Effects of environmental agents on the attainment of puberty: considerations when assessing exposure to environmental chemicals in the National Children's Study.

The apparent decline in the age at puberty in the United States raises a general level of concern because of the potential clinical and social consequences of such an event. Nutritional status, genetic predisposition (race/ethnicity), and environmental chemicals are associated with altered age at puberty. The Exposure to Chemical Agents Working Group of the National Children's Study (NCS) presents an approach to assess exposure for chemicals that may affect the age of maturity in children. The process involves conducting the assessment by life stages (i.e., in utero, postnatal, peripubertal), adopting a general categorization of the environmental chemicals by biologic persistence, and collecting and storing biologic specimens that are most likely to yield meaningful information. The analysis of environmental samples and use of questionnaire data are essential in the assessment of chemicals that cannot be measured in biologic specimens, and they can assist in the evaluation of exposure to nonpersistent chemicals. Food and dietary data may be used to determine the extent to which nutrients and chemicals from this pathway contribute to the variance in the timing of puberty. Additional research is necessary in several of these areas and is ongoing. The NCS is uniquely poised to evaluate the effects of environmental chemicals on the age at puberty, and the above approach will allow the NCS to accomplish this task. Key words: children, environmental chemicals, exposure assessment, hormonally active agents, National Children's Study, puberty. doi:10.1289/ehp.7615 available via http://dx.doi.org/[Online 12 May 2005]

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The purpose of the National Children's Study (NCS) is to evaluate the health risks to children in this country from environmental exposures by using a longitudinal cohort design (Children's Health Act of 2000). This effort is expected to improve our children's health by mitigating these health risks in our society. The approach of the NCS is to centralize its activities around hypotheses that evaluate the relationship between a wide array of environmental exposures (i.e., chemical, biologic, physical, and psychosocial factors) and priority health outcomes. The list of priority outcomes includes pregnancy, neurodevelopmental injury, asthma, and obesity and physical development. One of several NCS hypotheses addressing these concerns seeks to evaluate the effects of environmental agents on the age at puberty, which was discussed at an inter-work group meeting co-chaired by the Nutrition, Growth, and Pubertal Development Working Group and the Exposure to Chemical Agents Working Group (Baltimore, Maryland, 17-18 December 2002). This article presents a brief background on the factors associated with the age of maturity and discusses the assessment for exposure to environmental chemical agents in the developing child by using a life-stage approach. The articles in this mini-monograph describe the roles and efforts of the Exposure to Chemical Agents Working Group in the NCS. Portions of this discussion may refer the reader to these other articles for additional information.

Recent evidence suggests that puberty in U.S. children is starting at an earlier age compared with previous years (Anderson et al. 2003; Freedman et al. 2003; Herman-Giddens et al. 1997, 2001). This is of general interest because the extent to which this is occurring in this population has not been well characterized and because such findings have potential influences within our society. In cross-sectional studies of populations in the United States from 1988 through 1994, it is reported that the mean age of onset for breast development for girls was 9.5-9.7 years, which is approximately 1-2 years earlier than the observations of previous investigators (Lee et al. 2001). This approximates to 14% of the population achieving Tanner stage 2 or greater for breast development at the age of 8 years (Lee et al. 2001). In U.S. boys sampled from 1988 through 1994, the median age of onset for genital development ranged from 9.5 to 10.4 years, depending on race/ethnicity, which is about 1 year earlier than that previously described (Herman-Giddens et al. 2001). This corresponded to about 32-58% of the boys attaining Tanner stage 2 at 9 years of age. These observed changes in pubertal development are not unique to the United States but also have been noted in other countries (Huen et al. 1997; Karlberg 2002; Parent et al. 2003; Proos et al. 1993; Viner 2002). To define properly the significance of these findings to the population, more refined investigations are needed to characterize the time trend in pubertal development because of the limitations in study design of these prior reports (Lee et al. 2001; Reiter and Lee 2001; Viner 2002). Nutrition, genetic predisposition, and environmental chemical exposure are factors associated with pubertal change that can be evaluated during this process.

The factors associated with altered pubertal development are numerous (Parent et al. 2003). When early onset of puberty was originally noted in immigrant children, it was attributed to improved nutrition and well-being. Although these remain the primary factors determining the onset and progression of puberty, there are other considerations. These include genetic predisposition or host susceptibility, and environmental chemical exposure. Certain populations may be susceptible to variations in pubertal development base on observations among racial/ethnic groups (Anderson et al. 2003; Freedman et al. 2002; Sun et al. 2002; Wu et al. 2002). African-American girls were reported to develop either thelarche or pubarche at about the mean age of 8.8 years, 1-2 years earlier than non-Hispanic white girls (Herman-Giddens et al. 1997). This trend in race/ethnic groups for the age of onset for puberty was noted in boys as well (Herman-Giddens et al. 2001; Sun et al. 2002). Although obesity was associated with the early onset of puberty in these racial/ethnic groups, it was less of a determining variable in African-American girls, suggesting the involvement of other factors (Kaplowitz et al. 2001). In a study of serum leptin levels in girls 8-17 years of age, African-Americans girls were noted to have higher leptin levels than Caucasians after controlling for obesity, age, and serum insulin levels (Wong et al. 1998). Although serum leptin was shown to increase before gonadotropin levels in girls and boys (Garcia-Mayor et al. 1997), the signaling pathway for leptin in the development of puberty is not known, and further work is necessary to define this mechanism and the difference in leptin levels among racial/ethnicity groups (Danadian et al. 1999; Ruhl et al. 2004).

The early onset of puberty is clinically and socially important to the population. The early onset of thelarche is associated with an early diagnosis of breast cancer in susceptible populations (Hamilton and Mack 2003) and adult obesity (Biro et al. 2003). Breast cancer is associated with an early age of onset of menarche as well. Girls attaining menarche after 13 years of age were observed to have a one-third decreased risk [multivariate relative risk = 0.66; 95% confidence interval (CI), 0.44-0.99] for breast cancer compared with those attaining menarche at a younger age (Garland et al. 1998). This effect also was characterized by defining a 2-year delay in the age of menarche to a 10% decrease in risk for breast cancer (Hsieh et al. 1990). These observations imply the need for additional surveillance for these measures because they can affect diagnostic evaluation (Chalumeau et al. 2002; Dorn et al. 1999; Kaplowitz and Oberfield 1999). The social consequence is that children will need to learn how to adapt to their maturing bodies at an earlier age, and our public school system will need to re-evaluate the timing and structure of health education classes because the early onset of puberty has been associated with risky and unhealthy behaviors (Orr and Ingersoll 1995; Simon et al. 2003).

Environmental Chemicals

Several environmental chemicals are known to have hormonal activity, and their effects are well demonstrated in animal studies (Goldman et al. 2000; Stoker et al. 2000). The hormonal effects of these chemicals can be categorized by estrogenic, antiestrogenic, androgenic, antiandrogenic, and thyroidal activities. Depending on a chemical's hormonal activity, onset of puberty may be either delayed or accelerated. There is also experimental evidence demonstrating that chemical exposures during gestation can result in altered mammary gland development (Nikaido et al. 2004; Rayner et al. 2004) and age of pubertal onset (Casanova et al. 1999; Nikaido et al. 2004; Tou et al. 1998). The effects of these chemicals on the growth, development, and reproductive health of people are largely unknown because this remains a relatively new area of investigation. The chemical class that is best described in this area is polychlorinated biphenyls (PCBs). PCBs and their hydroxylated metabolites are known for their effects on the thyroid regulatory pathway, which can affect neurodevelopment (Jacobson and Jacobson 1996; Meerts et al. 2002). In addition to PCBs, several other chemicals with hormonal activity are associated with health effects after exposure, some of which are discussed below (Table 1). (Note that the discussion of these chemicals does not imply that the NCS has determined that these chemicals either cause these health effects or will be included in the NCS.)

