kajian Phthalates

Phthalates and other additives in plastics: human exposure and associated health outcomes

 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2873014/

Concern exists over whether additives in plastics to which most people are exposed, such as phthalates, bisphenol A or polybrominated diphenyl ethers, may cause harm to human health by altering endocrine function or through other biological mechanisms. Human data are limited compared with the large body of experimental evidence documenting reproductive or developmental toxicity in relation to these compounds. Here, we discuss the current state of human evidence, as well as future research trends and needs.

Because exposure assessment is often a major weakness in epidemiological studies, and in utero exposures to reproductive or developmental toxicants are important, we also provide original data on maternal exposure to phthalates during and after pregnancy (n = 242). Phthalate metabolite concentrations in urine showed weak correlations between pre- and post-natal samples, though the strength of the relationship increased when duration between the two samples decreased. Phthalate metabolite levels also tended to be higher in post-natal samples.

In conclusion, there is a great need for more human studies of adverse health effects associated with plastic additives. Recent advances in the measurement of exposure biomarkers hold much promise in improving the epidemiological data, but their utility must be understood to facilitate appropriate study design.

Keywords: bisphenol A, endocrine disruption, epidemiology, phthalate, polybrominated diphenyl ether, reproductive health

Advances in materials science and engineering in recent decades have led to the widespread and diverse use of plastics to provide cheaper, lighter, stronger, safer, more durable and versatile products and consumer goods that serve to improve our quality of life. Plastics can be designed to keep our foods fresher for longer periods of time, can provide therapeutic benefits through timed-release pharmaceuticals and other medical applications, and can prevent electronics and other household items from starting or spreading fires (see Andrady & Neal 2009; Thompson et al. 2009a,b). However, scientific, governmental and public concern exists over the potential adverse human health risks related to ubiquitous exposures to plastic additives among the general population. The leading hypothesis for these growing concerns is that certain chemicals, used in plastics to provide beneficial physical qualities, may also act as endocrine-disrupting compounds (EDCs) that could lead to adverse reproductive and developmental effects (NRC 1999). In men, environmental or occupational exposures to EDCs may be associated with or lead to declined reproductive capacity or possibly increased risk of testicular or prostate cancer (Fleming et al. 1999; Pflieger-Bruss et al. 2004; Toft et al. 2004). In fact, a number of studies have suggested the use of circulating reproductive hormone levels (follicle-stimulating hormone (FSH) and/or inhibin B) as a surrogate measure for semen quality or fecundity in epidemiologic studies (Jensen et al. 1997; Uhler et al. 2003; Mabeck et al. 2005), although other recent studies suggest hormone levels may lack sufficient ability to predict poor semen quality (Dhooge et al. 2007; Meeker et al. 2007a). Endocrine alterations in women resulting from environmental or occupational exposure may represent increased risk for endometriosis, reproductive and other endocrine-related cancers, or impaired oocyte competence, ovarian function or menstrual cycling (Nicolopoulou-Stamati & Pitsos 2001; Pocar et al. 2003; Windham et al. 2005). Effects of early life exposures to EDCs remain unclear, though it has been suggested that foetal or childhood exposure may lead to altered sex differentiation (Toppari & Skakkebaek 1998), effects on neurological and reproductive development (Tilson 1998; Teilmann et al. 2002; Colborn 2004, 2006; Swan et al. 2005) and increased risk of reproductive problems or cancer later in life (Damgaard et al. 2002; Aksglaede et al. 2006; Main et al. 2006a). Programming in early life can determine an individual's future health; therefore, early chemical exposures may have long-term impacts later on in life (Gluckman et al. 2008). A leading hypothesis for a collection of linked conditions in human males exposed to EDCs in utero is termed ‘testicular dysgenesis syndrome (TDS)'. TDS represents a number of reproductive disorders of varying severity that are associated with disturbed gonadal development, including cryptorchidism, hypospadias and smaller reproductive organs (Olesen et al. 2007). Later in life, the effects of TDS are hypothesized to manifest as a reduction in semen quality and infertility as well as an increased risk for testicular cancer.

