Presentation to the FIFRA SAP, April 19, 2016

Chlorpyrifos: Analysis of Biomonitoring Data

Internal, deliberative, not for distribution

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Dana Vogel, Director 

Health Effects Division 

Office of Pesticide Programs 

Environmental Protection Agency

vogel.dana@epa.gov

703-305-0874

Presentation to the FIFRA SAP, April 19, 2016

Chlorpyrifos: Analysis of Biomonitoring Data

Background and Overview 

Internal, deliberative, not for distribution

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• Organophosphate pesticide first registered in 1965 

• In June 2000:

• The technical registrants entered into an agreement with 

the Agency to eliminate and phase out nearly all uses that 

result in residential exposures 

• Some mitigation on food (apples & tomatoes)

• Some mitigation for workers (PPE & engineering controls for 

some uses)

• Registration Eligibility Decision (RED) in 2006

• September 2007 petition from NRDC & PANNA to revoke all 

tolerances & cancel registrations

Regulatory Context: Chlorpyrifos
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• FIFRA SAP 2008: New science on infants, children, & pregnant 

women from experimental laboratory toxicology & 

epidemiology studies since analysis for RED

• Evaluated both AChE & non-cholinergic modes of action

• FIFRA SAP 2009: Approach for risk assessment of semi-volatile 

pesticides 

• FIFRA SAP 2010: Development of the draft “Framework for 

Incorporating Human Epidemiologic & Incident Data in Health 

Risk Assessment” 

External Peer Reviews
5

• FIFRA SAP 2011: Chlorpyrifos Physiologically-Based 

Pharmacokinetic/Pharmacodynamic (PBPK/PD) Modeling 

linked to the Cumulative and Aggregate Risk Evaluation System 

(CARES)

• Federal Peer Review 2012: MRI and neurobehavioral testing in 

children to further clarify results obtained by examination of 

the epidemiological cohorts

• FIFRA SAP 2012: Further, more in-depth analysis on science on 

infants, children, & pregnant women from experimental 

laboratory toxicology & epidemiology studies. 

External Peer Reviews
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• December, 2014 Revised Risk Assessment

• RBC AChE inhibition as the critical effect for deriving the 

POD

• PBPK/PD model used to derive human specific PODs for 

various age groups, routes, and durations

• PBPK/PD model used to derive intra-species factor for some 

lifestages (not women of childbearing age)

• Retained FQPA 10X Safety Factor: uncertainty in the human 

dose-response relationship for neurodevelopmental effects 

Regulatory Context: Chlorpyrifos
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• EPA has been working to complete its human health risk assessment in 

registration review on a parallel track with its response to the 2007 PANNA 

& NRDC petition to revoke all chlorpyrifos tolerances and cancel all 

chlorpyrifos registrations.

• The timing of the petition response has been the subject of litigation. In 

2015, the 9th Circuit ordered EPA to respond to the petition by October 

31, 2015 and to issue any final action by December 30, 2016.

• Accordingly, consistent with EPA’s revised human health risk assessment 

and in accordance with the 9th Circuit’s order, in late 2015 EPA proposed to 

revoke all chlorpyrifos tolerances. EPA is now working to complete any 

final tolerance rule by the end of the year as ordered by the court.

Regulatory Context: Chlorpyrifos
• From the 2012 SAP Report, pp. 49-50 (emphasis added):

• The Panel acknowledged the limitations in the three 

longitudinal children’s cohort studies of estimating 

chlorpyrifos exposures (i.e., the Columbia study, the Mt. 

Sinai study, and the CHAMACOS study), based on the 

exposure measures collected, and was in general agreement 

that the data from these studies alone were not sufficient to 

derive a point of departure (POD) for purposes of 

quantitative risk assessment. 

• New information has emerged from the analysis that 

reduces some of the uncertainty in the biomonitoring data.

2012 FIFRA SAP Report

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• Per the 2012 SAP report, the agency was urged to find ways to use 

the epidemiology studies, in particular the CCCEH study, to inform 

the chlorpyrifos assessment (emphasis added):

• “...if one assumes that cord blood measurements reflect exposure levels 

during the critical prenatal period for induction of neurodevelopmental 

effects, then in theory, these would be the ideal data from which to 

derive the POD for chlorpyrifos in humans”

• In doing so, the agency was encouraged to make use of the PBPK 

model to further characterize CCCEH dose estimates

• “...a [PBPK] model could be used to further characterize the dose 

estimates in the epidemiology studies, for additional dose response 

analyses. Such a PBPK model will become even more important in the 

event that the agency might, at some point in the future, decide to 

move from using AChE inhibition to another outcome.”

2012 FIFRA SAP Report 
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• Used PBPK model & EPA-OPP’s exposure assessment 

approaches to characterize exposure profile to women in the 

CCCEH cohort.

• Exposure approaches have been previously reviewed by SAP 

and have used extensively in risk assessments.

• Results provide support for using cord blood data for POD.

• Case studies developed to illustrate how the PBPK model 

would be used to predict internal doses from existing 

exposures to chlorpyrifos. 

Risk Assessment of Pesticides
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• Background & Overview (Dana Vogel)

• Overview of EPA Epidemiological Literature Review 

(Elizabeth Holman)

• Interpreting Biomarker Data in the CCCEH Cohort Using a 

PBPK Model (Cecilia Tan)

• Evaluation of CCCEH Blood Data and Predicted Exposures 

(Wade Britton, Rochelle Bohaty, Danette Drew)

• Point of Departure, Intra-Species Extrapolation, the FQPA 

10X Safety Factor, and Case Studies (Anna Lowit)

Outline of Presentations
QUESTIONS?

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Elizabeth Holman, DrPH

Health Effects Division 

Office of Pesticide Programs 

Environmental Protection Agency

holman.elizabeth@epa.gov

703-603-8761

Presentation to the FIFRA SAP, April 19, 2016

Chlorpyrifos: Overview of EPA 

Epidemiological Literature Review

Internal, deliberative, not for distribution

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• Background and History of EPA review

• Overview of U.S. Prospective Cohort Studies

• Focus of Assessment

• Synthesis of Epidemiological Literature

• Columbia University Prospective Cohort Study

• Additional Background

• Highlights of Findings

Outline

Internal, deliberative, not for distribution

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• 2008 FIFRA SAP meeting - involved a preliminary review of the literature for chlorpyrifos, 

with a particular focus on women and children 

• “Chlorpyrifos likely played a role” but not “sole contributor” due to other co- exposures

• Epidemiology studies "should not be considered ...basis for characterization of PoD” 

• 2010 - OPP developed a draft “Framework for Incorporating Human Epidemiologic & Incident 

Data in Health Risk Assessment” 

• Provides foundation for evaluating multiple lines of scientific evidence, including 

epidemiology, in the context of the understanding adverse outcome pathway (or mode of 

action) 

• Includes two key components: problem formulation and use of the modes of 

action/adverse outcome pathways (MOA/AOP) frameworks

• Reviewed favorably by the SAP in 2010

• Consistent with updates to the World Health Organization/International Programme on 

Chemical Safety mode of action/human relevance framework highlighting importance of 

problem formulation and need to integrate information at different levels of biological 

organization (Meek et al., 2014) 

Background and History

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• 2011 - agency released “Chlorpyrifos: Preliminary Human 

Health Risk Assessment for Registration Review”:

• Focusing on the AChE inhibiting potential of chlorpyrifos

• Included assessment of exposures from dietary (food, 

water), occupational and residential pathways

• Focus consistent with recommendation from 2008 SAP - AChE data provide most appropriate endpoint and dose- response data for deriving points of departure (PODs) for 

purposes of risk assessment

• Also in 2011, the physiologically based pharmacokinetic- pharmacodynamic model (PBPK-PD) reviewed by FIFRA SAP

Background and History

Internal, deliberative, not for distribution
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• 2012 EPA updated and expanded targeted review of epidemiological data: 

• New etiologic papers (intelligence quotient)

• New methodological papers (measurement error)

• Socioeconomic status (SES)-Confounding Bias 

• Exposure characterization and validation

• Supplemental analyses by authors

• Draft Framework Analysis approach 

• 2012 SAP concurred with EPA ‘played a role’

• December 2014 – HHRA, FQPA 10x

• 2015 – EPA conducted updated epidemiological literature review

• Expanded beyond chlorpyrifos to other OPs 

• Did not change 2014 conclusions 

Background and History

Internal, deliberative, not for distribution
Internal, deliberative, not for distribution

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• Expanded beyond chlorpyrifos to other 

OPs

• New results from 3 U.S. cohorts (Mt. 

Sinai, Columbia, CHAMACOS)

• Non-U.S. cohorts (1 Mexico)

• Other study designs: 

• Case-control (1 U.S.)

• Cross-sectional (1 U.S., 2 China, 1 Canada)

• EPA Document (09/15/2015): 

“Literature Review on 

Neurodevelopment Effects & FQPA 

Safety Factor Determination for the 

Organophosphate Pesticides”

2015 Epidemiological Data Review

• Assessment focused on chlorpyrifos

• 3 U.S. Cohorts (Mt. Sinai, Columbia, 

CHAMACOS)

• 2012 SAP review – concurred with 

EPA conclusion that 3 U.S. cohorts is 

most robust available evidence

• EPA Document (12/29/2014): 

“Chlorpyrifos: Revised Human Health 

Risk Assessment for Registration 

Review”

2012/2014 Epidemiological Data Review

Background and History
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• Based on 2012 SAP review and available studies assessed 

in 2015, EPA continues to conclude that data from the 3 

U.S. cohorts are the most robust, relevant available 

epidemiological data

Background and History

Internal, deliberative, not for distribution
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• Three prospective birth cohorts to examine environmental 

exposures and adverse health outcomes 

• Columbia cohort (1998-2004) – denoted as CCCEH 

• NYC, multi-ethnic, Cord Blood Chlorpyrifos

• Mt. Sinai cohort (1998-2001)

• NYC, multi-ethnic, TCPy (fetal growth only), DAPs, PON1

• U of CA Berkeley Cohort (CHAMACOS) (1999-2002)

• CA-Ag region, Mexican-American, ag worker, TCPy/DAPs, PON1

• Children’s Environmental Health and Disease Prevention 

Research Centers (funded by EPA and NIEHS)

Overview of U.S. Cohorts

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Internal, deliberative, not for distribution

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• Multi-ethnic populations: 

African-American, Dominican, 

Hispanic, Mexican-American, 

White

• Younger mothers, ~25 years 

• Low prevalence risky health 

behaviors, co-morbidities 

(criteria)

Cohort Characteristics

• Sought prenatal care (<20-26 

weeks)

• Residency requirement

• Deliver participating 

hospitals 

• No diabetes, hypertension, 

HIV, history birth defects

• No smoking, illegal drug use

General Eligibility

3 Cohorts - Population Characteristics
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• “The Panel considers the Agency’s epidemiology review to be very 

clearly written, accurate, and to generally provide a very thorough 

review of the epidemiology literature.”