Environmental lead was recently observed to be associated with delayed age of onset of puberty in girls in the United States (Selevan et al. 2003; Wu et al. 2003). In a cross-sectional survey of 1,706 girls 8 through 16 years of age during 1988 through 1994, increased blood lead level was associated with a decreased likelihood for the attainment of either pubarche or menarche but not thelarche (Wu et al. 2003). Girls with lead levels in the ranges of 2.1-4.9 and 5-21.7 [micro]g/dL were approximately 50 and 80%, respectively, less likely to reach these measures of puberty than those with lead levels between 0.7 and 2.0 [micro]g/dL. In younger girls (8 through 12 years of age), the percentage of girls attaining pubarche by age decreased with increasing blood lead levels. When the same population was analyzed by racial/ethnicity groups, a higher blood lead level (3 vs. 1 [micro]g/dL) in African-American girls was associated with delayed onset of pubarche, thelarche, and menarche by 2-4 months (Selevan et al. 2003). There also was a decreased likelihood of approximately 60% for the attainment of a successive Tanner stage for breast and pubic hair development in African-American girls. Similar delays in thelarche and pubarche were observed in Mexican-American girls, except the delays were smaller in magnitude. No differences in blood lead levels were observed in pubertal outcomes in non-Hispanic white girls. Underlying differences in growth or hormonal regulation among the racial/ethnic groups may explain the observed pubertal effects in association with lead exposure (Wong et al. 1998).

Chronic exposure to lead was demonstrated to affect the hypothalamo-pituitary-gonadal axis by altering serum levels of gonadotropic and androgenic hormones. In occupationally exposed males, prolonged lead exposure at the workplace was associated with decreased serum testosterone (Rodamilans et al. 1988). Serum luteinizing hormone levels were found to be lower both in occupational males (McGregor and Mason 1990) and in children 11 through 13 years of age from the general population (Vivoli et al. 1993) with high lead levels. The effects of exposure to environmental lead on the levels of estrogens, androgens, and gonadotropins in younger children and their pubertal development remain to be determined. Future studies in this area need to pay particular attention to the timing of exposure during development and the contribution of race/ethnicity to such findings.

The effects of the exposure to persistent organohalogen chemicals on maturation have been evaluated in several epidemiologic investigations. Two studies evaluated the contribution of in utero and postnatal exposure to these chemicals. Polybrominated biphenyls (PBBs) were found to be associated with premature thelarche in girls who were breast-fed by mothers with high serum PBB levels ([greater than or equal to] 7 [micro]g/L) resulting from exposure to milk from cows given contaminated feed (Blanck et al. 2000). However, in girls who were not breast-fed by their PBB-exposed mothers, the maternal serum PBB level was not associated with early breast development, thus suggesting the contribution of postnatal exposure to altered pubertal development in this setting. There also was an increased risk (adjusted hazard ratio, 3.70; 95% CI, 1.41-9.71) for the earlier onset of menarche in girls breast-fed by mothers with high serum PBB levels compared with girls who were not breast-fed by mothers with low serum PBB levels (less than or equal to limit of detection). In the North Carolina Infant Feeding Study (1978 through 1982), in utero exposure to either PCBs or 1,1'-(2,2-dichloroethenylidene)-bis[4-chlorobenzene] [p,p'-DDE, a metabolite of dichlorodiphenyltrichloroethane (p,p'-DDT)] was not associated with the time to attainment of the Tanner stages in boys and girls (Gladen et al. 2000). However, increasing human milk levels of PCBs and p,p'-DDE were associated with increased body weight in girls and boys, respectively. Other studies evaluating the effects of exposure to PCBs and dioxin-like compounds (Den Hond et al. 2002) and to endosulfan (Saiyed et al. 2003) on the timing of pubertal milestones found either no effects or delayed effects, which varied by gender. These latter reports were limited by either the contribution of potential risk factors (e.g., congenital disorders) to the findings or their inability to relate their findings to prenatal versus postnatal exposure because the participants were evaluated only during puberty.

Finally, nonpersistent chemicals such as phthalates (Colon et al. 2000) and phytoestrogens have been investigated for their association with altered time of onset of puberty as well. Phytoestrogens are naturally occurring environmental chemicals found in fruits, vegetables, and legumes (e.g., soybeans) and have notable estrogenic activity. In a case-control study, the intake of soy-based formula was associated with an increased likelihood of premature thelarche in children younger than 2 years but not in children diagnosed with thelarche beyond this age (Freni-Titulaer et al. 1986). This suggests that various factors were most likely contributing to early breast development in this study population, depending upon the age of onset of the disorder. In both these areas of investigation, additional work is needed to clarify these earlier findings because of either their small sample sizes or the current availability of improved methodology (e.g., biomonitoring) in exposure assessment.

Despite the findings of the various studies presented in the preceding section, the extent to which environmental chemicals contribute to changes in human pubertal development remains largely unknown. This is because of limitations in the designs of the investigations from which these observations were made, the various approaches used in conducting exposure assessment, and the few number of chemicals studied to date that are related to populations. A focused investigation could attempt to answer some of the questions regarding environmental chemical exposure and maturation. For example, what is the significance to pubertal development of the timing of exposure to a chemical during a child's growth? Current studies in this area do not allow for such conclusions because of cross-sectional or retrospective design.

The observations from either cross-sectional or retrospective study designs are difficult to interpret as to the timing of chemical exposure during a child's development, and use less than perfect models to assess for overall exposure when a single level of a chemical in a biologic matrix is available. An example of when the latter situation becomes challenging is with human milk. When infant exposure is assessed from human milk consumption, both the changing level of the chemical in the milk during the breast-feeding period and the duration of breast-feeding must be considered. The timing of exposure to a chemical is extremely difficult to ascertain from these study designs. This is because the random sampling of blood or urine from an individual does not allow for a determination of when the exposure occurred, which could have been at any time before the procurement of the specimen. However, exposures to apparent events (e.g., the industrial chemical explosion in Seveso, Italy, in 1976) in combination with elevated levels are exceptions. Timing is more of a problem for a persistent chemical with a long half-life than for a nonpersistent chemical. This problem can make it difficult to assess either the occurrence of subacute exposures or the source of the exposure when location has changed. The lack of consideration for these concerns can lead to either misclassification of a study participant for a critical window of exposure, or increased variance in the population for a given effect. Additional concerns with these types of designs for investigating health effects include the difficulty in establishing a temporal relation between exposure and effect and the introduction of recall bias when collecting information from the participants.