Exposure to plastic additives and other EDCs may cause altered endocrine activity and reproductive development through a number of biological mechanisms, which can target different levels of the hypothalamic–pituitary–gonad/thyroid axis, ranging from effects on hormone receptors to effects on hormone synthesis, secretion or metabolism (Boas et al. 2006; Bretveld et al. 2006). The purpose of this manuscript is not to discuss the various biological pathways or the hundreds of animal and in vitro studies that have been conducted on plastic additives as reproductive and developmental toxicants, but rather to review the existing epidemiologic literature on human exposure to these compounds and the relationship with adverse reproductive or developmental endpoints. Because exposure assessment is a fundamental and frequently weak component in large epidemiological studies due to technical, logistic and financial constraints, and in utero exposures are among the exposure periods of greatest concern with regard to EDCs, we also provide original data on human exposure to a class of potential endocrine-disrupting plastic additives during and after pregnancy.

Despite the increasing concern for human health impacts associated with plastic additives, there remains a paucity of human studies that have investigated these relationships. While the clinical significance of some markers of endocrine disruption, reproductive health or altered development that commonly appear in the human research literature remains unclear, such as declines in semen quality or subclinical alterations in circulating hormone levels, there is a limited but growing body of evidence for such changes to be associated with environmental and occupational exposure to plastic additives and other potential EDCs. In addition, these markers may serve as intermediate indicators that altered endocrine function is the pathway linking environmental exposures to clinical reproductive and developmental effects. Also, because such a large number of people are exposed to background levels of a number of proven or suspected EDCs, even seemingly subtle epidemiologic associations may result in large increases in reproductive and other endocrine-related disease among populations and thus should be of great public health concern. The background material presented in this manuscript is meant as an introductory review of human studies conducted in this area to date. The reader is directed to the individual references for additional study detail. We focus here on three types of plastic additives—phthalates, bisphenol A (BPA) and polybrominated diphenyl ethers (PBDEs)—because there is laboratory evidence for reproductive or developmental effects in relation to exposure to these compounds. These chemicals were also chosen to be discussed here based on strong evidence for widespread human exposure (CDC 2005; Calafat et al. 2008; Sjodin et al. 2008).

PHTHALATES

(a) Exposure

The diesters of 1,2-benzenedicarboxylic acid (phthalic acid), commonly known as phthalates, are a group of man-made chemicals widely used in industrial applications. High-molecular weight phthalates (e.g. di(2-ethylhexyl) phthalate (DEHP)) are primarily used as plasticizers in the manufacture of flexible vinyl plastic which, in turn, is used in consumer products, flooring and wall coverings, food contact applications and medical devices (David et al. 2001; ATSDR 2002; Hauser & Calafat 2005). Manufacturers use low-molecular weight phthalates (e.g. diethyl phthalate (DEP) and dibutyl phthalate (DBP)) as solvents in personal-care products (e.g. perfumes, lotions, cosmetics), and in lacquers, varnishes and coatings, including those used to provide timed releases in some pharmaceuticals (ATSDR 1995, 2001; David et al. 2001).

As a result of the ubiquitous use of phthalates in personal-care and consumer products, human exposure is widespread. Exposure through ingestion, inhalation and dermal contact is considered important routes of exposure for the general population (Adibi et al. 2003; Rudel et al. 2003). For infants and children, added skin contact with surfaces and frequent mouthing of fingers and other objects (e.g. plastic toys) may lead to higher phthalate exposures, as might ingestion of phthalates present in breast milk, infant formula, cow's milk or food packaging (Sathyanarayana 2008). Frequent use of personal-care products may lead to higher exposures to the lower molecular weight phthalates, as increased exposures have been found among men reporting recent use of cologne and aftershave (Duty et al. 2005a) and among infants whose mothers reported recent use of certain infant-care products (lotions, powders and shampoos) (Sathyanarayana et al. 2008). Parenteral exposure from medical devices and products containing phthalates is also an important source of high exposure to phthalates, primarily DEHP (ATSDR 2002; Green et al. 2005; Weuve et al. 2006), for hospitalized populations.

