Skip to main content
NCBI home page
Environmental Health Perspectives logoLink to Environmental Health Perspectives
. 2015 Jul 1;123(7):A166–A168. doi: 10.1289/ehp.1510041

Optimal Exposure Biomarkers for Nonpersistent Chemicals in Environmental Epidemiology

Antonia M Calafat 1, Matthew P Longnecker 2, Holger M Koch 3, Shanna H Swan 4, Russ Hauser 5, Lynn R Goldman 6, Bruce P Lanphear 7, Ruthann A Rudel 8, Stephanie M Engel 9, Susan L Teitelbaum 4, Robin M Whyatt 10, Mary S Wolff 4,✉
PMCID: PMC4492274  PMID: 26132373

Summary

We discuss considerations that are essential when evaluating exposure to nonpersistent, semivolatile environmental chemicals such as phthalates and phenols (e.g., bisphenol A). A biomarker should be chosen to best represent usual personal exposures and not recent, adventitious, or extraneous exposures. Biomarkers should be selected to minimize contamination arising from collection, sampling, or analysis procedures. Pharmacokinetics should be considered; for example, nonpersistent, semivolatile chemicals are metabolized quickly, and urine is the compartment with the highest concentrations of metabolites. Because these chemicals are nonpersistent, knowledge of intraindividual reliability over the biologic window of interest is also required. In recent years researchers have increasingly used blood as a matrix for characterizing exposure to nonpersistent chemicals. However, the biologic and technical factors noted above strongly support urine as the optimal matrix for measuring nonpersistent, semivolatile, hydrophilic environmental agents.

Quantification of exposure biomarkers is increasingly used to provide an integrated measure of a person’s multiple chemical-specific exposures (Pirkle et al. 1995). Yet successful exposure characterization requires more than sophisticated analytical chemistry techniques—the biomarker and matrix are also key.

For general-population exposures, persistent organic pollutants (POPs) are usually measured in blood, and metabolites of nonpersistent chemicals such as phthalates and bisphenol A (BPA) are best measured in urine (Koch and Calafat 2009). In recent years, however, investigators have increasingly characterized exposure to nonpersistent chemicals by using other tissue matrices, particularly blood. To illustrate this trend, we identified 80 scientific articles, published in 2000–2014, that reported measuring BPA or phthalates in blood serum/plasma and other nonaqueous matrices, and that addressed topics on etiology, exposure, or metabolism (Figure 1).

Urine is the preferred matrix for most nonpersistent chemicals because of their pharmacokinetics. Nonpersistent chemicals are quickly transformed to hydrophilic, polar metabolites and excreted mainly in urine (Koch and Calafat 2009). The concentration of most urinary metabolites is 30–100 times greater than concentrations in blood (Engel and Wolff 2013; National Research Council 2008). With existing analytical techniques, higher urinary concentrations facilitate quantification, whereas relatively low blood concentrations of polar biomarkers increase the likelihood that external contamination obscures true exposures.

These are paramount considerations, given the increasingly recognized ubiquity of contamination. Controlled conditions of collection, storage, and processing of biospecimens are a long-acknowledged critical step for trace analyses of metals, volatile organic compounds, and POPs (Alcock et al. 1994; Ashley et al. 1992; Bolann et al. 2007). Unfortunately, the importance of preanalytic contamination sources is not as well understood for semivolatiles such as phthalates, phenols (e.g., BPA, parabens, triclosan), and similar modern nonpersistent chemicals.

Rather than reflecting a person’s usual exposure over months to years, detected biomarkers of chemicals such as phthalates and BPA can represent recent use of medical equipment or treatment near the time of biospecimen collection (Jaeger and Rubin 1970; Larson et al. 1977; Vandentorren et al. 2011; Yan et al. 2009). Extraneous contamination may occur during both the preanalytical and analytical phases. Phthalates and BPA can be detected even in the cleanest laboratories, from reagents, sampling equipment, and analytical apparatus (Fankhauser-Noti and Grob 2007; Longnecker et al. 2013; Marega et al. 2013; Markham et al. 2010; Vandenberg et al. 2014). Extraneous phenols may also come from sources such as plastics or paper products (BPA), soap (triclosan), moist towelettes (parabens), and preservatives (BPA, parabens) (Guidry et al. 2015; Longnecker et al. 2013; Ye et al. 2013). Moreover, in the field, during processing, or in the laboratory, beginning immediately after sample collection, phthalate diesters are hydrolyzed to monoesters by enzymes present in most biologic matrices (e.g., milk, serum, meconium, amniotic fluid, skin, saliva, sweat). These hydrolytic enzymes are not present in urine (Hines et al. 2009). In other words, monoesters are formed both from diesters absorbed in the body from a person’s general environment—the exposure we intend to assess—and from diesters introduced into the biospecimen itself from contaminated surfaces and medical devices.

