Article

Journal of Exposure Science and Environmental Epidemiology (2011) 21, 595–600; doi:10.1038/jes.2011.22; published online 25 May 2011

Plasma concentrations of parabens in postmenopausal women and self-reported use of personal care products: the NOWAC postgenome study

Torkjel M Sandangera,b, Sandra Hubera, Morten K Moea, Tonje Braathenb, Henriette Leknesc and Eiliv Lundb

  1. aNorwegian Institute for Air Research, Fram Centre, Tromsø, Norway
  2. bInstitute of Community Medicine, University of Tromsø, Tromsø, Norway
  3. cNorwegian Institute of Air Research, Kjeller, Norway

Correspondence: Dr. Torkjel M. Sandanger, NILU, Norwegian Institute for Air Research, Fram Centre, FRAM—High North Research Centre on Climate and Environment, Hjalmar Johansens gate 14, Tromsø, Norway 9296, USA. Tel.: +47 77750392. Fax: +47 77750376. E-mail: torkjel.sandanger@nilu.no

Received 30 November 2010; Accepted 8 February 2011; Published online 25 May 2011.

Top

Abstract

Parabens are used extensively in personal care products; however, their estrogenic properties have raised concern over risks to human health. High levels of total parabens, mainly as conjugates, have been reported in human plasma/serum, with limited data on native parabens. Our objective was to assess and link plasma concentrations of native common parabens to self-reported use of personal care products in women from the general population. The information was obtained from an extensive questionnaire on diet and lifestyle previously answered by the women in the NOWAC study. Plasma samples from 332 individuals were extracted and cleaned up by automated solid phase extraction and analyzed by ultra high performance liquid chromatography time-of-flight mass spectrometry. Native methyl paraben dominated and was detected in 63% of the samples, with a median level of 9.4ng/ml. Ethyl paraben (median <3ng/ml) and propyl paraben (median <2ng/ml) were detected in 22 and 29%, respectively. Butyl and benzyl parabens were not detected. For the first time, elevated levels of native parabens are reported in women from the general population. The concentrations were significantly associated with the use of skin lotions, indicating that frequent (daily or more) use maintain elevated concentrations despite the parabens short half-lives. These findings clearly emphasize the need to study potential health effects in the general population.

Keywords:

parabens; human plasma; skin care products; native form; human; personal care products

Top

Introduction

The alkyl esters of p-hydroxybenzoic acid, the parabens, are widely used as antimicrobial preservatives, where the antimicrobial activity increases with the alkyl chain length from methyl to n-butyl. The parabens are popular preservatives; for example, in personal care products (PCPs), pharmaceuticals and food and beverages, because of their low toxicity and cost, their broad inertness, and their worldwide regulatory acceptance (Elder, 1984; NTP, 2005; Soni et al., 2005; Andersen, 2008). Methyl (MP) and propyl (PP) parabens are by far the most used within this class of chemicals (Andersen, 2008). In PCPs, the permitted content of an individual paraben is limited to 0.4% (by volume) and the total paraben content may not exceed 0.8% (EU cosmetics directive 76/768/EEC).

Most studies have indicated that parabens are not mutagenic (Elder, 1984), but have been shown to bind to estrogen receptors from different animal species, including rodent uterus (Routledge et al., 1998; Perkins and Sheehan, 2000; Fang et al., 2001) and MCF/breast cancer cells (Byford et al., 2002; Darbre et al., 2002, 2003). Having the ability to bind to estrogen receptors, parabens might cause health effects at much lower concentrations and more specific than through non-receptor-mediated mechanisms. Further, the presence of parabens in human breast tumors (Darbre et al., 2004) have initiated a debate regarding their use in cosmetics and the incidence of breast cancer. Parabens have also been associated with genotoxicity, allergies, and antiandrogenic activity (Cashman and Warshaw, 2005; Darbre and Harvey, 2008).