• “Overall, the Panel concurs with the 2008 SAP and the Agency in 

concluding that chlorpyrifos likely plays a role in impacting the 

neurodevelopmental outcomes examined in the three cohort 

studies.” 

• “The strengths of the three studies support the Panel’s conclusion. 

There are nine strengths identified by the Panel which are discussed 

in the detailed response section of this report.”

2012 SAP Review of Epidemiological 

Literature

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“The conclusion is enhanced by the strengths of the three 

studies reviewed, specifically:

• The longitudinal designs which permitted clear indications of 

the temporal relation of chlorpyrifos exposure to adverse 

neurodevelopmental outcomes;

• The inclusion of biomarkers of exposure as well as self 

reported exposure;

• The relative consistency of findings in different populations 

but using similar standardized exposure and outcome 

measures;

• The strength of the associations found;

2012 SAP Review – Strengths of 3 U.S. cohorts

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• The use of objective measures of exposure and standardized, 

validated measures of outcomes;

• The control of multiple confounding variables including other 

environmental exposures and other pesticides;

• The suggestion of a dose-response effect;

• Minimization in bias in assessing outcomes and exposures and 

the likelihood that biases and misclassification of exposures 

and outcomes resulted in a bias to the null, i.e., under- estimation of effect; and

• Attempts to investigate genetic and phenotypic effect 

modification and dose-response effects.”

2012 SAP Review – Strengths of 3 U.S. cohorts

(Cont.)

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• Multiple outcomes assessed in these studies including

• Fetal Growth

• Neonatal Neurological Development

• Infant Motor and Mental Development 

• Attention Disorders

• Autism Spectrum Disorders

• Intelligence Measures

• Consistent with previous 2008 and 2012 SAP evaluations for 

chlorpyrifos, EPA is focusing this analysis on 

neurodevelopmental outcomes

Focus of Assessment

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• Fetal Growth not the focus of this assessment: 

• Inconsistent evidence of organophosphate (OP) exposure and 

association with adverse birth outcomes/fetal growth outcomes

• CCCEH observed evidence of an inverse association, i.e.:

• Increasing cord blood chlorpyrifos associated with decreased measures 

of birth weight and length (Whyatt et al., 2004)

• Mt. Sinai and CHAMACOS cohorts: 

• Reported either no association, or evidence of a positive relationship 

(Berkowitz et al., 2004; Eskenazi et al., 2004), respectively

• Given lack of consistency among cohorts for fetal growth metrics, 

proposed link between fetal growth and OP exposure is tenuous

• EPA continues to monitor literature for chlorpyrifos exposure and 

fetal growth outcomes 

Focus of Assessment

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Internal, deliberative, not for distribution

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Exposure Measurement, Confounding Adjustment, 

Statistical Analyses, Bias Considerations

Methods and Analysis
Exposure Assessment

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• Cohorts differing exposure profiles, pathways, routes

• Multiple biomarkers reflect different windows of exposure:

• Self-report questionnaire (each cohort)

• TCPy in urine (each cohort)

• TCPy meconium (validation study) (Columbia)

• Chlorpyrifos in cord blood (Columbia) at birth

• Chlorpyrifos in air (validation study) (Columbia)

• AChE and BuChE in cord blood (CHAMACOS)

• DAPs in urine (CDC methods) (Mt. Sinai, CHAMACOS)

• PON1 genotype and phenotype (Mt. Sinai, CHAMACOS)
Methods to Control Confounding

• Statistical Adjustment (individual-level)

• Race, education, income, housing quality

• Tested for statistical interaction

• Multilevel Model (Socioeconomic status, Columbia only)

• Account group-level and individual-level variability

• Restriction (design)

• Through recruitment techniques, similar population 

characteristics 29
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• Biomonitoring data were collected from individuals within 

each cohort 

• The unexposed children in the epidemiology studies are those 

whose biomonitoring data are low and often below the limit 

of detection (ie, so low as to not be measurable) 

• The unexposed children are derived from the same 

populations and location in the same living and economic 

conditions as the exposed or highly exposed children 

• In this way, important issues such as socioeconomic status are 

similar across the entire group of exposure and unexposed 

Restriction (Design) Methods to Control 

Confounding

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Internal, deliberative, not for distribution

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• Other exposures including:

• Polyaromatic hydrocarbons (PAH)

• Environmental tobacco smoke 

(ETS) 

• DDT/DDE 

• Polychlorinated biphenyls (PCBs) 

• Organochlorines/ 

Hexachlorobenzene 

• Lead 

• Demographic

• Socio-economic status

• Health-related

• HOME/child stimulation

• Postnatal environment

• Other pesticides including:

• Diazinon

• Propoxur

• Pyrethroid pesticides

Confounding Variables
Statistical Analysis

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• Statistical modeling decisions appropriate to the research 

question, characteristics of data etc.

• Missing data, <LOD

• (imputed, sensitivity analyses)

• Sample size adequate main effects

• Likely under-powered for effect modification

• Sensitivity analyses to evaluate alternative modeling 

options

• Cessation of exposure, loss to follow up
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• Errors in outcome and 

exposure measures occurred

• Design reduces differential bias

• Validation study 

• Confounding

• Several control methods

Information Bias

• Retention rate moderately 

high for birth cohorts 

(60-80% at 7 years)

• Included and excluded 

comparable

• Measured variables 

• Missing Data imputed

Selection Bias

BIAS (Threats to Validity)
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CCCEH Mt. Sinai CHAMACOS

Neonatal 

Neurodevelopment

(2-5 days old) 

No Yes Yes

Infant Neurodevelopment 

(6-36 months)

Yes Yes Yes

Attention Problems

(3-5 years old)

Yes No Yes

Autism Spectrum Disorders Yes Yes Yes

Intelligence Measures

(6-9 years old)

Yes Yes Yes

Outcomes Measured Across 3 U.S. Cohorts

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Neonatal Neurodevelopment

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• Brazelton Neonatal Behavioral Assessment Scale (BNBAS)

• Goal: provide information parents/caregivers

• 28 behavioral items-habituation, range of state, orientation

• 18 primitive reflexes

• Assessed 2-5 days post-partum

• Trained neonatologists administered

• Hospital setting

• BNBAS only measured in CHAMACOS and Mt. Sinai cohorts
Infant Neurodevelopment

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• Bayley Scales of Infant Development II

• Mental and Motor development index, 6-36 months

• Reliable and valid as screening tool, early intervention

• Measured in all 3 cohorts
Attention Problems

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• Measurement Tools

• Combination Maternal report and direct observation

• Child Behavior Checklist (maternal report)

• Direct Assessment of child (K-CPT)

• Hillside Psychometrician report

• DSM-IV ADHD symptoms

• Assessed at 3, 3.5 and 5 years in CHAMACOS and CCCEH 

cohorts 
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• Assessed in all 3 U.S. cohorts 

• The DSM-V defines ASD (autism spectrum disorder) which 

now encompasses several disorders that were different 

diagnoses in DSM-IV, including PDD (pervasive 

developmental disorder, a catch-all where the other 

categories didn’t fit) 

• Depending on when the study was conducted, the 

authors may use the PDD or ASD criteria and 

terminology 

Autism Spectrum Disorders

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Intelligence Measures 

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• Wechsler Intelligence Scale for Children IV

• Measured in all 3 U.S. cohorts 

• Verbal comprehension

• Perceptual Reasoning

• Working Memory (WM)

• Processing Speed

• Full Scale IQ (FSIQ)

• Standardized US population, norms English and Spanish speaking 

populations

• Age 6-9 years

• Cultural Bias has been noted

• Restricted sample population
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RESULTS
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• CHAMACOS and Mt. Sinai cohorts measured neurological 

effects 2-5 days after birth:

• Observed putative association with prenatal DAPs (Engel 

et al., 2007; Young et al., 2005) 

• Cross-sectional Chinese study (Zhang et al., 2014) 

measured neonatal neurodevelopment assessed 3 days 

after birth:

• Statistically significant associations with prenatal DAPs 

from prenatal OP pesticide exposure 

Neurological Effects Near Birth

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• 3 US cohorts (CCCEH, Mt. Sinai, CHAMACOS) each 

reported evidence of prenatal chlorpyrifos or DAPs 

exposure and impaired mental and psychomotor 

development:

• Albeit not consistent by age at time of testing (ranging 

from 6 month to 36 months across the three cohorts) 

• Cross-sectional Chinese study (Guodong et al. 2012) 

measured developmental quotient score for 23-25 

month old children:

• Observed no association with postnatal urinary DAPs

Mental/Psychomotor Problems

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• Attentional problems and ADHD reported suggestive or 

positive associations in 3 prospective cohorts (2 U.S., 1 

Mexican), and a U.S. cross-sectional study

• Rauh et al., 2006; Eskenazi et al., 2007; Marks et al., 2010; 

Fortenberry et al., 2014; and Bouchard et al, 2010 

• 3 cohorts – prenatal; Cross-sectional – postnatal

• Canadian cross-sectional study (Oulhote and Bouchard 

2013) did not observe association between postnatal DAPs 

and parentally reported attentional problems

• Outcome assessment may not have been sensitive enough 

Attentional Problems

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• 3 US cohorts and a U.S. case-control study reported suggestive or 

positive associations between OP exposure and autism spectrum 

disorders: 

• Rauh et al., 2006; Shelton et al., 2014; Eskenazi et al., 2007; 

Furlong et al., 2014

• Studies vary in the magnitude of the overall strength of association, 

but consistently observe positive association between OP exposure 

and ASD.

• For intelligence measures at age 7 years, CCCEH, Mt. Sinai, 

CHAMACOS reported an inverse relation between the respective 

prenatal measures of chlorpyrifos or DAPs

• Rauh et al., 2011; Engel et al., 2011; Bouchard et al., 2011.

