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SAP CHARGE

As part of the re-evaluation of the health effects of atrazine, three meetings of 

the FIFRA Scientific Advisory Panel (SAP) are scheduled in 2010. The first two 

meetings were held in February and April. The purpose of the September 

meeting is to solicit feedback from the SAP on the status and overall scientific 

direction of the Agency’s re-evaluation on several topics including our preliminary 

conclusions regarding the non-cancer epidemiology literature on atrazine, as well 

as the human health risk implications of the experimental toxicology data. The

Agency is also seeking the Panel’s feedback on the proposed updates to the 

dose-response assessment including the use of an internal dose metric and

EPA’s benchmark dose analysis. With respect to drinking water exposure, the 

Agency is seeking the Panel’s feedback on a general framework for designing 

monitoring studies along with basic approaches for analyzing sampling strategies 

and other methods for estimating exposures from less frequent existing 

monitoring data. Finally, the SAP is being asked to consider and comment on the 

implications of atrazine’s toxicity profile and it’s MOA on the development of a 

water monitoring strategy. 

Question 1.0: Non-cancer Epidemiology

Section 3.0 and Appendix B of the draft Issue Paper provide the Agency reviews 

and synthesis of the non-cancer epidemiology studies available for atrazine. 

These include studies on a variety of topics, notably female and male 

reproductive outcomes and birth outcomes in addition to other topics. Section 

4.0 integrates the findings of the epidemiology and experimental toxicology 

studies. 

Question 1.1

Please comment on the sufficiency of the Agency’s non-cancer epidemiology 

reviews with respect to identifying the major strengths and limitations of each 

study.

Question 1.2

At this time, atrazine’s non-cancer epidemiologic database is not robust enough 

for inclusion in quantitative risk assessment and does not support the ability to 

determine causal associations. There are several limitations present in the 

current database, and the quality of atrazine exposure assessment is paramount 

among the limitations. In Section 4.0 of the draft Issue Paper, the Agency 

describes the qualitative similarities and differences between the epidemiologic 

findings and the experimental toxicology database. In short, the observational 

studies – particularly those related to reproductive effects in adult females, as 

well as small for gestational age in newborns – lend further support for the 

human relevance of the laboratory animal findings. 
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a. Please comment on the scientific information that does and does not 

support the Agency’s conclusions (as described in Section 3.0) with 

respect to the characterization of quality, and limitations of the noncancer epidemiologic database and its utility in hazard 

characterization, dose response analysis, and quantitative risk 

assessment. 

b. Please comment on scientific information that does and does not 

support the integrative analysis and conclusions, contained in 

Section 4.0, with respect to the similarities, differences, and 

uncertainties of the experimental toxicology and epidemiologic 

findings. 

Question 2.0: Review of Studies on Mammary Gland Development

At the April SAP, the Agency evaluated effects of atrazine on a number of apical 

endpoints including neuroendocrine, neurotoxicity, and immunotoxicity effects. 

At that time, the Agency deferred review of experimental toxicology studies on 

mammary gland development to the September SAP meeting. Development of 

mammary tissue occurs in defined stages linked to sexual maturation and 

reproduction including embryonic, pre- and peripubertal periods, as well as 

pregnancy and lactation. Given that atrazine affects the hormonal environment 

and reproductive system, it is not unreasonable to hypothesize that it may also 

affect mammary gland development. The database of mammary gland 

development studies includes four studies---three from same the research group 

(Rayner et al., 2004; Rayner et al., 2005; Enoch et al., 2007) in addition to one 

conducted by Coder (2010). It is important to note that the key events leading to 

reported delays in mammary gland development in the young following 

gestational exposure to atrazine are not known and none of these studies 

propose a mechanism/mode of action. 

Question 2.1 

The studies from the two different research groups yielded different results 

regarding the impact of atrazine on mammary gland development. While the 

Rayner and Enoch studies report delays in mammary gland development, the 

Coder study does not. The basis of these different findings is unknown. 

However, one notable difference in the study design used by the two research 

groups is the scoring system. The Rayner et al. and Enoch et al. studies used a 

subjective scoring scale with a 1-4 scoring system (1 = poor 

development/structure; 4 = normal development/structure) with criteria which 

vary depending on the age of the pups. In contrast, Coder employed a 

quantitative morphometric analysis to evaluate mammary gland development. In 

addition, the Coder study also included the use of BrDU labeling to assess cell 

proliferation in the mammary tissue. 
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Please comment on the quality, strengths, and limitations of the mammary gland 

development studies by Rayner et al. (2004, 2005) and Coder (2010) studies. 