Following exposure and adsorption, parabens are rapidly metabolized (the biological half-life is less than 24h) to facilitate urinary excretion (Okereke et al., 1993; Kadry et al., 1995; Moss et al., 2000; Vokel et al., 2002; Ye et al., 2006). The parabens are converted to p-hydroxybenzoic acid by estereases present in the skin (phase I metabolism) or to a glycine, sulfate, or glucuronide conjugate (phase II metabolism) in the liver (Soni et al., 2005). p-Hydroxybenzoic acid is considered a weaker estrogen agonist than native parabens (Darbre and Harvey, 2008); however, the estrogenic agonist properties of the phase II metabolites are not known. Recently, butyl paraben (BuP) was shown to systemically beign absorbed in men after topical application, with high levels of native BuP being detected in plasma (Janjua et al., 2007). This study showed rapid skin penetration and absorption with maximum levels achieved only 3h after application to the whole body.

High levels of native parabens found in plasma shortly after application combined with their estrogenic properties, indicate the need for assessment of plasma concentrations from the general population and to assess potential health risks. Recent reviews (Boberg et al., 2010) concluded that improved studies on absorption, distribution, metabolism, and elimination (ADME) are required, specifically on uptake and metabolism in exposed individuals, as well as background human populations. Hitherto the most extensive background study has been on total paraben in urine from US citizens, where parabens were detected even in young children and adolescents (Calafat et al., 2010). In that study it was concluded that there is a considerable human exposure to parabens.

The aim of this study was to assess the levels of metyl-, ethyl (EP)-, and propyl-parabens in their native form in plasma from women of the general population and further investigate the possible link to self-reported use of personal care products.

Top

Methods

Study Participants and Collection of Blood Samples

The women taking part in the current study are all participants in the Norwegian Women And Cancer Study (NOWAC) (Lund et al., 2008), which consists of more than 1,72,000 women who have answered detailed questionnaires regarding their diet and lifestyle. From the original cohort, 50,000 women (born between 1943 and 1957) were randomly recruited in batches of 500 to the NOWAC postgenome study (Dumeaux et al., 2008). In addition to answering questionnaires regarding lifestyle and defined exposures, these women also later donated blood samples. From a randomly selected batch of 500 women, 332 blood samples (blood drawn in 2005) were analyzed. All samples were frozen within 3 days of collection, and the sampling date and time were noted. All women participating in the NOWAC study were randomly selected. What has previously been reported for the NOWAC women is that the external validity is good and thus the women are representative for Norwegian women and their age (Lund et al., 2003).

The women were asked the following questions related to their PCP use; “How often do you use the following PCPs; facial cream, hand cream, body lotion and perfume?” The questions had the following frequency alternatives for each product: “rarely/never, 1–3 times per month, once per week, 2–4 times per week, 5–6 times per week, once per day, twice or more per day.” The women were also asked about frequency of showering and whether they used soap or not when they showered.

As the NOWAC study was originally not designed to study PCPs and parabens, the women were not asked at the day of sampling whether they had used any of the products on the same day. It is therefore not possible to adjust for PCP use on the sampling day.

Total skin lotion use was assessed by combining facial cream, hand cream, and body lotion on the basis of the relative skin surface area of body, hands and face, and frequency of application. The Lund and Browder Chart for skin burns were used to estimate area of face (3.5%) and hands (6%) in relation to the total body surface area. For example, 100% per day equals applying skin lotion to your whole body once per day and 200% equals whole body twice per day.

Analytical Methods

All solvents and reagents were of suprasolv or lichrosolv grade and were purchased from Merck-Schuchard (Hohenbrunn, Germany), with exception of formic acid (98–100%), which was of p.a. grade. Ultragrade high-purity water was produced from a MilliQ-Advantage A10 water purification system (Millipore, Massachusetts, USA). Methyl paraben (MP) and butyl paraben (BuP) were purchased from Fluka (Steinheim, Germany), ethyl (EP), propyl (PP) and benzyl paraben (BzP) were purchased from Aldrich (Steinheim, Germany) and all were of 99% purity. Mass labeled compounds of 99% purity were used as internal standards and were purchased from Cambridge Isotope Laboratory (Massachusetts, USA): d4-MP (M+4) and 13C6-BuP (M+6). Branched perfluorodecanoic acid (bPFDcA), purchased from ABCR (Karsruhe, Germany), 97% purity, was used as a recovery standard.

