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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

OFFICE OF 

PREVENTION, PESTICIDES

AND TOXIC SUBSTANCES

MEMORANDUM

Date: 3 December 2009

Subject: Diquat Dibromide: Human Health Risk Assessment for the Section 18 Use on 

Canola in Oklahoma and Kentucky.

PC Code: 032201 DP Barcode: D355407

Decision No.: 398579 Registration No.: 100-1061

Petition No.: 08OK02 Regulatory Action: Section 18

Risk Assessment Type: NA Case No.: NA

TXR No.: NA CAS No.: 85-00-07

MRID No.: None 40 CFR: 180.226

From: Michael A. Doherty, Ph.D., Chemist

Shih-Chi Wang, Ph.D., Biologist

Risk Assessment Branch II

Health Effects Division 7509P

Through: William T. Drew, Chemist

Zaida Figueroa, Industrial Hygenist

Edward Scollon, Ph.D., Toxicologist

Christina Swartz, Branch Chief

Risk Assessment Branch II

Health Effects Division 7509P

Susan V. Hummel, Senior Scientist

Risk Assessment Branch IV

Health Effects Division 7509P

To: Libby Pemberton/Dan Rosenblatt (Team 05)

Risk Integration Minor Use and Emergency Response Branch

Registration Division 7505P
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Introduction

The Oklahoma and Kentucky Departments of Agriculture have declared crisis exemptions for the 

use of diquat (as Reglone Desiccant, EPA Reg. No. 100-1061) on canola as a harvest aid. 

Rainfall patterns in canola-growing areas of these states have resulted in secondary blooming of 

the canola crop. Without the use of a desiccant, the secondary blooming can result in significant 

yield loss. Pesticide use in Oklahoma was authorized to begin 9 June 2008 and discontinue after 

23 June 2008.

Agency Memoranda Used to Support this Section 18 Exemption Risk Assessment

D369480, 26 October 2009, M. Doherty, Diquat: Acute and Chronic Aggregate Dietary (Food 

and Drinking Water) Exposure and Risk Assessments for the Section 18 Registration 

Action for Use on Canola in Oklahoma and Kentucky.

D369862, 21 October 2009, B. Phillips and D. Berwald, Estimates of Crop Irrigation for 

Endothall Registration.

Assessment summary 

Diquat dibromide {6,7-dihydrodipyrido[1,2-a:2′,1′-c]pyrazinediium dibromide} is a quaternary

ammonium herbicide. It is registered for use on a number of crops, primarily non-bearing crops 

or crops grown for seed, with tolerances [40 CFR 180.226] ranging from 0.02 ppm to 20 ppm. 

Many of the tolerances are to cover irrigation of crops with diquat-treated water. The residue of 

concern for tolerance-enforcement and risk-assessment purposes is the diquat cation.

For the purposes of this Section 18, the toxicology data base is adequate to characterize the 

toxicity of diquat dibromide. Diquat dibromide exhibits low acute toxicity via the oral (Toxicity

Category II for technical, III for formulation) and inhalation (Toxicity Category III) routes of 

exposure but is moderately to severely toxic via the dermal route of exposure (Toxicity 

Categories I for technical and II for formulation). Diquat dibromide is not an acute skin irritant 

(Toxicity Category IV) or a dermal sensitizer, but it is considered a moderate-to-severe eye 

irritant (Toxicity Category II). Subchronic and chronic studies in several species indicate 

multiple target sites for diquat dibromide toxicity. In subchronic dermal exposure studies in rats, 

diquat dibromide showed evidence of severe systemic toxicity, i.e. high mortality and clinical 

signs. In a subchronic inhalation study in rats, the lung was determined to be the primary target 

site for toxicity. Chronic feeding studies in dogs, rats, mice, and rabbits indicate that target sites 

include the eyes and kidneys in both males and females and the adrenals and epididymides in 

males. Developmental toxicity was observed in rat, rabbit, and mouse studies, and reproductive 

toxicity was observed in the rat in both generations. Rat and rabbit studies provided evidence of 

maternal toxicity. Acute and subchronic studies in mice and rats provided no evidence of 

neurotoxicity. Available data provide no evidence of endocrine disruption following exposure to 

diquat dibromide. Carcinogenicity studies in rats and mice provided no evidence of an increased 

tumor incidence and diquat dibromide is classified as a Category E (evidence of noncarcinogenicity to humans), and the weight of the evidence is predominantly negative for 

mutagenicity. The data show no indication of increased sensitivity of rats, mice, or rabbits from

in utero and/or early postnatal exposure to diquat dibromide. The Health Effects Division 
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(HED) has determined that the FQPA safety factor should be reduced to 1X. The toxicological 

doses and associated endpoints for human health risk assessment are summarized in Tables 1a 

and 1b. The same study, with the same doses and effects, was selected to assess exposure from 

dermal and oral routes. Therefore, it is appropriate to combine these routes when assessing 

aggregate exposure. The effects associated with inhalation exposure are different from those 

being used to assess the dermal and oral routes; therefore, inhalation exposures should not be 

aggregated.

Table 1a. Summary of Toxicological Doses and Endpoints for Diquat for Use in Dietary and Non-Occupational 

Human Health Risk Assessments.

Exposure Scenario Dose Endpoint Study

Acute Dietary

general population

incl infants and 

children

NOAEL= 75 mg/kg 

UF = 100

LOAEL of 150 mg/kg based on clinical signs 

and decreased body-weight gain. 

Acute 

neurotoxicity

rat

FQPA = 1x Acute RfD = 0.75 mg/kg Acute PAD = 0.75 mg/kg

Chronic Dietary

NOAEL = 0.5 

mg/kg/day

UF = 100

LOAEL of 2.5 mg/kg/day based on cataracts in 

females and decreased adrenal and epididymides 

weights in males. 

chronic toxicity 

dog

FQPA = 1x Chronic RfD = 0.005 mg/kg/day Chronic PAD = 0.005 mg/kg/day

Short-Term Oral

(1day - 1 month)

NOAEL = 1 mg/kg/day 

MOE = 100

LOAEL of 3 mg/kg/day based on body-weight 

loss and decreased food consumption. 

Developmental 

toxicity -

rabbit

Short-Term Dermal a 

(1 day - 1 month)

NOAEL = 1 mg/kg/day 

MOE = 100

LOAEL of 3 mg/kg/ day based on body-weight 

loss and decreased food consumption.

Developmental 

toxicity -

rabbit

Short-Term

Inhalation

(1 day - 1 month)

NOAEL = 0.1 μg/L

(0.024 mg/kg/d male, 

0.026 mg/kg/d female)

MOE = 100

LOAEL of 0.49 μg/L (0.117 mg/kg/day male, 

0.128 female) due to increased mean lung 

weight in males, mottling and reddening of 

lungs in females, and lung lesions. 

21-day inhalation 

toxicity

rat

a Since an oral value was selected, route-to-route extrapolation should be followed. A dermal absorption factor is 

required for this risk assessment.