Phase I (e.g., phthalate oxidative metabolites) and phase II (e.g., phenol conjugates) metabolite biomarkers are least likely to arise from extraneous sources (Koch and Calafat 2009). Also, these metabolites exist at higher levels than the hydrolytic monoesters or free (unconjugated) phenols in urine (Hines et al. 2009; Teeguarden et al. 2013). For many phenols, no specific oxidative metabolites that can exclude contamination are currently used for biomonitoring. Instead, conjugated phenols (rather than the unconjugated species) are the most valid exposure biomarkers (Koch et al. 2012; Teeguarden et al. 2013).

The short half-life of nonpersistent chemicals presents additional challenges, in that a biomarker must meet the criterion of temporality. Regardless of the matrix, intraindividual variability exists in exposure metrics over short time periods, particularly when such exposures are episodic in nature (Frederiksen et al. 2013; Koch et al. 2014; Lassen et al. 2013; Preau et al. 2010; Ye et al. 2011). However, metabolites are detectable longer in urine than in other matrices. Acceptable biomarker variability can exist because exposures arise from common, quotidian sources. Consequently, reasonable reproducibility over months to years has been found for urinary biomarker concentrations (Baird et al. 2010; Engel et al. 2014; Hauser et al. 2004; Mahalingaiah et al. 2008; Marcus et al. 2010; Meeker et al. 2012; Nepomnaschy et al. 2009; Teitelbaum et al. 2008; Townsend et al. 2013). To improve exposure assessment, studies can also incorporate pooling of specimens or repeated measurements across the time window of interest, such as trimesters of pregnancy (Adibi et al. 2008; Braun et al. 2011; Cantonwine et al. 2014; Fisher et al. 2014; Irvin et al. 2010; Jusko et al. 2014; Meeker et al. 2013; Philippat et al. 2013; Quirós-Alcalá et al. 2013; Smith et al. 2012; Valvi et al. 2015).

In population research, environmental agents are selected for study based on biological relevance and exposure prevalence. Exposure biomarkers should be based on pharmacokinetics of the target chemicals and a suitable sampling matrix. The design should incorporate appropriate control procedures for collecting and processing specimens, validated analytical methods, and statistical analyses that account for sampling issues such as urine dilution (Barr et al. 2005) and collection times (Calafat and Needham 2009).

Technology has made it analytically possible to measure polar biomarkers at trace concentrations in many media, but these endeavors risk exposure misclassification due to low biomarker concentrations, short biologic half-lives, and threat of external contamination. Greater reliance on phthalate and phenol biomarker concentrations in urine will improve the return on investment in environmental research.

Figure 1.