Plasma samples were extracted using a solid phase extraction (SPE) method on a Rapidtrace Automated SPE workstation (Zymark, Hopkinton, MA, USA). Samples were extracted on an Oasis HLB (540mg; Waters, Milford, MA, USA) solid phase extraction (SPE) column. An internal standard mixture (20μl of 200pg/μl d4-MP and 240pg/μl C6-BuP in methanol) was added to 0.5g of plasma and vortexed for 10s. Thereafter, 0.5ml of formic acid and 0.5ml of ultra high-purity water was added and the mixture was finally vortexed for 1min.

The Oasis HLB column was conditioned with 6ml methanol and 3ml of a 5% methanol in 0.1M aqueous HCl solution before the sample was loaded onto the column. After drying the column with pressurized nitrogen (5.0 grade) for 15min, the analytes were eluted with 6ml methanol. The extract was reduced in volume to ~0.1ml on a Rapidvap system (Labonco, Kansas City, MO, USA) and transferred to autosampler vials, with 10μl of recovery standard (102pg/μl in methanol) and 100μl of ultra high-purity water being added to the extract. Analysis was carried out on a Acquity ultra performance Liquid Chromatograph (UPLC) from Waters coupled to a LCT Premier XE time-of-flight (TOF) mass spectrometer (MS) (Waters), with atmospheric pressure chemical ionization (APCI) in negative mode. Using a 10μl injection volume, separation was achieved on a Waters Acquity BEH Phenyl column (100mm × 2.1mm ID, 1.7μm particle size) using acetonitrile (A) and 10mM aqueous NH4OAc (B) as the mobile phase with a flow rate of 0.45ml/min. The following gradient program was applied: initial conditions 98% A/2% B, increased linearly over 6min to 1% A/99% B, which was kept for 2min, then reduced to 1% A/99% B over 0.1min 98% A/2% B, and finally equilibrated for 1.9min at 98% A/2% B, total run time of 10min. Ionization conditions were as follows: probe temperature 350 °C, corona current 5μA and N2 desolvation gas flow 200l/min. Cone voltage was kept at 50V with a source temperature of 120 °C and aperture 1 at 3V.

The MS was run in high-resolution (W) mode with R=10,000. Full scan spectra were recorded over the range m/z 100–1000. For quantification, extracted high-resolution mass chromatograms were used with a typical peak width of 50mDa. Following masses were monitored: m/z 151.139 (MP), m/z 165.164 (EP), m/z 179.192 (PP), m/z 193.219 (BuP), m/z 227.241 (BzP), m/z 155.171 (d4-MP), m/z 199.239 (13C6-BuP), and m/z 469.085 (bPFDcA). LC-MS instrument control and processing was carried out using the software MassLynx ver. 4.1 (Micromass) and Quanlynx ver. 4.1 for quantification.

Internal standard method was applied for quantification using isotope-labeled parabens. d4−MP was used for quantification of MP, EP, and PB, and 13C6-BP was used for quantification of BP and BzP. A five-point calibration curve (50, 100, 200, 400, and 600pg injected on column) was used. The recovery standard (bPFDcA) was used for monitoring the recoveries of the internal standards from the extraction method. Solvent injections were carried out regularly during analysis in order to monitor instrument background and carry over effects.

QA/QC

Considering the lack of certified reference materials for determining parabens in plasma, as well as the high risk of elevated procedural blanks several quality assurance steps were implemented. The procedural blank contribution from the extraction and evaporation method was assessed by including three samples of ultra high-purity water with each extraction batch of 30 samples. The method precision was assessed from 25 repeated measurements of a low-level pooled plasma sample (unspiked) and a 10ng/ml spiked pooled plasma sample over the 8-week period of analysis.

Great care was taken to prevent sample contamination. All personnel involved in sample handling and clean up used paraben-free PCP. All equipment, benches, and fume hoods were thoroughly washed with methanol before sample clean up. Furthermore, all personnel wore gloves and hair nets, and all paraben-containing products (e.g., soaps) were removed from the laboratory the week before the sample clean up was conducted.

Statistical Evaluation

SAS (version 9.1.3, SAS Institute, Cary, NC, USA) was used for the statistical analysis. Statistical assessment of differences between the different PCP user groups was carried out using the Jonckheere-Terpstra test, a non-parametric test for a monotone trend across ordered groups. For correlations the Spearman's rank correlation test was used. P<0.01—significant trend.