Table 1b. Summary of Toxicological Doses and Endpoints for Diquat for Use in Occupational Human Health Risk 

Assessments

Exposure Scenario Dose Endpoint Study

Short-Term Dermal a 

(1 day - 1 month)

NOAEL = 1 mg/kg/day 

MOE = 100

LOAEL of 3 mg/kg/ day based on body-weight 

loss and decreased food consumption.

Developmental 

toxicity -

rabbit

Short-Term

Inhalation

(1 day - 1 month)

NOAEL = 0.1 μg/L

(0.024 mg/kg/d male, 

0.026 mg/kg/d female)

MOE = 100

LOAEL of 0.49 μg/L (0.117 mg/kg/day male, 

0.128 female) due to increased mean lung 

weight in males, mottling and reddening of 

lungs in females, and lung lesions. 

21-day inhalation 

toxicity

rat

a Since an oral value was selected, route-to-route extrapolation should be followed. A dermal absorption factor is 

required for this risk assessment.

Data depicting residues of diquat in/on canola seed have been submitted as part of the Section 18 

request. Based on those data, the NAFTA MRL-Tolerance Harmonization Spreadsheet 

recommends a tolerance of 4.0 ppm. There are currently diquat residue limits in/on canola seed 

(rapeseed) of 2 ppm for Codex and 1 ppm for Canada. For harmonization purposes and since the 
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submitted data show most of the residue levels to be below 1 ppm, HED is recommending 1 ppm 

as the appropriate tolerance level for this Section 18. The acute dietary exposure assessment is

based on tolerance-level residues and assumes 100% crop treated. The analysis indicates that 

acute risks associated with diquat are approximately 1% of the acute population-adjusted dose 

(aPAD), which is well below HED’s level of concern (i.e., less than 100% of the aPAD). The

chronic assessment is based on tolerance-level residues and 100% crop treated for crops with 

direct diquat uses. For crops with tolerances to cover irrigation with diquat-treated water, 

anticipated residue levels from irrigation trials are used in conjunction with estimates of % of 

crops irrigated. For fish, average residues are assumed. The resulting chronic dietary risk 

estimates are below HED’s level of concern, ranging from 13 to 34% of the chronic populationadjusted dose (cPAD).

Diquat dibromide can be applied to turf, to recreational ponds & lakes, for general weed control 

in and around home & garden sites, and for landscape uses by residential handlers. These uses 

can result in oral, dermal, and inhalation exposures to diquat. MOEs for these exposures range 

from 140 to 11,000,000. None of these residential/recreational MOEs exceed HED’s level of 

concern (diquat MOEs of < 100 are considered to be of concern to HED).

Aggregate exposures from the agricultural, aquatic, and residential uses of diquat result in MOEs 

of 104 to 660, depending on the age group. These MOEs represent risk estimates that are below 

HED’s level of concern for all population subgroups.

For purposes of this Section 18, HED has assumed that diquat does not share a common 

mechanism of toxicity with another pesticide and, therefore, has not undertaken a cumulative 

risk assessment for this compound.

HED has assessed the risks associated with occupational exposure to diquat. Mixer, loader, 

applicator, flagger, and post-application scenarios have been assessed. Provided proper personal 

protective equipment (PPE) is used, there are no occupational MOEs that HED considers to be 

risks of concern.

For purposes of this Section 18, HED is recommending that the emergency exemption be granted 

and that time-limited tolerances be established for residues of diquat cation in/on canola seed at 

1.0 ppm and in/on canola meal at 3.0 ppm.

Toxicological Considerations

HED completed an FQPA human health risk assessment as part of the RED process for diquat 

(Diquat Tolerance Reassessment Eligibility Document, D281890, B. Daiss, 22 March 2002). At 

that time, it was determined that the toxicological database for diquat was adequate for risk 

assessment purposes. Due to changes in pesticide registration data requirements [40 CFR Part 

158], an immunotoxicity study (OPPTS 870.7800) is now required. HED is not requiring this

study as part of this Section 18 assessment. Should the registrant wish to pursue a Section 3 

registration for this use or any other new use of diquat, then the study will be required.

The following is from the 2002 TRED and does not reflect the need for newly required data:
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Subchronic and chronic studies in several species indicate multiple target sites for diquat 

dibromide toxicity. In subchronic dermal exposure studies in rats, diquat dibromide 

showed evidence of severe systemic toxicity, i.e. high mortality and clinical signs. While 

a dermal study was available, the dermal endpoint is based on a rabbit developmental 

gavage study because the dermal study was determined to be inappropriate for endpoint 

selection. Route specific endpoints are available for all other exposure pathways. In a 

subchronic inhalation study in rats, the lung was determined to be the primary target site 

for inhalation toxicity. Chronic feeding studies in dogs, rats, and mice indicate that target 

sites include the eyes and kidneys in both males and females and the adrenals and 

epididymides in males. Developmental toxicity was observed in rat, rabbit, and mouse 

studies, and reproductive toxicity was observed in the rat in both generations. Rat and 

rabbit studies provided evidence of maternal toxicity. The acute and subchronic

neurotoxicity studies in rats provided no evidence of neurotoxicity. Available data 

provide no evidence of endocrine disruption following exposure to diquat dibromide. 

Carcinogenicity studies in rats and mice provided no evidence of increase tumor 

incidence and diquat dibromide was classified as a Category E (evidence of noncarcinogenicity to humans) by the HED Reference Dose (RfD)/Peer Review Committee 

based on the 1999 EPA Draft Proposed Guidelines for Carcinogen Risk Assessment. The 

weight of the evidence was predominantly negative for mutagenicity. The data provided 

no indication of increased sensitivity of rats, mice, or rabbits to in utero and/or early 

postnatal exposure to diquat dibromide. The terminal residue of concern is the parent 

compound, diquat cation. There is no indication that metabolites are present in significant

quantities.

FQPA Considerations - HED’s FQPA committee determined that the FQPA safety factor 

could be removed (1X) in assessing the risk posed by this chemical because the 

toxicological database is complete for FQPA assessment, there is no indication of 

quantitative or qualitative increased susceptibility of rats or rabbits to in utero and/or 

postnatal exposure, a developmental neurotoxicity study is not required, and the dietary

(food and drinking water) and residential exposure assessments will not underestimate 

the potential exposures for infants and children. The HIARC and FQPA Committee

determined that a developmental neurotoxicity study was not required for diquat 

dibromide based on the fact that (1) there is no indication of abnormalities in the 

development of the fetal nervous system in prenatal developmental toxicity studies in 

rats, mice, and rabbits at oral dose levels that were maternally toxic, (2) there was no 

evidence of neuropathology in either the acute or subchronic neurotoxicity studies, (3) 

the clinical and functional observational battery observations in the acute neurotoxicity

study, which could not be unequivocally correlated to an effect on the nervous system, 

were not observed in the subchronic neurotoxicity study, and (4) no neurotoxic effects 

were observed in the brain weights or histopathology of the nervous system in the chronic

toxicity studies with diquat in several species.

