short- and intermediate-term exposure to

thiabendazole while performing seed treatment activities in commercial settings (mixing, 

loading, applying liquid formulations; bagging treated seed; sewing bags; and multiple 

activities). In addition, occupational handlers may experience short- and intermediate-term 

exposure while planting thiabendazole-treated seeds (for the purposes of regulatory seed 

treatment, planting treated seeds are referred to as ‘secondary handlers’). However, since the 

same dermal and inhalation endpoints were selected for short- and intermediate-term exposure 

durations, short-term exposure and risk estimates are considered to be protective of intermediate-
Thiabendazole/060101 Human Health Risk Assessment D425853

Page 19 of 25

term exposure. While the number of days treating the proposed seed crops could potentially 

exceed the typical intermediate-term duration used in HED’s occupational assessments, HED has 

no information to indicate that the same chemical is used consecutively for a long-term duration. 

Therefore, chronic occupational handler exposure for seed treatment was not assessed.

Dermal and inhalation risk estimates were combined in this assessment because the toxicological 

effects for these exposure routes were similar. 

This risk assessment relies in part on data from studies in which adult human subjects were 

intentionally exposed to a pesticide or other chemical. These data, listed in Appendix C, are (1) 

subject to ethics review pursuant to 40 CFR 26, (2) have received that review, and (3) are 

compliant with applicable ethics requirements.

The results of the occupational handler exposure and risk assessment indicate that short-/ 

intermediate-term dermal and inhalation risks are not of concern (i.e., MOEs ≥ 300). Total 

MOEs range from 3,200 to 35,000 for seed treaters and for seed planters, the MOE is 6,800. 

(Tables 9.1.1 and 9.1.2).
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Table 9.1.1 Summary of Short-/Intermediate-Term Handler Exposures/Risks for Thiabendazole Seed Treatment

Seed 

Type

Max 

Application 

Rate1

Unit Exposure2

(mg/lb ai)

Amount 

of Seed 

Treated3

Absorbed Dose

(mg/kg/day)4 MOE5

Total

MOE6

lb ai/lb seed Dermal Inhal. lb 

seed/day Dermal Inhal. Dermal Inhal.

LOADER/APPLICATOR (single layer, no gloves)

succulent 

pea

0.00033 0.079 0.00034 339,500 5.5E-04 4.7E-04 18,000 21,000 9,800

SEWER (single layer, no gloves)

succulent 

pea

0.00033 0.0062 0.00023 339,500 4.3E-05 3.2E-04 230,000 31,000 28,000

BAGGER (single layer, no gloves)

succulent 

pea

0.00033 0.0091 0.00016 339,500 6.3E-05 2.2E-04 160,000 45,000 35,000

MULTIPLE ACTIVITIES

(combination of loader/applicator without gloves + no coveralls, sewer without gloves, stacker without gloves)

succulent 

pea

0.00033 0.134 0.0016 339,500 9.3E-04 2.2E-03 11,000 4,500 3,200

1 Application Rate based on proposed label use for thiabendazole (see Table 3.3.1).

2 Unit Exposures from HED Exposure Science Advisory Council Policy 14: Standard Operating Procedures for Seed Treatment. In cases where available 

data represents workers wearing chemical-resistant gloves. Unit exposures representing workers without chemical-resistant gloves were calculated by 

increasing exposure to the hands-only portion of the total dermal exposure measurement assuming chemical-resistant gloves provide 90% protection (i.e., 

dividing non-gloved hand exposures by 0.1), the standard protection factor assumption for chemical-resistant gloves.

3 HED default for lb seed treated per day from HED Exposure Science Advisory Council Interim Policy 15.1 and the BEAD memo “Acres Planted Per Day 

and Seeding Rates of Crops Grown in the United States” (J. Becker, March 2011).

4 Daily Absorbed Dose (mg/kg/day) = daily unit exposure (mg/lb ai) × application rate (lb ai/lb seed) × amount treated (lb seed/day) × Absorption (0.5 % 

for dermal) ÷ body weight (80 kg.

5 MOE = NOAEL (dermal = 10 mg/kg/day for short-/intermediate-term exposure; inhalation = 10 mg/kg/day for short-/intermediate-term) ÷ Absorbed 

Dose (mg/kg/day). LOC = 300.

6 Total MOE = 1 ÷[(1 ÷ Dermal MOE) + (1 ÷ Inhalation MOE)]

Table 9.1.2. Summary of Secondary Handler (Planter) Exposures and Risks for Thiabendazole 

(Single Layer, No Gloves during Loading Only).