Please discuss in your comments factors which could lead to different findings. 

Please comment on the Agency’s conclusions regarding these studies on 

atrazine.

Question 2.2.

In three of the studies considered in Question 2.1 (Rayner et al., 2004 and 2005; 

Coder, 2010), effects have only been observed at an atrazine dose of 100 

mg/kg/day. In contrast, Enoch et al. (2007) report delays in mammary gland 

development as low as 0.09 mg/kg/day following exposure to atrazine and a 

mixture of several metabolites/degradates (DEA, DIA, DACT, hydroxy-atrazine). 

The Agency’s review of this mixture study is provided in Appendix A (Section 

A.10) Please comment on the Enoch et al. (2007) study and the degree to 

which the Agency’s review accurately reflects the strengths and limitations of the 

study. Please include in your comments a consideration of the study design of 

the mixtures experiment and how this design impacts the interpretation of the 

results. 

Question 2.3.

As described in the Introduction of the draft Issue Paper (Section 1.0), the 

Agency’s problem formulation is being conducted in a step-wise manner. One 

important component of this problem formulation is the evaluation of the overall 

database and the quality of the available experimental toxicology studies. In the 

case of mammary gland development, the Agency is proposing to acknowledge 

these four studies (Rayner et al., 2004; Rayner et al., 2005; Enoch et al., 2007; 

Coder, 2010) in the hazard characterization but place little emphasis on them in 

its proposed updates to the dose-response assessment for atrazine. The only 

findings reported in the Rayner et al., (2004), Rayner et al., (2005), and Coder 

(2010) studies, occurred at a high dose (100 mg/kg/day) which is approximately 

50-fold higher than the PoD of 1.86 for LH attenuation being proposed by the 

Agency. In the Enoch et al. (2007) study, there are significant limitations with the 

conduct and reporting the Agency believes which preclude the study from use in 

quantitative risk assessment. 

In light of Panel discussion on Questions 2.1 and 2.2, please comment on the 

manner in which the Agency has proposed to use the mammary gland 

development studies in the hazard assessment for atrazine.
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Question 3.0: Proposed Updates to the Dose-Response Assessment

At the April, 2010 SAP, the Panel made two key recommendations related to 

dose-response assessment: 1) consider pursuing a dose-response analysis 

based on an internal dose metric, as an alternative to the administered dose 

metric used in the previous risk assessment; and 2) identify the sensitive 

endpoint associated with functional impairment and perform benchmark dose 

(BMD) modeling. The Agency conducted both recommended analyses (Section 

5.0 of the draft Issue Paper) in response to these recommendations. LH 

attenuation, as a key event in atrazine’s MoA, is the most sensitive internal dose 

metric that is associated with a number of endocrinopathies

Question 3.1

Given that there is no robust pharmacokinetic model yet available, a noncompartmental analysis of the existing pharmacokinetic data was conducted and 

described in Section 5.2 of the draft Issue Paper. EPA thinks this represents the 

best method to estimate an internal dose metric without exceeding the limits of 

the available data. 

a. Please comment on the Agency’s analysis of the pharmacokinetic data 

contained in Thede (1987) to estimate internal dose reflective of atrazine 

and its metabolites in rat plasma. 

b. Based on Figure 5.5 in the draft Issue Paper, the Agency has preliminarily 

concluded that plasma levels of atrazine and its metabolites reach (or 

nearly reach) pharmacokinetic pseudo-steady state in the rat plasma after 

approximately four daily oral exposures over a wide range of doses. 

Please comment on the extent to which the data do and do not support 

this finding.

c. Figure 5.8 of the draft Issue Paper shows a plot of LH attenuation data 

versus administered atrazine across a range of exposure durations (i.e., 

four days up to six months). The data contained on this figure were 

collected from several different laboratories and involved two modes of 

oral administration (dietary, oral gavage) and two different rat strains. 

Attenuation of LH, as measured by percent of control, is remarkably 

similar across studies, strains, laboratories, and most notably duration of 

exposure. The Agency has preliminarily concluded that the findings on 

Figure 5.8 strongly support the hypothesis that pseudo-steady state is 

achieved (or nearly achieved) in adult rats after four daily consecutive oral 

exposures. The Agency also believes that pseudo-steady state is strongly 

associated with the attenuation of the LH surge following atrazine 

exposure in rats. Please comment on the analysis shown in Figure 5.8 

and the Agency’s preliminary conclusions related to these findings in rats. 
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Question 3.2

In Section 5.3, the Agency describes a benchmark (BMD) analysis conducted for 

LH attenuation studies. As described in the draft Issue Paper, the Agency 

initially considered a wide range of toxicological endpoints for inclusion in the 