No new information has been submitted to the Agency that would change the toxicological 

conclusions described in the 2002 TRED. Based on the available data, which do not show any

indications of immunotoxicity, and for purposes of this Section 18 only, HED does not believe 
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that the newly required data will result in lower doses for assessing risk and, therefore, is not 

adding a database uncertainty factor at this time.

Occupational Exposure Considerations

This Section 18 emergency exemption is being requested for the end-use product containing 

diquat dibromide as the active ingredient. The proposed action is to use Reglone Desiccant

(EPA Reg. No. 100-1061, liquid, 2 lbs. diquat cation per gallon) as a harvest aid on canola grown 

in Oklahoma and Kentucky. The product will be applied at a rate of 24 to 30 fl. oz per acre (30 

fl. oz/acre = 0.468 lb. ai/acre) by ground or aerial equipment when the crop is at the 60-75% seed 

turn (green to brown) stage. 

Handlers

Equations/Calculations

The following equations are used to estimate handler exposure and risk:

Dermal Dose (mg/kg/day) = Rate (lb ai/A) x UE (mg/lb ai) x DA (0.003) x Acre Treated (A/day)

 BW (kg)

Inhalation Dose (mg/kg/day) = Rate (lb ai/A) x UE (mg/lb ai) x Acre Treated (A/day)

 BW (kg)

Where:

Rate (Application Rate) = Maximum application rate on product label (lb ai/A)

UE (Unit Exposure) = Exposure value derived from August 1998 PHED Surrogate 

Exposure Table (mg/lb ai handled)

DA = Dermal absorption factor (0.3%, from 2002 RED)

Acre Treated = Maximum area treated per day (A/day)

BW = Body weight (kg)

Dermal MOE = Dermal NOAEL (1 mg/kg/day)

Dermal Daily Dose (mg/kg/day)

Inhalation MOE = Inhalation NOAEL (0.024 mg/kg/day)

Inhalation Daily Dose (mg/kg/day)

Exposure Scenarios

There are 5 handler scenarios that are expected to result in the highest exposure for the proposed 

uses:

 Mixing/loading liquid for ground application (Scenario 1)

 Mixing/loading liquid for aerial application (Scenario 2)

 Applying sprays using ground equipment (Scenario 3) 

 Applying sprays using fixed-wing aerial equipment (Scenario 4)

 Flagging during aerial application (Scenario 5)
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Application Rate

The maximum application rate provided by the Oklahoma Department of Agriculture, Food, and 

Forestry is 0.468 lb ai per acre.

The Amount of Pesticide Handled per Day

Based on HED’s Exposure Science Advisory Council Policy Number 9.1, (1) 80 acres/day for 

application using ground-boom and (2) 350 acres/day for application using fixed-wing aircraft 

are assumed. 

Body Weight

The average body weight for general population (70 kg) is used for this assessment. 

Exposure Frequency

No data on the number of exposure days per year was provided. For this risk assessment, it is 

assumed that handlers would be exposed for less than 30 days per year (short-term exposure). 

Intermediate- and long-term exposures are not expected. 

Unit Exposures

The unit exposures are based on the Pesticide Handler’s Exposure Database (PHED) Version 1.1 

as presented in the August 1998 PHED Surrogate Exposure Guide. PHED was designed by a 

task force of representatives from the U.S. EPA, Health Canada, the California Department of 

Pesticide Regulation, and member companies of the American Crop Protection Association. 

PHED is a software system consisting of two parts–a database of measured exposure values for 

workers involved in the handling of pesticides under actual field conditions and a set of 

computer algorithms used to subset and statistically summarize the selected data. Currently, the 

database contains values for over 1,700 monitored individuals (i.e., replicates).

Users select criteria to subset the PHED database to reflect the exposure scenario being 

evaluated. The subsetting algorithms in PHED are based on the central assumption that the 

magnitude of handler exposures to pesticides is primarily a function of activity (e.g., 

mixing/loading, applying), formulation type (e.g., wettable powders, granulars), application 

method (e.g., aerial, ground-boom), and clothing scenarios (e.g., gloves, double layer clothing). 

The Surrogate Exposure Guide, which contains the most commonly used subsets of PHED, was 

used to develop exposure estimates.

Once the data for a given exposure scenario have been selected, the data are normalized for the 

amount of pesticide handled, resulting in standard unit exposures (milligrams of exposure per 

pound of active ingredient handled). Following normalization, the data are statistically

summarized. The distribution of exposure values for each body part (e.g., chest, upper arm) is 

categorized as normal, lognormal, or “other” (i.e., neither normal nor lognormal). A central 

tendency value is then selected from the distribution of the exposure values for each body part. 
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These values are the arithmetic mean for normal distributions, the geometric mean for lognormal 

distributions, and the median for all “other” distributions. Once selected, the central tendency

values for each body part are composited into a “best fit” exposure value representing the entire 

body.

There are three basic risk mitigation approaches considered appropriate for controlling 

occupational exposures. These include administrative controls, the use of PPE, and the use of 

engineering controls. Occupational handler exposure assessments are often completed by HED 

using baseline, PPE, and engineering controls. [Note: Administrative controls available generally 

involve altering application rates for handler exposure scenarios. These are typically not utilized 

for completing handler exposure assessments.] The baseline clothing level scenario for 

occupational exposure scenarios is generally an individual wearing long pants, a long-sleeved 

shirt, no chemical resistant gloves, and no respirator. The first level of mitigation generally 

applied is PPE. As reflected in the calculations included herein, PPE may involve the use of an 

additional layer of clothing, chemical-resistant gloves, and a respirator. The next level of 

mitigation considered in the risk assessment process is the use of appropriate engineering 

controls which, by design, attempt to eliminate the possibility of human exposure. Examples of 

commonly used engineering controls include enclosed tractor cabs and cockpits, closed 

mixing/loading/transfer systems, and water-soluble packets.

Handlers’ Exposure and Risk

HED’s level of concern (LOC) for occupational exposure to diquat is 100 (i.e., MOE>100 not of 

concern). All dermal handler scenarios result in MOEs greater than 100 if chemical resistant 

gloves are worn in addition to the baseline attire. All inhalation scenarios, except one, result in 

MOEs greater than 100 if dust-mist respirators are worn. A closed system is needed for the 

mixer/loader of aerial application systems to result in MOEs greater than 100. HED 

recommends that RD ensures proper PPE & engineering controls are on the final label.

A summary of the exposures/risks for handlers is presented in Table 2.

The handler exposure estimates in this assessment are based on a central tendency estimate of 

unit exposure and an upper-percentile assumption for the application rate, and are assumed to be 

representative of high-end exposures. The uncertainties associated with this assessment stem 

from the use of surrogate exposure data (e.g., differences in use scenario and data confidence), 

and assumptions regarding the amount of chemical handled. The estimated exposures are 

believed to be reasonable high-end estimates based on observations from field studies and 

professional judgment.