Seed Type

Max 

Application 

Ratea

Unit Exposureb

(mg/lb ai)

Amount of 

Seed 

Planted 

Per Dayc

Absorbed Dose

(mg/kg/day) d MOEe Total

MOEf

lb ai/lb seed Dermal Inhal. lb seed/day Dermal Inhal. Dermal Inhal.

succulent pea 0.00033 1.51 0.0034 32,960 1.0E-03 4.6E-04 9,900 22,000 6,800

a Application Rate based on proposed label use for thiabendazole (see Table 3.3.1).

b Unit Exposures from HED Exposure Science Advisory Council Policy 14: Standard Operating Procedures for Seed Treatment. Available data 

represents workers wearing chemical-resistant gloves. Unit exposures representing workers without chemical-resistant gloves were calculated 

by increasing exposure to the hands-only portion of the total dermal exposure measurement assuming chemical-resistant gloves provide 90% 

protection (i.e., dividing non-gloved hand exposures by 0.1), the standard protection factor assumption for chemical-resistant gloves.

c HED default for lb seed planted per day from HED Exposure Science Advisory Council Interim Policy 15 and the BEAD memo “Acres

Planted Per Day and Seeding Rates of Crops Grown in the United States” (J. Becker, March 2011).

d Daily Absorbed Dose (mg/kg/day) = daily unit exposure (mg/lb ai) × application rate (lb ai/lb seed) × amount treated (lb seed/day) × 

Absorption (0.5% for dermal) ÷ body weight (80 kg).

e MOE = NOAEL (dermal = 10 mg/kg/day for short-/intermediate-term exposure; inhalation = 10 mg/kg/day for short-/intermediate-term) ÷

Absorbed Dose (mg/kg/day). LOC = 300.

f Total MOE = 1 ÷ [(1 ÷ Dermal MOE) + (1 ÷ Inhalation MOE)]
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9.2 Short-/Intermediate Post-Application Risk

HED uses the term post-application to describe exposures that occur when individuals are 

present in an environment that has been previously treated with a pesticide (also referred to as reentry exposure). Such exposures may occur when workers enter previously treated areas to 

perform job functions, including activities related to crop production, such as scouting for pests 

or harvesting. The potential for dermal and inhalation post-application exposures following the 

planting of thiabendazole-treated seeds is unlikely because sustained levels of contact with 

treated seed after it has been placed in the soil or other planting media would not be expected 

because no routine cultural practice required for the production of agricultural commodities 

involves such an activity as defined in the no/low contact criteria in the WPS. Therefore, while 

the requirement of a 12 hour restricted entry interval (REI) for potential contact with treated seed 

following planting applies based on the acute toxicity of technical thiabendazole, so does the 

potential exception to this requirement per WPS guidance for situations where contact with 

residue is not expected. 

Similarly, a post-application inhalation exposure assessment is not required, as exposure is 

expected to be negligible. However, the seed treatment handler assessments provide quantitative 

inhalation exposure assessments for seed treaters and planters, both of which would be protective 

of most post-application inhalation exposure scenarios. 

10.0 References

Becker, J. and Ratnayake, S. 3/24/11. No DP Barcode. Acres Planted per Day and Seeding Rates 

of Crops Grown in the United States.

Cropp-Kohlligian, B. 3/10/14. D416470. Thiabendazole: Acute Probabilistic and Chronic 

Aggregate Dietary (Food and Drinking Water) Exposure and Risk Assessments for Section 3 

Registration for New Seed Treatment Uses on Assorted Vegetables/Small Grains and Increased 

Seed Treatment Use Rate on Soybean.

Hummel, S. 2/25/14. D410807, D410833, D313507. Thiabendazole. PP# 3F8166. New Seed 

Treatment Uses on Assorted Vegetables and Small Grains (vegetable, root (except sugar beet); 

subgroup1B, onion, bulb, subgroup 3-07A; Brassica, head and stem, subgroup 5A; vegetable, 

cucurbit group 9; alfalfa; spinach (including baby); and small grains (wheat, barley, oats, rye, 

and triticale). Summary of Analytical Chemistry and Residue Data. Multiresidue Method Data.