These study design issues are best resolved with a prospective longitudinal design, which can be accomplished for environmental chemicals during the evaluation of other factors more closely associated with pubertal development. The assessment for exposure in this setting may be more resource intensive than in the aforementioned designs, but this can be reduced by constructing nested case-control studies. Nutrition and genetic predisposition are important contributors to growth and maturity, which require further investigations. Although it is generally accepted that nutrition affects pubertal development, unresolved issues remain because this association is variable among studies, and recent evidence suggests that birth size contributes to puberty as well (Luo et al. 2003). The extent to which birth height and body mass index relate to nutritional status in determining the timing of puberty is not known. As more work is done to resolve such questions, it would be important to control for environmental chemical exposure.

Strategies for Testing Hypotheses

Sampling Periods

The timing for the collection of exposure data (environmental, questionnaire, and biologic) depends on two general factors: the health effect being monitored and the environmental chemical that is under evaluation. In the evaluation of sexual maturation, it is important to monitor for the formative periods of the sexual organs (in utero) when a significant amount of chemical exposure is expected, that is, postnatal (breast-feeding), pre- and peripuberty, and the completion of puberty. These are discussed to a greater extent elsewhere (Pryor et al. 2000). Recruitment of participants during preconception will ensure that exposures in the first trimester of gestation will be characterized, and data collected at this time may serve as a baseline comparison throughout the pregnancy and the postpartum period.

The successful completion of sexual maturation is dependent on proper anatomic development of the organs and hormonal regulation leading to their function. The anatomic development of organs and sexual differentiation of the fetus occurs during pregnancy or the in utero period. Organogenesis takes place in the first trimester (weeks 4-8) of gestation, and disruption during this time can lead to disorders that may be apparent at birth or not apparent until adulthood. Some notable examples of this include diethylstilbestrol (DES) and thalidomide. DES was used in the early 1940s and 1950s for the treatment of irregular vaginal bleeding and threatened abortions. DES is associated with the occurrence of cervical or vaginal clear-cell adenocarcinoma in the [F.sub.1] generation when exposure was before 18 weeks of gestation (Hatch et al. 1998). Thalidomide was used in the late 1950s to treat nausea and vomiting associated with pregnancy, causing severe limb malformations in the newborn when exposure occurred during days 34-50 of gestation (Smithells and Newman 1992). The in utero exposure to various stimuli leading to altered pubertal development was demonstrated in animal experiments using intrauterine artery ligation and chemical treatments. Intrauterine ligation during gestation was used to simulate malnutrition in a rat model and was shown to cause a delay in the onset of puberty in males and females (Engelbregt et al. 2000). Bisphenol A (Howdeshell et al. 1999) and octylphenol (Bogh et al. 2001; Wright et al. 2002) treatments in animals were demonstrated to advance puberty by shortening the time to either the first estrus or its equivalent. For example, the gilts of sows treated with octylphenol from days 23-85 of gestation were observed to have a shorter period to first estrous compared with controls (mean [+ or -] SD, 211.5 [+ or -] 14.5 days vs. 246.0 [+ or -] 36.7 days) (Bogh et al. 2001). However, octylphenol did not alter testicular size in boars that were exposed to octylphenol as fetuses (Bogh et al. 2001). When ewes were treated with octylphenol from day 70 of gestation to birth, the times to the dates of first estrous and first progesterone rise in the lambs were decreased compared with controls by an average of about 20 days and 44 days, respectively (Wright et al. 2002). In the same study the lambs of ewes treated through birth and to weanling had shorter times to puberty compared with controls as well, but they were not different from lambs from ewes treated to birth. Atrazine delayed mammary gland development in female rats treated either as fetuses during gestational days 15-19 or during lactation (Rayner et al. 2004). In the same model, vaginal opening was delayed only in rat pups that were exposed to atrazine by lactation. Phytoestrogens, such as genistein (Casanova et al. 1999) and the lignan-containing flaxseed (Tou et al. 1998), advanced the time of onset of puberty in female rats when high doses were initiated during the gestational period and continued posnatally. Of note, when flaxseed was administered at a lower dose, it delayed the onset of puberty (Tou et al. 1998). No observed pubertal effects (i.e., time of onset, testicular weight) were found in males in these phytoestrogen models. The effects of in utero exposure to environmental chemicals on pubertal development were evaluated in several epidemiologic studies. For PCBs and p,p'-DDE this route of exposure was compared with that from breast-feeding in one study (Table 1). The significant difference between these two exposure periods is that the amount of chemical delivered to the infant from lactation is usually greater than the amount of chemical delivered to the fetus by the placenta (Jacobson and Jacobson 1996). The importance of these experimental findings in animals and data on human exposure to chemicals is that they lend support to the biologic plausibility or likelihood that a chemical can either cause or modify the outcome of the purported health effect. It will be important to consider mechanisms of action, timing of exposure, and existing chemical levels in the general population from background exposure when candidate chemicals are reviewed for this hypothesis.

During the in utero period, the fetus should be assessed for exposures that coincide with the developmental windows of the primary and secondary sexual organs (Pryor et al. 2000) (Table 2). For example, mammary gland development begins at about week 4 and continues through week 10 of gestation. The primary mammary bud is formed during this period. From weeks 10 through 15 of gestation, formation of secondary buds, the nipple, and the areola occurs. The critical window of development for the external genitalia is from weeks 7 through 12 of gestation. Because it is important to assess for exposure during this early period of gestation, it is recommended to recruit study participants before conception to ensure that this critical period during gestation is not missed.

Once the sexual organs are formed, their activation for reproduction is determined by hormonal (gonadotropins, estrogens, androgens) regulation. In females this is largely determined by estrogenic activity and in males, by androgens. In the prepubertal female, estrogen levels are low and stable until puberty, when a surge in estrogen level occurs, resulting from gonadotropin stimulation. The pituitary gland begins to secrete luteinizing hormone at approximately 1-3 years before the onset of puberty. The prepubertal period is an important time to assess for environmental chemical exposure because of several reported cases of girls developing premature thelarche after being exposed to estrogen compounds (Teilmann et al. 2002). Thus, the sampling would include in utero (gestation), postnatal (for breast-feeding infants), before puberty (pre- and peripuberty), and postpuberty (or the completion) periods. It is important to monitor puberty to its completion because the time of onset of puberty and the duration for its completion may vary independently. Further definitions of the frequency for monitoring during the prepubescent period may be gained from reviewing longitudinal studies on puberty (Biro et al. 1995; Lee 1980; Roche et al. 1995).

Populations with High Exposures

Certain populations that may be susceptible or vulnerable to environmental chemical exposure will need to be considered in the study design. The reasons are variable and include genetic predisposition, nutrition, and socioeconomic factors. For example, persons frequently ingesting fish or whale meals are likely to have elevated levels of PCBs, polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs). This was observed in persons residing near the Great Lakes region of the United States and members of the Inuit tribe in Canada. Children residing in homes built before 1946 were found to have approximately a 5-fold increased incidence of blood lead levels [greater than or equal to] 10 [micro]g/dL compared with children residing in homes built after 1973 (Pirkle et al. 1998). Low-income status and African-American race/ethnicity were associated with elevated blood lead levels ([greater than or equal to] 10 [micro]g/dL) in families living in homes built before 1946.