Post-application 

Based on the requested use, there is potential post-application exposure to workers performing 

hand harvesting of canola. The transfer coefficient (TC) for hand harvesting of canola assigned 

in the HED’s Science Advisory Council for Exposure Policy Number 3.1 is 2,500 cm2/hr. Based 

on this TC, the post-application dermal MOE for hand harvesting canola on Day 0 is 1,100 

which indicates the risk is not of concern. The calculations are shown in Table 3.
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The State of Oklahoma indicated that the emergency exemption will specify a pre-harvest 

interval (PHI) of 7 days. Given the properties of diquat, the inhalation exposure on the 8

th day

after application is considered to be very limited. 

Review of Human Research

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, which comprise the PHED, 

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

The studies in PHED were considered appropriate (or ethically conducted) for use in risk 

assessments.

Table 2. Non-cancer Risks Estimates for Diquate Dibromide Handlers.

Exposure 

Scenario 

(Scenario #)

Mitigation 

Levela

Dermal 

Unit 

Exposureb

(mg/lb ai)

Inhalation 

Unit 

Exposurec 

(μg/lb ai)

Applic.

Rate

(lb ai/A)

Amount 

Treatedd

(A/day)

Daily 

Dermal

Dosee

(mg/kg/day)

Daily

Inhalation

Dosef

(mg/kg/day)

Dermal

MOEg

Inhalation

MOEh

Mixer/Loader

Liquid for Ground 

application (1)

Baseline 2.9 1.2 0.468 80 0.0047 0.00064 210 38

Dust-Mist

Respirator 1.2 x 20% 0.468 80 --- 0.000128 --- 190

Liquid for Aerial 

application (2)

Baseline 2.9 1.2 0.468 350 0.020 0.0028 50 9

Baseline 

+Gloves 0.023 0.468 350 0.00016 --- 6,300 ---

Org. Vapour

Respirator 1.2 x 10% 0.468 350 --- 0.00028 --- 90

Closed System, 

single layer 

+gloves

0.0086 0.083 0.468 350 0.00006 0.000194 17,000 120

Applicator

Sprays with 

Groundboom (3)

Baseline 0.014 0.74 0.468 80 0.000022 0.00040 45,000 60

Dust-Mist

Respirator 0.74 x 20% 0.468 80 --- 0.000082 --- 300

Sprays with FixWing aircraft (4) Eng. Cont 0.0050 0.068 0.468 350 0.000035 0.00016 29,000 150

Flagger

Flagging during 

aerial applications 

(5)

Baseline 0.012 0.35 0.468 350 0.000084 0.00082 12,000 29

Dust-Mist

Respirator 0.35 x 20% 0.468 350 --- 0.000164 --- 150

a Baseline consists of long-sleeve shirt, long pants, shoes, and socks and no respirator. Eng. Cont. consists of enclosed cockpit.

b Baseline Dermal Unit Exposure represents long pants, long sleeved shirt, no gloves, open mixing/loading, and open cab tractors, as 

appropriate. Eng. Cont. Dermal Unit Exposure represents enclosed cockpit. 

c Baseline Inhalation Unit Exposure represents no respiratory protection, open mixing/loading, and open cab tractors, as appropriate. Eng. 

Cont. Inhalation Unit Exposure represents enclosed cockpit. 

d Daily acres treated values are from EPA estimates of acreage that could be treated in a single day for each exposure scenario of concern. 

Exposure SAC Policy 9, 7/5/2000. 

e Daily dermal dose (mg/kg/d) = [unit dermal exposure (mg/lb ai) x dermal absorption (0.003) x application rate (lb ai/acre) x daily acres 

treated / body weight (70kg).

f Daily inhalation dose (mg/kg/d) = (unit exposure (μg/lb ai) x (1mg/1000 μg) conversion x appl. rate (lb ai/acre) x daily acres treated / body 

weight (70kg).

g Dermal MOE = Dermal NOAEL (1 mg/kg/d) / Dermal daily dose. UF = 100.

h Inhalation MOE = Inhalation NOAEL (0.024 mg/kg/d) / Inhalation daily dose. UF = 100.
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Table 3. Post-Application Exposure and Risk Estimates for Hand Harvesting Diquat-treated canola.

Crop Application Rate 

(lb ai/A) Work Activity

Transfer 

Coefficientsa

(cm2/hr)

Postapplication 

Dayb

DFRc

(μg/cm2)

Daily Dosed

(mg/kg/day)

MOE e

Field/row Crop, 

low/medium,

Canola

0.468 Hand 

Harvesting 2,500 0 1.050 0.00090 1,100

a Transfer coefficient from Science Advisory Council for Exposure: 01/31/08.

b Day after treatment represents approximately 12 hours following application when sprays have dried.

c DFR = Application Rate (lb ai/acre) x Fraction of active ingredient that remains on the foliage when sprays have dried x 4.54E8 μg/lb x 24.7E9 acre/cm2

.

d Daily dose = DFR (μg/cm2) x TC (cm2/hr) x conversion factor (1 mg/1,000 μg) x dermal absorption (0.003) x exposure time (8 hrs/day) / body 

weight (70 kg).

e MOE = NOAEL (1 mg/kg/day) / daily dose (mg/kg/day).

Residue Chemistry Considerations

The reregistration requirements for plant metabolism are fulfilled. The qualitative nature of the 

residue in plants is adequately understood and the terminal residue of concern in plants is the 

diquat cation. Similarly, the qualitative nature of the residue in livestock is adequately 

understood based on acceptable poultry, ruminant, and fish metabolism studies, and the residue 

of concern is the diquat cation

The Pesticide Analytical Manual (PAM) Vol. II. lists a spectrophotometric method, designated 

as Method A, as available for the enforcement of tolerances for residues of diquat in/on plant and 

in livestock commodities. The limit of detection is 0.01 ppm. FDA’s multiresidue methods are 

unlikely to be successful for the analysis of diquat.

Data depicting residues of diquat in/on canola seed, cake, and oil were submitted with the 

Section 18 materials. Data are from Canadian and European trials, conducted at application rates 

ranging from 0.2 to 3.8 kg/ha (0.18 to 3.4 lb/A) with PHIs ranging from 0 to 32 days. The use 

pattern associated with the Section 18 is 0.52 kg/ha (0.47 lb/A) per season with a PHI of 7 days. 

Using the subset of the submitted data which most closely matches the use pattern results in 

residues ranging from <0.05 ppm to 1.3 ppm. The average residue for this subset is 0.5 ppm. 

Using the NAFTA Tolerance/MRL Calculator, assuming that <0.05 ppm equals 0.05 ppm, gives 

a recommended tolerance of 4 ppm (Attachment 1).

Maximum Residue Limits (MRLs) are listed for Codex and Canada at 2 ppm and 1 ppm, 

respectively (Attachment 2). For purposes of harmonization, HED is recommending a timelimited tolerance of 1.0 ppm in canola seed.