Figure 1

Open in a new tab

Number of publications per year using blood or other non-urine biomarkers for bisphenol A (BPA) or phthalates, by type of study, from 2000 through 2014 (n = 80). The search strategy aimed to capture studies with measurements of free BPA or phthalate made on a human matrix, published from 2000 through 2014. We excluded 16 papers that were exclusively about the analytical chemistry methods of assay. We searched for English-language articles using the terms “phthalate” or “BPA,” measured in human serum, plasma, semen, adipose, milk, or saliva. The following list of PubMed identifiers, pasted into PubMed, retrieves the 80 articles we assessed, of which 38 are etiologic and 42 are exposure assessment: 25371878[uid] or 25337790[uid] or 25296284[uid] or 25268510[uid] or 25227326[uid] or 25213476[uid] or 25048886[uid] or 25036990[uid] or 24974312[uid] or 24816463[uid] or 24724919[uid] or 24720399[uid] or 24550655[uid] or 24503621[uid] or 24378374[uid] or 24336026[uid] or 24255718[uid] or 24025997[uid] or 23941471[uid] or 23904340[uid] or 23761051[uid] or 23710608[uid] or 23710174[uid] or 23667484[uid] or 23651625[uid] or 23506159[uid] or 23441348[uid] or 23411151[uid] or 23404131[uid] or 23377699[uid] or 23347089[uid] or 23213291[uid] or 23145999[uid] or 24524038[uid] or 22805002[uid] or 22722103[uid] or 22578698[uid] or 22498808[uid] or 22402483[uid] or 22381621[uid] or 22267833[uid] or 22050967[uid] or 21875366[uid] or 21705716[uid] or 21527603[uid] or 21440837[uid] or 24278551[uid] or 22953188[uid] or 21193545[uid] or 20822678[uid] or 20579427[uid] or 19706995[uid] or 19555962[uid] or 19444800[uid] or 19426969[uid] or 19165392[uid] or 18577445[uid] or 18273031[uid] or 18245696[uid] or 17822133[uid] or 17689919[uid] or 17661831[uid] or 17049806[uid] or 16603434[uid] or 16451866[uid] or 15995852[uid] or 15947000[uid] or 15893743[uid] or 15847671[uid] or 15644579[uid] or 14594632[uid] or 12869118[uid] or 12566679[uid] or 12417499[uid] or 12407035[uid] or 12401500[uid] or 11829464[uid] or 11604266[uid] or 10964036[uid] or 10716589[uid].

The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention. This research was supported in part by the Intramural Research Program of the National Institute of Environmental Health Sciences (NIEHS), National Institutes of Health, Department of Health and Human Services. We thank Sandra Chambers at the NIEHS for help in identifying articles listed in Figure 1.

Footnotes

The authors declare they have no actual or potential competing financial interests.