Autism Spectrum Disorders (ASD) and Intelligence 

Measures

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• Epidemiology, pre-natal: 

• Numerous epidemiological investigations observed a link between pre- natal exposure to chlorpyrifos or OPs (measured as chlorpyrifos, TCPy, or 

DAPs) and adverse effects on neurodevelopment through age 7 years, 

with additional more limited evidence up through approximately age 11

• Results across numerous studies show relatively consistent elevated 

risks associated with prenatal OP exposure:

• Autism spectrum disorders, intelligence decrements, attentional 

problems and ADHD, neurological effects near birth, and 

mental/psychomotor delays (6 months-11 years) 

• Results not always consistent with magnitude of association and ages 

where associations observed across studies 

Prenatal vs. Postnatal Exposure

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• Epidemiology, post-natal: 

• Smaller number of studies have assessed postnatal exposure to OPs (measured as DAPs) 

• Postnatal exposure assessed in the CHAMACOS cohort (Eskenazi et al., 2007; Young et al., 2005; 

Bouchard et al., 2011) and 3 cross-sectional studies (Guodong et al., 2012; Bouchard et al., 2010; 

Oulhote and Bouchard, 2013) 

• Postnatal exposures not assessed in the CCCEH and Mt. Sinai studies (Rauh et al., 2011; 

Engel et al., 2011)

• Given that the major source of exposure (residential use) was cancelled partway through the 

CCCEH study, this limits the ability of this study to inform the impacts of long-term postnatal 

exposure to chlorpyrifos

• Bouchard et al. (2010) looked at U.S. children age 8–15 years in the 2000–2004 National 

Health and Nutrition Examination Survey (NHANES), and observed a positive association 

between attention and behavior problems and DAPs

• No other neurodevelopmental associations were found between postnatal biomarkers

Prenatal vs. Postnatal Exposure

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Synthesis of Evidence

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• Strength of observed associations high in some cases, but 

not all

• Consistency in Brazelton (abnormal reflexes), ADHD-like 

behavior problems, Bayley (particularly MDI), and IQ 

(WM)

• Alternative explanations investigated

• Modeling, transformations, confounding variables
Synthesis of Evidence

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• Prospective studies (primary basis of conclusions)

• Temporal relation

• Issue of prenatal vs. postnatal exposure – datagap

• Differential misclassification less likely 

• Biological plausibility

• MOA unclear, several plausible hypotheses

• Influence of cessation of residential exposure on 

association

• Limited evidence (low power), not designed to assess
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Highlights of CCCEH Findings
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• Mothers and Newborn Study of North Manhattan and South Bronx 

conducted by Columbia University – referred to as CCCEH study 

• CCCEH study - measured parent chlorpyrifos in cord blood, and other 

indicators (e.g., air sampling, behavioral information) 

• Other two birth cohorts generally measured non-specific urinary 

metabolites of chlorpyrifos and other OPs (TCPy, DAPs)

• Therefore, EPA considers the CCCEH Mothers and Newborn Study 

research results as most relevant to the chlorpyrifos HHRA and is 

thus the focus of this analysis 

• The other two cohorts and additional epidemiological literature 

provide important supporting information

• 2008/2012 SAP concurred with this conclusion 

Additional CCCEH Background

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Internal, deliberative, not for distribution

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• CCCEH study time period -

spanned before and after when 

pesticide manufacturers 

voluntarily cancelled the use of 

chlorpyrifos in the home 

environment 

• Researchers able to show change in 

exposure before (high use period) 

and after (low/no use period) the 

period of removal of chlorpyrifos 

products from the residential 

marketplace 

From Whyatt et al (2004)

Additional CCCEH Background
52

CCCEH Blood Levels

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Year Percentile Cord Blood Chlorpyrifos 

(pg/g)

Maternal Blood

Chlorpyrifos (pg/g)

1998-1999

25 <LOD 2.60

50 3.78 6.70

75 8.80 9.55

90 12.1 16.0

95 15.1 19.4

2001

25 <LOD <LOD

50 <LOD <LOD

75 <LOD <LOD

90 2.45 2.44

95 2.58 2.61

Distribution of chlorpyrifos blood levels

<LOD = Less than limit of detection
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• Data from CCCEH suggest maternal blood reasonable 

surrogate for cord blood 

• Levels of chlorpyrifos in maternal blood and umbilical cord 

blood were highly correlated:

• Whyatt et al. (2003) r = 0.76, p<0.001, n=180 

• Whyatt et al. (2009) r = 0.9, p<0.001, n = 64

• These together suggest that tracking the blood concentrations 

of the mother is a reasonable surrogate for the fetus

• Will hear more about this surrogacy issue in later 

presentations 

Correlation Between Cord Blood and 

Maternal Blood

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• Rauh et al. (2006) - 254 children followed through age 3 years 

and used a dichotomized statistical approach:

• ‘Low group’: ≤6.17 pg/g of CPF in cord blood 

• ‘High group’: >6.17 pg/g of CPF in cord blood

• Statistically significant deficits of 6.5 points on the Bayley 

Psychomotor Development Index (PDI) at 3 years of age when 

comparing high to low exposure groups 

• PDI decrements persist even after adjustment for group and 

individual level socioeconomic variables (Lovasi et al., 2011)

CCCEH Findings

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• At age 3 years, when comparing high to low exposure groups

(Rauh et al, 2006) observed increased odds of: 

• Mental delay (OR=2.4; 95% CI: 1.1–5.1) 

• Psychomotor delay (OR=4.9; 95% CI: 1.8–13.7) 

• Attention disorders (OR=11.26; 95% CI: 1.79–70.99) 

• ADHD (OR=6.50; 95% CI: 1.09–38.69)

• PDD (OR=5.39; 95% CI: 1.21–24.11)

• In a follow-up study at age 11, increased odds of mild to 

moderate tremor when comparing high to low exposure 

groups (Rauh et al., 2015)

CCCEH Findings

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• Rauh et al. (2011) evaluated relationship between prenatal chlorpyrifos 

exposure and neurodevelopment:

• 265 of the CCCEH cohort participants - 7 years old 

• Complete set of data including prenatal maternal interview data, 

prenatal chlorpyrifos marker levels from maternal and/or cord blood 

samples at delivery, postnatal covariates, and neurodevelopmental 

outcome data

• Described the log of Working Memory Index (WMI) of children as linearly 

associated with concentration of chlorpyrifos (CPF) in cord blood: 

• Slope = -0.006 (95% CI = -0.01, -0.002) 

• For each standard deviation increase in exposure (4.61 pg/g), observed 

a 1.4% reduction in Full-Scale IQ and a 2.8% reduction in Working 

Memory 

CCCEH Findings

Internal, deliberative, not for distribution
Conclusions

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• Current database strengthens the 2008/2012 SAP conclusion 

that “chlorpyrifos likely plays a role” in observed adverse 

effects on child neurodevelopment

• Given study conduct, design, methods EPA believes more 

likely under- than over-estimate associations

• New Data (2012 and 2015) support and extend previous 

conclusion

• Retained FQPA 10x in HHRA (2014)
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Questions
Cecilia Tan, Research Physical Scientist

Health Effects Division 

Office of Pesticide Programs 

Environmental Protection Agency

tan.cecilia@epa.gov

919-541-2542

Presentation to the FIFRA SAP, April 19, 2016

Chlorpyrifos: Interpreting Biomarker Data 

Use of a Physiologically Based 

Pharmacokinetic (PBPK) Model

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• Linking external exposures to internal doses

• Background of PBPK modeling 

• PBPK model for chlorpyrifos

• Addressing uncertainties in interpreting biomarker data

• Likely exposure scenario

• Time between exposure and sampling

Outline

60
• Pharmacokinetics: Absorption, distribution, metabolism, and 

excretion (ADME) of chemicals in the body

• PBPK Models: A series of mathematical representations of 

biological tissues and physiological processes in the body that 

simulate the ADME of chemicals that enter the body*

• PBPK Modeling in Risk Assessment: PBPK modeling is a 

scientifically sound and robust approach to estimating the internal 

dose of a chemical at a target site and as a means to evaluate and 

describe the uncertainty in risk assessments*

Physiologically Based Pharmacokinetic 

(PBPK) Model

61 *Approaches for the Application of Physiologically Based Pharmacokinetic 

(PBPK) Models and Supporting Data in Risk Assessment (EPA, 2006)
• PBPK models incorporate physiological and biochemical 

determinants of chemical disposition 

• PBPK models can be used to simulate the time course of 

internal dosimetry (e.g., blood concentrations) under 

various exposure scenarios (e.g., daily oral exposures) in 

different animal species.

PBPK Modeling

62
Underlying assumption: equivalent biological response occur at 

equal tissue dose, not at equal external dose

• Organize available mechanistic data

• Identify data gaps and suggest new experiments

• Predict chemical disposition under new/inaccessible 

conditions (e.g., high to low dose, inter-species, intra-species, 

route-to-route extrapolations)

• Quantify uncertainty and variability in physiology and ADME

• Can be linked to PD models to predict biological endpoints

Applications of PBPK Modeling in 

Risk Assessment

63
• Linking biomarker data to exposure

• Require knowledge of exposure scenarios and 

biomarker sampling procedures

• Forward vs. reverse dosimetry

• Linking biomarker data to health effects

• Biomarkers measured in accessible media may not 

be a good surrogate for target tissue dose

Applications of PBPK Modeling in 

Biomarker Interpretation

64
65

• Forward dosimetry: Compare observed biomarker 

data with predicted values from likely exposure 

scenarios and known temporal relationship 

between exposure and sampling

• Reverse dosimetry: Estimate exposure 

concentrations from an assumed exposure scenario 

and temporal relationship between exposure and 

sampling

Forward vs. Reverse Dosimetry
66

• What is the likely 

exposure scenario (e.g., 

which time course curve 

is reasonable)?

• Time between exposure 

and biomarker sample 

collection?

Two Major Sources of Uncertainty
• December 2014 Human Health Risk Assessment

• Using the PBPK-PD model to derive point-of-departure 

based on 10% RBC AChE inhibition by oxon

• Food, water (oxon-only exposure), currently registered 

residential use (golf course, mosquitocide application), 

worker exposure (applicators, post-application)

• April 2016 SAP 

• Using the PBPK model to simulate chlorpyrifos 

concentrations in blood (NOT to derive PoD)

• Food, water, worker exposure (applicators, post-application)

Using the Chlorpyrifos Model

67
Chlorpyrifos PBPK-PD Model

68

Poet 2013 (MRID 49074901)
• Originally developed in 2002, refined over the years 

as more data had become available.

• The latest version includes multi-route exposures 

(Poet et al., 2014).

• The model was reviewed by the FIFRA SAP in 2011. 

The Agency has continued to evaluate the refined 

model.