Environmental Chemicals

Many environmental chemicals are observed to have endocrine or hormonal activity (National Research Council 1999). However, only a few of these chemicals are associated with effects on the age at puberty in humans, including lead (Selevan et al. 2003; Wu et al. 2003), PBBs (Blanck et al. 2000), PCBs (Den Hond et al. 2002; Gladen et al. 2000), PCDDs/PCDFs (Den Hond et al. 2002), p,p'-DDT/p,p'-DDE (Gladen et al. 2000; Krstevska-Konstantinova et al. 2001), endosulfan (Saiyed et al. 2003), phthalates (Colon et al. 2000), and phytoestrogens (Freni-Titulaer et al. 1986). To encompass all these chemicals into the assessment strategy, the chemicals can be categorized as either biologically persistent or nonpersistent (Table 2). Generally, persistent chemicals have long half-lives (months to years) and are measured in the blood. Although this may seem advantageous from a sampling perspective (i.e., wide window of opportunity), it does pose a challenge when critical windows of exposure are of concern and when subacute exposures may have occurred. Thus, if exposure occurs during critical periods of a child's development, then specimens should be obtained proximate to those periods. Questionnaires and environmental assessment would be necessary if the pathways of exposure are important, because biologic monitoring would not be able to identify the exposure pathway or the exposure location. For example, the presence of a persistent chemical in the blood of a person who recently moved to a new location would most likely represent an exposure from the former location. However, this would not be known by the blood test alone. If documentation of exposure of a persistent chemical is all that is necessary, then biologic monitoring should be adequate. Nonpersistent chemicals have short half-lives (hours to days) and are usually measured in the urine. For nonpersistent organic chemicals, increased reliance is needed on questionnaires and environmental samples to document exposure. This is because of the relatively short time that these chemicals exist in the body.

Measurements

Exposure assessment. The choice of the biologic matrix to measure for the chemicals is largely determined by the properties of the chemical (e.g., lipid solubility, protein-binding capacity, and ionic charge), which governs the distribution of the chemical in the body. Blood and urine are the standard specimens used for the measurement of chemicals, and water-soluble chemicals are commonly quantified in urine. In addition, the target organ or organism needs to be considered. For example, if the interest is in fetal exposure, then cord blood is the preferred specimen. If the route of exposure is important, for example, identify either the source of exposure or the degree to which one source may be more important in determining a health outcome. If two sources are involved, then several matrices will have to be sampled. For example, an infant's exposure can occur either during the in utero period through the placenta or during breast-feeding. However, neither the amount of chemical exposure nor the determined health effect is consistent by either of these routes of exposure. In the case of PCB exposure, it was determined that breastfeeding contributed to a larger amount of exposure to the infant than did placental transfer; however, the latter was deemed more consequential to delayed neurodevelopment (Jacobson and Jacobson 1996). If the extent to which a chemical partitions into various biologic matrices is known, then alternative specimens may be used if the desired specimen is not adequate. For example, maternal blood, cord blood, and maternal milk may be used to assess for fetal exposure to persistent organic chemicals. The assessment of the infant to exposure to chemicals from human milk requires multiple milk samplings during the breast-feeding period because of the expected decline in the level of persistent chemicals in human milk from maternal depuration. Additional consideration is needed to characterize the difference between infants who are exclusively breast-fed and those who are partially breast-fed because the volume of milk intake will vary by these practices of breast-feeding. Meconium is a biologic matrix that is currently under investigation as an indicator of chemical exposure. The interest in this matrix is because of its potential to reflect chemical exposures to the fetus as early as the second trimester.

Researchers may need to customize questionnaires and environmental sampling based on the use patterns of particular classes of chemicals. Phthalates, for example, are commonly found in personal care products (e.g., cosmetics, shampoos); therefore, researchers should ask specific questions regarding the use of these products (Tiwary and Ward 2003). Similarly, changes in location throughout the life stages (home, school, work site) may warrant adjustments of these assessment tools. In the design of these exposure assessment tools, it is important to validate them before applying them to health effect studies.

Biologic markers of health effect. These measurements will be used to monitor the onset and completion of puberty. Various tools can be used, including biochemical measures (e.g., hormones), physical examination, and diagnostic scans (Rockett et al. 2004) (Table 3). Dual-energy X-ray absorptiometry is a noninvasive method to assess body fat mass, body fat-free mass, and bone mineral mass and density for skeletal maturity.

Potential Risk Factors for Altered Pubertal Development

Several factors are known to cause or are associated with altered pubertal development that need to be considered in the study because they may bias the estimate of the relationship between chemical exposure and the age of onset of puberty. Some of these items are listed in Table 4.

Conclusion

Evidence exists that nutritional status, genetic predisposition (race/ethnicity), and environmental chemical exposure are associated with altered age at puberty. The assessment for exposure to environmental chemicals that may affect the attainment of puberty in the developing child is challenged by several factors, including the various developmental periods when an exposure can affect the maturation process, limited access to certain biologic specimens during select periods of human development, and the many chemicals with varying properties. The recommended approach is to conduct the assessment by life stages (i.e., in utero, postnatal, pre- and peripubertal). Study participants need to be recruited at preconception to ensure that the "critical window" of gestation, that is, the first trimester, is included in the exposure assessment process. Chemicals should be categorized by biologic persistence, and biologic specimens that are most likely to yield meaningful information should be collected and stored. The latter will allow for the judicious use of specimens of limited quantity for special investigations (e.g., nested case-control studies) and time to develop new and improved analytical methods. For chemicals that either cannot be measured in biologic specimens or have short biologic half-lives, the analysis of environmental samples and use of questionnaire data are necessary to complete the assessment for exposure. Food and dietary data may assist in determining the extent to which nutrients and chemicals from this pathway contribute to the variance in the timing of puberty. Factors that are either known causes of, or associated with, altered pubertal development need to be controlled for during the assessment of health outcomes. The National Children's Study is uniquely poised to evaluate the effects of environmental chemicals on the age at puberty, and the above approach will allow the NCS to accomplish this task.

Table 3. Health effect measures to assess for pubertal
development. (a)

Onset of puberty
  Sexual maturity rating (Tanner staging)
    Performed by observer or by self-reporting
    Frequency: annually
  Stature
  Hormones [luteinizing hormone, estradiol, testosterone,
  dehydroepiandrostendione-sulfate (DHEA-S)]
    Salivary hormone levels
    Thyroid-stimulating hormone

Later stages of puberty
  Voice pattern change (males)
  Menarche
  Menstrual history
  Dual-energy X-ray absorptiometry
    At age 6 and 12 years for females and at age 6 and
      14 years for males

(a) Discussed at the NCS inter-work group meeting "Obesity
and Physical Development." This meeting was co-chaired
by the Nutrition, Growth, and Pubertal Development and
the Exposure to Chemical Agents working groups in
Baltimore, Maryland (USA), on 17-18 December 2002.

Table 4. Potential risk factors affecting pubertal development.