Top

Results

QA/QC

The concentrations of parabens determined in an unspiked and a 10ng/ml spiked plasma pool are presented with the calculated method detection limits (MDLs) in Table 1. MP and PP was present in 100 and 70% of the blank samples, respectively. Thus the MDL was calculated using three times the blank levels for these two parabens. EP was not detected in any of the blank samples, therefore MDL for EP was reported as the instrumental detection limit by the QuanLynx software (3 × signal-to-noise ratio). The average blank levels were not subtracted from the measured levels, as the 10ng/ml spike solution only overestimated the levels by ~15%.


The semiautomated extraction procedure and sample handling precautions described previously resulted in only minor variations in the blank levels for MP and PP, with an average of 2.2 (SD 0.9, N=15) and 0.4ng/ml (SD 0.5, N=15), respectively.

The relative standard deviation (RSD), an estimate of inter- and intra-day variability of the method, ranged from 1.5–11.9% (Table 1). The recoveries of the internal standards varied between 45–67%.

Study Group Characteristics

The women randomly selected for this study were on average 55 years old (range: 48–62 years). As the NOWAC study was not designed to study PCP use and parabens, bias in terms of product use and pre-selection of women choosing to participate was avoided. In all, 25% of women sampled used body lotion at least once a day with 55 and 28% reporting use of facial cream “once” and “twice or more per day”, respectively. For hand lotion 24 and 32%, applied it “once” and “twice or more per day”, respectively in (Table 2).


Self-reported use of hand cream, facial cream, and body lotion was correlated; facial cream/hand cream r2=0.30 (P<0.001), facial cream/body lotion r2=0.27 (P<0.001), hand cream/body lotion r2=0.39 (P<0.001).

Concentrations of Parabens and Self-Reported use of PCPs

All parabens were detected in their native form above MDL in 63% (MP), 29% (PP), and 22% (EP) of the plasma samples from the women participating in this study. The overall median level was above MDL only for MP (9.4ng/ml) and below MDL for EP and PP. The concentrations of all three parabens were significantly correlated. For the analysis of BuP and BzP, the interferences in the chromatograms were too large for an accurate determination and they are thus not included in this study.

The median level of MP was higher in the groups of individuals reporting a more frequent use of PCP compared with a lower frequency (Table 3). PP and EP were not included in the table, as PP was only detected above MDL in women who used body lotion “once a day” (2.2ng/ml) and “twice or more per day” (4.0ng/ml). EP was only observed above MDL for the latter group of body lotion users (2.4ng/ml). The median level of EP and PP for the use of hand and facial cream was below MDL in all user groups.


Using the non-parametric Jonckheere-Terpstra test for statistical trends with increasing use of the different PCP in question, the change for MP, EP, and PP was highly significant (P<0.0001) for all PCP, except for PP and facial cream (P=0.09). For some of the infrequent user groups, the maximum levels were comparable to the maximum levels observed for the frequent users.

The concentrations of all three parabens and total skin care product use (% of skin area creamed per day) is indicated in Table 4. The median level of MP is graphically illustrated in Figure 1.

Figure 1.
Figure 1 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

Median MP concentration (ng/ml) and the area of skin (%) creamed per day. (X-axis: area of skin creamed per day, Y-axis: median concentration (ng/ml) of MP.

Full figure and legend (36K)


Figure 1 shows a clear increase in the median level of MP by increasing percentage of skin area creamed per day. The observed trend is significant (non-parametric Jonckheere-Terpstra test) for all three parabens (P<0.001), but increase for EP and PP only appear among the participants applying skin lotion to more than 100% of their skin surface per day (Table 4). High concentrations were also observed among infrequent users. The maximum level of MP (142.9ng/ml) was observed in the 150–200% group. The maximum level for EP (45.9ng/ml) and PP (43.9ng/ml) was observed in the 50–100% group.

Showering (with and without soap), perfume, age, and body mass index (BMI) did not significantly affect the concentrations of parabens detected. There was no significant association with concentration of any of the parabens and sampling time (hour of the day).

Top

Discussion

The calculation of “total body area creamed per day” from the frequency of use of body, hand, and face lotions, resulted in one cumulative exposure variable. This clearly gave a better average estimate of exposure than analyzing the three variables separately. Further it increased the exposure range and thus the likelihood of identifying true significant associations.