Chlorpyrifos PBPK-PD Model

69
Model Calibration/Evaluation

70

Gray highlights: Data available for model calibration/evaluation

Blue highlights: Chlorpyrifos concentrations measured in humans
• A version of the PBPK model that describes physiological and 

biochemical changes during pregnancy is available

• Not peer-reviewed

• No time course data available for pregnant women

• The model was built based on the best knowledge available 

• Preliminary results from the pregnancy model suggested that 

the combined effects from multiple physiological changes 

during pregnancy resulted in slightly lower chlorpyrifos 

concentrations in blood during the third trimester.

Using 75 kg Females as Surrogates for 

Pregnant Women

71
72

EPA evaluated the exposures likely to have occurred in the 

CCCEH cohort, and whether these exposures could have 

resulted in the observed cord blood measures using the 

PBPK model

• Food 

• Drinking water

• Residential use: indoor application

Uncertainty: Exposure Scenarios?
73

Likely Exposure Scenario: Residential Indoor 

Use, Post-Application Exposure
74

• For CCCEH cohort, chlorpyrifos was measured in umbilical 

cord blood collected close to delivery.

• Reported time for labor and delivery for the first 

pregnancy range from 8 to 20 hours (early labor phase 6-

12 hours, active labor phase 4-8 hours), and may become 

faster for subsequent pregnancies.

• Time between last exposure and biomarker sampling can 

be bounded.

Uncertainty: Time Between 

Exposure and Sampling?
75

Time Course of Blood Concentrations

Biphasic Clearance

Time Conc (pg/g)

Peak on the 30th day 62.7

4 hours after the last peak 22.5

8 hours after the last peak 14.3

20 hours after the last peak 11.0

2 days after the last peak 9.6

Chlorpyrifos is removed from blood through (1) distribution to tissues, and (2) metabolism and excretion of chlorpyrifos. 

Rapid decrease immediately after 

exposure, half-life ~3 to 4 hours

Gradual decrease several hours after 

exposure, half-life ~120 hours
76

• The PBPK model for chlorpyrifos has been evaluated using 

human data, including chlorpyrifos concentrations in blood.

• Two major uncertainties in linking biomarker data to exposures:

• Exposure scenarios

• Time between exposures and biomarker sampling

• The PBPK model was used to simulate time course of 

chlorpyrifos concentrations in blood

• Likely exposure scenario: residential indoor use, post- application exposure

• Time between exposures and biomarker sampling: ~4 hours to 

2 days

Summary
Internal, deliberative, not for distribution

77

Questions
Wade Britton, MPH

Health Effects Division

Office of Pesticide Programs

U.S. EPA

britton.wade@epa.gov

703-308-0139

Chlorpyrifos: Evaluation of CCCEH 

Blood Data and Predicted Exposures

78

Rochelle Bohaty, PhD

Environmental Fate and 

Effects Division

Office of Pesticide Programs

U.S. EPA

bohaty.rochelle@epa.gov

703-305-6381

Danette Drew

Health Effects Division

Office of Pesticide Programs

U.S. EPA

drew.danette@epa.gov

703-305-6028

Presentation to the FIFRA SAP, April 19, 2016
79

•Introduction • Exposure Scenarios •Drinking Water • Food • Residential • Conclusions

Outline
80

• The 2012 SAP report urged the agency to find ways to use the 

epidemiology studies to inform the chlorpyrifos assessment, in 

particular the CCCEH study

• “...if one assumes that cord blood measurements reflect exposure levels 

during the critical prenatal period for induction of neurodevelopmental 

effects, then in theory, these would be the ideal data from which to 

derive the POD for chlorpyrifos in humans”

• In doing so, the agency was encouraged to make use of the PBPK 

model to further characterize CCCEH dose estimates

• “...a [PBPK] model could be used to further characterize the dose 

estimates in the epidemiology studies, for additional dose response 

analyses. Such a PBPK model will become even more important in the 

event that the agency might, at some point in the future, decide to 

move from using AChE inhibition to another outcome.”

Introduction
81

• Mothers and Newborn Study of North Manhattan and South Bronx 

conducted by Columbia University – referred to as CCCEH study 

• CCCEH study measured parent chlorpyrifos in cord blood, and other 

indicators (e.g., air sampling, behavioral information)

• Other two birth cohorts generally measured non-specific urinary 

metabolites of chlorpyrifos and other OPs (TCPy, DAPs)

• Therefore, while the other two cohorts provide important supporting 

information, EPA considers the CCCEH Mothers and Newborn Study 

research results as most relevant to the chlorpyrifos HHRA, and is 

thus the focus of this analysis

• 2008/2012 SAP concurred with this conclusion

Introduction: CCCEH Study Background
82

Introduction: Distribution of CCCEH Blood 

Concentrations

Percentile

Cord Blood Chlorpyrifos 

(pg/g)

Maternal Blood Chlorpyrifos 

(pg/g)

1998-1999 2001 1998-1999 2001

25 < LOD < LOD 2.60 < LOD

50 3.78 < LOD 6.70 < LOD

75 8.80 < LOD 9.55 < LOD

90 12.1 2.45 16.0 2.44

95 15.1 2.58 19.4 2.61

< LOD = Less than Limit of Detection (0.5 and 1.0 pg/g)
83

• Chlorpyrifos was one of the most widely used insecticides 

during the time of the CCCEH cohort and exposures were 

likely to have occurred

• Used on a variety of food and feed crops

• Results in dietary exposures from drinking water and food

• Until 2000, when nearly all residential uses were 

voluntarily cancelled, chlorpyrifos was one of the major 

insecticides in indoor residential settings

Introduction: Chlorpyrifos Regulatory History
84

• EPA has evaluated likely exposures in the CCCEH cohort 

and whether they could have resulted in the observed 

blood measures using the PBPK model

1. Defined exposures (drinking water, food, and 

residential) to the women in the cohort 

2. Quantified the magnitude of exposure/dose estimates 

from likely chlorpyrifos exposures

3. Compared PBPK model predicted blood levels 

resulting from these exposures to blood levels 

reported by CCCEH

Introduction: Evaluation
85

Drinking Water Exposure
86

• 99% is sourced from Croton 

and Catskill-Delaware 

systems

• Watershed protection 

contains objectives to 

prevent pollution from both 

agricultural (watershed 

overlap) and non- agricultural use sites

• Does not completely 

eliminate the potential for 

exposure to chlorpyrifos 

and chlorpyrifos-oxon

Drinking Water: New York City

New York City Surface Water Intakes

2001 National Land Cover Dataset

New York City Surface Water Watershed
87

Chlorpyrifos Transformation
88

• Surface water monitoring data for Croton and Catskill-Delaware watersheds 

• NYC Community Water System Reports – no detections

• NYC Community Water System processing includes chlorination and source 

water mixing

• Potential exposure to chlorpyrifos is virtually eliminated

Drinking Water: New York City

Year 1997 1998 1999 2000 2001 2002 2003 2004

Concentration 

(μg/L) 0.002 0.015 0.015 0.06 0.0423 0.0025 0.25 0.25

Bold font indicates measured concentrations. Other referenced concentrations are 1⁄2 the limit of detection. 

Treatment Method Free Chlorine

Percent Reduction Study 1a 90.3 (pH 6.6) 85.7 (pH 8.6)

Percent Reduction Study 2b 100 pH 8.0

a. Chamberlain 2014 – Experimental time was representative of typical drinking water treatment condition

b. Ormad 2008 – Experimental time not reported
89

Food Exposure
90

• 2014 Chlorpyrifos HHRA 

• Combined dietary exposure from all foods with a 

U.S. tolerance for chlorpyrifos

• Chlorpyrifos is the residue of concern in food; the 

oxon metabolite is not expected to occur in food

• Acute (one day) and Steady State (21 day) 

durations

• Females of childbearing age, infants and children

Food Exposure: Background
91

• Dietary methods have undergone multiple FIFRA SAP 

reviews

• Dietary Exposure Evaluation ModelTM (DEEM) with the 

Food Commodity Intake Database (FCID), Version 3.16

• Calendex-FCIDTM Model

• Both models incorporate 2003-2008 consumption data 

from USDA’s National Health and Nutrition Examination 

Survey, What We Eat in America, (NHANES/WWEIA)

Food Exposure: Methods
92

• The food exposure results are reflective of those

expected in the CCCEH study cohort

• Food exposure results and the PBPK model have 

been used to:

• Predict chlorpyrifos blood levels that may occur from 

food exposure

• Evaluate the contribution of food as a source of 

chlorpyrifos exposure for women in the CCCEH cohort

Food Exposure: Application of Results
93

• The PBPK model was used to predict chlorpyrifos blood 

concentrations from food exposure for the following scenario:

• Female, 72.9 kg

• Following a single, repeated daily exposure to chlorpyrifos in 

food

• Performed using the food exposure results for a range of 

percentiles of exposure

• Predictions for the blood levels at 10 hours and 24 hours after 

peak exposure are compared to the blood levels measured in 

the CCCEH cohort

Food Exposure: PBPK Modeling
94

Percentile of 

Exposure

PBPK-Predicted

10 Hour 

Blood Chlorpyrifos 

From Food (pg/g)

PBPK-Predicted

24 Hour 

Blood Chlorpyrifos 

From Food (pg/g)

2001 

Maternal Blood 

Chlorpyrifos(pg/g)

10 < LOD < LOD --

25 -- -- < LOD

50 < LOD < LOD < LOD

70 < LOD < LOD --

75 -- -- < LOD

90 -- -- 2.44

95 < LOD < LOD 2.61

99.9 1.490 < LOD --

Food Exposure Results
95

Residential Exposure
96

• “Characterization of chlorpyrifos exposure experienced by 

women in the Columbia cohort, particularly during the precancellation period, remains an important uncertainty in using 

these data in quantitative risk assessment.”

• “...the actual level of such exposure particularly during any 

critical window(s) of susceptibility is not known, and a better 

understanding of the range of possible exposures and the 

degree to which they may or may not have elicited inhibition 

of AChE, remains a key scientific question.”

