11
Approaches to Pesticide Approaches to Pesticide Cumulative Risk Cumulative Risk
Assessment: Policy, Assessment: Policy, Practice, ExperimentationPractice, Experimentation
Anna B. Lowit, Ph.D.Anna B. Lowit, Ph.D.U.S. Environmental Protection AgencyU.S. Environmental Protection Agency
Office of Pesticide ProgramsOffice of Pesticide Programs
Virginia Moser, Ph.D.Virginia Moser, Ph.D.U.S. Environmental Protection AgencyU.S. Environmental Protection AgencyOffice of Research and DevelopmentOffice of Research and Development
22
Outline: Policy & PracticeOutline: Policy & Practice
1.1. Introduction: regulatory context, Introduction: regulatory context, guidance documents, key principlesguidance documents, key principles
2.2. Hazard Assessment: relative potency Hazard Assessment: relative potency factor approach factor approach
3.3. Exposure Assessment: food, water, Exposure Assessment: food, water, residentialresidential
4.4. Cumulative assessment & ‘Track Back’Cumulative assessment & ‘Track Back’
5.5. SummarySummary
33
Introduction Introduction
EPA’s Office of Pesticide Programs is a EPA’s Office of Pesticide Programs is a licensing program regulating pesticide products licensing program regulating pesticide products in the U.S.in the U.S.• Review effects of pesticides on human and ecological Review effects of pesticides on human and ecological
healthhealth
Food Quality Protection Act (FQPA, 1996) Food Quality Protection Act (FQPA, 1996) • Requires EPA to take into account when setting Requires EPA to take into account when setting
pesticide tolerances: pesticide tolerances: ““available evidence concerning the cumulative effects on available evidence concerning the cumulative effects on
infants and children of such residues and other substances infants and children of such residues and other substances that have a that have a common mechanism of toxicitycommon mechanism of toxicity.” .”
44
IntroductionIntroduction
Under FQPA (1996), cumulative risk is Under FQPA (1996), cumulative risk is defined as: defined as: • The risk associated with a group of chemicals The risk associated with a group of chemicals
that are toxic by a common mechanism from that are toxic by a common mechanism from all pathways all pathways
• Multi-chemical & Multi-pathway Multi-chemical & Multi-pathway Food, drinking water, consumer uses Food, drinking water, consumer uses Routes of exposure (oral, dermal, inhalation)Routes of exposure (oral, dermal, inhalation)
55
Introduction: CRA GuidanceIntroduction: CRA Guidance
OPP developed guidance OPP developed guidance document for cumulative risks document for cumulative risks assessments under FQPAassessments under FQPA• Established core principles for Established core principles for
performing cumulative risk performing cumulative risk assessmentsassessments
• Developed tools for calculating Developed tools for calculating multichemical and multipathway multichemical and multipathway risk estimatesrisk estimates
• Not a ‘recipe book’Not a ‘recipe book’
http://www.epa.gov/oppfead1/trac/science/#common
66
Introduction: Key PrinciplesIntroduction: Key Principles Appropriately Integrate Toxicology & Exposure DataAppropriately Integrate Toxicology & Exposure Data
• Time-Frame ConsiderationsTime-Frame Considerations Time to peak effect? Time to recovery?Time to peak effect? Time to recovery? When does the exposure occur? What is the duration of exposure?When does the exposure occur? What is the duration of exposure?
Strive for Realistic & Accurate AssessmentsStrive for Realistic & Accurate Assessments• Use Representative DataUse Representative Data• Avoid Compounding ConservatismsAvoid Compounding Conservatisms
Preserve and Maintain Geographic, Temporal & Preserve and Maintain Geographic, Temporal & Demographic SpecificityDemographic Specificity• Calendar-Base ApproachCalendar-Base Approach
Be Able to “Track Back” Sources of Exposures & Be Able to “Track Back” Sources of Exposures & Perform Sensitivity AnalysesPerform Sensitivity Analyses• Major Risk Contributors Major Risk Contributors
Emphasis of presentation at CRA Workshop
77
Basic Steps in a Pesticide Basic Steps in a Pesticide Cumulative Risk AssessmentCumulative Risk Assessment
Identify common mechanism group (CMG) Identify common mechanism group (CMG) Determine relevant exposure scenarios/pathwaysDetermine relevant exposure scenarios/pathways Identify cumulative assessment group (CAG)Identify cumulative assessment group (CAG) Consider appropriate method(s) & data sourcesConsider appropriate method(s) & data sources Conduct assessmentConduct assessment
• Characterize & select common mechanism endpoint(s), Characterize & select common mechanism endpoint(s), determine chemical potency & select index chemicaldetermine chemical potency & select index chemical
• Convert pesticide residues to equivalents of the index Convert pesticide residues to equivalents of the index chemicalchemical
• Combine/integrate food, water, & residential exposures Combine/integrate food, water, & residential exposures on an internally consistent manner which incorporates on an internally consistent manner which incorporates demographic & temporal-spatial factorsdemographic & temporal-spatial factors
88
Introduction: CMG Guidance Introduction: CMG Guidance
Mechanism of Toxicity-Mechanism of Toxicity--Major steps leading to an -Major steps leading to an adverse health effect following interaction of a pesticide adverse health effect following interaction of a pesticide with biological targets. All steps leading to an effect do with biological targets. All steps leading to an effect do not need to be specifically understoodnot need to be specifically understood
Common Mechanism-Common Mechanism--Two or more pesticide chemicals -Two or more pesticide chemicals that cause a common toxic effect…by the same, or that cause a common toxic effect…by the same, or essentially the same, sequence of major biochemical essentially the same, sequence of major biochemical eventsevents
http://www.epa.gov/fedrgstr/EPA-PEST/1999/February/Day-05/6055.pdf
Group via Common
Mechanism
Pesticides
1010
Common Mechanism of Toxicity?Common Mechanism of Toxicity?
Three general principles to guide common Three general principles to guide common mechanism determinations: mechanism determinations: • Act on the same molecular target at the same Act on the same molecular target at the same
target tissue,target tissue,• Act by the same biochemical mechanism of Act by the same biochemical mechanism of
action, possibly sharing a common toxic action, possibly sharing a common toxic intermediateintermediate
• Cause the same critical toxic effectCause the same critical toxic effect Called the common toxic effect Called the common toxic effect
1111
Common Mechanism of Toxicity?Common Mechanism of Toxicity?
Is there concordance in dose response Is there concordance in dose response and timing between the major steps and and timing between the major steps and the toxic effect?the toxic effect?