The data submitted for the canola press cake shows that diquat concentrates in the cake relative 

to levels in the seed. Due to limitations in the data presentation, it is difficult to determine the 

exact processing factor for any given data set; nevertheless, the maximum factor appears to be on 

the order of 3X. Therefore, HED is recommending a time-limited tolerance for residues of 

diquat in/on canola meal at 3.0 ppm. The data show no detectable residues of diquat in canola 

oil, as would be expected for an ionic compound. A tolerance in canola oil is not needed.
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Drinking Water Considerations

Diquat is registered for aquatic weed control and, as such, may be applied directly to bodies of 

water. The maximum contaminant level (MCL) for diquat established by the Office of Water is 

20 ppb (0.02 ppm). As per the 2002 Tier 1 drinking water assessment1, the MCL of 20 ppb (0.02 

ppm) was assumed for both the acute and chronic assessments, based on results of diquat 

monitoring data. As a point of characterization, the FQPA Index Reservoir Screening Tool 

(FIRST) model was used to estimate environmental concentrations in drinking water from 

surface water contaminated by terrestrial uses of diquat. The following scenarios were modeled: 

trees/vines/small fruits/vegetables (maximum use rate of 1 lb a.i./A, single application, 87 PCA); 

potatoes (0.5 lb a.i./A, 2 applications, 14 day application interval, 87 PCA); and 

alfalfa/clover/non-crop (0.5 lb a.i./A, 1 application, single application, 87 PCA) . The surface 

water estimated environmental concentrations (EECs) generated by FIRST ranged from 6.3- 13.2 

ppb (ug/L) for peak exposure and 0.2-0.4 ppb for annual average exposure. The model and its 

description are available at the EPA internet site: http://www.epa.gov/oppefed1/models/water/.

Dietary Exposure Analysis

Acute and chronic aggregate dietary (food and drinking water) exposure and risk assessments 

were conducted using the Dietary Exposure Evaluation Model DEEM-FCIDTM, Version 2.03 

which uses food consumption data from the U.S. Department of Agriculture’s Continuing 

Surveys of Food Intakes by Individuals (CSFII) from 1994-1996 and 1998. The analyses were 

performed to support a Section 18 use of diquat on canola. Default processing factors from 

DEEM v.7.81 were used in the analysis.

Acute Dietary (Food and Drinking Water) Exposure Results and Characterization

The acute dietary assessment assumes 100% crop treated and tolerance-level residues of diquat 

for all foods with registered uses. For drinking water, the acute assessment uses the Office of 

Water’s MCL (0.02 ppm), which is considered to be a worst-case estimate. Acute dietary risk 

estimates are below HED’s LOC (i.e., < 100% of the aPAD) for all population subgroups (Table 

4).

Chronic Dietary (Food and Drinking Water) Exposure Results and Characterization

The chronic assessment is based on tolerance-level residues and 100% crop treated for crops 

with direct diquat uses. For crops with tolerances to cover irrigation with diquat-treated water, 

anticipated residue levels from irrigation trials are used in conjunction with estimates of % of 

crops irrigated. For fish, average residues are assumed. Default processing factors from DEEM 

v.7.81 were used in the analysis. For drinking water, the chronic assessment uses the Office of 

Water’s MCL (0.02 ppm), which is considered to be a worst-case estimate. Chronic dietary risk 

estimates are below HED’s LOC (i.e., < 100% of the cPAD) for all population subgroups (Table 

4).

 

1 Tier I Drinking Water and Aquatic Ecological Exposure Assessments for Diquat Dibromide. J Breithaupt (5 March 

2002, D281199).
Page 12 of 21

Diquat has been classified as not carcinogenic to humans; therefore, cancer risk is not a concern 

for this compound.

Table 4. Summary of Dietary (Food + Drinking Water) Exposure and Risk for Diquat

Population Subgroup

Acute Dietary

(95th Percentile)

Chronic Dietary

Dietary Exposure 

(mg/kg/day) % aPAD Dietary Exposure(mg/kg/day) % cPAD

General U.S. Population 0.007339 1 0.000848 17

All Infants (< 1 year old) 0.006505 1 0.001752 35

Children 1-2 years old 0.010082 1 0.001688 34

Children 3-5 years old 0.009391 1 0.001444 29

Children 6-12 years old 0.006599 1 0.000975 20

Youth 13-19 years old 0.005566 1 0.000670 13

Adults 20-49 years old 0.007337 1 0.000761 15

Adults 50+ years old 0.007101 1 0.000756 15

Females 13-49 years old 0.005907 1 0.000709 14

Non-Dietary, Non-Occupational Exposure

Diquat dibromide can be applied to turf, to recreational ponds & lakes, for general weed control 

in and around home & garden sites, and for landscape uses by residential handlers. Application 

rates for diquat dibromide range from 0.25 to 4.0 pounds active ingredient per acre. Diquat 

dibromide is formulated for residential uses as a ready-to-use liquid and as a soluble concentrate/ 

liquid that can be mixed with water and then applied with a low-pressure handwand, backpack 

sprayer or with a trigger-pump sprayer. This residential exposure and risk assessment is based 

on the previous assessment (Tom Brennan, D279507, 14 December 2001) with the following 

changes to reflect: (1) the use of a new dermal absorption rate of 0.3%, (2) the use of PHED unit 

exposures for some handler’s exposure calculations, and (3) the corrections of miscalculations 

found in exposure estimations. 

Handlers 

There are four residential handler scenarios that are expected from the residential uses:

● Mixing, loading, and applying with a low-pressure handwand;

● Mixing, loading, applying with a backpack sprayer;

● Applying with a ready-to-use product in an aerosol can; and

● Applying with a trigger pump sprayer.

The following exposure assumptions are made in the exposure calculations:

● Average body weight of an adult handler is 70 kg/day.

● The highest application rate of 0.018 lb ai/gallon of spray solution is used. 

● Backpack sprayers and low pressure handwands are applied at a rate of 5 gallons per day. 

● Aerosol cans and triggers sprayers are applied at a rate of 0.125 gallons per day.

● No protective clothing is worn for these residential handler exposure/risk scenarios.

● Exposure frequency - a short-term exposure duration (less than 30 days).
Page 13 of 21

Low-Pressure Handwand

This scenario estimates exposure and risk to a handler who performs mixing, loading, and 

applying diquat dibromide with a low-pressure handwand to control weeds in and around lawns, 

gardens, around buildings, driveways, fence lines and other such edge areas. The dermal and 

inhalation unit exposures for this scenario are based on the PHED Version 1.1 (8/98). The 

dermal MOE is 2,600 and the inhalation MOE is 620 (Table 5). These MOEs indicate that risks 

are not of concern.

Backpack sprayer

This scenario estimates exposure and risk to a handler who performs mixing, loading, and 

applying diquat dibromide with a backpack sprayer to control weeds in and around lawns, 

gardens, around buildings, driveways, fence lines and other such edge areas. The dermal and 

inhalation unit exposures for this scenario are based on the PHED Version 1.1 (8/98). The 

dermal MOE is 100,000 and the inhalation MOE is 620 (Table 5). These MOEs indicate that 

risks are not of concern.