SAP Recommendations for Residential Analysis
97

• 2014: Dose reconstruction analysis evaluated indoor 

exposures that were likely at the time of the CCCEH 

cohort and whether these could have elicited 10% RBC 

AChE inhibition, the regulatory endpoint for chlorpyrifos

• 2016: Biomarker analysis used the PBPK model under a 

variety of expanded exposure conditions to predict 

chlorpyrifos blood concentrations for comparison to blood 

levels reported in the CCCEH cohort

Residential Analyses Completed in 2014 and 2016
98

• In 1997, all chlorpyrifos residential indoor broadcast and total 

release aerosol/fogger uses were voluntarily cancelled

• Existing stocks could be phased out, or applied until depleted

• In 2000, the technical registrants agreed to phase out of nearly 

all remaining residential uses (including home lawn and indoor 

crack & crevice uses) 

• The phase out during the time of the CCCEH cohort implies 

indoor residential exposures could have occurred because 

chlorpyrifos products could have remained in the chain of 

trade and been legally used

Regulatory History of Residential Chlorpyrifos Use
99

• Chlorpyrifos was one of the most widely used insecticides during 

the time of the CCCEH cohort and indoor residential exposures 

likely occurred

• Insecticide usage information reported in the Columbia study 

publications is very limited 

• Whyatt et al. (2002) reported that pest control measures were 

used by 85% of respondents, treatment by themselves, or by 

housing superintendent, pest control operator (PCO)

• Whyatt et al. (2009) reported that, of those who used pesticides, 

these were used once monthly

Residential Exposures in the CCCEH Cohort
100

• EPA compared chlorpyrifos products and application types 

available during the time of the CCCEH cohort to reported 

pest control methods to determine which of these may 

have been used by the study participants 

• Whyatt et al. (2002): Broadcast and crack & crevice (perimeter) 

treatment by a pest control operator (PCO), 39%; aerosol can 

sprays, 26%; and total release foggers/bombs, 5% 

• Products registered as both 0.5% and 1.0% active ingredient (ai) 

formulations

Residential Exposures in the CCCEH Cohort
101

• Screening-level analysis conducted using the 2012 Residential 

SOPs and available chlorpyrifos-specific exposure data to assess 

exposures from likely/reported uses

• Handler (adult) and post-application (adults and children 

1 to < 2 years old) exposures considered

• Exposure parameters: 

• Available exposure data includes registrant-submitted studies and 

open literature; primarily related to chlorpyrifos indoor crack & 

crevice and broadcast treatments

2014 Dose Reconstruction Analysis

• Maximum, 1.0% ai formulation

• Exposure duration (8 hours exposure on 

carpeted floors, 2 hours on hard flooring)

• Amount handled (1 entire spray 

can for broadcast treatment, 1/2 

can for crack and crevice)
102

• PBPK model used to estimate RBC AChE inhibition from these 

estimated exposures

• Adult handler and post-application: dermal and inhalation exposures

• Children 1 to < 2 years old post-application: dermal, incidental oral, and 

inhalation exposures

• Conservative PBPK model inputs: 

• Less than 10% peak RBC AChE inhibition for all exposure assessed

• Post-application exposures following broadcast and perimeter treatments, 

resulted in the highest exposure potential of all product types assessed

2014 Dose Reconstruction Analysis

• 24 hour daily exposure

• No adjustment for bathing/showering

• Residence not left for 14 days

• Daily residue dissipation (10%) 
103

• 2016 analysis is conceptually similar analysis to the 2014 dose 

reconstruction assessment

• Both use the 2012 Residential SOPs and chemical-specific exposure data

• Whereas the 2014 residential analysis used the PBPK/PD model to 

evaluate whether 10% RBC AChE inhibition could have occurred in 

the cohort, the 2016 analysis used the PBPK model to evaluate 

chlorpyrifos blood concentrations for comparison to CCCEH reported 

blood levels

• 2016 analysis assesses only post-application exposures

• PBPK model used under a variety of expanded exposure conditions

2016 Analysis
104

• A total of 6 post-application exposure scenarios were developed to establish 

ranges of possible exposure levels to women in the CCCEH cohort and 

compare predicted internal dosimetry levels to those reported by the CCCEH 

investigators

• Two post-application exposure scenarios mimic potential chlorpyrifos use 

patterns likely to have occurred and represent high-end indoor exposures

• Broadcast application of liquid chlorpyrifos to the entire hard floor surface

• The highest of all assessed exposure scenarios

• Perimeter application of liquid chlorpyrifos in a band or strip around a 

carpeted room

• Likely the predominate treatment type during the CCCEH cohort given 

the 1997 broadcast cancellation

2016 Analysis Exposure Scenarios
105

• Four post-application exposure scenarios were assessed with use of the 

highest CCCEH blood value reported by Rauh et al. (2011), 63 pg/g

• Establish an upper and lower exposure bound with use of this blood 

value

• Two post-application exposure scenarios (hard and carpeted flooring) 

that would have resulted in a peak concentration of ~ 60 pg/g on the 

final (30th) day of PBPK simulation

• Daily peak exposures back-calculated from the final day of exposure

• Two post-application exposure scenarios (hard and carpeted flooring) 

that would have resulted in a peak concentration of ~60 pg/g on day 

of application (1st day of PBPK simulation)

• Simulates exposures following Day 1 peak exposure

2016 Analysis Exposure Scenarios
106

• Some of the 2014/2016 analysis PBPK exposure inputs differ 

in order to model more realistic exposure conditions

• Daily shower following exposure

• Once monthly (30 day) pesticide application

• 8 and 2 hour daily exposure duration for contact with 

carpeted and hard flooring, respectively 

• 2014/2016 analysis PBPK model exposure inputs which are 

the same

• Daily post-application indoor exposures 

• Single pesticide application

• 10% daily residue dissipation

2016 Analysis: PBPK Exposure Inputs
107

• PBPK model simulations show a saw tooth pattern of daily, rapid increases in 

internal chlorpyrifos dose during exposure periods followed by a rapid decline 

after exposure

• Peak internal dose for all simulations occurred during the final hour of daily 

exposure, just prior to the showering event

• A rapid decline is seen following the showering event and continues to decline 

for 24 hours, just before initiation of the subsequent day’s exposure event

• Internal dose increases once again immediately following initiation of the 

exposure event

• Internal doses peak on the initial day of exposure and decline step-wise with 

each subsequent day modeled due to 10% daily dissipation input

• Following the final day of exposure, internal dose continues to decline during 

terminal clearance phase

Results: 2016 Analysis PBPK Modeling
108

Results: Perimeter and Broadcast PBPK Modeling

-1000

0

1000

2000

3000

4000

5000

6000

7000

8000

0 200 400 600 800 1000

Blood Chlorpyrifos (pg/g)

Time (hours)

Perimeter Application to Carpeted Flooring - 8 hours Broadcast Application to Hard Flooring – 2 hours

-200

0

200

400

600

800

1000

1200

0 500 1000 1500

Blood Chlorpyrifos (pg/g)

Time (hours)
109

Results: 60 pg/g Chlorpyrifos in Blood on the Day of Application 

(Day 1) and on the Last (30th) Day; 2 Hour Daily Duration

-200

0

200

400

600

800

1000

1200

0 200 400 600 800 1000

Blood Chlorpyrifos (pg/g)

Time (hours)

60 pg/g

-10

0

10

20

30

40

50

60

70

0 200 400 600 800 1000

Blood Chlorpyrifos (pg/g)

Time (hours)

60 pg/g
110

Model Run

Highest Peak 

Blood Concn 

(pg/g)

24 Hour Blood 

Concn (pg/g)

Blood Concn 10 Hours 

After the Peak on Day 

30 (pg/g)

Blood Concn 24 Hours 

After the Peak on Day 

30 (pg/g)

Broadcast (Hard Floor; 

2 hours) 7,179 41 42.6 31.7

Perimeter (Carpeted 

Floor; 8 hours) 1,053 13 12.9 10.7 

60 pg/g Peak 30th Day

(2 hours) ~ 1,030

4.3 6.44 4.79

60 pg/g Peak 30th Day

(8 hours) 12.8 12.7 10.6

60 pg/g Peak on Day of 

Application (2 hours) 60

0.25 0.37 0.28

60 pg/g Peak on Day of 

Application (8 hours) 0.74 0.73 0.61

Results: 2016 Analysis PBPK Modeling
111

Results: 

Peak 

Chlorpyrifos 

Blood 

Concentrations 

on Day 1

0

1000

2000

3000

4000

5000

6000

7000

8000

Broadcast

hard floor

(2h/day)

Perimeter

carpeted

floor

(8h/day)

Peak at 60

pg/g on

Day 30

(2h/day)

Peak at 60

pg/g on

Day 30

(8h/day)

Peak at 60

pg/g on

Day 1

(2h/day)

Peak at 60

pg/g on

Day 1

(8h/day)

7179

1053 1030 1030

60 60

Blood Chlorpyrifos (pg/g)

Peak Concentration on Day 1 (pg/g)
112

Results: 

Chlorpyrifos 

Blood 

Concentrations 

on Day 1 at 

Hour 24 

0

5

10

15

20

25

30

35

40

45

Broadcast

hard floor

(2h/day)

Perimeter

carpeted

floor

(8h/day)

Peak at 60

pg/g on

Day 30

(2h/day)

Peak at 60

pg/g on

Day 30

(8h/day)

Peak at 60

pg/g on

Day 1

(2h/day)

Peak at 60

pg/g on

Day 1

(8h/day)

41

13

4.3

13

0.25 0.74

Blood Chlorpyrifos (pg/g)

Concentration at Hour 24 (pg/g)
113

Results: 

Chlorpyrifos 

Blood 

Concentrations 

10 Hours and 

24 Hours After 

the Peak on 

Day 30

0

5

10

15

20

25

30

35

40

45

Broadcast

hard floor

(2h/day)

Perimeter

carpeted

floor

(8h/day)

Peak at 60

pg/g on

Day 30

(2h/day)

Peak at 60

pg/g on

Day 30

(8h/day)

Peak at 60

pg/g on

Day 1

(2h/day)

Peak at 60

pg/g on

Day 1

(8h/day)

42.6

12.9

6.44

12.7

0.37

0.73

31.7

10.7

4.79

10.6

0.28 0.61

Blood Chlorpyrifos (pg/g)

Blood Chlorpyrifos (pg/g)

10 hours After the Day 30 peak 24 hours After the Day 30 peak
114

Distribution of CCCEH Blood Concentrations

Percentile

Cord Blood Chlorpyrifos 

(pg/g)

Maternal Blood Chlorpyrifos 

(pg/g)

1998-1999 2001 1998-1999 2001

25 < LOD < LOD 2.60 < LOD

50 3.78 < LOD 6.70 < LOD

75 8.80 < LOD 9.55 < LOD

90 12.1 2.45 16.0 2.44

95 15.1 2.58 19.4 2.61

< LOD = Less than Limit of Detection (0.5 and 1.0 pg/g)
115

Last Day of 30 Day Simulation for Perimeter 

Carpet Exposure Scenario

First Day of 30 Simulation for Perimeter 

Carpet Exposure Scenario

Residential Estimates Compared to CCCEH Blood Data

0

10

20

30

40

50

60

70

680 700 720 740 760 780

Blood Chlorpyrifos (pg/g)