Upon exposure, phthalates are rapidly metabolized and excreted in urine and faeces (ATSDR 1995, 2001, 2002). Owing to the ubiquitous presence of phthalates in indoor environments and concern for sample contamination when measuring the parent diesters in biological samples, the most common approach for investigating human exposure to phthalates is the measurement of urinary concentrations (biomarkers) of phthalate metabolites. The Centers for Disease Control and Prevention's (CDC) Third National Report on Human Exposure to Environmental Chemicals showed that the majority of people in the USA have detectable concentrations of several phthalate monoesters in urine (mono-ethyl phthalate (MEP), mono-(2-ethylhexyl) phthalate (MEHP), mono-butyl phthalate (MBP) and mono-benzyl phthalate (MBzP)), reflecting widespread exposure to the parent diester compounds among the general population (CDC 2005). Two oxidative metabolites of DEHP, mono-(2-ethyl-5-hydroxylhexyl) phthalate (MEHHP) and mono-(2-ethyl-5-oxohexyl) phthalate (MEOHP) were present in most subjects at urinary concentrations higher than those of MEHP, the hydrolytic metabolite of DEHP (CDC 2005).

More recently, a larger study using urinary levels of phthalate metabolites was conducted by Duty et al. (2003a), with follow-up analysis reported by Hauser et al. (2006). Study subjects consisted of male partners of subfertile couples that presented to an infertility clinic in Massachusetts, USA. At the time of the clinic visit, one sample of semen and urine were collected. In the initial study, there were dose–response relationships (after adjusting for age, abstinence time and smoking status) between MBP and below World Health Organization (WHO) reference value sperm motility and sperm concentration among 168 men (Duty et al. 2003a). There was also a dose–response relationship between MBzP (the primary hydrolytic metabolite of BBzP) and below WHO reference value sperm concentration. In a recent follow-up study including these 168 men, plus an additional 295 men newly recruited into the study, Hauser et al. (2006) confirmed the associations between MBP and increased odds of below-reference sperm concentration and motility. The relationships appeared to follow dose-dependent patterns, where greater odds ratios were calculated among increasing phthalate metabolite quartiles. However, there was only a suggestive association between the highest MBzP quartile and low sperm concentration (p = 0.13), which was not fully consistent with the results of the preliminary analysis (Duty et al. 2003a).

In the Massachusetts (USA) study, semen samples from 379 men were also cryogenically frozen and sperm cells later analysed for DNA damage using the neutral comet assay (Hauser et al. 2007). Sperm DNA damage measurements included comet extent (CE), percentage of DNA in tail (tail%) and tail distributed moment (TDM). In multivariate linear regression models adjusted for age and smoking, significant positive associations were found for at least one of the three DNA damage measures with MEP (CE, TDM), MBP (tail%), MBzP (CE, TDM) and MEHP (tail%). For MEP, the significant association with CE and TDM confirmed previous findings among an earlier and smaller subset from the same study population (Duty et al. 2003b). Another interesting finding was that MEHP was strongly associated with all three DNA damage measures after adjustment for the oxidative DEHP metabolites, which may serve as phenotypic markers of DEHP metabolism to ‘less toxic’ metabolites and lower susceptibility to exposure-related effects compared with those individuals with low concentrations of oxidative DEHP metabolites relative to MEHP concentration (Hauser 2008). Metabolism of phthalates depends on the size and structure of the diester, and can occur via two steps: phase I (e.g. hydrolysis, oxidation) followed by phase II (conjugation) (Frederiksen et al. 2007). Since the monoester metabolite may be the more bioactive form of the phthalate, individuals who are predisposed to form and retain more monoester may have a heightened sensitivity to phthalate exposure.

In summary, the epidemiologic data on semen quality and/or sperm cell integrity in relation to phthalate exposure remain limited and inconsistent. Additional studies are critically needed to help elucidate possible explanations for differences across studies, and most importantly, to address whether phthalate exposure alters semen quality, sperm function and male fertility.