References

  1. Adibi JJ, Whyatt RM, Williams PL, Calafat AM, Camann D, Herrick R, et al. 2008Characterization of phthalate exposure among pregnant women assessed by repeat air and urine samples. Environ Health Perspect 1164467–473.; 10.1289/ehp.10749 [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Alcock RE, Halsall CJ, Harris CA, Johnston AE, Lead WA, Sanders G, et al. 1994Contamination of environmental samples prepared for PCB analysis. Environ Sci Technol 28111838–1842.; 10.1021/es00060a013 [DOI] [PubMed] [Google Scholar]
  3. Ashley DL, Bonin MA, Cardinali FL, McCraw JM, Holler JS, Needham LL, et al. Determining volatile organic compounds in human blood from a large sample population by using purge and trap gas chromatography/mass spectrometry. Anal Chem. 1992;64(9):1021–1029. doi: 10.1021/ac00033a011. [DOI] [PubMed] [Google Scholar]
  4. Baird DD, Saldana TM, Nepomnaschy PA, Hoppin JA, Longnecker MP, Weinberg CR, et al. 2010Within-person variability in urinary phthalate metabolite concentrations: measurements from specimens after long-term frozen storage. J Expo Sci Environ Epidemiol 202169–175.; 10.1038/jes.2009.17 [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Barr DB, Wilder LC, Caudill SP, Gonzalez AJ, Needham LL, Pirkle JL.2005Urinary creatinine concentrations in the U.S. population: implications for urinary biologic monitoring measurements. Environ Health Perspect 1132192–200.; 10.1289/ehp.7337 [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Bolann BJ, Rahil-Khazen R, Henriksen H, Isrenn R, Ulvik RJ.2007Evaluation of methods for trace-element determination with emphasis on their usability in the clinical routine laboratory. Scand J Clin Lab Invest 674353–366.; 10.1080/00365510601095281 [DOI] [PubMed] [Google Scholar]
  7. Braun JM, Kalkbrenner AE, Calafat AM, Bernert JT, Ye X, Silva MJ, et al. 2011Variability and predictors of urinary bisphenol A concentrations during pregnancy. Environ Health Perspect 1191131–137.; 10.1289/ehp.1002366 [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Calafat AM, Needham LL.2009What additional factors beyond state-of-the-art analytical methods are needed for optimal generation and interpretation of biomonitoring data? Environ Health Perspect 117101481–1485.; 10.1289/ehp.0901108 [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Cantonwine DE, Cordero JF, Rivera-Gonzalez LO, Anzalota Del Toro LV, Ferguson KK, Mukherjee B, et al. 2014Urinary phthalate metabolite concentrations among pregnant women in Northern Puerto Rico: distribution, temporal variability, and predictors. Environ Int 621–11.; 10.1016/j.envint.2013.09.014 [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Engel LS, Buckley JP, Yang G, Liao LM, Satagopan J, Calafat AM, et al. 2014Predictors and variability of repeat measurements of urinary phenols and parabens in a cohort of Shanghai women and men. Environ Health Perspect 1227733–740.; 10.1289/ehp.1306830 [DOI] [PMC free article] [PubMed] [Google Scholar]
  11. Engel SM, Wolff MS.2013Causal inference considerations for endocrine disruptor research in children’s health. Annu Rev Public Health 34139–158.; 10.1146/annurev-publhealth-031811-124556 [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Fankhauser-Noti A, Grob K.2007Blank problems in trace analysis of diethylhexyl and dibutyl phthalate: investigation of the sources, tips and tricks. Anal Chim Acta 5822353–360.; 10.1016/j.aca.2006.09.012 [DOI] [PubMed] [Google Scholar]
  13. Fisher M, Arbuckle TE, Mallick R, LeBlanc A, Hauser R, Feeley M, et al. 2015Bisphenol A and phthalate metabolite urinary concentrations: daily and across pregnancy variability. J Expo Sci Environ Epidemiol 253231–239.; 10.1038/jes.2014.65 [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Frederiksen H, Kranich SK, Jorgensen N, Taboureau O, Petersen JH, Andersson AM.2013Temporal variability in urinary phthalate metabolite excretion based on spot, morning, and 24-h urine samples: considerations for epidemiological studies. Environ Sci Technol 472958–967.; 10.1021/es303640b [DOI] [PubMed] [Google Scholar]