Time (hours)

63 pg/g

95th-%ile, 

19.4 pg/g

90th-%ile, 

16.0 pg/g

75th-%ile, 

9.55 pg/g

0

200

400

600

800

1000

1200

0 10 20 30

Blood Chlorpyrifos (pg/g)

Time (hours)

95th-%ile, 

19.4 pg/g

90th-%ile, 

16.0 pg/g
116

Residential Estimates Compared to CCCEH Blood Data

• The simulations, 

perimeter carpet 

exposure scenario and 

the 60 pg/g as the peak 

at Day 30, are very 

similar

• Maximum peak

• Lowest peak (~60 pg/g)

• 24 hour blood 

concentration following 

Day 1 peak

• 10/24 hour blood levels 

after the final day peak 

exposure

0

200

400

600

800

1000

1200

0 200 400 600 800 1000

Venous blood CPY (pg/g)

Time (hr)

60 pg/g Final Day Scenario Perimeter Use Scenario
117

• The approaches used by EPA for estimation of drinking 

water, food, and residential exposures are supported by 

multiple SAPs 

• These exposure estimates make use of chemical-specific 

exposure and monitoring data, where available

• The simulations of predicted chlorpyrifos blood 

concentrations for each exposure pathway were 

conducted using the robust, highly refined PBPK model

• Integrating these approaches has allowed for predictions 

about the women in the CCCEH cohort

Summary
118

• Chlorpyrifos blood concentrations predicted with the 

PBPK model are well within the range of levels reported in 

the CCCEH cohort

• The watershed protection plan and water processing 

virtually eliminates all chlorpyrifos drinking water 

exposure

• Internal blood concentrations of chlorpyrifos from food 

exposures are consistent with the CCCEH blood levels 

post-cancellation

• Higher levels of blood concentrations in 98-99 were likely 

from chlorpyrifos residential indoor usage

Conclusions
119

Questions
Anna Lowit, Ph.D.

Senior Toxicologist, Health Effects Division 

Office of Pesticide Programs 

Environmental Protection Agency

Lowit.anna@epa.gov

703-308-4135 (work), 703-258-4209 (cell)

Presentation to the FIFRA SAP, April 19, 2016

Point of Departure, Intra-species 

Extrapolation, the FQPA 10X Safety 

Factor, and Case Studies 

Internal, deliberative, not for distribution

120
Point of Departure, 

Intra-species Extrapolation, & 

FQPA 10X Safety Factor

Internal, deliberative, not for distribution

121
• From the 2012 SAP Report, pp. 49-50 (emphasis added):

• The Panel acknowledged the limitations in the three 

longitudinal children’s cohort studies of estimating 

chlorpyrifos exposures (i.e., the Columbia study, the Mt. 

Sinai study, and the CHAMACOS study), based on the 

exposure measures collected, and was in general agreement 

that the data from these studies alone were not sufficient to 

derive a point of departure (POD) for purposes of 

quantitative risk assessment. 

• New information has emerged from the analysis that 

reduces some of the uncertainty in the biomonitoring data.

Introduction 

Internal, deliberative, not for distribution

122
123

• Per the 2012 SAP report, the agency was urged to find ways to use 

the epidemiology studies, in particular the CCCEH study, to inform 

the chlorpyrifos assessment (emphasis added):

• “...if one assumes that cord blood measurements reflect exposure levels 

during the critical prenatal period for induction of neurodevelopmental 

effects, then in theory, these would be the ideal data from which to 

derive the POD for chlorpyrifos in humans”

• In doing so, the agency was encouraged to make use of the PBPK 

model to further characterize CCCEH dose estimates

• “...a [PBPK] model could be used to further characterize the dose 

estimates in the epidemiology studies, for additional dose response 

analyses. Such a PBPK model will become even more important in the 

event that the agency might, at some point in the future, decide to move 

from using AChE inhibition to another outcome.”

Introduction

Internal, deliberative, not for distribution
Internal, deliberative, not for distribution

124

• The dose-response point that marks 

the starting point for low-dose 

extrapolation. The POD may be a 

NOAEL/LOAEL, but ideally is 

established from BMD modeling of 

the experimental data, and 

generally corresponds to a selected 

estimated low-level of response.

• Some form of extrapolation below 

the POD may be employed for 

estimating low-dose risk or the POD 

may be divided by a series of 

uncertainty factors to arrive at a 

reference dose (RfD). 

Point of Departure
Internal, deliberative, not for distribution

125

Risk Metrics

MOE = Point of Departure 

Exposure

Target MOE = Total Uncertainty Factors

RfD = Point of Departure 

Uncertainty Factors 
126

• Dermal exposure to female worker to achieve 10% RBC AChE inhibition

• External dose from exposure 8 hours/day, 5 day/week for 3 weeks

• Blood concentration peaked at 120,000 pg/g

• Blood concentration still above 100 pg/g at 32 days after the last 

exposure. 

• Food exposure for adult females: once daily exposure, steady state to 

achieve 10% RBC AChE inhibition

• Blood concentration approximately 7000 pg/g at the daily peaks.

• Blood concentration never goes below 100 pg/g over the continuous 21- day exposure simulation 

• Arnold et al. (2015) recently Concentrations of chlorpyrifos in blood 

associated with the predicted RBC AChE inhibition of less than or equal to 

10% ranged from 6100 to 64000 pg/g (5th to 95th percentile).

PoDs from 2014 HHRA: 10% RBC AChE Inhibition

Internal, deliberative, not for distribution
127

• Option 1: Lower limit of the top tertile (>6.17 pg/g cord blood) 

derived from Rauh et al. (2006). 

• Includes 254 children followed through age 3 years and used a 

dichotomized statistical approach of ≤6.17 pg/g cord blood for the 

‘low group’ and >6.17 pg/g cord blood as the ‘high’ group.

• Statistically significant deficits of 6.5 points on the Bayley 

Psychomotor Development Index (PDI) at 3 years of age when 

comparing high to low exposure groups. 

• Increased odds of mental delay (OR=2.4; 95% CI: 1.1–5.1) and 

psychomotor delay (OR=4.9; 95% CI: 1.8–13.7) at age 3 years when 

comparing high to low exposure groups. 

Options for Point of Departure

Internal, deliberative, not for distribution
128

• Option 1: Lower limit of the top tertile (>6.17 pg/g cord 

blood) derived from Rauh et al. (2006), cont’d. 

• Large odds ratios for attention disorders (OR=11.26; 95% 

CI: 1.79–70.99), ADHD (OR=6.50; 95% CI: 1.09–38.69), and 

PDD (OR=5.39; 95% CI: 1.21–24.11) at age 3 years when 

comparing high to low chlorpyrifos exposure groups. 

• If the agency pursued this study for deriving a PoD, the 

appropriate value would be 6.17 pg/g cord blood.

Options for Point of Departure

Internal, deliberative, not for distribution
129

• Option 2: Benchmark Dose estimates derived from linear regression 

reported in Rauh et al. (2011) for deficits in Working Memory. 

• Evaluated the relationship between prenatal chlorpyrifos exposure and 

neurodevelopment among 265 of the CCCEH cohort participants who 

had reached the age of 7 years and had a complete set of data.

• Described the log of Working Memory Index (WMI) of children as 

linearly associated with concentration of chlorpyrifos (CPF) in cord blood 

with a slope = -0.006 (95% CI = -0.01, -0.002). 

• Each standard deviation increase in exposure (4.61 pg/g) there is a 1.4% 

reduction in Full-Scale IQ and a 2.8% reduction in Working Memory. 

• The concentration levels of chlorpyrifos in cord blood (and its 95% lower 

confidence limit, one-sided) can be estimated using the estimated slope 

and its 95% lower confidence limit (one-sided). 

Options for Point of Departure

Internal, deliberative, not for distribution
Internal, deliberative, not for distribution

130

Percent 

Reduction of 

WMI (%)

Estimated 

concentration of 

CPF in cord 

bloodc (pg/g):

BMD

BMDL 95% 

lower limit 

(one-sided) of 

estimated 

concentration of 

CPF in cord 

blood (pg/g)

1 1.68 1.07

2 3.37 2.16

3 5.08 3.26

4 6.80 4.36

5 8.55 5.48

• At the 1% change: BMDL values are close to the LODs of 

0.5/1.0 pg/g for measuring chlorpyrifos residues in cord 

blood & thus have more uncertainty 

• At 3-5% change, concentrations are near or above the 

6.17 pg/g which represents the lower limit of the top 

tertile where neurodevelopmental outcomes have been 

associated with chlorpyrifos exposure (Rauh et al., 2006, 

2015) & thus may not be protective. 

• Therefore a 2% working memory reduction which 

corresponds to an internal dose measure of 2.16 pg/g 

chlorpyrifos in cord blood is proposed for use as POD.

• Quantitatively near the value reported by Rauh (2.8% 

reduction in working memory) 

• Within the range of levels measured in the blood samples 

collected by CCCEH.

Option 2: Benchmark Dose estimates derived from linear regression reported in 

Rauh et al. (2011) for deficits in Working Memory. 
131

• In typical risk assessments, PoDs are derived directly from laboratory 

animal studies and inter- and intra-species extrapolation is accomplished by 

the use of 10X factors. 

• The proposed PoDs are derived from human information obviating the need 

for the inter-species extrapolation. 

• The intra-species uncertainty factor (UFH):

• to account for variations in susceptibility within the human population 

(human variability) 

• and the possibility (given a lack of relevant data) that the database 

available is not representative of the dose/exposure 

response- relationship in the groups of the human population that are 

most sensitive to the health hazards of the chemical being assessed.

• pharmacodynamics & pharmacokinetics can be considered separately.

Intra-Species Extrapolation

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Internal, deliberative, not for distribution

132

• Some form of extrapolation 

below the POD may be 

employed for estimating lowdose risk or the POD may be 

divided by a series of 

uncertainty factors to arrive at 

a reference dose (RfD). 

Intra-Species Extrapolation
133

Pharmacodynamics (PD): 

• AOP(s) for neurodevelopmental outcomes related to chlorpyrifos exposure are 

not known, although there are many potential pathways suggested in the 

literature. By extension, the window(s) of susceptibility and associated 

duration(s) of exposure are not known. 