Concentrations of parabens were significantly associated with self-reported use of PCP, which was most clearly demonstrated for MP (Figure 1), but also for PP and EP. This clearly shows that despite short half-lives, elevated plasma concentrations of native MP, EP, and PP can result from continuous use of skin lotions. This study focused on the native parabens as an assessment of internal dose, and did not determine the total amount of parabens (i.e. the sum of native parabens, p-hydroxybenzoic acid and phase II conjugates).

The automated analytical extraction method enabled a fast and cost-efficient determination of MP, EP, and PP in small volumes of human plasma (0.5ml). Despite elevated MDLs, the method resulted in reproducible blank contamination with little risk of finding false positives, with a RSD of less than 12% for all parabens. Lower blank values have previously been achieved using an online SPE system coupled to a HPLC–MS system (Ye et al., 2008). An online technique may be considered a preferred option in order to avoid contamination.

High levels of MP, EP, and PP were detected in a number of individuals even though relatively few participants had concentrations above MDL for EP (22%) and PP (29%). MP in its native form was detected in as many as 63% of the participants. This study provides data on native paraben plasma concentrations within the general population, which has not yet been published (Boberg et al., 2010). Furthermore, parabens were analyzed in their native form in order to assess the actual levels of the unchanged form, which is also more relevant for the estrogenic effects (Soni et al., 2005).

Native and conjugated parabens have previously been investigated in human serum (Ye et al., 2008). In 15 individuals, low levels of native parabens were identified; however, more than 87% of the detected parabens were present as conjugated metabolites. The reported median level of native MP (9.4ng/ml) in this study was considerably higher than the value (0.2ng/ml) reported by Ye et al. (2008), but similar to their value reported for total MP (10.9ng/ml). These levels of native MP are supported by recent findings in human milk, in which a maximum concentration of 8ng/ml was reported (Schlumpf et al., 2010).

The most comprehensive study on parabens conducted by the US NHANES investigated both native and conjugated form in urine samples (Calafat et al., 2010). However, little is known about the metabolism (i.e. pharmacokinetics) of parabens for a proper assessment of internal dose on the basis of urinary concentrations. Therefore, direct comparisons between plasma and urine cannot be made.

This study presents the first report of native MP, EP, and PP in plasma from the general population. The high concentrations detected seem plausible considered that high mean concentrations of BuP (135±11ng/ml) were detected 3h after whole-body application with cream containing 2% BuP (Janjua et al., 2007). Although maximum concentrations for PP (43.9ng/ml) and MP (129.3ng/ml) were lower in our study, it could be due to differences in adsorption rates and/or lower paraben content in the products used. In commercially available PCP the total paraben content is supposed to be lower than 0.8% within the EU, which is 40% of the cream used by Janjua et al. (2007). The high concentration of native parabens identified in this study is not likely caused by hydrolysis of conjugates as paraben conjugates in human serum have been shown to be stable over 30 days when stored at 37°C (Ye et al., 2009). The contribution of conjugate hydrolysis is therefore considered negligible to the values reported.

Highly significant trends were observed for paraben levels (with the exception of PP and facial cream) and frequency of skin care product use. Linear trends were observed for MP, whereas the trends for EP and PP was most significant in groups with highest use (“once per day” and “twice or more per day”). The impact of shower frequency and whether the women used soap or not when they showered was also investigated, but no significant impact on the paraben levels was observed. The shower frequency was highly correlated with the use of skin care products, and it seems clear that the amounts of lotion or parabens in the lotion masks the potential effects of products used when showering.

Despite the significant trends, high concentrations were also detected among some of the infrequent users. However, this could be explained by a number of factors like application of other paraben containing products or lotion the same day. There was no connection with the use of vaginal creams or sun lotions, but there could be other products like medicine and food that there was no information available for. The participants would also apply different amounts of products and the questionnaire does not consider this nor lotion use on the day of the sampling. Questionnaire information was obtained several months before they actually donated the blood sample and it is thus impossible to know which women applied skin care products on the day of donating the blood samples. On the basis of results by Janjua et al. (2007), it is evident that high concentrations of the native form of the parabens can be expected shortly after whole-body application (i.e. within hours). The strong association between concentrations and questionnaire information indicates consistent use of PCP with time by the participants in the NOWAC study. This can be used in future follow-up studies to classify the women based on their paraben exposure. The calculation of total body area creamed on the basis of frequency and percent area of hands, face, and body was considered a valid approach as the relationship between use and concentrations became more evident.