Precocious
  Gonadotropin-dependent (central precocious puberty)
    Brain pathology (e.g., hypothalamic hamartoma, tumors,
      hydocephalus, severe head trauma)
    Hypothyroidism, untreated
  Gonadotropin-independent precocious puberty
    McCune-Albright syndrome in girls
    Familial male precocious puberty (testotoxicosis)
    Tumors (ovarian, adrenocortical, Leydig cell, chorionic
      gonadotropin-secreting tumors)
    Exogenous pharmaceutical estrogen or androgen use
  Isolated premature thelarche, premature pubarche/adrenarche,
    premature menarche

Delayed
  Poor nutrition
  Chronic illness
  Constitutional growth delay
  Intense physical training
General categories associated with altered pubertal development
  General nutrition
  General health
  Brain injury
  Obesity
  Socioeconomic status
  Immigration/adoption
  Race/ethnicity
  Pharmaceuticals [estrogenic, androgenic, estrogen blocker (i.e.,
    aromatase inhibitors), and androgen blocker (e.g., flnasteride,
    flutamide)]
  Genetics
  Gestational age

REFERENCES

Anderson SE, Dallal GE, Must A. 2003. Relative weight and race influence average age at menarche: results from two nationally representative surveys of US girls studied 25 years apart. Pediatrics 111(4 pt 1):844-850.

Biro FM, Lucky AW, Huster GA, Morrison JA. 1995. Pubertal staging in boys [published erratum J Pediatr 127:674]. J Pediatr 127:100-102.

Biro FM, Lucky AW, Simbartl LA, Barton BA, Daniels SR, Striegel-Moore R, et al. 2003. Pubertal maturation in girls and the relationship to anthropometric changes: pathways through puberty. J Pediatr 142:643-646.

Blanck HM, Marcus M, Tolbert PE, Rubin C, Henderson AK, Hertzberg VS, et al. 2000. Age at menarche and Tanner stage in girls exposed in utero and postnatally to polybrominated biphenyl. Epidemiology 11:641-647.

Bogh IB, Christensen P, Dantzer V, Groot M, Thofner IC, Rasmussen RK, et al. 2001. Endocrine disrupting compounds: effect of octylphenol on reproduction over three generations. Theriogenology 55:131-150.

Casanova M, You L, Gaido KW, Archibeque-Engle S, Janszen DB, Heck HA. 1999. Developmental effects of dietary phytoestrogens in Sprague-Dawley rats and interactions of genistein and daidzein with rat estrogen receptors alpha and beta in vitro. Toxicol Sci 51:236-244.

Chalumeau M, Chemaitilly W, Trivin C, Adan L, Breart G, Brauner R. 2002. Central precocious puberty in girls: an evidence-based diagnosis tree to predict central nervous system abnormalities. Pediatrics 109:61-67.

Children's Health Act of 2990. Public Law 106-310, 2000.

Colon I, Caro D, Bourdony C J, Rosario O. 2000. Identification of phthalate esters in the serum of young Puerto Rican girls with premature breast development. Environ Health Perspect 108:895-900.

Danadian K, Suprasongsin C, Janosky JE, Arslanian S. 1999. Leptin in African-American children. J Pediatr Endocrinol Metab 12:639-644.

Den Hond E, Roels HA, Hoppenbrouwers K, Nawrot T, Thijs L, Vandermeulen C, et al. 2002. Sexual maturation in relation to polychlorinated aromatic hydrocarbons: Sharpe and Skakkebaek's hypothesis revisited. Environ Health Perspect 110:771-776.

Dorn LB, Hitt SF, Rotenstein D. 1999. Biopsychological and cognitive differences in children with premature vs. on-time adrenarche. Arch Pediatr Adolesc Med 153:137-146.

Engelbregt MJ, Houdijk ME, Popp-Snijders C, Delemarre-van de Waal HA. 2000. The effects of intra-uterine growth retardation and postnatal undernutrition on onset of puberty in male and female rats. Pediatr Res 48:803-807.

Freedman DS, Khan LK, Serdula MK, Dietz WH, Srinivasan SR, Berenson GS. 2002. Relation of age at menarche to race, time period, and anthropometric dimensions: the Bogalusa Heart Study. Pediatrics 110:1-7.

Freedman DS, Khan LK, Serdula MK, Dietz WH, Srinivasan SR, Berenson GS. 2003. The relation of menarcheal age to obesity in childhood and adulthood: the Bogalusa Heart Study. BMC Pediatr 3:1-9.

Freni-Titulaer LW, Cordero JF, Haddock L, Lebron G, Martinez R, Mills JL. 1986. Premature thelarche in Puerto Rico. A search for environmental factors. Am J Dis Child 140:1263-1267.

Garcia-Mayor RV, Andrade MA, Rios M, Lage M, Dieguez C, Casanueva FF. 1997. Serum leptin levels in normal children: relationship to age, gender, body mass index, pituitary-gonadal hormones, and pubertal stage. J Clin Endocrinol Metab 82:2849-2855.

Garland M, Hunter DJ, Colditz GA, Manson JE, Stampfer MJ, Spiegelman D, et al. 1998. Menstrual cycle characteristics and history of ovulatory infertility in relation to breast cancer risk in a large cohort of US women. Am J Epidemiol 147:639-643.

Gladen BC, Ragan NB, Rogan WJ. 2000. Pubertal growth and development and prenatal and lactational exposure to polychlorinated biphenyls and dichlorodiphenyl dichloroethene. J Pediatr 136:490-496.

Goldman JM, Laws SC, Balchak SK, Cooper RL, Kavlock RJ. 2000. Endocrine-disrupting chemicals: prepubertal exposures and effects on sexual maturation and thyroid activity in the female rat. A focus on the EOSTAC recommendations. Crit Rev Toxicol 30:135-196.

Hamilton AS, Mack TM. 2003. Puberty and genetic susceptibility to breast cancer in a case-control study in twins. N Engl J Med 348:2313-2322.

Hatch EE, Palmer JR, Titus-Ernstoff L, Noller KL, Kaufman BH, Mittendorf R, et al. 1998. Cancer risk in women exposed to diethylstilbestrol in utero. JAMA 280:630-634.

Herman-Biddens ME, Slora EJ, Wasserman RC, Bourdony CJ, Bhapkar MV, Koch (SG, et al. 1997. Secondary sexual characteristics and menses in young girls seen in office practice: a study from the Pediatric Research in Office Settings network. Pediatrics 99:505-512.

Herman-Giddens ME, Wang L, Koch G. 2001. Secondary sexual characteristics in boys: estimates from the National Health and Nutrition Examination Survey III, 1988-1994. Arch Pediatr Adolesc Med 155:1022-1028.

Howdeshell KL, Hotchkiss AK, Thayer KA, Vandenbergh JG, vom Saal FS. 1999. Exposure to bisphenol A advances puberty. Nature 401:763-764.

Hsieh CC, Trichopoulos O, Katsouyanni K, Yuasa S. 1990. Age at menarche, age at menopause, height and obesity as risk factors for breast cancer: associations and interactions in an international case-control study. Int J Cancer 46:796-800.

Huen KF, Leung SS, Lau JT, Cheung AY, Leung NK, Chiu MC. 1997. Secular trend in the sexual maturation of southern Chinese girls. Acta Paediatr 86:1121-1124.

Jacobsen JL, Jacobsen SW. 1996. Intellectual impairment in children exposed to polychlorinated biphenyls in utero. N Engl J Med 335:783-789.

Kaplowitz PB, 0berfield SE. 1999. Reexamination of the age limit for defining when puberty is precocious in girls in the United States: implications for evaluation and treatment. Drug and Therapeutics and Executive Committees of the Lawson Wilkins Pediatric Endocrine Society. Pediatrics 104:936-941.