Is it biologically/chemically plausible?Is it biologically/chemically plausible? What are strengths & uncertainties of the What are strengths & uncertainties of the
available data?available data? Could there be other an alternative Could there be other an alternative
mechanism(s) of action?mechanism(s) of action?
1212
Relative Potency Factor MethodRelative Potency Factor Method PBPK models would be preferred PBPK models would be preferred
• In vivoIn vivo and and in vitroin vitro pharmacokinetic data not available pharmacokinetic data not available at this timeat this time
• Multi-chemical, multi-pathway models not availableMulti-chemical, multi-pathway models not available Relative toxic potency of each chemical is Relative toxic potency of each chemical is
calculated in comparison to “index chemical” calculated in comparison to “index chemical”
Exposure equivalents of index chemical are Exposure equivalents of index chemical are combined in the cumulative risk assessmentcombined in the cumulative risk assessment
=RPFIndex ChemicalBMD
Chemical nBMD
OP CRA Hazard & Dose Response OP CRA Hazard & Dose Response
• Collaborative effort with EPA-ORD
• Rat data collected from studies at 21 days or longer where inhibition is no longer changing (ie, steady state)
• Use of multiple studies provides robust estimate of pesticide potency & incorporates variability across studies
Relative Potency Factors from OP Cumulative Risk Assessment
1515
NMC CRA Hazard & NMC CRA Hazard & Dose ResponseDose Response
Collaborative effort with ORDCollaborative effort with ORD• Benchmark modeling and dose-response and time course Benchmark modeling and dose-response and time course
laboratory studieslaboratory studies Relative potencies are estimated along with recovery Relative potencies are estimated along with recovery
half lives from acute (single dose) rat dose-time half lives from acute (single dose) rat dose-time response data at or near peakresponse data at or near peak
Dose & Time Course Model UsedDose & Time Course Model Used• Dose-response portion of model is similar to that used for AChE Dose-response portion of model is similar to that used for AChE
inhibition by organophosphatesinhibition by organophosphates• Time course model reflects an exponential decay of inhibitionTime course model reflects an exponential decay of inhibition
Rapid nature of NMC toxicity----Exposure assessment Rapid nature of NMC toxicity----Exposure assessment on single day exposures onlyon single day exposures only
Example: Oxamyl Dose-Time Example: Oxamyl Dose-Time ResponseResponse
1616
Dose of Mixture (mg/kg)
0 2 4 6 8 10 12 14 16
Cho
lines
tera
se a
ctiv
ity (
% c
ontr
ol)
20
30
40
50
60
70
80
90
100
110
120
Expected Values (+ 95% confidence limits)Actual Data (+ sem)
Carbamate Mixture Study:Brain Cholinesterase Activity
Dose-Additive Design
Equipotent Mixture
Control
1717
1818
Exposure Assessment & Exposure Assessment & Probabilistic TechniquesProbabilistic Techniques
Probablistic exposure techniques are routinely Probablistic exposure techniques are routinely applied by OPP for virtually all its pesticide risk applied by OPP for virtually all its pesticide risk assessmentsassessments• More accurate estimate of the entire range of More accurate estimate of the entire range of
exposures and their associated probabilities exposures and their associated probabilities
OPP’s Cumulative Risk Assessments rely on OPP’s Cumulative Risk Assessments rely on probabilistic (Monte-Carlo) techniques to probabilistic (Monte-Carlo) techniques to evaluate exposureevaluate exposure• Food, drinking water, residential uses, multi-pathwayFood, drinking water, residential uses, multi-pathway
1919
Exposure Assessment Exposure Assessment Software & ModelingSoftware & Modeling
Development of probabilistic models that Development of probabilistic models that permit time-based integration of residential, permit time-based integration of residential, food, and water exposures to pesticidesfood, and water exposures to pesticides
• ““Time-Based Integration” = Calendar-based Time-Based Integration” = Calendar-based approachapproach
• Allow probabilistic combining of exposures Allow probabilistic combining of exposures through multiple pathways and routesthrough multiple pathways and routes Single chemical or Multi-chemicalSingle chemical or Multi-chemical Food, Drinking Water, ResidentialFood, Drinking Water, Residential Ingestion, Inhalation, Dermal absorptionIngestion, Inhalation, Dermal absorption
2020
Exposure Assessment Exposure Assessment Software & ModelingSoftware & Modeling
Key concept: Must track potentially Key concept: Must track potentially exposed persons on a daily basis in a way exposed persons on a daily basis in a way that preserves all appropriate linkages and that preserves all appropriate linkages and appropriately allows for co-occurring appropriately allows for co-occurring exposuresexposures• Age, sex, behavior, region, etc.Age, sex, behavior, region, etc.
2121
Exposure Assessment SoftwareExposure Assessment Software OPP has used several software models to OPP has used several software models to
perform its risk assessmentsperform its risk assessments
Presented to FIFRA SAP by OPP along with Presented to FIFRA SAP by OPP along with model development teamsmodel development teams• LifelineLifeline• CARESCARES• DEEM/CalendexDEEM/Calendex• SHEDSSHEDS
All four models All four models • conform to EPA & OPP guidance conform to EPA & OPP guidance • have undergone peer reviewhave undergone peer review• are publicly availableare publicly available
2222
Exposure Assessment Exposure Assessment Software & ModelingSoftware & Modeling
Inputs includeInputs include• Toxicity information (e.g RfD, BMD, NOAEL)Toxicity information (e.g RfD, BMD, NOAEL)• Exposure informationExposure information
ResiduesResidues Food consumption (from USDA’s CSFII)Food consumption (from USDA’s CSFII) Behavior information (e.g., hand to mouth behavior)Behavior information (e.g., hand to mouth behavior)
Output includesOutput includes• Exposure levels (mg/kg bwt/day)Exposure levels (mg/kg bwt/day)• Risk metric (% RfD occupied, Margin of Exposure)Risk metric (% RfD occupied, Margin of Exposure)• Risk “drivers” Risk “drivers”
chemical(s), commodities, or residential uses which chemical(s), commodities, or residential uses which contribute significantly to riskcontribute significantly to risk
2323
Exposure Assessment Exposure Assessment Software & ModelingSoftware & Modeling
Use data from well-known surveys to Use data from well-known surveys to generate and evaluate specific daily generate and evaluate specific daily exposures for individualsexposures for individuals• Use available databases to address each Use available databases to address each
component of simulationcomponent of simulation• Incorporates seasonal and other aspectsIncorporates seasonal and other aspects
2424
Populations Groups AssessedPopulations Groups Assessed
Separate assessments were based on survey Separate assessments were based on survey information on the following age groups: information on the following age groups: • Infants <1Infants <1• Children 1 - 2 years oldChildren 1 - 2 years old• Children 3 - 5 years oldChildren 3 - 5 years old• Children 6 - 12 years oldChildren 6 - 12 years old• Youths 13 - 19 years oldYouths 13 - 19 years old• Adults 20 - 49 years oldAdults 20 - 49 years old• Adults 50+ years oldAdults 50+ years old• Females 13 - 49Females 13 - 49
2525
Regions AssessedRegions Assessed
2626
Software Inputs: Software Inputs: CSFII 1994-96/1998 Food CSFII 1994-96/1998 Food
Consumption SurveyConsumption Survey Nationally Representative/Statistically-BasedNationally Representative/Statistically-Based
• Intakes of individuals residing in 50 states and D.C. Intakes of individuals residing in 50 states and D.C. • 21,662 individual participants interviewed over the period 21,662 individual participants interviewed over the period
1998 Supplemental Children’s Survey 1998 Supplemental Children’s Survey • ~5000 children ~5000 children • birth through 9 years old birth through 9 years old • integrated into 1994-96 CSFIIintegrated into 1994-96 CSFII
Consisted of:Consisted of:• 2 non-consecutive days using in-person 24 hour recalls (ca. 2 non-consecutive days using in-person 24 hour recalls (ca.