Aerosol Can

This residential scenario estimates exposure and risk to a handler who applies diquat dibromide 

with an aerosol spray can to control weeds in and around lawns, gardens, around buildings, 

driveways, fence lines and other such edge areas. The dermal and inhalation unit exposures for 

this scenario are based on the PHED Version 1.1 (8/98). The dermal MOE is 55,000 and the 

inhalation MOE is 570 (Table 6). These MOEs indicate that risks are not of concern. 

Trigger-Pump Sprayer

This residential scenario estimates exposure and risk to a handler who applies diquat dibromide 

with a trigger-pump sprayer to control weeds in and around lawns, gardens, around buildings, 

driveways, fence lines and other such edge areas. The dermal and inhalation unit exposures for 

this scenario are based on a carbaryl study (ORETF study MRID # 44459801) on ready-to-use 

insect sprayer application to home garden vegetables. The dermal MOE is 200,000 and the 

inhalation MOE is 11,000,000 (Table 6). These MOEs indicate that risks are not of concern.

Post-Application

Diquat dibromide is used on dormant turf grass for weed control. There are potential short-term 

post-application exposures (1 to 30 days) associated with this type of use because of residents’ 

activities on treated lawns in the initial hours and days following treatment. Post-application 

exposure to golfers who play on a treated golf course is also assessed. Additionally, a postapplication assessment for swimmers is assessed to address diquat use in recreational ponds and 

lakes for aquatic weed control.

Six residential post-application scenarios are assessed: 
Page 14 of 21

● Dermal exposure to treated turf grass (adults and children); 

● Recreational exposure from playing golf (adults);

● Incidental ingestion of treated turf grass via object-to-mouth activity (toddlers);

● Incidental ingestion of residue via hand-to-mouth activity (toddlers);

● Incidental ingestion of soil from treated area (toddlers); and

● Recreational exposure from swimming in treated ponds and lakes (adults and children).

Dermal Exposure Scenarios

Following assumptions are made in the dermal exposure calculations:

● The highest application rate of 0.50 lbs ai/acre is used.

● Turf transferable residue is equal to 5 % of the application rate.

● Turf transfer coefficient is 14,500 cm2/hr for adult and 5,200 cm2/hr for children.

● Exposure duration is 2 hours for residential lawns, and 4 hours for golf course turf.

● Body weight is 70 kg for adults, and 15 kg for toddlers.

● No protective clothing is assumed for these post-application exposure/risk scenarios.

Dermal Exposure in Residential Lawns

This scenario estimates dermal exposure and risk to adults (females were assessed due to the 

developmental toxicity endpoint) and toddlers from dermal contact with turf treated with diquat 

dibromide when they enter treated yards for recreation, yard work, or other homeowner 

activities. The adult MOE is 32,000 and toddler MOE is 19,000 (Table 6). These MOEs 

indicate that risks are not of concern.

Dermal Exposure at a Golf Course.

This scenario estimates dermal exposure and risk to adult golfers (females were assessed due to 

the developmental toxicity endpoint) golfers from dermal contact with diquat-treated turf grass 

when they enter the golf course after diquat dibromide sprays have dried and then play four 

hours of golf. The MOE for golfers is 480,000 (Table 6) and indicates that the risk is not of 

concern.

Incidental Oral Exposure Scenarios

Following assumptions are made in the incidental oral exposure calculations:

● The highest application rate of 0.50 lbs ai/acre is used.

● 5 % of the application rate is available for transfer from treated turf to wet hands and objects.

● Surface area of the portion of the hand put into the mouth is 20 cm2.

● Hand-to-mouth exposure frequency is 20/times per hour.

● Body weight is 15 kg for toddlers.

● Exposure time is 2 hours.

● Saliva extract factor (for hand-to-mouth only) is 50 percent.
Page 15 of 21

● No protective clothing is assumed for these post-application exposure/risk scenarios.

Toddler Ingestion of Treated Turf Grass via Object-to-Mouth Activities

This scenario estimates doses to toddlers from incidental ingestion of diquat-treated turf grass 

while they play in the treated areas. The MOE for object-to-mouth is 6,300 (Table 7) and 

indicates that the risk is not of concern.

Toddler Ingestion of Residue via Hand-to-Mouth Activities

This scenario estimates doses to toddlers from incidental ingestion of diquat residues as a result 

of hand-to-mouth transfer when they play on treated yards. The MOE for hand-to-mouth is 140 

(Table 7) and and indicates that the risk is not of concern.

Toddler Ingestion of Soil from Treated Area

This scenario estimates doses to toddlers from incidental ingestion of soil containing pesticide

residues while they play in treated areas. The MOE for ingesting treated soil is 800,000 (Table 

7) and indicates that the risk is not of concern.

Swimming Exposure Scenario

The potential exposures to swimmers who enter treated ponds and lakes are based on the 

Swimmer Exposure Assessment Model (SWIMODEL) which was developed for estimating the 

human exposure doses to the pesticides and toxic pollutants in swimming pools. This model is a 

modification of a study used by J. A. Beech (1980) for estimating exposure to Trihalomethanes 

(THMs) in swimming pools. Clearly in many ways swimming in ponds and lakes is different 

from that in pools; however, the basic exposure to chemicals in the water column is similar 

enough to warrant using this model for this assessment. 

The model is based on exposure routes and age-specific contact factors, exposure duration and 

frequency, chemical/physical properties of the pollutant, and pollutant concentration. With these 

parameters, total exposure doses can be approximated by the model. For this assessment, noncompetitive children swimmers (age 7-10), non-competitive adult female swimmers, and a 

diquat dibromide concentration of 260 ppb (the highest value based on nationwide monitoring 

data - reported by FEAD) in the lake are modeled. 

The MOE for non-competitive children swimmers age 7-10 is 180 (Table 8). The MOE for noncompetitive swimming adult females is 770 (Table 9). These MOEs indicate that risks are not of 

concern.

The calculations for post-application exposure to swimmers are described in the previous diquat 

assessment (D279507, T. Brennan, 14 December 2001). The SWIMODEL runs, including 

detailed analysis for each of the model inputs, assumptions, and results, are provided in the 

Appendix 1 of D279507.
Page 16 of 21

Table 5. Residential Handler’s Short-term Exposure and Risk Estimates Associated with Diquat Dibromide.