Kaplowitz PB, Slora EJ, Wasserman RC, Pedlow SE, Herman-Giddens ME. 2001. Earlier onset of puberty in girls: relation to increased body mass index and race. Pediatrics 108(2): 347-353.

Karlberg J. 2002. Secular trends in pubertal development. Horm Res 57(suppl 2):19-30.

Krstevska-Konstantinova M, Charlier C, Craen M, Du Caju M, Heinrichs C, de Beaufort C, et al. 2001. Sexual precocity after immigration from developing countries to Belgium: evidence of previous exposure to organochlorine pesticides. Hum Reprod 16:1020-1026.

Lee PA. 1980. Normal ages of pubertal events among American males and females. J Adolesc Health Care 1:26-29.

Lee PA, BUD SS, Kulin HE. 2001. Age of puberty: data from the United States of America. APMIS 109:81-88.

Luo ZC, Cheung YB, He Q, Albertsson-Wikland K, Karlberg J. 2003. Growth in early life and its relation to pubertal growth. Epidemiology 14:66-73.

McBregor A J, Mason HJ. 1990. Chronic occupational lead exposure and testicular endocrine function. Hum Exp Toxicol 9:371-376.

Meerts IA, Assink Y, Cenijn PH, Van Den Berg JH, Weijers BM, Bergman A, et al. 2002. Placental transfer of a hydroxylated polychlorinated biphenyl and effects on fetal and maternal thyroid hormone homeostasis in the rat. Toxicol Sci 68:361-371.

National Research Council 1999. Hormonally Active Agents in the Environment. Washington, DC:National Academy Press, 27-54.

Nikaido Y, Yoshizawa K, Danbara N, Tsujita-Kyutoku M, Yuri T, Uehara N, et al. 2004. Effects of maternal xenoestrogen exposure on development of the reproductive tract and mammary gland in female CD-1 mouse offspring. Reprod Toxicol 18:803-811.

Orr DP, Ingersoll GM. 1995. The contribution of level of cognitive complexity and pubertal timing to behavioral risk in young adolescents. Pediatrics 95:528-533.

Parent AS, Teilmann G, Juul A, Skakkebaek NE, Toppari J, Bourguignon JP. 2003. The timing of normal puberty and the age limits of sexual precocity: variations around the world, secular trends, and changes after migration. Endocr Rev 24:666-693.

Pirkle JL, Kaufmann RB, Brody D J, Hickman T, Bunter EW, Paschal DC. 1998. Exposure of the U.S. population to lead, 1991-1994. Environ Health Perspect 106:745-750.

Proos LA, Karlberg J, Hofvander Y, Tuvemo T. 1993. Pubertal linear growth of Indian girls adopted in Sweden. Acta Paediatr 82:641-844.

Pryor JL, Hughes C, Foster W, Hales BF, Robaire B. 2000. Critical windows of exposure for children's health: the reproductive system in animals and humans. Environ Health Perspect 108(suppl 3):491-503.

Rayner JL, Wood C, Fenton SE. 2004. Exposure parameters necessary for delayed puberty and mammary gland development in Long-Evans rats exposed in utero to atrazine. Toxicol Appl Pharmacol 195:23-34.

Reiter EO, Lee PA. 2001. Have the onset and tempo of puberty changed? Arch Pediatr Adolesc Med 155:988-989.

Roche AF, Wellens R, Attie KM, Siervogel RM ]995. The timing of sexual maturation in a group of US white youths. J Pediatr Endocrinol Metab 8:11-18.

Rockett JC, Lynch CD, Buck GM. 2004. Biomarkers for assessing reproductive development and health. Part 1: pubertal development. Environ Health Perspect 112:106-112.

Rodamilans M, Osaba MJ, To-Figueras J, Rivera Fillat F, Marques JM, Perez P, et al. 1988. Lead toxicity on endocrine testicular function in an occupationally exposed population. Hum Toxicol 7:126-128.

Ruhl CE, Everhart JE, Ding J, Goodpaster BH, Kanaya AM, Simonsick EM, et al. 2004. Health, Aging, and Body Composition Study. Serum leptin concentrations and body adipose measures in older black and white adults. Am J Clin Nutr 80:576-583.

Saiyed H, Dewan A, Bhatnagar V, Shenoy U, Shenoy R, Rajmohan H, et al. 2003. Effect of endosulfan on male reproductive development. Environ Health Perspect 111:1958-1962.

Selevan SG, Rice DC, Hogan KA, Euling SY, Pfahles-Hutchens A, Bethel J. 2003. Blood lead concentration and delayed puberty in girls. N Engl J Med 348:1527-1536.

Simon AE, Wardle J, Jarvis MJ, Steggles N, Cartwright M. 2003. Examining the relationship between pubertal stage, adolescent health behaviours and stress. Psychol Med 33:1369-1379.

Smithells RW, Newman CB. 1992. Recognition of thalidomide defects. J Med Genet 29:716-723.

Stoker TE, Parks LG, Gray LE, Cooper RL. 2000. Endocrine-disrupting chemicals: prepubertal exposures and effects on sexual maturation and thyroid function in the male rat. A focus on the EDSTAC recommendations. Endocrine Disrupter Screening and Testing Advisory Committee. Crit Rev Toxicol 30:197-252.

Sun SS, Schubert CM, Chumlea WC, Roche AF, Kulin HE, Lee PA, et al. 2002. National estimates of the timing of sexual maturation and racial differences among US children. Pediatrics 110:911-919.

Teilmann B, Juul A, Skakkebaek NE, Toppari J. 2002. Putative effects of endocrine disrupters on pubertal development in the human. Best Pract Res Clin Endocrinol Metab 16:105-121.

Tiwary CM, Ward JA. 2003. Use of hair products containing hormone or placenta by US military personnel. J Pediatr Endocrinol Metab 16:1025-1032.

Tou JC, Chen J, Thompson LU. 1998. Flaxseed and its lignan precursor, secoisolariairesinol diglycoside, affect pregnancy outcome and reproductive development in rats. J Nutr 128:1861-1868.

Viner R. 2002. Splitting hairs. Arch Dis Child 86(1):8-10.

Vivoli G, Fantuzzi G, Bergomi M, Tonelli E, Gatto MR, Zanetti F, et al. 1993. Relationship between low lead exposure and somatic growth in adolescents. J Expo Anal Environ Epidemiol 3(suppl 1):201-209.

Wong WW, Nicolson M, Stuff JE, Butte NF, Ellis KJ, Hergenroeder AC, et al. 1998. Serum leptin concentrations in Caucasian and African-American girls. J Clin Endocrinol Metab 83:3574-3677.

Wright C, Evans AC, Evans NP, Duffy P, Fox J, Boland MP, et al. 2002. Effect of maternal exposure to the environmental estrogen, octylphenol, during fetal and/or postnatal life on onset of puberty, endocrine status, and ovarian follicular dynamics in ewe lambs. Biol Reprod 67(6):1734-1740.

Wu T, Buck GM, Mendola P. 2003. Blood lead levels and sexual maturation in U.S. girls: the Third National Health and Nutrition Examination Survey, 1988-1994. Environ Health Perspect 111:737-741.