3-10 days apart)3-10 days apart) Covers all seasons of year and all days of weekCovers all seasons of year and all days of week
2727
USDA Pesticide Data Program USDA Pesticide Data Program (PDP) Residue Data(PDP) Residue Data
Statistically-reliable sampling procedure designed to be Statistically-reliable sampling procedure designed to be representative of US food supply representative of US food supply • Approximately 600 samples per commodity per yearApproximately 600 samples per commodity per year
Samples collected at terminal markets and distribution Samples collected at terminal markets and distribution centerscenters• Samples prepared as if for consumptionSamples prepared as if for consumption
PDP has tested more than 50 different commodities and PDP has tested more than 50 different commodities and more than 300 pesticides/metabolitesmore than 300 pesticides/metabolites• Fresh/frozen/canned fruits & vegetables, fruit juices, milk, grains, Fresh/frozen/canned fruits & vegetables, fruit juices, milk, grains,
meat/poultry/pork, corn syrup, etc. meat/poultry/pork, corn syrup, etc. • Emphasis on children’s foodsEmphasis on children’s foods
Reliable analytical methods with low limits of detectionReliable analytical methods with low limits of detection
2828
““Track Back” in Food ExposureTrack Back” in Food Exposure
0%
5%
10%
15%
20%
25%
30%
Pot
ato
Str
awbe
rry
Pea
ch
Wat
erm
elon
Can
talo
upe
Cuc
umbe
r
Spi
nach
Nec
tarin
e
Pea
r
Lettu
ce, h
ead
Gra
pe
App
le
Bea
n, s
nap
Lettu
ce, l
eaf
Cab
bage
Ora
nge
Che
rry
Thiodicarb
Pirimicarb
Oxamyl
Methomyl
Methiocarb
Formetanate hydrochloride
Carbofuran
Carbaryl
Aldicarb
Sum of H X F
Food
PARENTPESTICIDE
2929
Cumulative DW AssessmentCumulative DW Assessment
Regional level screenRegional level screen Watershed-based modeling for surface Watershed-based modeling for surface
water sourceswater sources Shallow ground water for private wellsShallow ground water for private wells ““Typical” usage patternsTypical” usage patterns Daily distribution over multiple yearsDaily distribution over multiple years Estimates compared with, calibrated Estimates compared with, calibrated
against monitoringagainst monitoring
3030
For DW, Each Regional Location For DW, Each Regional Location Reflects …Reflects …
Geographic area with high potential for Geographic area with high potential for combined (cumulative) exposurecombined (cumulative) exposure• Influenced by both use and relative toxicitiesInfluenced by both use and relative toxicities
Location-specific conditionsLocation-specific conditions• environmental data (soil/site, weather, crops)environmental data (soil/site, weather, crops)• Major crop-pesticide combinations within that Major crop-pesticide combinations within that
areaarea Vulnerable drinking water sources within Vulnerable drinking water sources within
the regionthe region
3131
Residential Exposure Residential Exposure AssessmentAssessment
Extensive use of survey data and other pesticide use Extensive use of survey data and other pesticide use informationinformation
Use of distributions for residues and behavior/activity Use of distributions for residues and behavior/activity elementselements
• Hand-to-mouth activitiesHand-to-mouth activities
• Choreographed adult activities/Non-scripted playChoreographed adult activities/Non-scripted play
• Transfer Coefficients/Dislodgeable Foliar ResidueTransfer Coefficients/Dislodgeable Foliar Residue Use of a calendar based model to address the temporal Use of a calendar based model to address the temporal
use of residential usesuse of residential uses Region-specific analysesRegion-specific analyses
3232
Residential Exposure Residential Exposure AssessmentAssessment
Assessment performed for the following Assessment performed for the following uses:uses:• Indoor UsesIndoor Uses• Pet UsesPet Uses• Home Lawn and GardenHome Lawn and Garden• Golf CourseGolf Course• Public Health UsesPublic Health Uses
Cumulative MOEs for Children 1-2 Region A Seven Day Rolling Average Analysis
1
10
100
1000
10000
100000
1000000
1 10 19 28 37 46 55 64 73 82 91 100
109
118
127
136
145
154
163
172
181
190
199
208
217
226
235
244
253
262
271
280
289
298
307
316
325
334
343
352
361
Julian Days
Food MOE PRZM-EXAMS Water MOE Total MOE Inhalation MOE Dermal MOE Oral (non-dietary) MOE
Day of the Year
Example of Time based exposure Example of Time based exposure profile: Organophosphatesprofile: Organophosphates
Children 1-27-day
Inhalation + Total
Food
Water
Oral (non-dietary)
Dermal
MO
Es
3333
3434
Public Participation ProcessPublic Participation Process Numerous Public Technical Briefings on methods Numerous Public Technical Briefings on methods
and approaches for cumulative risk assessment and approaches for cumulative risk assessment and resultsand results
FIFRA Science Advisory Panel meetings on FIFRA Science Advisory Panel meetings on methods and approachesmethods and approaches• More than 20More than 20
Preliminary assessment –public comment and Preliminary assessment –public comment and Science Advisory Panel meetings Science Advisory Panel meetings
Revised assessment(s)–public commentRevised assessment(s)–public comment Website dedicated to cumulative risk assessment Website dedicated to cumulative risk assessment
http://www.epa.gov/pesticides/cumulative/
3535
Pesticide Cumulative RisksPesticide Cumulative Risks
Organophosphates (OP) Organophosphates (OP) NN-methyl carbamates -methyl carbamates Triazines Triazines ChloroacetanilidesChloroacetanilides
3636
Pesticide Cumulative RisksPesticide Cumulative Risks
Pyrethroids—Work has only just begunPyrethroids—Work has only just begun• Draft common mechanism grouping Draft common mechanism grouping
reviewed & supported by SAP, June 2009reviewed & supported by SAP, June 2009• OPP & ORD developing PBPK models for OPP & ORD developing PBPK models for
use in the pyrethroid cumulative risk use in the pyrethroid cumulative risk assessmentassessment
• Linkage between probabilistic exposure Linkage between probabilistic exposure assessment (SHEDS) and PBPK modelsassessment (SHEDS) and PBPK models
3737
Key PrinciplesKey Principles Appropriately Integrate Toxicology & Exposure DataAppropriately Integrate Toxicology & Exposure Data
• Time-Frame ConsiderationsTime-Frame Considerations Time to peak effect? Time to recovery?Time to peak effect? Time to recovery? When does the exposure occur? What is the duration of exposure?When does the exposure occur? What is the duration of exposure?
