Phthalate exposure is of potential concern for reproductive age women because the developing fetus may be susceptible to endocrine modulating effects. However, phthalate exposures in women of child-bearing age are not well characterized, particularly among underserved populations and in specific ethnic groups that cannot be captured in nationally representative surveys such as the National Health and Nutrition Examination Survey (NHANES). In our study, we investigated temporal variability of phthalate exposures in a socio-economically disadvantaged minority population of Southeast Asian women living in the Green Bay, Wisconsin area who emigrated to the U.S. from Laos and Thailand in the 1970’s and thereafter. We found detectable urinary concentrations of 9 of the 10 phthalate metabolites measured in more than 80% of the samples analyzed with the highest concentrations observed for MEP.
Five previous studies have evaluated the temporal variability of phthalate metabolites in various populations over time periods ranging from days to months (Adibi, et al., 2008
; Fromme, et al., 2007
; Hauser, et al., 2004
; Hoppin, et al., 2002
; Teitelbaum, et al., 2008
). Hoppin et al. (2002)
assessed agreement of phthalate measurements using first-morning urine samples collected on two consecutive days from 46 African-American women. Hauser et al. (2004)
reported the temporal variability of phthalate concentrations among 11 men providing 9 samples each over a 3 month period. These earlier studies each assessed MEP, MBP, MBzP and MEHP but did not measure the oxidative metabolites of DEHP. More recently, Fromme et al. (2007)
determined phthalate metabolites in the morning urine of 50 German men and women age 14–60 across eight consecutive days. Teitelbaum et al. (2008)
collected two to seven urine samples over 6 months in 35 Hispanic and Black children age 6 to 10 years old. Adibi et al. (2008)
described phthalate measurements in 28 pregnant Dominican and African-American women who gave two to four urine samples over a six week period during the third trimester. In accordance with ICCs reported previously for creatinine-adjusted MEHP, MEHHP, MEOHP, and MECPP over 8 days to 6 months (Adibi, et al., 2008
; Fromme, et al., 2007
; Teitelbaum, et al., 2008
), our evaluation of measurement agreement over a one month period found these metabolites to have poor reliability (ICC<0.22). We found MBzP to be the most reproducible metabolite. Furthermore, the magnitude of the ICC for MBzP has been relatively consistent across previous studies (range 0.53 – 0.64) suggesting that exposures to BBzP may be fairly consistent over time and within particular age and racial/ethnic groups. Since BBzP is commonly used in home furnishings such as vinyl floor tile, vinyl wallpaper and carpet backing, it would seem reasonable that leaching or evaporation into indoor air would be a relatively consistent source of exposure via inhalation and ingestion for individuals living or working around such materials. Compared to MBzP, the reproducibility of measurements for MEP, MnBP and MiBP is less consistent across previous studies, but patterns of decreasing ICCs with increasing sampling intervals are not observed across studies. Our results, however, show moderately strong reproducibility for these biomarkers over a one month sampling interval. Furthermore, the similarly strong magnitude of the MnBP, MCPP, MBzP and MEP correlations between spouses suggests that the primary sources of such exposures may originate from the shared home environment and common lifestyle habits.
In agreement with the surrogate category analyses reported by Hauser et al. (2004)
and Teitelbaum et al. (2008)
, a single urine sample was found to provide a reasonable prediction of high, medium and low categories of exposure to MEP, MBzP, MnBP, and MiBP as well as MCPP, which was only assessed by Teitelbaum. Unlike Teitelbaum, our results did not provide good support for the use of a single sample to indicate accurate exposure categories for DEHP metabolites. This may reflect differences in the timing of urine collection (first morning versus convenience sampling) or differences in phthalate exposure patterns among children compared to women of reproductive age. Although Hauser et al. (2004)
did not measure the oxidative metabolites of DEHP, MEHP was reported as the least predictive metabolite of those evaluated. The MEHP monoester of DEHP is further metabolized by oxidation to several oxidative metabolites including MEOHP, MEHHP, and MECPP. Thus, the more complex metabolism of higher molecular weight phthalates such as DEHP could lead to greater within-subject variability.