• CCCEH cohort: self-identified as African-American or Dominican, were 18–35 

years of age, and who lived in specific urban areas (northern Manhattan 

(Central Harlem or Washington Heights/Inwood) or the South Bronx) for a year 

or more prior to pregnancy. This study population was of generally low 

socioeconomic status, with approximately 85% of the population having annual 

incomes of $30,000 or less. 

• Do not have adequate data to assess the relative sensitivity of CCCEH cohort 

compared with other groups. 

• Given the current state of knowledge, the PD variability cannot be quantified 

and is therefore unknown. The default 3X is retained. 

Intra-Species Extrapolation

Internal, deliberative, not for distribution
134

Pharmacokinetics (PK):

• There is a robust PBPK model which accounts for population 

variability for infants, but a fully vetted, validated PBPK model is not 

available for pregnant women. 

• For PK variation, a data-derived factor cannot be derived and the 

default 3X is retained. 

Total Intra-species Factor:

• Proposed to use a 10X intra-species extrapolation factor: 3X for PK 

variability and 3X for PD variability 

• Consistent with that used for methyl mercury. 

Intra-Species Extrapolation

Internal, deliberative, not for distribution
135

• The FQPA (1996) instructs EPA, in making its “reasonable certainty of no 

harm” finding, that in “the case of threshold effects, an additional tenfold 

margin of safety for the pesticide chemical residue and other sources of 

exposure shall be applied for infants and children to take into account 

potential pre- and postnatal toxicity and completeness of data with 

respect to exposure and toxicity to infants and children.” 

• Section 408 (b)(2)(C) further states that “the Administrator may use a 

different margin of safety for the pesticide chemical residue only if, on the 

basis of reliable data, such margin will be safe for infants and children.” 

• Note: The magnitude of the FQPA Safety Factor is a policy decision made 

by the agency. 

• The science underlying the policy decision is included in Charge 

Questions 6b & 6c.

FQPA 10X Safety Factor

Internal, deliberative, not for distribution
Pre- and Post-Natal Toxicity Database

• Experimental Laboratory Animals:

• There are numerous animal studies (rats and mice) in the 

literature that report changes in aspects of neurobehavioral 

function from adolescence to adulthood following 

developmental exposure.

• Adverse outcomes in several neurological domains including cognitive, 

anxiety and emotion, social interactions, and neuromotor function

• Extensive differences in study designs hinder comparability 

across studies

• Species; age, gender; exposure levels, timing, and route; test methods, 

equipment; procedures, endpoints; analyses, interpretations
Pre- and Post-Natal Toxicity Database

• Studies involve early and/or late gestational and/or early 

and/or late postnatal dosing 

• Timing of nervous system development not directly 

comparable across species. 

• Although, early postnatal periods in rodents generally 

correspond to last trimester in human development but later 

post-natal studies correspond more closely to early human 

infant.

• No clear evidence of critical period of exposure: some 

neurobehavioral outcomes observed across all windows of 

exposure, others not. 

• Systematic comparisons across exposures, doses, and tests are lacking.
138

• Epidemiology, pre-natal: 

• Numerous epidemiological investigations have observed a link between 

pre-natal exposure to chlorpyrifos or OPs (measured as chlorpyrifos, 

TCPy, or DAPs) and adverse effects on neurodevelopment through age 7 

years, with additional more limited evidence up through approximately 

age 11.

• Results across numerous studies show relatively consistent elevated 

risks associated with OP exposure:

• Autism spectrum disorders, intelligence decrements, attentional problems and 

ADHD, neurological effects near birth, and mental/ psychomotor delays (6 

months-11 years). 

• Results not always consistent with magnitude of association and ages where 

associations observed across studies. 

Pre- and Post-Natal Toxicity Database

Internal, deliberative, not for distribution
139

• Epidemiology, post-natal: 

• Smaller number of studies have assessed postnatal exposure to OPs (measured as DAPs) 

• Postnatal exposure assessed in the CHAMACOS cohort (Eskenazi et al., 2007; Young et al., 

2005; Bouchard et al., 2011) and 3 cross-sectional studies (Guodong et al., 2012; Bouchard 

et al., 2010; Oulhote and Bouchard, 2013) 

• Postnatal exposures not assessed in the CCCEH and Mt. Sinai studies (Rauh et al., 2011; 

Engel et al., 2011)

• Given that the major source of exposure (residential use) was cancelled partway through 

the CCCEH study, this limits the ability of this study to inform the impacts of long-term 

postnatal exposure to chlorpyrifos

• Bouchard et al. (2010) looked at U.S. children age 8–15 years in the 2000–2004 National 

Health and Nutrition Examination Survey (NHANES), and observed a positive association 

between attention and behavior problems and DAPs

• No other neurodevelopmental associations were found between postnatal biomarkers

Prenatal vs. Postnatal Exposure
140

• In sum, given that the extensive experimental laboratory 

animal database suggests that the post-natal period may 

be a potential susceptible time, the lack of postnatal 

exposure assessment in the CCCEH and Mt. Sinai studies is 

a source of uncertainty in the epidemiology database. 

Pre- and Post-Natal Toxicity Database

Internal, deliberative, not for distribution
141

• In the 2012 SAP report, the Panel noted the CCCEH studies had:

• “Relatively modest sample sizes which limited the statistical power to 

classify some meaningful differences as statistically significant and to 

examine the effect of modification by race/ethnicity and other 

characteristics.”

• “Relatively moderate to large exposure differences needed to see 

significant effects, likely due to the modest sample sizes used.”

• The modest sample size in the CCCEH study makes it difficult to say that the 

dose-response relationship between chlorpyrifos and neurodevelopmental 

outcomes in the overall U.S. population has been fully characterized. 

• The magnitude of the PoD in the general U.S. population of infants and 

children may be higher or lower than that estimated using the CCCEH study 

results, and the shape of the dose-response curve may also be different. 

Impact of Sample Size on CCCEH Findings

Internal, deliberative, not for distribution
142

• There are remaining uncertainties surrounding 

• the lack of information on post-natal exposure and evaluation of such exposure 

to neurodevelopmental outcomes in the CCCEH and Mt. Sinai cohorts 

• the modest sample sizes which limited the statistical power and exposure 

differences required to see statistically significant results. 

• As such, there is sufficient uncertainty which prevents the agency from reducing 

or removing the statutory 10X FQPA Safety Factor. 

• EPA therefore intends to continue to retain the FQPA 10X Safety Factor for 

assessing risk from chlorpyrifos for infants, children, youths, and women of 

childbearing age for all exposure scenarios.

• Note: The magnitude of the FQPA Safety Factor is a policy decision made by 

the agency. 

FQPA 10X Safety Factor

Internal, deliberative, not for distribution
143

• Charge question 5a-c: Derivation of POD

• 5a: Change from AChE inhibition based POD to 

neurodevelopmental outcomes

• 5b: POD options

• 5c: Calculation approach to deriving POD from Rauh et al 

(2011)

• Charge question 6a-c 

• 6a: Proposal to retain 10X Intraspecies Extrapolation Factor

• 6b-c: Science that underlies the policy to retain the FQPA 

10X Safety Factor

Issues for Panel Discussion

Internal, deliberative, not for distribution
Case Studies:

Linking Exposure Assessment with 

PBPK Modeling to Derive 

Predicted Internal Doses

Internal, deliberative, not for distribution

144
145

• EPA will need re-assess exposure from food, water, 

occupational scenarios along with other remaining uses 

with the new POD.

• Case studies developed to illustrate how the PBPK model 

would be used to predict internal doses from existing 

exposures to chlorpyrifos. 

• Exposure approaches have been previously reviewed by 

SAP and have used extensively in risk assessments.

Risk Assessment of Pesticides

Internal, deliberative, not for distribution
146

• Proposed an internal concentration PoD of 2.16 pg/g 

adjusted by 10X based on the intra-species factor and 10X 

FQPA Safety Factor. 

• For assessing dietary (food & water): Internal 

concentration RfD is 0.022 pg/g. 

• For residential, occupational exposure assessment 

which uses MOE as the risk metric, the internal PoD of 

2.16 pg/g will be used with a target MOE of 100. 

Conducting a Risk Assessment Using Internal 

Dose as the POD

Internal, deliberative, not for distribution
147

• PK profile for chlorpyrifos in blood is characterized by rapid 

increases and decreases during exposure periods and 

immediately after exposure, followed by slower decrease 

during the terminal clearance phase. 

• The CCCEH cord blood data are likely to represent data from 

the low points on the PK profile and likely in terminal 

clearance phase. 

• The agency will, to the extent possible, compare internal 

concentrations of chlorpyrifos at 10 and 24 hours after the 

last peak in each simulation, specifically for food exposure and 

occupational/residential exposure scenarios. 

Conducting a Risk Assessment Using 

Internal Dose as the POD

Internal, deliberative, not for distribution
Internal, deliberative, not for distribution

148

Time Course of Blood Concentrations

Biphasic Clearance

Time Conc (pg/g)

Peak on the 30th day 62.7

4 hours after the last peak 22.5

8 hours after the last peak 14.3

20 hours after the last peak 11.0

2 days after the last peak 9.6

Rapid decrease immediately after 

exposure, half-life ~3 to 4 hours

Gradual decrease several hours after 

exposure, half-life ~120 hours

Compare internal concentrations of chlorpyrifos at 10 and 24 hours after the last peak in each 

simulation, specifically for food exposure and occupational/residential exposure scenarios.
• For CCCEH cohort, chlorpyrifos was 

measured in umbilical cord blood 

collected close to delivery & in 

mothers up to 2 days after delivery.

• Reported time for labor and 

delivery for the first pregnancy 

range from 8 to 20 hours (early 

labor phase 6-12 hours, active 

labor phase 4-8 hours), and may 

become faster for subsequent 

pregnancies.

• Time between last exposure and 

biomarker sampling can be 

estimated.