Strive for Realistic & Accurate AssessmentsStrive for Realistic & Accurate Assessments• Use Representative DataUse Representative Data• Avoid Compounding ConservatismsAvoid Compounding Conservatisms
Preserve and Maintain Geographic, Temporal & Preserve and Maintain Geographic, Temporal & Demographic SpecificityDemographic Specificity• Calendar-Base ApproachCalendar-Base Approach
Be Able to “Track Back” Sources of Exposures & Be Able to “Track Back” Sources of Exposures & Perform Sensitivity AnalysesPerform Sensitivity Analyses• Major Risk Contributors Major Risk Contributors
3838
Thank You!Thank You!
Approaches to Pesticide Approaches to Pesticide Cumulative Risk Assessment: Cumulative Risk Assessment:
Policy, Practice, Policy, Practice, ExperimentationExperimentation
Ginger Moser, Ph.D., D.A.B.T.Ginger Moser, Ph.D., D.A.B.T.TAD/NHEERL/ORD/US EPATAD/NHEERL/ORD/US EPA
[email protected] 14, 2009July 14, 2009
3939
AcknowledgementsAcknowledgements
Statistical expertise: Virginia Statistical expertise: Virginia Commonwealth University Commonwealth University • Drs. Chris Gennings, Hans Carter, Jr.Drs. Chris Gennings, Hans Carter, Jr.• Graduate students including but not limited to Graduate students including but not limited to
Michelle Casey, Adam HammMichelle Casey, Adam Hamm
Technical collaborations: US EPATechnical collaborations: US EPA• Drs. Dave Herr, Stephanie Padilla, Anna Drs. Dave Herr, Stephanie Padilla, Anna
Lowit, Jane Ellen SimmonsLowit, Jane Ellen Simmons• Pam Phillips, Kathy McDaniel, RenPam Phillips, Kathy McDaniel, Renéée e
MarshallMarshall4040
BackgroundBackground
Humans are exposed to multiple Humans are exposed to multiple chemicalschemicals
Effects of chemical mixtures may not be Effects of chemical mixtures may not be adequately predicted by studying adequately predicted by studying individual chemicalsindividual chemicals
Component-based mixtures risk Component-based mixtures risk assessment is aided by experimental assessment is aided by experimental design combining:design combining:• exposure evaluationsexposure evaluations• quantitative chemical informationquantitative chemical information• appropriate statistical analyses appropriate statistical analyses 4141
Theories of AdditivityTheories of Additivity
TerminologyTerminology• Zero interaction = additivityZero interaction = additivity• Synergy, antagonism = response greater, less Synergy, antagonism = response greater, less
than predicted under additivitythan predicted under additivity
Dose additivity = chemicals interacting as if Dose additivity = chemicals interacting as if they were dilutions of one anotherthey were dilutions of one another• Does not require same shape of dose-responseDoes not require same shape of dose-response• Does not require common mechanism of actionDoes not require common mechanism of action• Combinations of sub-threshold doses may be Combinations of sub-threshold doses may be
activeactiveBerenbaum, J. Theor. Biol. 114:413-431,1985 4242
Isobolographic ApproachIsobolographic Approach
Classic method of describing dose-additivityClassic method of describing dose-additivity• Isobols of equi-effective dosesIsobols of equi-effective doses• Requires multiple dose-response determinations with Requires multiple dose-response determinations with
different dose combinations of each chemicaldifferent dose combinations of each chemical• Data intensiveData intensive
Isobolographic Analysis
Chemical A Dose
0 2 4 6 8
Che
mic
al B
Dos
e
0
5
10
15
20
A ED50
B ED50
ED50 isobol
Isobolographic Analysis
Chemical A Dose
0 2 4 6 8
Che
mic
al B
Dos
e
0
5
10
15
20
ED50
ED20ED10
4343
Ray ApproachRay Approach Dose-response along ray of mixture with fixed proportions Dose-response along ray of mixture with fixed proportions
of componentsof components Uses individual chemical dose-response curves plus Uses individual chemical dose-response curves plus
mixture curvemixture curve Inferences limited to mixing ratio testedInferences limited to mixing ratio testedIsobol = curve fitted to Isobol = curve fitted to points with fixed points with fixed responseresponseRay = curve fitted to Ray = curve fitted to points with fixed ratiospoints with fixed ratios
Ray Design
Chemical A Dose
0 2 4 6 8
Che
mic
al B
Dos
e
0
5
10
15
20
ED50
ED20ED10
1:5 ray
2:1 ray
4444
Advantages of Ray DesignsAdvantages of Ray Designs
Useful for any number of chemicalsUseful for any number of chemicals Economical and efficient design to test for Economical and efficient design to test for
interactionsinteractions Provides statistical test of additivityProvides statistical test of additivity Mixture of study can be tailored to address Mixture of study can be tailored to address
experimental question(s)experimental question(s) Hypothesis-testing as well as -generatingHypothesis-testing as well as -generating
4545
General Methodology for Additivity General Methodology for Additivity Analysis using Ray DesignsAnalysis using Ray Designs
Dose-response model is fit to single chemical dataDose-response model is fit to single chemical data Additivity model (predicted) along fixed ray is Additivity model (predicted) along fixed ray is
generated based on single-chemical data and generated based on single-chemical data and mixing ratio of each chemical mixing ratio of each chemical
Dose-response model (observed) is fit to Dose-response model (observed) is fit to experimental mixture dataexperimental mixture data
Fitted models (predicted vs observed) are tested for Fitted models (predicted vs observed) are tested for departure from additivity, departure from additivity, e.g.e.g., , • Equality of parameters for experimental and additivity Equality of parameters for experimental and additivity