Kidwell, J et al. 8/26/14. D416471. Thiabendazole: Human Health Risk Assessment for the 

Requested Increase in the Currently Registered Seed Treatment Use Rate on Soybeans and the 

New Section 3 Uses of Thiabendazole for Seed Treatment on Assorted Vegetables and Small 

Grains Including: Vegetable, Root (Except Sugar Beet), Subgroup1B; Radish Tops; Onion, 

Bulb, Subgroup 3-07A; Brassica, Head and Stem, Subgroup 5A; Vegetable, Cucurbit Group 9; 

Alfalfa; Spinach; and a Number of Small Grains (Barley, Oats, Rye, and Triticale).

Leighton, T. 6/23/10. D379421. Thiabendazole: Antimicrobial Residential Uses for 

Consideration in the Aggregate Assessment to be Developed by USEPA/OPP/HED.
Thiabendazole/060101 Human Health Risk Assessment D425853

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O’Rourke, K. 2/19/14. D416472. Thiabendazole: Occupational/Residential Exposure 

Assessment for Proposed Seed Treatment Uses on Alfalfa, Brassica, Bulb Vegetables, Cucurbits, 

Root Vegetables, Small Grain Cereals, Spinach and Soybeans.

Wolf, J., Hetrick, J., et al. 2/24/14. D410805. Thiabendazole: Drinking Water Assessment for 

Proposed New Uses of the Fungicide Mertect® 340-F (EPA Reg. No. 100-889) as a Seed 

Treatment.
Thiabendazole/060101 Human Health Risk Assessment D425853

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Appendix A. International Residue Limits

Thiabendazole (PC Code 060101)

Summary of US and International Tolerances and Maximum Residue Limits 

Residue Definition:

US Canada Mexico1 Codex2

40 CFR 180.242

Plants: 2-(4-thiazolyl)benzimidazole) 

and its metabolite benzimidazole (free 

and conjugated)

2-(4-thiazolyl)benzimidazole Plants: thiabendazole

Commodity Tolerance (ppm) /Maximum Residue Limit (mg/kg)

US Canada Mexico1 Codex2

Pea, succulent shelled 0.02

Completed: M. Negussie; 03/17/15

1 Mexico adopts US tolerances and/or Codex MRLs for its export purposes.

2 * = absent at the limit of quantitation; Po = postharvest treatment, such as treatment of stored grains. PoP = 

processed postharvest treated commodity, such as processing of treated stored wheat. (fat) = to be measured on the 

fat portion of the sample. MRLs indicated as proposed have not been finalized by the CCPR and the CAC.
Thiabendazole/060101 Human Health Risk Assessment D425853

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Appendix B. Physical/Chemical Properties

Table C1. Physicochemical Properties of the Technical Grade Test Compound: Thiabendazole. 

Parameter Value Reference

Melting point/range 304-305 °C Thiabendazole RED 10/02

pH 5.16-5.97 Thiabendazole RED 10/02

Density 25-30 cc (tapped) Thiabendazole RED 10/02

Water solubility 0.028-0.030 mg/mL at 25 °C Thiabendazole RED 10/02

Solvent solubility

acetone

benzene

chloroform

dimethylformamide

dimethyl sulfoxide

ethanol 

ethyl acetate

methanol

25 oC

4.2 g/L

230 mg/L

80 mg/L

39 g/L

80 g/L

7.9 g/L

2.1 g/L (R.T.)

9.3 g/L

The Pesticide Manual, 8th edition

Vapor pressure 4 x 10 -9 mm Hg at 25 °C Thiabendazole RED 10/02

Dissociation constant, pKa pKa1 = 4.73; pKa2 = 12.00 Thiabendazole RED 10/02

Octanol/water partition coefficient Log(KOW) = 240-285 at pH 7 Thiabendazole RED 10/02

UV/visible absorption spectrum Neutral solution

32,170 L/mol*cm3at 205.5 nm

Acidic solution

41,520 L/mol*cm3at 303.5 nm

Neutral solution

40,553 L/mol*cm3at 215.5 nm

MRID 462868011

1 Data were referenced in MRID 47705121. 
Thiabendazole/060101 Human Health Risk Assessment D425853

Page 25 of 25

Appendix C. Studies Reviewed for Ethical Conduct

This risk assessment relies in part on data from studies in which adult human subjects were 

intentionally exposed to a pesticide or other chemical. These studies were determined to require 

a review of their ethical conduct, have received that review and have been determined to be 

ethical.

HED Exposure Science Advisory Council Policy 14: Standard Operating Procedures for Seed Treatment.