BMD analysis. In considering all of the data, this analysis focused on studies 

measuring LH attenuation where female rats were exposed orally for four days 

and/or longer. This was not only the most sensitive endpoint but it’s temporal and 

dose response are well defined experimentally. The LH effect originates from 

atrazine’s effect on the hypothalamic control of pituitary function through its 

interference with GnRH neurotransmitters. Thus, protection from LH attenuation 

as a precursor event to several functional impairments would be protective of 

atrazine’s neuroendocrine effects. 

a. Please comment on the BMD analysis summarized in Section 5.3 of the 

draft Issue Paper with details provided in Appendix C. 

b. The selection of a suitable benchmark response (BMR) is an important 

component of conducting a BMD analysis. As described in Section 5.3, 

for continuous endpoints, like LH attenuation, the BMR most often 

represents an X% change from background levels (or untreated controls). 

Typically, the BMR is selected on the basis of a combination of biological 

(mode of action, quantitative link between key events and adverse 

outcomes, historical/concurrent controls) and statistical considerations 

(sample size, variability, etc). LH attenuation may be potentially 

associated with several adverse outcomes and the level of attenuation of 

the LH surge considered to be adverse is a function of several factors 

including the functional outcome and the life-stage. There is an absence 

of information concerning the level of response (or % change) associated 

with the various functional impairments. Also, the differences in 

reproductive cycles/aging between rodents and humans add an additional 

level of complexity to establishing a specific BMR value. When an X% 

change can not be defined, the Agency’s draft BMD guidance suggests 

that the BMD and BMDL corresponding to a change in the mean response 

equal to one standard deviation from the control mean be used as the 

BMR. This approach was applied to atrazine. Please comment on the 

scientific factors important for establishing a BMR for LH attenuation as 

part of the BMD analysis for atrazine. 
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Question 4.0: Approaches To Evaluating Water Sampling Strategies And 

Frequency Of Monitoring

The ultimate utility of pesticide monitoring data for drinking water exposure 

estimations in human health risk assessments depends on how well the 

monitoring data characterize the spatial and temporal variability of pesticide 

concentrations in drinking water, with an emphasis on the upper end of the 

exposure distribution. In addition, the method used to estimate concentrations 

from monitoring data depends on the duration of concern and how critical it is to 

estimate peak concentrations.

Question 4.1

A well-designed drinking water monitoring study takes into account both spatial 

and temporal patterns of exposure. Please comment on the USEPA’s 

recommended framework for designing a monitoring study by targeting the most 

vulnerable areas, targeting seasonal times for more intensive sampling, basing 

sampling frequency on the toxicological exposure duration of concern, and using 

autosamplers to supplement monitoring data (see Section 7.2 of the draft Issue 

Paper).

Question 4.2

The April 2010 SAP recommended that simulations of candidate monitoring 

strategies and evaluations of the utility of different exposure estimation methods 

be benchmarked against intensive empirical data that cover a representative 

range of sites. In response, the USEPA proposes using more intensively 

sampled ambient water monitoring for flowing waters and PRZM/EXAMS 

modeling for static water bodies, matching them to the CWS based on similar 

chemograph shapes (see Section 7.3). 

a. Please comment on the Agency’s proposal to use chemograph shapes 

(number, duration, and magnitude of peaks) to match CWS with more 

intensively monitored datasets. In particular, the Agency is interested in 

recommendations on approaches for matching chemograph shapes given 

the loss of short-duration peaks that occurs with less frequent sampling.

b. Please comment on the strengths and weaknesses of using more 

intensively sampled datasets from Heidelberg and AEEMP for analysis of

flowing water and PRZM/EXAMS for lakes and reservoirs.
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Question 4.3

Once the magnitude and duration of exposure of toxicological concern are 

identified, the USEPA will determine which method(s) to use to analyze the 

existing atrazine CWS monitoring data for use in drinking water exposure 

estimates. Based on the April 2010 SAP recommendations, the Agency is 

focusing on regression-based modeling combined with random function modeling 

(for example, a revised WARP model coupled with kriging or the USGS 

SEAWAVE-Q model).

Please comment on the strengths and weaknesses of these proposed 

approaches and provide any recommendations for improving the model 

applications. 