modelmodel• Experimental model fits within confidence limits of Experimental model fits within confidence limits of
predicted modelpredicted model• Equality of statistically derived thresholdsEquality of statistically derived thresholds
4646
Considerations Using Ray DesignsConsiderations Using Ray Designs Adequately characterize shape of individual Adequately characterize shape of individual
and mixture dose responseand mixture dose response Dose-response characteristicsDose-response characteristics
• Maximal responsesMaximal responses• Slopes Slopes
Focus on chemical selections, Focus on chemical selections, combinations, and mixing ratios of combinations, and mixing ratios of interestinterest
Also important: dose-rate, sequence and Also important: dose-rate, sequence and route of administrationroute of administration
4747
Mixture of 5 Organophosphorus Mixture of 5 Organophosphorus PesticidesPesticides
Why OPs? Why OPs? • Widely used pesticides, still Widely used pesticides, still • Potential for human exposure to multiple OPs through Potential for human exposure to multiple OPs through
use on foods and other commercial crops, pets, use on foods and other commercial crops, pets, garden, homegarden, home
• Common mode of action (inhibition of Common mode of action (inhibition of acetylcholinesterase)acetylcholinesterase)
• Epidemiological studies implicate OPs for Epidemiological studies implicate OPs for neurological adverse effects not predicted by neurological adverse effects not predicted by individual chemicalsindividual chemicals
Why 5 OPs?Why 5 OPs?• Monitoring data show 99% of food products have Monitoring data show 99% of food products have 5 5
pesticide residues (USDA Pesticide Data Program)pesticide residues (USDA Pesticide Data Program)4848
Mixture of 5 Organophosphorus Mixture of 5 Organophosphorus PesticidesPesticides
Which OPs?Which OPs?• Relevance based on potential human exposures, Relevance based on potential human exposures,
usage patterns, food residuesusage patterns, food residues• Overlapping geographical usageOverlapping geographical usage• Chlorpyrifos, diazinon, malathion, acephate, Chlorpyrifos, diazinon, malathion, acephate,
dimethoatedimethoate These were among top 10 OPs in use in USThese were among top 10 OPs in use in US
What ratios?What ratios?• Proportions based on predicted dietary Proportions based on predicted dietary
exposures estimated by Dietary Exposure exposures estimated by Dietary Exposure Estimate Model (DEEMEstimate Model (DEEMTMTM))
4949
Environmentally Relevant Environmentally Relevant Proportions (Ratios)Proportions (Ratios)
Dose ratios Dose ratios 0.031 (chloryprifos) 0.002 (diazinon)0.031 (chloryprifos) 0.002 (diazinon) 0.825 (malathion) 0.04 (acephate)0.825 (malathion) 0.04 (acephate) 0.102 (dimethoate)0.102 (dimethoate)
Chlorpyrifos DEEM values
Acute Population Adjusted Dose – based on RfD5050
http://water.usgs.gov/nawqa/pnsp/usage/maps/
2002 Pesticide 2002 Pesticide Usage MapsUsage Maps
5151
Mixture of Organophosphorus Mixture of Organophosphorus PesticidesPesticides
Would we expect dose-additivity?Would we expect dose-additivity?• Default assumption, but…Default assumption, but…• Old literature (50’s, 60’s) shows non-additive Old literature (50’s, 60’s) shows non-additive
interactions in about half of binary OP interactions in about half of binary OP combinationscombinations
• Well-known OP interactions with malathion Well-known OP interactions with malathion due to inhibition of detoxifying enzymesdue to inhibition of detoxifying enzymes
• Several potential kinetic sites for interactionsSeveral potential kinetic sites for interactions• Recent data suggest non-additivity dependent Recent data suggest non-additivity dependent
on sequence of administrationon sequence of administration5252
ApproachApproach Use multiple endpoints to fully characterize Use multiple endpoints to fully characterize
interactionsinteractions• BrainBrain and blood ChE inhibition, and blood ChE inhibition, motor activitymotor activity, ,
gait score, tail pinch responsegait score, tail pinch response Evaluate influence of malathion in the Evaluate influence of malathion in the
mixture by removing it (reduced ray)mixture by removing it (reduced ray) Acute oral dosing, tested at 4 hr (time of Acute oral dosing, tested at 4 hr (time of
peak effect), male peak effect), male
Long-Evans rats, Long-Evans rats,
n=10/dosen=10/dose
Adult, PND17Adult, PND175353
Brain CholinesteraseBrain Cholinesterase
Malathion had no effect up to 500 mg/kgMalathion had no effect up to 500 mg/kg
ACEPHATE
Dose (mg/kg)
0 3 10 30 60 120P
erce
nt C
ontr
ol C
hE A
ctiv
ity (
X±S
EM
)
0
20
40
60
80
100
| | | | | |
CHLORPYRIFOS
Dose (mg/kg)
012 5 10 20 25 30 50Per
cent
Con
trol
ChE
Act
ivity
(X
±SE
M)
0
20
40
60
80
100
||| |||
DIAZINON
Dose (mg/kg)
05 25 50 75 125
150
250P
erce
nt C
ontr
ol C
hE A
ctiv
ity (
X±S
EM
)
0
20
40
60
80
100
||||||
DIMETHOATE
Dose (mg/kg)
0 5 10 25 50 75Per
cent
Con
trol
ChE
Act
ivity
(X
±SE
M)
0
20
40
60
80
100
| | | | | |
Moser et al., Tox. Sci. 86:101-115, 2005
5454
Brain CholinesteraseBrain CholinesteraseMixture DataMixture Data
BRAIN CHOLINESTERASE: FULL RAY
Total Mixture Dose (mg/kg)
010 55 100
200
300
450
Per
cent
Con
trol
ChE
Act
ivity
(X
±SE
M)
0
20
40
60
80
100Predicted additivityExperimental mixture
The full and reduced rays showed synergism, and were not different from each otherThe full and reduced rays showed synergism, and were not different from each other1.5 to 2.1-fold shift in ED20 or ED50; 6 to 19-fold shift in thresholds1.5 to 2.1-fold shift in ED20 or ED50; 6 to 19-fold shift in thresholds
BRAIN CHOLINESTERASE: REDUCED RAY
Total Mixture Dose (mg/kg)
0.01.