The findings of elevated levels of native parabens in plasma from the general population give reason for concern. The strong and significant association with use of skin care products clearly shows that repetitive use of these products increases the exposure to native parabens. These findings clearly emphasize the need for epidemiological studies on the possible health effects of continuous paraben exposure in the general population.

Top

Notes

Disclaimer

The findings and conclusions are the authors and do not necessarily reflect the view of NILU or the University of Tromsø. The authors declare no conflict of interest.

Top

References

  1. Andersen F.A. Final amended report on the safety assessment of methylparaben, ethylparaben, propylparaben, isopropylparaben, butylparaben, isobutylparaben, and benzylparaben as used in cosmetic products. Int J Toxicol 2008: 27: 1–82. | ISI |
  2. Boberg J., Taxvig C., Christiansen S., and Hass U. Possible endocrine disrupting effects of parabens and their metabolites. Reprod Toxicol 2010: 30: 301–312. | Article | PubMed | ISI |
  3. Byford J.R., Shaw L.E., Drew M.G.B., Pope G.S., Sauer M.J., and Darbre P.D. Oestrogenic activity of parabens in MCF7 human breast cancer cells. J Steroid Biochem Mol Biol 2002: 80: 49–60. | Article | PubMed | ISI | ChemPort |
  4. Calafat A.M., Ye X.Y., Wong L.Y., Bishop A.M., and Needham L.L. Urinary concentrations of four parabens in the US population: NHANES 2005–2006. Environ Health Perspect 2010: 118: 679–685. | Article | PubMed | ISI |
  5. Cashman A.L., and Warshaw E.M. Parabens: a review of epidemiology, structure, allergenicity, and hormonal properties. Dermatitis 2005: 16: 57–66. | Article | PubMed | ISI |
  6. Darbre P.D., Aljarrah A., Miller W.R., Coldham N.G., Sauer M.J., and Pope G.S. Concentrations of parabens in human breast tumours. J Appl Toxicol 2004: 24: 5–13. | Article | PubMed | ISI | ChemPort |
  7. Darbre P.D., Byford J.R., Shaw L.E., Hall S., Coldham N.G., Pope G.S., and Sauer M.J. Oestrogenic activity of benzylparaben. J Appl Toxicol 2003: 23: 43–51. | Article | PubMed | ISI | ChemPort |
  8. Darbre P.D., Byford J.R., Shaw L.E., Horton R.A., Pope G.S., and Sauer M.J. Oestrogenic activity of isobutylparaben in vitro and in vivo. J Appl Toxicol 2002: 22: 219–226. | Article | PubMed | ISI | ChemPort |
  9. Darbre P.D., and Harvey P.W. Paraben esters: review of recent studies of endocrine toxicity, absorption, esterase and human exposure, and discussion of potential human health risks. J Appl Toxicol 2008: 28: 561–578. | Article | PubMed | ISI |
  10. Dumeaux V., Borresen-Dale A.L., Frantzen J.O., Kumle M., Kristensen V.N., and Lund E. Gene expression analyses in breast cancer epidemiology: the Norwegian Women and Cancer postgenome cohort study. Breast Cancer Res 2008: 10.
  11. Elder R.L. Final report on the safety assessment of methylparaben, ethylparaben, propylparaben, and butylparaben. J Am Coll Toxicol 1984: 3: 147–209.
  12. Fang H., Tong W.D., Shi L.M., Blair R., Perkins R., Branham W., Hass B.S., Xie Q., Dial S.L., Moland C.L., and Sheehan D.M. Structure-activity relationships for a large diverse set of natural, synthetic, and environmental estrogens. Chem Res Toxicol 2001: 14: 280–294. | Article | PubMed | ISI |
  13. Janjua N.R., Mortensen G.K., Andersson A.M., Kongshoj B., Skakkebaek N.E., and Wulf H.C. Systemic uptake of diethyl phthalate, dibutyl phthalate, and butyl paraben following whole-body topical application and reproductive and thyroid hormone levels in humans. Environ Sci Technol 2007: 41: 5564–5570. | Article | PubMed | ISI | ChemPort |