Question 5.0: Scientific Considerations in Potential Sensitivity of Infants & 

Children 

As described in Section 6.0 in the draft Issue Paper, the FFDCA, as amended by 

the FQPA (1996), requires the Agency to give special attention to infants and 

children by placing emphasis on the availability of toxicology and exposure 

information to estimate the potential risk to these age groups. The 2003 

Registration Eligibility Decision (RED) notes that the Agency reduced the FQPA 

Safety Factor for those localities which are part of the drinking water monitoring 

program conducted by Syngenta, as required by EPA. This reduction in the 

Safety Factor was based on more intensive monitoring required in the monitoring 

program that reduced the uncertainty for estimating concentrations of atrazine 

and its metabolites in drinking water for the 90-day rolling average identified in 

the previous risk assessment as the appropriate duration of concern. Based on 

the newest studies, our understanding of atrazine’s temporal pharmacokinetic 

and pharmacodynamic characteristics has changed and it appears that shorter 

durations may be appropriate. In light of the potential need to shorten the critical 

duration of exposure and as discussed by the Panel in Questions 4.1-4.3, the 

Agency is evaluating appropriate methods for assessing the drinking water 

monitoring data. 

Previously, the Agency noted data gaps related to “information on Atrazine 

concerning exposure throughout all critical developmental periods (i.e., 

gestation through puberty in both sexes), in particular, early in development.... 

(emphasis added, FQPA memo, 2002).” At the time of the previous risk 

assessment, there were only a few available studies which included measures 

sensitive to endocrine disruption on specific early life exposure periods (e.g., 

peripubertal). Since that time new studies have become available (Section 6.0) 

which add to the existing database and support the findings of the older studies. 

In addition, new tissue dosimetry studies provide information on the transfer of 

atrazine and its chlorinated metabolites to the fetus and to the lactating pup. 

Further, a study evaluating exposure to atrazine from multiple critical 
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developmental periods identified previously by the Agency as a data gap is ongoing. The results of this research will inform our understanding of the potential 

for differential life stage sensitivity.

In the coming months, as the drinking water exposure analysis develops further 

and the on-going toxicology studies become available, the Agency will work 

towards completing the scientific analysis that will inform whether or not a new 

FQPA Safety Factor should be applied in the atrazine risk assessment. 

The Agency requests the Panel to comment on important scientific factors for the 

Agency to consider in its analysis. Please include in your comments specific 

consideration of uncertainties in estimating drinking water exposures and 

remaining uncertainties in atrazine’s toxicological profile across life stages, 

particularly as they pertain to assessing risk to infants and children.

Question 6.0: Implications of MOA & Toxicity Profile on Water Monitoring

As described in the Introduction of the draft issue Paper (Section 1), the goals of 

the current atrazine re-evaluation are: 1) to determine the extent to which new 

science indicates a need for the Agency to develop a revised human health risk 

assessment for atrazine and 2) to re-consider, as appropriate, the frequency of 

drinking water monitoring needed. Two important issues related to achieving 

these goals are determining the point of departure and identifying the critical 

effect and its associated duration of exposure. Proposed updates to the point of 

departure are considered in Questions 3.1 and 3.2. With respect to the critical 

duration of concern, the frequency of drinking water monitoring is related to the 

temporal pattern of the toxicological endpoint of concern used for the risk 

assessment. Generally, longer durations of toxicological concern (e.g., a longterm chronic effect) require a less frequent drinking water sampling design to 

approximate longer term exposures. However, as the duration of concern 

shortens, the frequency and timing of sampling become more important in 

determining how well the sample data capture short-duration exposures. 

Observation epidemiology studies raise hypotheses and suggest possible links 

between atrazine exposure and reproductive and developmental outcomes, but 

these epidemiology studies suffer from limitations which prevent firm 

conclusions. Although certain studies some provide qualitative support for the 

human relevance of the endpoints identified through the experimental 

toxicological database, they provide little to no information on the critical duration 

of exposure. In addition, the MOA and PK database are also lacking in human 

specific information on the effects of atrazine which could be used to 

quantitatively extrapolate between species. As such, the information available to 

evaluate the critical duration of exposure lacks quantitative precision. Thus, the 

critical duration of exposure is instead derived by inferring generic knowledge 

from a variety of scientific disciplines. 
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The Agency has used a variety of approaches to extrapolate findings from 

experimental animal data to humans including allometric scaling and humanspecific information on the physiology of the menstrual cycle inferred from the 

IVF literature. According to the Agency’s analysis of the pharmaceutical data 

and allometric scaling of the rodent pharmacokinetics data, the potential 

durations of human exposure that would correspond to the exposure period of 

interest in rats lie between a few days to approximately 4 weeks of exposure. 

Please comment on the Agency’s analysis and preliminary conclusions contained 

in Section 8.0 of the draft Issue Paper as it relates to the potential critical 

windows of exposure. Please include in your comments additional or alternative 

approaches or data that may inform this issue.