Wu T, Mendola P, Buck GM. 2002. Ethnic differences in the presence of secondary sex characteristics and menarche among US girls: the Third National Health and Nutrition Examination Survey, 1986-1994. Pediatrics 110:752-757.

Richard Y. Wang, Larry L. Needham, and Dana B. Barr

National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, Georgia, USA

This article is part of the mini-monograph "Assessing Exposures to Environmental Agents during the National Children's Study."

Address correspondence to R. Wang, National Center for Environmental Health, Centers for Disease Control and Prevention, 4770 Buford Highway, MS-F17, Atlanta, GA 30341 USA. Telephone: [phone]. Fax: [phone]. E-mail: [e-mail]

The authors declare they have no competing financial interests.

Received 20 September 2004; accepted 14 March 2005.

Table 1. Human studies on the effects of environmental chemical
exposure on pubertal development.

                                       Developmental
                                     window or route of
Chemical          Reference          exposure assessed

p,p'-DDE          Gladen et al.         Intrauterine
                  2000                   Postnatal
                                      (breastfeeding)
                  Krstevska-              Puberty
                  Konstantinova
                  et al. 2001
Dioxin-like       Den Hord et al.         Puberty
compounds         2002
Endosulfan        Saiyed et al.           Puberty
                  2003
Lead              Selevan et al.          Puberty
                  2003
                  Wu et al.               Puberty
                  2003
PBBs              Blanck et al.         Intrauterine
                  2000
PCBs              Blanck et al.         Intrauterine
                  2000
                  Den Hond et al.         Puberty
                  2002
                  Gladen et al.         Intrauterine
                  2000                   Postnatal
                                      (breastfeeding)
Phthalates        Colon et al.            Puberty
                  2000
Phytoestrogens    Freni-Titulaer          Puberty
                  et al. 1986

Chemical            Exposure measure            Level of exposure

p,p'-DDE          Milk level at birth      Median, 2.4 pg/g lipids
                  Lactational exposure     Range, 0.3-23.8 pg/g lipids
                    index
                  Serum level at or        Immigrant/adopted, 1.04
                    postpuberty              [micro]g/L (median)
                                           Immigrant/nonadopted,
                                             1.20 [micro]g/L (median)
                                           Native, 0.13 [micro]g/L
                                             (median)
Dioxin-like       Serum level at           Exposures: range of means,
compounds           puberty                  0.11-0.21 ng TEQ/L
Endosulfan        Serum level at           Exposed, 7.47 [+ or -] 1.19
                    puberty                  [micro]g/L (mean [+ or -]
                                             SD)
                                           Nonexposed, 1.37 [micro]
                                             0.40 [micro]g/L
                                             (mean [+ or -] SD)
Lead              Blood level at           Non-Hispanic whites:
                    puberty                  geometric mean, 1.4
                                             [micro]g/dL; 95% CI,
                                             1.2-1.5
                                           African Americans:
                                             geometric mean, 2.1
                                             [micro]g/dL; 95% CI,
                                             1.9-2.3
                                           Mexican Americans:
                                             geometric mean, 1.7
                                             [micro]g/dL, 95% CI,
                                             1.6-1.9 Range, 0.7-21.7
                  Blood level at             [micro]g/dL
                    puberty
PBBs              Extrapolated             Mean [+ or -] SD, 17.3 pg/L
                    maternal serum           [+ or -] 107.8; range, ND
                    level postevent (b)      (a) to 1,142 [micro]g/L
PCBs              Maternal serum           Mean [+ or -], 5.6
                    level postevent (b)      [micro]g/L [+ or -] 5.5;
                                             range, NDI (c)to 78
                                             [micro]g/L
                  Serum level at           Exposures: range of PCB-138
                    puberty                  means, 94-227 ng/L
                                           Exposures: range of PCB-153
                                             means, 166-332 ng/L
                                           Exposures: range of PCB-180
                                             means, 95-202 ng/L
                  Milk level at birth      Median, 1.7 pg/g lipids,
                                           Range, 0.5-5.5 [micro]g/g
                                             lipids
                  Lactational exposure
                    index
Phthalates        Serum level at           Cases: DEHP, 450 pg/L
                    puberty                  (mean [+ or -] SD not
                                             given)
                                           Controls: DEHP, 70 pg/L
                                             (mean [+ or -] SD not
                                             given)
Phytoestrogens    Questionnaire

                              Associated health effects

Chemical                    Males                      Females

p,p'-DDE          No effect on attainment      No effect on time of
                    of puberty by DDE level      onset, height, or body
                    from intrauterine            weight at puberty by
                    exposure                     DDE level from
                  Increased height and           intrauterine exposure
                    body weight at puberty
                    with increasing DDE
                    level from intrauterine
                    exposure
                  No effect on attainment
                    of puberty by DDE level
                    from lactational
                    exposure
                  Immigrant children with idiopathic precocious
                    puberty had higher DDE levels than native
                    children with precocious puberty, adoption
                    status was not associated with DDE level

Dioxin-like       No effect on likelihood      Decreased likelihood to
compounds           to attain either             attain thelarche with
                    genital maturity or          increasing TEQ level
                    pubarche by TEQ level
Endosulfan        Delayed pubarche and
                    genital maturity with
                    exposure

Lead                                           No effect on pubarche,
                                                 menarche, or thelarche
                                                 by lead level for non-
                                                 Hispanic whites
                                               Delayed pubarche,
                                                 thelarche, and
                                                 menarche with
                                                 increasing lead level
                                                 for African Americans
                                               Delayed pubarche and
                                                 thelarche with
                                                 increasing lead level
                                                 for Mexican Americans
                                               Decreased height with
                                                 increasing lead level,
                                                 regardless of
                                                 race/ethnicity
                                               Delayed pubarche
                                                 and menarche with
                                                 increasing lead level,
                                                 no effect on attainment
                                                 of thelarche by lead
                                                 level
PBBs                                           Accelerated attainment
                                                 of menarche with in-
                                                 creasing intrauterine
                                                 PBB level and breast-
                                                 feeding
PCBs                                           No effect on age at
                                                 menarche or attainment
                                                 of thelarche or
                                                 pubarche by intraute-
                                                 rine PCB level
                  Decreased likelihood to      No effect on likelihood
                    attain genital maturity      to attain either
                    with increasing PCB-138      thelarche or pubarche
                    level                        by PCB-138 level
                  Decreased likelihood to      No effect on likelihood
                    attain pubarche with         to attain either
                    increasing PCB-153           thelarche or pubarche
                    level                        by PCB-153 level
                  No effect on likelihood      No effect on likelihood
                    to attain either             to attain either
                    genital maturity or          thelarche or pubarche
                    pubarche by PCB-180          by PCB-180 level
                    level
                  No effect on time of         No effect on time of
                    onset of puberty or          onset of puberty by
                    height or body weight        PCB level from
                    at puberty by PCB level      intrauterine exposure
                    from intrauterine          Increased body weight at
                    exposure                     puberty with
                                                 increasing PCB level
                                                 from intrauterine
                                                 exposure in Caucasian
                                                 girls
                  No effect on time of         No effect on time of
                    onset of puberty or          onset, height, or body
                    height or body weight        weight at puberty by
                    at puberty by PCB level      PCB level from
                    from lactational             lactational exposure
                    exposure
Phthalates                                     Premature thelarche

Phytoestrogens                                 Use of soy-based formula
                                                 increased likelihood
                                                 for onset of premature
                                                 thelarche before
                                                 2 years of age

Abbreviations: DEHP, di-(2-ethylhexyl) phthalate; ND, not detected;
p,p'-DDE, pp, dichlorodiphenyldichloroethylene; TED, toxic equivalent.
(a) PBB limit of detection, 1 [micro]g/L. (b) Cows contaminated by feed
containing PBB in 1973. (c) PCB limit of detection, 5 [micro]g/L.