Internal, deliberative, not for distribution

149

Biomarker Sampling Time – Considering Labor & 

Delivery
150

Percentile 

of 

Exposure

Food

Exposure 

from 

Calendex

Max Blood Levels 

of CPFOS (pg/g)

10 hour Blood 

Levels Post 

Exposure Values of 

CPFOS (pg/g)

24 hour Blood 

Levels Post 

Exposure Values of 

CPFOS (pg/g)

10

0.003 

ug/kg/day 0.29 0.060 0.021

50

0.007 

ug/kg/day 0.67 0.139 0.048

70

0.009 

ug/kg/day 0.86 0.179 0.062

95

0.018 

ug/kg/day 1.71 0.358 0.124

99

0.029 

ug/kg/day 2.76 0.576 0.200

99.9

0.075 

ug/kg/day 7.14 1.490 0.517

Food Exposure: Results PBPK Modeling
Internal, deliberative, not for distribution

151

Last Day of Dietary Exposure to Adult 

Female at 99.9th-ile for 40 consecutive days

Dietary Exposure to Adult Female at 99.9thile for 40 consecutive days

Example Food Scenario: Adult Female at the 99.9th-ile

0

1

2

3

4

5

6

7

8

970 975 980 985 990 995 1000 1005 1010

Blood (pg/g)

Time (hours)

10 hours 

post-peak 24 hours 

post-peak

0

1

2

3

4

5

6

7

8

0

316293

124155186217248279310341372403434465496527558589620651682713744775806837868899930961992

Blood (pg/g)

Time (hours)
-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 200 400 600 800 1000

Blood (pg/g)

Time (hours)

Internal, deliberative, not for distribution

152

Last Day of Dietary Exposure to Adult 

Female at 10th-ile for 40 consecutive days

Dietary Exposure to Adult Female at 10th-ile 

for 40 consecutive days

Example Food Scenario: Adult Female at the 10th-ile

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

970 975 980 985 990 995 1000 1005 1010

Blood (pg/g)

Time (hours)

10 hours postpeak

24 hours postpeak
153

• The agency evaluated all potential occupational exposure 

scenarios as part of the 2014 HHRA. 

• ‘Low’ occupational scenarios were provided in the issue 

paper for purpose of illustrating how occupational 

handler exposures and PBPK modeling can be integrated.

Example Worker Scenarios

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Internal, deliberative, not for distribution

154

Last day of exposure resulting from the occupational 

handler exposure scenario, mixing/loading liquids 

(EC) for cole crops at 1 lb ai/A. 

Full profile resulting from the occupational 

handler exposure scenario, mixing/loading 

liquids (EC) for cole crops at 1 lb ai/A.

0

50

100

150

200

250

300

350

261265269273277281285289293297301305309313317321325329333337341345349

Blood (pg/g)

Time (hours)

10 hours 

post-peak

24 hours 

post-peak

0

50

100

150

200

250

300

350

0

13263952657891

104117130143156169182195208221234247260273286299312325338

Blood (pg/g)

Time (hours)

Example Worker Scenario: Cole Crop, Handler
155

• Drinking water treatment methods have varying degree of chlorpyrifos 

removal.

• Depending on the treatment method used by a given community water 

system exposure may be to chlorpyrifos or chlorpyrifos-oxon or a 

combination. 

Drinking Water Exposure Potential

Internal, deliberative, not for distribution

Treatment 

Methoda

Chlorine

Chloramines

Chlorine 

dioxide

Potassium 

permanganate 

Ultraviolet light 

Hydrogen 

Peroxide

Ozone

Softening

pH 

6.6

pH 

8.6

pH 

6.6

pH 

8.6

pH 

6.6

pH 

8.6

pH 

6.6

pH 

8.6

pH 

6.6

pH 

8.6

pH 

6.6

pH 

8.6

pH 

6.6

pH 

8.6 pH 12

Percent 

Reductionb 90.3 85.7 8.7 9.2 34.3 27.5 15.3 5.2 14.5 1.9 7.6 3.1 60.9 30.3 100.0

a. Experimental time was representative of typical drinking water treatment condition
Internal, deliberative, not for distribution

156

Measured Concentrations

(Orestimba Creek, California)

Estimated Concentrations

(lower bound scenario 1 lb a.i./ac./year) 

• 120 day subset used for PBPK modeling

Drinking Water Exposure Scenarios
Internal, deliberative, not for distribution

157

Input: Estimated Concentrations Input: Measured Concentrations

Females of Childbearing Age (13-49 years old)

The PBPK model was used to predict the time course of chlorpyrifos 

concentrations in venous blood 

• Adult female weight 72.9 kg (160.7 lbs.); consumption 4x/d for total of 1.71062 L (57.8 oz) 

During the maximum 4 month period:

• [peak blood]: 6.24 pg/g

During the maximum 4 month period:

• [peak blood]: 1.97 pg/g
Internal, deliberative, not for distribution

158

Input: Estimated Concentrations Input: Measured Concentrations

Infants: Formula Fed (with water) 

The PBPK model was used to predict the time course of chlorpyrifos 

concentrations in venous blood 

• Infant fixed weight 4.8 kg (10.6 lbs.); consumption 6x/d for total of 0.688557 L (23.3 oz)

During the maximum 4 month period:

• [peak blood]: 20.66 pg/g

During the maximum 4 month period:

• [peak blood]: 6.24 pg/g
159

• Linking Exposure Assessment with PBPK Modeling to 

Derive Predicted Internal Doses:

• Charge question 7. 

Case Studies

Internal, deliberative, not for distribution
• From the 2012 SAP Report, pp. 49-50 (emphasis added):

• The Panel acknowledged the limitations in the three 

longitudinal children’s cohort studies of estimating 

chlorpyrifos exposures (i.e., the Columbia study, the Mt. 

Sinai study, and the CHAMACOS study), based on the 

exposure measures collected, and was in general agreement 

that the data from these studies alone were not sufficient to 

derive a point of departure (POD) for purposes of 

quantitative risk assessment. 

• New information has emerged from the analysis that 

reduces some of the uncertainty in the biomonitoring data.

Summary 

Internal, deliberative, not for distribution

160
161

• Although in 2012, the SAP identified uncertainties precluding the use of 

the biomonitoring data for POD derivation, the SAP did recommend the 

agency find ways to use the epidemiology studies, in particular the CCCEH 

study, to inform the chlorpyrifos assessment (emphasis added):

• “...if one assumes that cord blood measurements reflect exposure levels during 

the critical prenatal period for induction of neurodevelopmental effects, then in 

theory, these would be the ideal data from which to derive the POD for 

chlorpyrifos in humans”

• In doing so, the agency was encouraged to make use of the PBPK model to 

further characterize CCCEH dose estimates

• “...a [PBPK] model could be used to further characterize the dose estimates in the 

epidemiology studies, for additional dose response analyses. Such a PBPK model 

will become even more important in the event that the agency might, at some 

point in the future, decide to move from using AChE inhibition to another 

outcome.”

Summary

Internal, deliberative, not for distribution
Uncertainty Identified by the FIFRA SAP (2012, p. 48) Section Where 

Considered

Comment

Relatively modest sample sizes which limited the 

statistical power to classify some meaningful differences 

as statistically significant and to examine the effect of 

modification by race/ethnicity and other characteristics.

8.2. FQPA 10X Safety 

Factor for Infants & 

Children

These points increase 

the uncertainty that 

the PoD based on the 

CCCEH cord blood data 

are not fully 

representative and 

have been used as 

support to retain the 

FQPA 10X Safety Factor.

Relatively moderate to large exposure differences needed 

to see significant effects, likely due to the modest sample 

sizes used.

Exposure at one point in prenatal time with no additional 

information regarding postnatal exposures.

162
Uncertainty Identified by the FIFRA SAP 

(2012, p. 48)

Section Where 

Considered

Comment

Lack of clarity regarding a linear doseresponse instead of a potential threshold 

effect.

7.2 Options for PoD 

Based on the CCCEH 

Biomonitoring Data

This remains an uncertainty since 

CCCEH investigators have not 

released the raw data publicly. 

However, given the modest sample 

size and that the voluntary 

cancellation occurred in 2000, the 

data may not support a different 

empirical model (i.e, non-linear 

model). This point adds support to 

the selection of the 2% change in 

working memory as this value is 

within the measured data (albeit at 

the low end).

163
164

Uncertainty Identified by the FIFRA SAP 

(2012, p. 48) Section Where Considered Comment

Use of a single or average sample for 

exposure. Although Whyatt et al. (2009) 

noted moderate but significant 

correlations between meconium and 

cord and maternal blood and average 

urine TCPy, the representativeness of a 

single point exposure is still unclear. 

Time-varying exposures or the ability to 

define cumulative exposures would be 

preferable.

6.3 Residential Exposure to 

the CCCEH Cohort

7.1 Uncertainties with Using 

Biomarker for the PoD

The agency notes similar uncertainties in 

Section 7.1. The agency has developed 

an analysis of the range of possible 

exposures to the CCCEH cohort. This 

analysis uses the best available 

information on pesticide use and 

chlorpyrifos applications from 1998-

2004, including Whyatt et al (2003) 

which indicates that pesticide 

applications occurred approximately 

monthly. The agency’s predicted blood 

concentrations closely match those 

reported by CCCEH. 

Lack of specificity of a critical window of 

effect and the potential for 

misclassification of individual exposure 

measures.

164
165

Uncertainty Identified by the FIFRA 

SAP (2012, p. 48)

Section Where 

Considered

Comment

External generalizability of the cohorts 

given their unique racial/ethnic and 

socioeconomic characteristics. 

However, it should be noted that their 

exposures were within the range of 

those seen in NHANES.

8.1. Intra-species 

Extrapolation

The agency concurs there are remaining 

uncertainties about the degree to which 

the CCCEH is more or less sensitive to 

other population groups. Thus, this issue 

is part of the rationale for retaining the 

10X intra-species factor. 

165
166

Uncertainty Identified by the FIFRA 

SAP (2012, p. 48)

Section Where 

Considered

Comment

Questions about biologic plausibility 

due to lack of clarity on mechanism of 

action, particularly at the low 

exposure levels seen in the cohorts 

and the limited and mixed results of 

animal studies showing 

neurodevelopmental effects.

Appendix 3.0 (3.1.1, 

3.1.2, 3.2) Background & 

Summary of 

Experimental and 

Epidemiology Studies on 

Neurodevelopmental 

Effects 

6.3 Residential Exposure 

to the CCCEH Cohort

The agency agrees that the AOP/MOA for 

neurodevelopmental outcomes are not 

known. The agency has conducted a 

systematic review of the laboratory 

animal literature. There is qualitative 

similarity with respect to the three major 

neurodevelopmental domains 

(specifically, cognition, motor control, and 

social behavior) between the laboratory 

animal studies and the epidemiology 

findings. The CCCEH exposure 

characterization analysis provides 

plausibility to the low blood levels. 

166
167

• Characterized predicted exposures to women in the 

CCCEH prior to giving birth. 

• Proposed approach for using the CCCEH cord blood data 

to derive a PoD for use in quantitative risk assessment. 

• Proposes an innovative approach to using the PBPK model 

to integrate external and internal dose from food, water, 

and occupational exposure assessment. 

Summary

Internal, deliberative, not for distribution
QUESTIONS?

168