89.
617
.535
.052
.578
.8P
erce
nt C
ontr
ol C
hE A
ctiv
ity (
X±S
EM
)0
20
40
60
80
100
Predicted additivityExperimental mixture
Moser et al., Tox. Sci. 86:101-115, 2005
5555
Motor ActivityMotor ActivityACEPHATE
Dose (mg/kg)
0 3 10 30 60 120
Act
ivity
Cou
nts
(X±S
EM
)
0
40
80
120
160
200
240
| | | | | |
CHLORPYRIFOS
Dose (mg/kg)
012 5 10 20 25 30 50
Act
ivity
Cou
nts
(X±S
EM
)
0
40
80
120
160
200
240
| | | | | |
DIAZINON
Dose (mg/kg)
05 25 50 75 125
150
250
Act
ivity
Cou
nts
(X±S
EM
)
0
40
80
120
160
200
240
||||||
DIMETHOATE
Dose (mg/kg)
0 5 10 25 50 75
Act
ivity
Cou
nts
(X±S
EM
)
0
40
80
120
160
200
240
| | | | | |
Malathion had no effect up to 500 mg/kgMoser et al., Tox. Sci. 86:101-115, 2005 5656
Motor ActivityMotor ActivityMixture DataMixture Data
MOTOR ACTIVITY: FULL RAY
Total Mixture Dose (mg/kg)
010 55 100
200
300
450
Act
ivity
Cou
nts
(X±S
EM
)
0
40
80
120
160
200
240
Predicted additivityExperimental mixture
MOTOR ACTIVITY: REDUCED RAY
Total Mixture Dose (mg/kg)
0.01.
89.
817
.535
.052
.578
.8
Act
ivity
Cou
nts
(X±S
EM
)
0
40
80
120
160
200
240
Predicted additivityExperimental mixture
The full and reduced rays showed synergism, and were not different from each otherThe full and reduced rays showed synergism, and were not different from each other1.2 to 1.7-fold shift in ED20 or ED50; >3-fold shift in thresholds1.2 to 1.7-fold shift in ED20 or ED50; >3-fold shift in thresholds
Moser et al., Tox. Sci. 86:101-115, 2005
5757
Adult Mixture SummaryAdult Mixture SummaryEndpointEndpoint Full Ray*Full Ray* Reduced Reduced
Ray*Ray*Full vs. Full vs. Reduced**Reduced**
ED20/50 ED20/50 DifferenceDifference
Brain ChEBrain ChE YesYes YesYes NoNo 1.5-2.1X1.5-2.1X
6-19X threshold 6-19X threshold shiftshift
Blood ChEBlood ChE YesYes YesYes YesYes 1.2-1.9X1.2-1.9X
Motor ActivityMotor Activity YesYes YesYes NoNo 1.2-1.7X1.2-1.7X
>3X threshold >3X threshold shiftshift
Gait ScoreGait Score YesYes NoNo YesYes 1.6-1.7X1.6-1.7X
Tail Pinch Tail Pinch ResponseResponse
NoNo NoNo -- -- ----
* significantly different from additivity, greater-than-additive (synergism)* significantly different from additivity, greater-than-additive (synergism)** significant difference between full and reduced rays** significant difference between full and reduced rays
Moser et al., Tox. Sci. 86:101-115, 2005 5858
PND17 Brain CholinesterasePND17 Brain CholinesteraseAcephate
Dose (mg/kg)
0 3 10 30 60 120
% C
on
tro
l Ch
E A
ctiv
ity (
X±S
EM
)
0
20
40
60
80
100 Chlorpyrifos
Dose (mg/kg)
0 1 2 5 10 20%
Co
ntr
ol C
hE
Act
ivity
(X
±SE
M)
0
20
40
60
80
100 Diazinon
Dose (mg/kg)
0 2 6 15 30 6025
% C
on
tro
l Ch
E A
ctiv
ity (
X±S
EM
)
0
20
40
60
80
100
Dimethoate
Dose (mg/kg)
0 5 10 25 50 75
% C
on
tro
l Ch
E A
ctiv
ity (
X±S
EM
)
0
20
40
60
80
100 Malathion
Dose (mg/kg)
010 50 100 250 500125
% C
on
tro
l Ch
E A
ctiv
ity (
X±S
EM
)
0
20
40
60
80
100
Moser et al., Tox. Sci. 92:235-245, 2006 5959
Brain CholinesteraseBrain CholinesterasePND17 Mixture DataPND17 Mixture Data
BRAIN CHOLINESTERASE: FULL RAY
Total Mixture Dose (mg/kg)
0 10 20 40 60 100 165
% C
on
tro
l Ch
E A
ctiv
ity (
X±S
EM
)
0
20
40
60
80
100
Experimental mixturePredicted additivity
BRAIN CHOLINESTERASE: REDUCED RAY
Total Mixture Dose (mg/kg)
0.0 1.8 3.5 7.0 10.5 17.5 28.9
% C
on
tro
l Ch
E A
ctiv
ity (
X±S
EM
)
0
20
40
60
80
100
Experimental mixturePredicted additivity
The full and reduced rays showed synergism, and they were different from each otherThe full and reduced rays showed synergism, and they were different from each other1.3 to 2-fold difference in ED20 or ED501.3 to 2-fold difference in ED20 or ED50
Moser et al., Tox. Sci. 92:235-245, 2006 6060
PND17 Mixture SummaryPND17 Mixture Summary
EndpointEndpoint Full Ray*Full Ray* Reduced Ray*Reduced Ray* Full vs. Full vs. Reduced**Reduced**
ED20/50 ED20/50 DifferenceDifference
Brain ChEBrain ChE YesYes YesYes YesYes 1.5-2.1X1.5-2.1X
Blood ChEBlood ChE YesYes YesYes YesYes 1.7-2.3X1.7-2.3X
Motor ActivityMotor Activity YesYes NoNo YesYes 1.3-2.6X1.3-2.6X
Gait ScoreGait Score YesYes YesYes YesYes 2.2-3X2.2-3X
Tail Pinch Tail Pinch ResponseResponse
YesYes NoNo YesYes 3.5X3.5X
* significantly different from additivity, greater-than-additive (synergism)* significantly different from additivity, greater-than-additive (synergism)** significant difference between full and reduced rays** significant difference between full and reduced rays
Moser et al., Tox. Sci. 92:235-245, 2006 6161
Summary of OP MixturesSummary of OP Mixtures
Greater-than-additive interactions (Greater-than-additive interactions (i.e.i.e., , synergism) detected with both mixtures at synergism) detected with both mixtures at both agesboth ages
• Interactions depended on endpointInteractions depended on endpoint• Significant differences at low end of dose-Significant differences at low end of dose-
response (threshold)response (threshold)• Comparing the reduced to the full ray indicated Comparing the reduced to the full ray indicated
an influence of malathion on most endpointsan influence of malathion on most endpoints Degree of influence depended on endpointDegree of influence depended on endpoint
6262
Mixture of 7Mixture of 7 N N-Methyl Carbamate -Methyl Carbamate PesticidesPesticides
Why carbamates?Why carbamates?• Broad agricultural and residential usesBroad agricultural and residential uses• Exposures from food residues, drinking water, Exposures from food residues, drinking water,
and home – dermal, inhalation, oraland home – dermal, inhalation, oral• Common mode of action (inhibition of Common mode of action (inhibition of
acetylcholinesterase)acetylcholinesterase) Why 7 carbamates?Why 7 carbamates?