  15. Guidry VT, Longnecker MP, Aase H, Eggesbo M, Zeiner P, Reichborn-Kjennerud T, et al. 2015Measurement of total and free urinary phenol and paraben concentrations over the course of pregnancy: assessing reliability and contamination of specimens in the Norwegian Mother and Child Cohort study. Environ Health Perspect 1237705–711.; 10.1289/ehp.1408325 [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Hauser R, Meeker JD, Park S, Silva MJ, Calafat AM.2004Temporal variability of urinary phthalate metabolite levels in men of reproductive age. Environ Health Perspect 112171734–1740.; 10.1289/ehp.7212 [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. Hines EP, Calafat AM, Silva MJ, Mendola P, Fenton SE.2009Concentrations of phthalate metabolites in milk, urine, saliva, and serum of lactating North Carolina women. Environ Health Perspect 117186–92.; 10.1289/ehp.11610 [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Irvin EA, Calafat AM, Silva MJ, Aguilar-Villalobos M, Needham LL, Hall DB, et al. 2010An estimate of phthalate exposure among pregnant women living in Trujillo, Peru. Chemosphere 80111301–1307.; 10.1016/j.chemosphere.2010.06.048 [DOI] [PubMed] [Google Scholar]
  19. Jaeger RJ, Rubin RJ.1970Plasticizers from plastic devices extraction, metabolism, and accumulation by biological systems. Science 1703956460–462.; PMID: [DOI] [PubMed] [Google Scholar]
  20. Jusko TA, Shaw PA, Snijder CA, Pierik FH, Koch HM, Hauser R, et al. 2014Reproducibility of urinary bisphenol A concentrations measured during pregnancy in the Generation R Study. J Expo Sci Environ Epidemiol 245532–536.; 10.1038/jes.2014.23 [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Koch HM, Aylward LL, Hays SM, Smolders R, Moos RK, Cocker J, et al. 2014Inter- and intra-individual variation in urinary biomarker concentrations over a 6-day sampling period. Part 2: personal care product ingredients. Toxicol Lett 2312261–269.; 10.1016/j.toxlet.2014.06.023 [DOI] [PubMed] [Google Scholar]
  22. Koch HM, Calafat AM.2009Human body burdens of chemicals used in plastic manufacture. Philos Trans R Soc Lond B Biol Sci 36415262063–2078.; 10.1098/rstb.2008.0208 [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. Koch HM, Kolossa-Gehring M, Schroter-Kermani C, Angerer J, Bruning T.2012Bisphenol A in 24 h urine and plasma samples of the German Environmental Specimen Bank from 1995 to 2009: a retrospective exposure evaluation. J Expo Sci Environ Epidemiol 226610–616.; 10.1038/jes.2012.39 [DOI] [PubMed] [Google Scholar]
  24. Larson DF, Mayall B, Anderson RM. Hemolysis due to chemical contamination of clinical perfusion apparatus. Ann Thorac Surg. 1977;23(4):374–375. doi: 10.1016/s0003-4975(10)64146-x. [DOI] [PubMed] [Google Scholar]
  25. Lassen TH, Frederiksen H, Jensen TK, Petersen JH, Main KM, Skakkebæk NE, et al. 2013Temporal variability in urinary excretion of bisphenol A and seven other phenols in spot, morning, and 24-h urine samples. Environ Res 126164–170.; 10.1016/j.envres.2013.07.001 [DOI] [PubMed] [Google Scholar]
  26. Longnecker MP, Harbak K, Kissling GE, Hoppin JA, Eggesbo M, Jusko TA, et al. 2013The concentration of bisphenol A in urine is affected by specimen collection, a preservative, and handling. Environ Res 126211–214.; 10.1016/j.envres.2013.07.002 [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Mahalingaiah S, Meeker JD, Pearson KR, Calafat AM, Ye X, Petrozza J, et al. 2008Temporal variability and predictors of urinary bisphenol A concentrations in men and women. Environ Health Perspect 1162173–178.; 10.1289/ehp.10605 [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Marcus M, Christensen KY, Manatunga A, Rudra CB, Brock JW, Small CM.2010Variability of phthalate monoester levels in daily first-morning urine from adult women: a pilot study. Rev Environ Health 254359–368.; 10.1515/REVEH.2010.25.4.359 [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Marega M, Grob K, Moret S, Conte L.2013Phthalate analysis by gas chromatography-mass spectrometry: blank problems related to the syringe needle. J Chromatogr A 1273105–110.; 10.1016/j.chroma.2012.11.076 [DOI] [PubMed] [Google Scholar]
  30. Markham DA, Waechter JM, Jr, Wimber M, Rao N, Connolly P, Chuang JC, et al. 2010Development of a method for the determination of bisphenol A at trace concentrations in human blood and urine and elucidation of factors influencing method accuracy and sensitivity. J Anal Toxicol 346293–303.; 10.1093/jat/34.6.293 [DOI] [PubMed] [Google Scholar]
  31. Meeker JD, Calafat AM, Hauser R.2012Urinary phthalate metabolites and their biotransformation products: predictors and temporal variability among men and women. J Expo Sci Environ Epidemiol 224376–385.; 10.1038/jes.2012.7 [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Meeker JD, Cantonwine DE, Rivera-Gonzalez LO, Ferguson KK, Mukherjee B, Calafat AM, et al. 2013Distribution, variability, and predictors of urinary concentrations of phenols and parabens among pregnant women in Puerto Rico. Environ Sci Technol 4773439–3447.; 10.1021/es400510g [DOI] [PMC free article] [PubMed] [Google Scholar]