While caution needs to be exercised in making comparisons across studies that employ different study designs or target different populations, our study confirms previous reports of detectable concentrations of urinary phthalate metabolites for the general U.S. population (Centers for Disease Control and Prevention, 2005
; Silva, et al., 2004
) and other female populations which included pregnant women (Adibi, et al., 2003
; Adibi, et al., 2008
; Swan, et al., 2005
; Wolff, et al., 2008
; Ye, et al., 2008
), middle-aged African-American women (Hoppin, et al., 2002
), young girls (Wolff, et al., 2007
), and German females (Koch, et al., 2003
) (). Consistent with these findings, MEP was the phthalate metabolite detected in urine at the highest median concentrations, although in our study median MEP concentrations (60.6 μg/g creatinine) were markedly lower than background levels in the U.S., averaging one-third the concentrations reported for females (≥ age 6 y) in NHANES 2001–2002 (171 μg/g creatinine) (Centers for Disease Control and Prevention, 2005
). Increased MEP concentrations have been previously linked with smoking and use of personal care products such as perfumes (Duty, et al., 2005
). While Hmong women have a lower prevalence of smoking compared to other ethnic groups, the degree to which lower MEP exposures in this population may be attributed to less frequent use of fragranced or other personal care products was not evaluated since identifying sources of phthalate exposure was beyond the scope of this paper. While MEP concentrations increased markedly with age in a clear dose-response fashion, we did not observe similar trends for any of the other phthalate metabolites. Although age patterns are not directly evaluated in the NHANES data, our data are consistent with the suggestion of an age trend as demonstrated by increasing MEP concentrations across children, adolescent and adult categories (Centers for Disease Control and Prevention, 2005
Median urinary phthalate metabolite concentrations (μg/g creatinine) reported in female populations
In contrast to our observations for MEP, median concentrations of MiBP and MBzP were approximately 2.7-fold (7.3 compared to 2.7 μg/g creatinine) and 1.6-fold (24.1 compared to 15.1 μg/g creatinine) greater in our study population compared to the general U.S. population. All other phthalate metabolites were present in our study population at concentrations similar to or slightly lower than those reported for the NHANES 2001–2002 population, except MECPP which was not measured in NHANES 2001–2002. Our median MECPP concentrations, however, were comparable to median MECPP concentrations calculated from 2003–2004 NHANES Laboratory Files data provided at http://www.cdc.gov/nchs/about/major/nhanes/nhanes2003-2004/lab03_04.htm
(31.1 μg/g creatinine compared to 28.7 μg/g creatinine).
Consistent with previous reports (Barr, et al., 2003
; Koch, et al., 2003
) the oxidative metabolites of DEHP exceeded the urinary concentrations of MEHP by four- to ten-fold. All DEHP metabolites were strongly correlated (r≥0.92) (Barr, et al., 2003
; Koch, et al., 2003
; Silva, et al., 2006
) as expected since they derive from a common parent compound. Similarly, the positive correlation we observed between MnBP and MBzP concentrations (r=0.54) is consistent with evidence that these metabolites arise from a common diester, BBzP. BBzP predominantly metabolizes to MBzP, with lesser quantities eliminated as MnBP (Anderson, et al., 2001
). DBP, however, is the primary source of MnBP excretion (Anderson, et al., 2001
); thus, exposure to common sources of BBzP and DBP may contribute to the observed correlation. Likewise, the strong positive correlation between MnBP and its structural isomer MiBP (r=0.54) may be attributed to the use of DBP and di-isobutyl phthalate in similar commercial applications such as in paints, printing inks, adhesives, insecticides, nail polish and cosmetics. Although the magnitude of associations are somewhat attenuated, MnBP (r=0.44) and MiBP (r=0.44) are also positively correlated with MCPP concentrations which reflects the metabolic breakdown of DBP into MnBP and small quantities of MCPP (Centers for Disease Control and Prevention, 2005
; Silva, et al., 2007
Data regarding sociodemographic characteristics associated with urinary phthalate metabolite concentrations are scarce. Using data collected as part of the NHANES III examination of urinary phthalate monoester concentrations (Blount, et al., 2000
), Koo et al. examined the association between various sociodemograhic characteristics and phthalate exposures (Koo, et al., 2002
). Lower household income levels, defined as less than $1500 during the month prior to sampling, were associated with increased estimates of exposure to DEHP and BBzP. In our sample, in which over half of the women (n=25) reported household incomes of less than $25,000 annually, there was no clear pattern between income and creatinine-adjusted phthalate metabolite concentrations, with the exception of marginally significant increases in the concentrations of DEHP oxidative metabolites MECPP, MEHHP, and MEOHP in the middle income group.