Exposure 

Scenario 

Dermal 

Unit 

Exposure 

(mg/lb ai)

Inhalation 

Unit 

Exposure 

(mg/lb ai)

Crop Application 

Rate

Amount 

Treated

Dermal

Daily Dose 

(mg/kg/day)

c

Dermal 

MOEd

Inhalation 

Daily Dose 

(mg/kg/day)e

Inhalation 

MOEf

Mixer/Loader/Applicator

Mixing/Loading/

Applying Liquids 

for Low-Pressure 

Handwand 

applicationa

100 0.03 Turf 0.018 lb ai per 

gallon

5 Gallons 

per day 0.000386 2,600 0.0000386 620

Mixing/Loading/

Applying Liquids 

for Backpack 

sprayer 

applicationa

2.5 0.03 Turf 0.018 lb ai per 

gallon

5 Gallons 

per day 0.0000096 100,000 0.0000386 620

Applicator

Aerosol can 

applicationa190 1.3 Turf 0.018 lb ai per 

gallon

0.125 

Gallons 

per day

0.000018 55,000 0.000042 570

Trigger Sprayerb53 0.000067 Turf 0.018 lb ai 

gallon

0.125 

Gallons 

per day

0.0000051 200,000 0.000000002 11,000,000

a Unit exposure values from PHED V1.1.

b Unit exposure values from ORETF study MRID # 44459801.

c Dermal daily dose (mg/kg/day) = daily unit exposure (mg/lb ai) x application rate (lb ai/gallon) x amount handled per day (gallons) x 

dermal absorption factor (0.3%) / body weight (70 kg).

d Dermal MOE = Dermal NOAEL (1 mg/kg/day) / Dermal daily dose (mg/kg/day).

e Inhalation daily dose (mg/kg/day) = inhalation unit exposure (mg/lb ai) x application rate (lb ai/gallon) x amount handled per day (gallons) /

body weight (70 kg). 

f Inhalation MOE = Inhalation NOAEL (0.024 mg/kg/day) / Inhalation daily dose (mg/kg/day).

Table 6. Post-Application Dermal Exposure and Risk Estimates for Toddlers and Adults on Day 0.

Scenario

Application

Rate1

(lb ai/acre)

Fraction of 

Residue

Retained

Transfer

Coefficie

nt

(cm2/hr)

Exposure

Duration

(hours)

Body

Weight

(kg)

Daily

Dermal Dose2 

(mg/kg/day)

Dermal 

MOE3

Toddler 0.5 0.05 5,200 2 15 0.000052 19,000

Adult 0.5 0.05 14,500 2 70 0.000031 32,000

Golfer -

Adult 0.5 0.05 500 4 70 0.0000021 480,000

1

The application rate is based on the highest labeled rates EPA Reg. No. 10182-404. 

2 Daily Dermal Dose on day “t” PDRt (mg/kg/day) = DFRt x CF1 x Tc x ET x DA / BW where: DFRt = dislodgeable foliar residue on day 

“t” (ug/cm2), CF1 = weight unit onversion factor to convert ug units in the DFR to mg for the daily dose (0.001 mg/ug), Tc = transfer 

coefficient (cm2/hr), ET = exposure time (hr/day), DA = Dermal Absorption factor (0.3%), BW = body weight (kg); and DFRt = AR x F x 

(1-D)t x CF2 x CF3 where: AR = application rate (lbs ai/ft2or lb ai/acre), F = fraction of ai retained on foliage (unitless), D = fraction of 

residue that dissipates daily (unitless), t = post-application day on which exposure is being assessed, CF2 = weight unit conversion factor 

to convert the lbs ai in the application rate to ug for the DFR value (4.54E8 ug/lb), CF3 = area unit conversion factor to convert the surface 

area units (ft2) in the application rate to cm2for the DFR value (1.08E-3 ft2/cm2or 24.7E-9 acre/cm2if the application rate is per acre)

3

Post-application dermal MOE = Dermal NOAEL (1 mg/kg/day) / Daily Dermal Dose (mg/kg/day)
Page 17 of 21

Table 7. Post-Application Incidental Oral Exposure and risk estimates for Toddlers Entering Treated Lawns.

Type of

Exposure

Applic.

Rate1

(lb ai/acre)

Fraction

of Residue

Available 

on Foliage

Ingestion 

Rate or Other 

Assumptions

Saliva 

Extraction 

Factor 

(%ile)

Exposure

Duration

(hours)

Body

Wgt

(kg)

Daily

Oral

Dose2

(mg/kg/day)

MOE 3

Object-to-mouth 

activities While 

on Treated Turf 

Grass

0.5 0.20 25 cm2/day

ingestion

NA 2 15 0.00016 6,300

Hand-to-Mouth 

Activity While on 

Treated Turf 

Grass

0.5 0.05 20 events/hr;

20 cm2

exposed 

area/event

50% 0.0074 140

Ingestion of Soil 

from Treated 

Area

0.5 0.05 100 mg/day

ingestion

NA 0.00000125 800,000

Combined 130

1

The application rate is based on the highest labeled rates EPA Reg. No. 10182-404. 

2 Object-to-mouth potential dose PDRt (mg/kg/day) = GRt x IgR x CF1 / BW where: GRt = grass (and plant matter) residue on day “t” 

(ug/cm2), IgR = ingestion rate of grass (cm2/day), CF1 = weight unit conversion factor to convert ug units in the DFR to mg for the daily 

dose (0.001 mg/ug), BW = body weight (kg); and GRt = AR x F x (1-D)t x CF2 x CF3 where: AR = application rate (lbs ai/ft2or lb 

ai/acre), F = fraction of ai available on the grass (unitless), D = fraction of residue that dissipates daily (unitless), t = post-application day on 

which exposure is being assessed, CF2 = weight unit conversion factor to convert the lbs ai in the application rate to ug for the grass residue 

value (4.54E8 ug/lb), CF3 = area unit conversion factor to convert the surface area units (ft2) in the application rate to cm2for the grass 

residue value (1.08E-3 ft2/cm2or 24.7E-9 acre/cm2if the application rate is per acre). 

Hand-to-mouth potential dose PDRt (mg/kg/day) = DFRt x SA x FQ x SEF x ET x CF1 / BW where: DFRt = dislodgeable foliar residue on 

day “t” (ug/cm2turf), SA = surface area of the hands (cm2/event), SEF = saliva extraction factor (%), FQ = frequency of hand-to-mouth 

activity (events/hr), ET = exposure time (hr/day), CF1 = weight unit conversion factor to convert ug units in the DFR value to mg for the 

daily exposure (0.001 mg/ug), BW = body weight (kg); and DFRt = AR x F x (1-D)t x CF2 x CF3 where: AR = application rate (lbs ai/ft2

or lb ai/acre), F = fraction of ai retained on foliage (unitless), D = fraction of residue that dissipates daily (unitless), t = post-application day 

on which exposure is being assessed, CF2 = weight unit conversion factor to convert the lbs ai in the application rate to ug for the DFR value 

(4.54E8 ug/lb), CF3 = area unit conversion factor to convert the surface area units (ft2) in the application rate to cm2for the DFR value 

(1.08E-3 ft2/cm2or 24.7E-9 acre/cm2if the application rate is per acre).