• Of 10 carbamates being regulated, these 7 Of 10 carbamates being regulated, these 7 had high contribution to cumulative risk had high contribution to cumulative risk assessmentassessment
6363
Mixture of 7 Mixture of 7 NN-Methyl Carbamate -Methyl Carbamate PesticidesPesticides
Which carbamates?Which carbamates?• High usage and contribution to cumulative risk High usage and contribution to cumulative risk
assessmentassessment• Carbaryl, carbofuran, formetanate HCl, Carbaryl, carbofuran, formetanate HCl,
methiocarb, methomyl, oxamyl, propoxurmethiocarb, methomyl, oxamyl, propoxur What ratios?What ratios?
• Proportions based on relative potencies using Proportions based on relative potencies using BMD10 (10% brain ChE inhibition)BMD10 (10% brain ChE inhibition)
• Proportions based on California database of Proportions based on California database of tonnage sold in 2005tonnage sold in 2005 Surrogate for total (aggregate) exposuresSurrogate for total (aggregate) exposures
6464
Relative PotenciesRelative Potencies
Ald
icar
bFo
rmet
anat
eC
arbo
fura
nO
xam
ylTh
iodi
carb
Met
hom
ylM
ethi
ocar
bC
arba
ryl
Pro
poxu
rP
irim
icar
b
RP
Fs
(95%
co
nfi
den
ce li
mit
s)
0.01
0.1
1
10
N-Methyl Carbamate Cumulative Risk Assessment, 2007
6565
California DatabaseCalifornia Database
http://www.cdpr.ca.gov/docs/pur/purmain.htm 6666
Carbamate ProportionsCarbamate Proportions
Relative Potency Relative Potency Factor MixtureFactor Mixture
CarbarylCarbaryl .42.42 PropoxurPropoxur .29.29 MethiocarbMethiocarb .20.20 MethomylMethomyl .05.05 FormetanateFormetanate .02.02 Oxamyl Oxamyl .01.01 CarbofuranCarbofuran .01.01
CA Environmental CA Environmental MixtureMixture
MethomylMethomyl .41.41 CarbarylCarbaryl .39.39 Oxamyl Oxamyl .13.13 CarbofuranCarbofuran .04.04 Formetanate Formetanate .03.03 MethiocarbMethiocarb .003.003 PropoxurPropoxur .002.002
6767
ApproachApproach Use several endpoints to characterize Use several endpoints to characterize
interactionsinteractions• BrainBrain and RBC ChE inhibition, and RBC ChE inhibition, motor activitymotor activity
Evaluate influence of mixing ratiosEvaluate influence of mixing ratios Acute oral dosing, tested at 40 min (time of Acute oral dosing, tested at 40 min (time of
peak effect), male Long-Evans rats,peak effect), male Long-Evans rats,
n=10/dosen=10/dose
Adult, (PND17)Adult, (PND17)
6868
Brain CholinesteraseBrain Cholinesterase
PROPOXUR
Dose (mg/kg)
0.00.
31.
03.
010
.020
.0
Per
cent
Con
trol
ChE
Act
ivity
(X
±SE
M)
0
20
40
60
80
100
CARBARYL
Dose (mg/kg)
0.0
3.0
7.5
15.0
30.0
50.0
Per
cent
Con
trol
ChE
Act
ivity
(X
±SE
M)
0
20
40
60
80
100METHIOCARB
Dose (mg/kg)
0.00.
52.
05.
012
.025
.0
Per
cent
Con
trol
ChE
Act
ivity
(X
±SE
M)
0
20
40
60
80
100
METHOMYL
Dose (mg/kg)
0.0
0.1
0.3
0.6
1.3
2.5
Per
cent
Con
trol
ChE
Act
ivity
(X
±SE
M)
0
20
40
60
80
100
6969
Motor ActivityMotor Activity
PROPOXUR
Dose (mg/kg)
0.00.
31.
03.
010
.020
.0
Act
ivity
Cou
nts
(X±S
EM
0
0
50
100
150
200
CARBARYL
Dose (mg/kg)
0.0
3.0
7.5
15.0
30.0
50.0
Act
ivity
Cou
nts
(X±S
EM
)
0
50
100
150
200
METHIOCARB
Dose (mg/kg)
0.00.
52.
05.
012
.025
.0
Act
ivity
Cou
nts
(X±S
EM
)
0
50
100
150
200
METHOMYL
Dose (mg/kg)
0.00
0.10
0.25
0.60
1.25
2.50
Act
ivity
Cou
nts
(X±S
EM
)
0
50
100
150
200
7070
Brain CholinesteraseBrain CholinesteraseRPF Carbamate MixtureRPF Carbamate Mixture
N-Methyl Carbamate Cumulative Risk Assessment, 2007
Confidence limits analysis suggests additivity7171
Brain CholinesteraseBrain CholinesteraseRPF Carbamate MixtureRPF Carbamate Mixture
Test of additivity not rejected (p=0.066)Test of additivity not rejected (p=0.066)
BRAIN CHOLINESTERASE: RELATIVE POTENCY MIXTURE
Dose (mg/kg)
0.00.
31.
33.
78.