  14. Kadry A.M., Okereke C.S., AbdelRahman M.S., Friedman M.A., and Davis R.A. Pharmacokinetics of Benzophenone-3 After Oral-Exposure in Male-Rats. J Appl Toxicol 1995: 15: 97–102. | Article | PubMed | ISI |
  15. Lund E., Dumeaux V., Braaten T., Hjartaker A., Engeset D., Skeie G., and Kumle M. Cohort profile: The Norwegian women and cancer study — NOWAC — Kvinner og kreft. Int J Epidemiol 2008: 37: 36–41. | Article | PubMed | ISI |
  16. Lund E., Kumle M., Braaten T., Hjartaker A., Bakken K., Eggen E., and Gram T.I. External validity in a population-based national prospective study--the Norwegian Women and Cancer Study (NOWAC). Cancer Causes Control 2003: 14: 1001–1008. | Article | PubMed |
  17. Moss T., Howes D., and Williams F.M. Percutaneous penetration and dermal metabolism of triclosan (2,4,4′-trichloro-2′-hydroxydiphenyl ether). Food Chem Toxicol 2000: 38: 361–370. | Article | PubMed | ISI |
  18. NTP. Butyl paraben. [CAS No. 94-26-8]. Review of Toxicological Literature. National Toxicology Program 2005 Research Triangle Park, NC http://ntp.niehs.nih.gov/ntp/h
    tdocs/Chem_Background/ExSumPdf/Butylparaben.pdf
    .
  19. Okereke C.S., Kadry A.M., AbdelRahman M.S., Davis R.A., and Friedman M.A. Metabolism of Benzophenone-3 in Rats. Drug Metabol Dispos 1993: 21: 788–791. | ISI |
  20. Perkins R., and Sheehan D.M. The estrogen receptor relative binding affinities of 188 natural and xenochemicals: structural diversity of ligands. Toxicol Sci 2000: 54: 138–153. | Article | PubMed | ISI |
  21. Routledge E.J., Parker J., Odum J., Ashby J., and Sumpter J.P. Some alkyl hydroxy benzoate preservatives (parabens) are estrogenic. Toxicol Appl Pharmacol 1998: 153: 12–19. | Article | PubMed | ISI |
  22. Schlumpf M., Kypke K., Wittassek M., Angerer J., Mascher H., Mascher D., Vokt C., Birchler M., Lichtensteiger W. Exposure patterns of UV filters, fragrances, parabens, phthalates, organochlor pesticides, PBDEs, and PCBs in human milk: correction of UV filters with use of cosmetics. Chemosphere 2010: 81: 1171–1183. | Article | PubMed | ISI |
  23. Soni M.G., Carabin I.G., and Burdock G.A. Safety assessment of esters of p-hydroxybenzoic acid (parabens). Food Chem Toxicol 2005: 43: 985–1015. | Article | PubMed | ISI | ChemPort |
  24. Vokel W., Colnot T., Csanady G.A., Filser J.G., and Dekant W. Metabolism and kinetics of bisphenol A in humans at low doses following oral administration. Chem Res Toxicol 2002: 15: 1281–1287. | Article | PubMed | ISI | ChemPort |
  25. Ye X., Tao L.J., Needham L.L., and Calafat A.M. Automated on-line column-switching HPLC-MS/MS method for measuring environmental phenols and parabens in serum. Talanta 2008: 76: 865–871. | Article | PubMed | ISI | ChemPort |
  26. Ye X.Y., Bishop A.M., Reidy J.A., Needham L.L., and Calafat A.M. Parabens as urinary biomarkers of exposure in humans. Environ Health Perspect 2006: 114: 1843–1846. | PubMed | ISI | ChemPort |
  27. Ye X.Y., Wong L.Y., Jia L.T., Needham L.L., and Calafat A.M. Stability of the conjugated species of environmental phenols and parabens in human serum. Environ Int 2009: 35: 1160–1163. | Article | PubMed | ISI |
Top

Acknowledgements

We are very grateful to all participants of the NOWAC study. We also thank Bente Augdal for her efforts in sample handling. This project was funded by “Sparebank1 Nord-Norge” research Funds.