Ingestion of soil potential dose PDRt (mg/kg/day) = SRt x IgR x CF1 / BW where: SRt = soil residues on day “t” (ug/g), IgR = ingestion 

rate of soil (mg/day), CF1 = weight unit conversion factor to convert ug units in the residues on the soil to g for the daily dose (1.0E-6 g/ug), 

BW = body weight (kg); and SRt = AR x F x (1-D)tx CF2 x CF3 x CF4 where: AR = application rate (lbs ai/ft2or lb ai/acre), F = fraction 

of ai available in uppermost cm of the soil (fraction/cm), D = fraction of residue that dissipates daily (unitless), t = post-application day on 

which exposure is being assessed, CF2 = weight unit conversion factor to convert the lbs ai in the application rate to ug for the soil residue

value (4.54E8 ug/lb), CF3 = area unit conversion factor to convert the surface area units (ft2) in the application rate to cm2for the SR value 

(1.08E-3 ft2/cm2or 24.7E-9 acre/cm2if the application rate is per acre), CF4 = volume to weight unit conversion factor to convert the volume 

units (cm3) to weight units for the SR value (0.67 cm3/g soil).

3

Post-application oral MOE = Oral NOAEL(1 mg/kg/day)/Daily Oral Dose(mg/kg/day). 

Table 8. Swimmers’ Exposure and Risk Estimates from a Treated Lake on Day 0.

Exposed 

Population

Concentration 

in Lake (ppb)1

Total Exposure 

(mg/event)2

Body Weight 

(kg)3

Total Dose 

(mg/kg/day)4 MOE5

Child (age 7-10) 260 0.21 37.8 0.0055 180

Adult (female) 260 0.11 84.4 0.0013 770

1

 The concentration in the lake was run at 260 ppb (the highest value based on nationwide monitoring data –

reported by FEAD)

2 Total Exposure is a combination of exposures via the following routes: oral, dermal, buccal/sublingual, 

orbital/nasal,

 aural, and inhalation.

3

 Body weights represent the 90% for the population being modeled.

4 Total dose (mg/kg/day) = Total exposure (mg/event) / body weight (kg)

5 MOE = Oral NOAEL 1 mg/kg/day / Total dose (mg/kg/day)
Page 18 of 21

Aggregate Risk

Aggregate risk estimates that combine exposures from agricultural, residential, and recreational 

sources of diquat have been calculated for this Section 18 assessment. There are no use patterns 

that would result in intermediate- or long-term residential exposure; therefore, acute and chronic 

aggregate risks are equivalent to the dietary risk discussed above. Short-term MOEs are 

presented in Table 9. All MOEs are greater than 100 and, therefore, represent risks that are not 

of concern.

Table 9. Short-Term Aggregate Risk Estimates for Diquat

Population 

Subgroup

Chronic 

Dietary, 

mg/kg/day

Short-Term

NOAEL, 

mg/kg/day

MOE

Dietary a

Mixer/ 

Loader/ 

Applicator b

Post-Application

Swimmer b Aggregate c Dermal b

Incidental 

Oral b

General U.S. 

Population 0.000848 1 1100 570 32000 -- 770 250

All Infants (< 1 

year old) 0.001752 1 570 -- 19000 130 -- 105

Children 1-2 

years old 0.001688 1 590 -- 19000 130 -- 106

Children 3-5 

years old 0.001444 1 690 -- 19000 -- -- 660

Children 6-12 

years old 0.000975 1 1000 -- 19000 -- 180 150

Youth 13-19 

years old 0.000670 1 1400 570 32000 -- 770 260

Adults 20-49 

years old 0.000761 1 1300 570 32000 -- 770 260

Adults 50+ 

years old 0.000756 1 1300 570 32000 -- 770 260

Females 13-49 

years old 0.000709 1 1400 570 32000 -- 770 260

a Dietary MOE = Short-term NOAEL ÷ Chronic Dietary Exposure

b From Tables 5-8.

c Aggregate MOE = 1 ÷ [(1 ÷ MOEDietary) + (1 ÷ MOEMLA) + (1 ÷ MOEDermal) + (1 ÷ MOEIncidental Oral) + (1 ÷ 

MOESwimmer)]

Cumulative Risk

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 for 

diquat and any other substances, and diquat does not appear to produce a toxic metabolite 

produced by other substances. For the purposes of this action, therefore, EPA assumed that 

diquat does not have 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 OPP 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/.
Page 19 of 21

Conclusions

HED is recommending in favor of the emergency exemption for the use of diquat dibromide on 

canola in Oklahoma and Kentucky, and is recommending that time-limited tolerances be 

established for residues of the diquat cation in/on canola commodities as follows:

Canola, seed 1.0 ppm

Canola, meal 3.0 ppm.
Page 20 of 21

Attachment 1

Regulator: EPA

Chemical: Diquat

Crop: Canola

PHI: Approx. 7 days

App. Rate: Approx. 0.5 kg/ha

Submitter:

n: 25

min: 0.05

max: 1.30

median; 0.41

average: 0.49

95th Percentile 99th Percentile 99.9th Percentile

1.2 1.4 1.7

(1.4) (1.8) (--)

1.9 4.0 9.0

(4.0) (9.0) (--)

1.7

1.7

UPLMedian95th 2.5

0.9256

p-value > 0.05 : Do not reject lognormality assumption

Approximate

Shapiro-Francia 

Normality Test 

EU Method II

Distribution-Free

California Method

μ + 3σ

EU Method I

Normal

EU Method I

Log Normal

Lognormal Probability Plot

0.1 1 2 5 10 20 30 50 70 80 90 95 98 99 99.9

2.0000

1.0000

0.5000

0.2500

0.1250

0.0625

0.0313

y = 1.044x - 1.1381

R

2

 = 0.9256

Percentiles

Concentrations

EPA Diquat Canola Approx. 7 days Approx. 0.5 kg/ha
Page 21 of 21

Attachment 2

INTERNATIONAL RESIDUE LIMIT STATUS

Chemical Name: 6,7-

dihydrodipyrido[1,2-

a:2′,1′-c]pyrazinediium 

dibromide

Common Name: 

Diquat

X Proposed tolerance

 Reevaluated tolerance

 Other

Date: 12/4/08

Codex Status (Maximum Residue Limits) U. S. Tolerances

 No Codex proposal step 6 or above

 No Codex proposal step 6 or above for the crops 

requested

Petition Number: PP#08OK02

DP Barcode: D355407

Other Identifier:

Residue definition (step 8/CXL): Diquat cation.

Generally available as dibromide.

Reviewer/Branch: M. Doherty/RAB2

Residue definition: Diquat cation

Crop (s) MRL (mg/kg) Crop(s) Tolerance (ppm)

Rape seed 2. Canola 4.0

Limits for Canada Limits for Mexico

 No Limits

 No Limits for the crops requested

 No Limits

 No Limits for the crops requested

Residue definition: 9,10-dihydro-8a,10adiazoniaphenanthrene ionResidue definition: diquat

Crop(s) MRL (mg/kg) Crop(s) MRL (mg/kg)

Rapeseed 1.0

Notes/Special Instructions: 

S. Funk, 12/04/2008.