Only a few studies have reported on the association between various phthalate monoesters and anthropometric measures, and two were restricted to male subjects (Duty, et al., 2005
; Hatch, et al., 2008
; Stahlhut, et al., 2007
). One study examining the relationship between usage of various personal products and phthalate exposures among men reported a weak correlation between BMI and MEP concentrations (Spearman correlation coefficient = 0.1, p < 0.05) (Duty, et al., 2005
). A recent study also found statistically significant positive correlations between the urinary concentrations of four phthalate metabolites, MBzP, MEHHP, MEOHP, and MEP, and increased abdominal adiposity in male NHANES participants (Stahlhut, et al., 2007
). In a separate analysis of NHANES data, Hatch et al. (2008)
reported increasing BMI and waist circumference with increasing MEP quartiles in adolescent girls, with a similar but somewhat weaker association observed in women between the ages of 20 and 59. In our sample of women with 60% (n=27) classified as overweight or obese, we observed an association between BMI and MEP (Spearman correlation = 0.26, p=0.08), utilizing the mean MEP concentrations for each woman. Correlations between BMI and other phthalate metabolites were not observed. There was a pattern of slightly higher geometric mean concentrations (creatinine-adjusted) for most of the metabolites, most notably for MEP, among overweight versus normal weight women. The results, however, may be the result of overweight women having less muscle mass and therefore lower creatinine levels. This pattern was not sustained among women classified as obese; although, this may be due to the very small numbers in this category (n=9). Future investigations evaluating larger sample sizes are needed to further explore the relationship between BMI and phthalate metabolites.
This study has several important strengths. Our study population is a distinct minority population in the U.S., which is characterized by a unique set of cultural factors including an unusually high birth rate that make this group an important population in which to investigate the effects of environmental exposures that may impair reproductive health or fetal development. To our knowledge, this is the first study to evaluate phthalate exposure among the Hmong population. In restricting the window of urine sample collection to first morning voids, we minimized a potential source of inter-individual variability in concentrations of phthalate metabolites. We also collected repeated measurements on over one-half of our study participants; thus, we could examine temporal variability and the effect of exposure determinants on urinary phthalate metabolites while incorporating intra-individual variability in phthalate concentrations in the analyses that were conducted. Finally, a greater number of urinary phthalate metabolites were evaluated in our investigation as compared to several previous studies (see ), which provided an opportunity to better characterize phthalate exposure in this understudied population.
While it is informative to compare the distribution of phthalate metabolites concentrations observed in this minority population with reference levels for females in the general U.S. population, these comparisons should be interpreted with caution due to our small sample size, differences in the timing of urine collection, and the restricted age distribution of our sample. Differences in phthalate metabolite concentrations between Hmong women and the general population could be attributed to differences in exposure patterns, geographic or diurnal variations, population characteristics or pharmacokinetic factors influenced by age or race/ethnicity. The Hmong population was largely comprised of nonsmokers with limited educational attainment and low household income. The lack of variability by smoking status and recent fish consumption restricted our ability to assess these factors. Furthermore, insufficient proportions of individuals with more than a high school education or household income of $40 000 or above limited our assessment of phthalate exposure patterns across all levels of education and income. Finally, assessment of environmental tobacco smoke exposure was self-reported and limited to exposure in the home.
The findings of this study support evidence that environmental phthalate exposures are also prevalent among women of reproductive age in underserved populations. Phthalate metabolites concentrations are reproducible over a one month sampling interval for most metabolites measured, but caution should be exercised when using single samples to estimate exposure to DEHP. Sociodemographic and lifestyle factors that increase the likelihood of exposure have not been well delineated and should be further explored. Given the limited number of studies in reproductive-aged women, future investigations are required to determine if adverse reproductive outcomes are associated with phthalate exposures at levels that have been commonly observed in the population.
The authors would like to thank Dr. Jane Hoppin for sharing phthalate exposure assessment questionnaires for adaptation for this study. We would also like to thank Donna Gasior and the staff of the FRIENDS study for their data collection and data management efforts, and Ella Samandar, James Preau and John A. Reidy (CDC, Atlanta, GA) for technical assistance in measuring the concentrations of phthalate metabolites. This research was supported by grants P30-ES09106 and ES011263 from the National Institute of Environmental Health Sciences, R82939001 from the U.S. Environmental Protection Agency, TS000008 from the Agency for Toxic Substances and Disease Registry, and the Women’s Studies Program Women’s Interdisciplinary Seed Grant funded by the Texas A&M University Office of the Vice President for Research. The authors declare no conflicts of interest.