  33. Nepomnaschy PA, Baird DD, Weinberg CR, Hoppin JA, Longnecker MP, Wilcox AJ.2009Within-person variability in urinary bisphenol A concentrations: measurements from specimens after long-term frozen storage. Environ Res 1096734–737.; 10.1016/j.envres.2009.04.004 [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. National Research Council. Washington, DC: National Academies Press; 2008. Phthalates and Cumulative Risk Assessment: The Tasks Ahead. [PubMed] [Google Scholar]
  35. Philippat C, Wolff MS, Calafat AM, Ye X, Bausell R, Meadows M, et al. 2013Prenatal exposure to environmental phenols: concentrations in amniotic fluid and variability in urinary concentrations during pregnancy. Environ Health Perspect 121101225–1231.; 10.1289/ehp.1206335 [DOI] [PMC free article] [PubMed] [Google Scholar]
  36. Pirkle JL, Needham LL, Sexton K. Improving exposure assessment by monitoring human tissues for toxic chemicals. J Expo Anal Environ Epidemiol. 1995;5(3):405–424. [PubMed] [Google Scholar]
  37. Preau JL, Jr, Wong LY, Silva MJ, Needham LL, Calafat AM.2010Variability over 1 week in the urinary concentrations of metabolites of diethyl phthalate and di(2-ethylhexyl) phthalate among eight adults: an observational study. Environ Health Perspect 118121748–1754.; 10.1289/ehp.1002231 [DOI] [PMC free article] [PubMed] [Google Scholar]
  38. Quirós-Alcalá L, Eskenazi B, Bradman A, Ye X, Calafat AM, Harley K.2013Determinants of urinary bisphenol A concentrations in Mexican/Mexican-American pregnant women. Environ Int 59152–160.; 10.1016/j.envint.2013.05.016 [DOI] [PMC free article] [PubMed] [Google Scholar]
  39. Smith KW, Braun JM, Williams PL, Ehrlich S, Correia KF, Calafat AM, et al. 2012Predictors and variability of urinary paraben concentrations in men and women, including before and during pregnancy. Environ Health Perspect 120111538–1543.; 10.1289/ehp.1104614 [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Teeguarden J, Hanson-Drury S, Fisher JW, Doerge DR.2013Are typical human serum BPA concentrations measurable and sufficient to be estrogenic in the general population? Food Chem Toxicol 62949–963.; 10.1016/j.fct.2013.08.001 [DOI] [PubMed] [Google Scholar]
  41. Teitelbaum SL, Britton JA, Calafat AM, Ye X, Silva MJ, Reidy JA, et al. 2008Temporal variability in urinary concentrations of phthalate metabolites, phytoestrogens and phenols among minority children in the United States. Environ Res 1062257–269.; 10.1016/j.envres.2007.09.010 [DOI] [PubMed] [Google Scholar]
  42. Townsend MK, Franke AA, Li X, Hu FB, Eliassen AH.2013Within-person reproducibility of urinary bisphenol A and phthalate metabolites over a 1 to 3 year period among women in the Nurses’ Health Studies: a prospective cohort study. Environ Health 12180; 10.1186/1476-069X-12-80 [DOI] [PMC free article] [PubMed] [Google Scholar]
  43. Valvi D, Monfort N, Ventura R, Casas M, Casas L, Sunyer J, et al. 2015Variability and predictors of urinary phthalate metabolites in Spanish pregnant women. Int J Hyg Environ Health 2182220–231.; 10.1016/j.ijheh.2014.11.003 [DOI] [PubMed] [Google Scholar]
  44. Vandenberg LN, Gerona RR, Kannan K, Taylor JA, van Breemen RB, Dickenson CA, et al. 2014A round robin approach to the analysis of bisphenol A (BPA) in human blood samples. Environ Health 13125; 10.1186/1476-069X-13-25 [DOI] [PMC free article] [PubMed] [Google Scholar]
  45. Vandentorren S, Zeman F, Morin L, Sarter H, Bidondo ML, Oleko A, et al. 2011Bisphenol-A and phthalates contamination of urine samples by catheters in the Elfe pilot study: implications for large-scale biomonitoring studies. Environ Res 1116761–764.; 10.1016/j.envres.2011.05.018 [DOI] [PubMed] [Google Scholar]
  46. Yan X, Calafat A, Lashley S, Smulian J, Ananth C, Barr D, et al. 2009Phthalates biomarker identification and exposure estimates in a population of pregnant women. Hum Ecol Risk Assess 153565–578.; 10.1080/10807030902892554 [DOI] [PMC free article] [PubMed] [Google Scholar]
  47. Ye X, Wong LY, Bishop AM, Calafat AM.2011Variability of urinary concentrations of bisphenol A in spot samples, first morning voids, and 24-hour collections. Environ Health Perspect 1197983–988.; 10.1289/ehp.1002701 [DOI] [PMC free article] [PubMed] [Google Scholar]
  48. Ye X, Zhou X, Hennings R, Kramer J, Calafat AM.2013Potential external contamination with bisphenol A and other ubiquitous organic environmental chemicals during biomonitoring analysis: an elusive laboratory challenge. Environ Health Perspect 1213283–286.; 10.1289/ehp.1206093 [DOI] [PMC free article] [PubMed] [Google Scholar]

Articles from Environmental Health Perspectives are provided here courtesy of National Institute of Environmental Health Sciences

RESOURCES