Document ID: EPA-HQ-OPP-2009-0920-0008
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2010-05-26T04:00Z

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

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 non-carcinogenicity 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 (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 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
population-adjusted 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:

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 non-carcinogenicity 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 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  tc \l2 "5.1   For Crop Protection Uses   5.1.1.  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)

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.  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. 

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 8th 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 Fix-Wing 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.



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)	Post-application
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
24.7E-9 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 time-limited 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.

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 assessment, 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:  
HYPERLINK "http://www.epa.gov/oppefed1/models/water/" 
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-FCID™, 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).

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).

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 post-application 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: 

●   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.

●   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,
non-competitive 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 non-competitive 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. 

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 applicationa	190	1.3	Turf	0.018 lb ai per gallon	0.125
Gallons per day	0.000018	55,000	0.000042	570

Trigger Sprayerb	53	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

Coefficient

(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/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 cm2 for the DFR value (1.08E-3 ft2/cm2
or 24.7E-9 acre/cm2 if the application rate is per acre)	

3 	Post-application dermal MOE = Dermal NOAEL (1 mg/kg/day) / Daily
Dermal Dose (mg/kg/day)



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/ft2 or 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 cm2 for
the grass residue value (1.08E-3 ft2/cm2 or 24.7E-9 acre/cm2 if 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/cm2 turf),  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 cm2 for the DFR
value (1.08E-3 ft2/cm2 or 24.7E-9 acre/cm2 if 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)t x CF2 x CF3 x
CF4  where:  AR = application rate (lbs ai/ft2 or 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 cm2 for the SR value (1.08E-3 ft2/cm2 or 24.7E-9
acre/cm2 if 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)

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. 

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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/.

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.

Attachment 1

Attachment 2

INTERNATIONAL RESIDUE LIMIT STATUS

: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

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

摧唣m

h

hydro-8a,10a-diazoniaphenanthrene ion

	

Residue definition: diquat

Crop(s)	

MRL (mg/kg)	

Crop(s)	

MRL (mg/kg)

Rapeseed	

1.0	

	

Notes/Special Instructions:  

S. Funk, 12/04/2008.

 Tier I Drinking Water and Aquatic Ecological Exposure Assessments for
Diquat Dibromide. J Breithaupt (5 March 2002, D281199).

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