Table 2. Sampling periods for the assessment of exposure to persistent
and nonpersistent chemicals.

                                         Pre-
                                      conception
                                         (a)            In utero

Persistent chemicals
  Urineb, (c,d)                          M/F       M (10-15 weeks (e))
  Serum (b,f)                            M/F       M (10-15 weeks (e))
  Whole blood (b,f)                      M/F       M (10-15 weeks (e))
  Hair (b)
  Human milk (b)                        2 weeks--2 months postpartum
  Cord serum (b)
  Cord whole blood (b)
  Meconium (b)
  Dietary assessment (g)                  x        M (10-15 weeks (e))
  Home air sample (b,g)                   x                 x
  Home composite dust sample (b,g)        x                 x
  Other environmental samples (b,g)    Special       Special studies
                                       studies
  Questionnaire (g)                       x                 x
  Ecologic analysis (e.g., GIS) (g)       x                 x
Nonpersistent chemicals
  Urineb (b,c,d)                         M/F       M (10-15 weeks (e))
  Serum (b,f)                            M/F       M (10-15 weeks (e))
  Human milk (b)                        2 weeks--2 months postpartum
  Cord serum (b)
  Meconium (b)
  Dietary assessment (g)                  x        M (10-15 weeks (e))
  Home air sample (b,g)                   x                 x
  Home composite dust sample (b,g)        x                 x
  Other environmental samples (b,g)    Special       Special studies
                                       studies
  Questionnaire (g)                       x                 x
  Ecologic analysis (e.g., GIS) (g)       x                 x

                                                       Postpartum
                                      Perinatal      (18-24 months)

Persistent chemicals
  Urineb, (c,d)                           M                 x
  Serum (b,f)                             M                 x
  Whole blood (b,f)                       M                 x
  Hair (b)                                M
  Human milk (b)                       2 weeks--2 months postpartum
  Cord serum (b)                          x
  Cord whole blood (b)                    x
  Meconium (b)                            x
  Dietary assessment (g)                  x                 x
  Home air sample (b,g)                   x                 x
  Home composite dust sample (b,g)        x                 x
  Other environmental samples (b,g)    Special       Special studies
                                       studies
  Questionnaire (g)                       x        At 6, 12, 24 and 36
                                                  months, then annually
                                                          until
  Ecologic analysis (e.g., GIS) (g)       x                 x
Nonpersistent chemicals
  Urineb (b,c,d)                          M                 x
  Serum (b,f)                             M                 x
  Human milk (b)                       2 weeks--2 months postpartum
  Cord serum (b)                                            x
  Meconium (b)                                              x
  Dietary assessment (g)                  x                 x
  Home air sample (b,g)                   x                 x
  Home composite dust sample (b,g)        x                 x
  Other environmental samples (b,g)    Special       Special studies
                                       studies
  Questionnaire (g)                       x        At 6, 12, 24 and 36
                                                  months, then annually
                                                      until puberty
  Ecologic analysis (e.g., GIS) (g)       x                 x

                                      Prepuberty
                                      (5-6 years)     Midpuberty

Persistent chemicals
  Urineb, (c,d)                            x               x
  Serum (b,f)                              x               x
  Whole blood (b,f)                        x               x
  Hair (b)
  Human milk (b)                      2 weeks--2 months postpartum
  Cord serum (b)
  Cord whole blood (b)
  Meconium (b)
  Dietary assessment (g)                   x               x
  Home air sample (b,g)                    x               x
  Home composite dust sample (b,g)         x               x
  Other environmental samples (b,g)     Special     Special studies
                                        studies
  Questionnaire (g)                    At 6, 12, 24 and 36 months,
                                           then annually until
  Ecologic analysis (e.g., GIS) (g)        x               x
Nonpersistent chemicals
  Urineb (b,c,d)                           x               x
  Serum (b,f)                              x               x
  Human milk (b)                      2 weeks--2 months postpartum
  Cord serum (b)
  Meconium (b)
  Dietary assessment (g)                   x               x
  Home air sample (b,g)                    x               x
  Home composite dust sample (b,g)         x               x
  Other environmental samples (b,g)     Special     Special studies
                                        studies
  Questionnaire (g)                    At 6, 12, 24 and 36 months,
                                       then annually until puberty
  Ecologic analysis (e.g., GIS) (g)        x               x

                                                Postpuberty

Persistent chemicals
  Urineb, (c,d)                                      x
  Serum (b,f)                                        x
  Whole blood (b,f)                                  x
  Hair (b)
  Human milk (b)                        2 weeks--2 months postpartum
  Cord serum (b)
  Cord whole blood (b)
  Meconium (b)
  Dietary assessment (g)                             x
  Home air sample (b,g)                              x
  Home composite dust sample (b,g)                   x
  Other environmental samples (b,g)           Special studies
  Questionnaire (g)                   At 6, 12, 24 and 36 months, then
                                               annually until
  Ecologic analysis (e.g., GIS) (g)                  x
Nonpersistent chemicals
  Urineb (b,c,d)                                     x
  Serum (b,f)                                        x
  Human milk (b)                        2 weeks--2 months postpartum
  Cord serum (b)
  Meconium (b)
  Dietary assessment (g)                             x
  Home air sample (b,g)                              x
  Home composite dust sample (b,g)                   x
  Other environmental samples (b,g)           Special studies
  Questionnaire (g)                   At 6, 12, 24 and 36 months, then
                                           annually until puberty
  Ecologic analysis (e.g., GIS) (g)                  x

GIS, geographic information system; F, father; M, mother; x, period
when samples are to be collected.

(a) At the time of enrollment. (b) Media with extant laboratory methods
for likely target chemical agent. Note that for nonpersistent
chemicals, multiple samples of biologic specimens may be needed during
this time because of possible variability in biologic exposure level
due to the short half-life of the chemical. This is particularly
important if a critical exposure period is being considered.
Alternatively, pilot data may demonstrate the lack of need for frequent
sampling if constant exposure occurred in a stable environment.
(c) Timed specimen collection; morning void specimen with creatinine
measurement. (d) Pediatric urine bag or diaper sample for
non-toilet-trained children. If not diaper, spot samples or multiple
spots. (e) For example, weeks 10-15 of gestation are a critical period
for breast development. (f) When blood collection is at a young age,
piggyback on lead screen at 12 and 24 months that is recommended by
the Centers for Disease Control and Prevention. (g) Environmental
sampling, dietary assessment, questionnaires, and ecologic analysis are
necessary for persistent chemicals to determine either route or pathway
of exposure; otherwise, exposure can be established through the
analysis of biologic specimens. Information should be obtained
proximate to the sampling period for biologic specimens.