615
.9
Per
cent
Con
tro
l ChE
Act
ivity
(X
±SE
M)
0
20
40
60
80
100 Predicted additivityExperimental mixture
7272
Motor ActivityMotor ActivityRPF Carbamate MixtureRPF Carbamate Mixture
Test of additivity not rejectedTest of additivity not rejected
MOTOR ACTIVITY: RELATIVE POTENCY MIXTURE
Mixture dose (mg/kg)
0.00.
31.
33.
78.
615
.9
Act
ivity
Cou
nts
(X±S
EM
)
0
50
100
150
200
250Predicted additivityExperimental mixture
7373
Brain CholinesteraseBrain CholinesteraseCA Carbamate MixtureCA Carbamate Mixture
BRAIN CHOLINESTERASE: CALIFORNIA ENVIRONMENTAL MIXTURE
Dose (mg/kg)
0.00.
10.
61.
42.
86.
0
Per
cent
Con
trol
ChE
Act
ivity
(X
±SE
M)
0
20
40
60
80
100 Predicted additivityExperimental mixture
Preliminary statistical analyses reveal non-additivity (synergy)Preliminary statistical analyses reveal non-additivity (synergy) 7474
Motor ActivityMotor ActivityCA Carbamate MixtureCA Carbamate Mixture
MOTOR ACTIVITY: CALIFORNIA ENVIRONMENTAL MIXTURE
Dose (mg/kg)
0.00.
10.
61.
42.
86.
0
Act
ivity
Co
un
ts (
X±S
EM
)
0
50
100
150
200
Predicted additivityExperimental mixture
Preliminary statistical analyses reveal non-additivity (synergy)Preliminary statistical analyses reveal non-additivity (synergy) 7575
Summary of 7-Carbamate MixturesSummary of 7-Carbamate Mixtures
Additivity (zero interaction) with mixture Additivity (zero interaction) with mixture ratios based on relative potency factorsratios based on relative potency factors
Greater-than-additive interactions (Greater-than-additive interactions (e.g.e.g., , synergy) with mixture ratios based on synergy) with mixture ratios based on amounts sold in Californiaamounts sold in California
Differences in outcome based on mixing Differences in outcome based on mixing ratios of componentsratios of components
7676
Other Studies, Similar MethodologyOther Studies, Similar Methodology 4 DBPs producing hepatotoxicity (mice)4 DBPs producing hepatotoxicity (mice)11
• Ratio based on average seasonal proportion of 35 water treatment Ratio based on average seasonal proportion of 35 water treatment facilitiesfacilities
• No departure from additivityNo departure from additivity 18 PHAHs decreasing serum T4 (rats)18 PHAHs decreasing serum T4 (rats)22
• Ratio based on average concentrations found in human breast milk Ratio based on average concentrations found in human breast milk and food sourcesand food sources
• Concentrations in range of human body burdensConcentrations in range of human body burdens• Synergy emerged as dose increasedSynergy emerged as dose increased
6 synthetic estrogens producing estrogenic actions (6 synthetic estrogens producing estrogenic actions (in vitroin vitro ER-ER-αα reporter gene and reporter gene and in vivoin vivo uterotrophic assays) uterotrophic assays)33
• With and without phytoestrogensWith and without phytoestrogens• Ratios based on relative potenciesRatios based on relative potencies• Interactions depended on concentrations and components of mixture Interactions depended on concentrations and components of mixture
1 Gennings et al., J Agr Biol Environ Stat, 2:198-211, 19972 Crofton et al., Env Health Perspec 113:1549-1554, 20053 Charles et al., Tox Appl Pharm 218:280-288, 2007 7777
Considerations for Environmentally Considerations for Environmentally Relevant Mixture ResearchRelevant Mixture Research
Appropriate experimental design and statistical Appropriate experimental design and statistical analysesanalyses• Specify dose- or response-additivity hypotheses, design Specify dose- or response-additivity hypotheses, design
and analyze experiment appropriatelyand analyze experiment appropriately
Strategically select specific exposure scenariosStrategically select specific exposure scenarios• Potentially worrisome chemicals, Potentially worrisome chemicals, e.g.e.g., high-use, , high-use,
environmentally persistent environmentally persistent • Rational mixing ratios, Rational mixing ratios, e.g.e.g., reflecting potential or known , reflecting potential or known
human exposurehuman exposure• Site-specific combinations and ratiosSite-specific combinations and ratios
Use of fixed-ratio ray designs can provide efficient Use of fixed-ratio ray designs can provide efficient and focused research of mixturesand focused research of mixtures 7878
Cadillac of Mixture AssessmentsCadillac of Mixture Assessments
Quantitative component-based mixtures risk Quantitative component-based mixtures risk assessment that includes:assessment that includes:• Exposure scenarios reflective of human exposuresExposure scenarios reflective of human exposures• Environmental relevance in composition of mixtureEnvironmental relevance in composition of mixture• Defined dose-response data addressing common Defined dose-response data addressing common
toxic pathwaytoxic pathway• Experimental data on actual mixturesExperimental data on actual mixtures• Evaluation of additional influences, Evaluation of additional influences, e.g.e.g., age, gender, , age, gender,
etc.etc.• Biologically based modeling (Biologically based modeling (e.g.e.g., PBPK) to describe , PBPK) to describe
interactionsinteractions
7979
Volkswagen of Mixture AssessmentsVolkswagen of Mixture Assessments
Less data-intensive approaches for Less data-intensive approaches for (partially) defined mixtures(partially) defined mixtures
Given exposure data and some measure Given exposure data and some measure of acceptable level (of acceptable level (e.g.e.g., RfD/C), RfD/C)• Hazard quotient (HQ) or index (HI)Hazard quotient (HQ) or index (HI)• Target organ toxicity dose (TTD)Target organ toxicity dose (TTD)• Toxicity equivalence factor (TEF)Toxicity equivalence factor (TEF)
Given only compositionGiven only composition• Analysis of sufficiently similar mixturesAnalysis of sufficiently similar mixtures
8080
ChallengesChallenges Determination of key events of componentsDetermination of key events of components
• Target organ and toxicityTarget organ and toxicity No complete dose-response data of componentsNo complete dose-response data of components
• Some statistical methods address thisSome statistical methods address this Not all components are identifiedNot all components are identified
• Evaluate data for similar mixtureEvaluate data for similar mixture• Evaluate known partial mixture with and without Evaluate known partial mixture with and without
undefined fractionundefined fraction Exposure and responseExposure and response
• ChronicityChronicity• TimingTiming• Aggregated routesAggregated routes• Dose-dependent transitionsDose-dependent transitions
These are research areas being proposed in NHEERL
8181
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