(ii) Other reproductive/endocrine effects

Several human studies have investigated associations between exposure to phthalates and circulating hormone levels. In a study of workers producing PVC flooring with high exposure to DEHP and DBP, urinary concentrations of metabolites of these phthalates were inversely associated with free testosterone levels (Pan et al. 2006). A report on 295 men from the Massachusetts (USA) infertility clinic study found a suggestive inverse association between MEHP and testosterone, along with a positive association between urinary MBP and inhibin B (a glycoprotein hormone produced by the gonads that has an inhibitory effect on pituitary FSH production), and an inverse association between urinary MBzP and FSH (Duty et al. 2005b). However, the significant results for MBP and MBzP and hormone levels were in patterns inconsistent with the authors’ hypotheses. It is interesting to note that although MEHP concentrations in the Massachusetts study were several orders of magnitude lower than those measured in the exposed Chinese workers (Pan et al. 2006), the evidence for decreased testosterone in relation to DEHP/MEHP was consistent between the two studies. It is also interesting to note that the inverse association between MBP and testosterone in the study of exposed Chinese workers (Pan et al. 2006) appears to be consistent with the male infant studies described earlier, where MBP concentrations were inversely associated with anogenital index (a measure of androgen activity) and free testosterone (Swan et al. 2005; Main et al. 2006b). On the other hand, the study of 234 young Swedish men found an inverse association between urinary MEP and LH but no association between MEP, MBP, MEHP or other phthalate metabolites in urine and FSH, testosterone, oestradiol or inhibin B (Jonsson et al. 2005).

Owing to the documented anti-androgenic effects of certain phthalates in animal models, and recent observations that low testosterone in adult males may be associated with an increased prevalence of obesity and type 2 diabetes (Ding et al. 2006; Selvin et al. 2007), Stahlhut et al. (2007) explored the relationship between phthalate exposure and waist circumference in a large cross-sectional study carried out among a subset of adult male participants in the 1999–2002 US National Health and Nutrition Examination Survey (NHANES). The authors reported significant associations between urinary phthalate monoester concentrations (MBzP, MEHHP, MEOHP and MEP) and increased insulin resistance (measured through homeostatic model assessment), and positive associations between MBP, MBzP and MEP and waist circumference. These findings provide preliminary evidence of a potential contributing role for phthalates in the overall population burden of insulin resistance, obesity and related clinical conditions, but additional studies are needed.

The potential for phthalates to affect thyroid function has been demonstrated in animal studies, but human studies are limited to two recent investigations: one within the Massachusetts (USA) male infertility clinic study (Meeker et al. 2007b) and another among pregnant Taiwanese women in their second trimester (Huang et al. 2007). In the Massachusetts study, phthalate metabolite concentrations were measured in urine and thyroid hormones were measured in serum from 408 men. MEHP was inversely associated with free T4 and total T3, but was not associated with thyroid-stimulating hormone (TSH). The inverse association between MEHP and free T4 became stronger when also taking into account the concentrations of oxidative DEHP metabolites that were positively associated with free T4. As with the findings from the study of sperm DNA damage (Hauser et al. 2007), these results may reflect metabolic susceptibility to the adverse effects of MEHP among individuals who less efficiently oxidize DEHP and/or MEHP (Meeker et al. 2007b; Hauser 2008). Among 76 pregnant Taiwanese women, Huang et al. (2007) reported an inverse association between MBP and both total and free levels of T4. Unlike the study among US men, they did not find an inverse association with MEHP, but there were considerable differences between the design of the two studies. In addition to having a smaller study size and a vastly different study population, the Taiwanese study also did not take into account concentrations of oxidative DEHP metabolites, which served to strengthen the associations between MEHP and thyroid hormones in the US study. More study is needed on the association between phthalate exposure and thyroid function, which plays an important role in many human systems including reproduction and foetal neurodevelopment.

Conclusions

A number of chemicals used in plastics for property enhancement are emerging environmental contaminants of concern (see also the discussion in Oehlmann et al. (2009), Koch & Calafat (2009), Thompson et al. (2009b) and this paper). Although the epidemiological data on the plastic additives described here suggest that there may be associations with altered endocrine function and reproductive or developmental effects, the number of human studies is currently limited and the quantity and quality of the data available for the different compounds are varied. Also, for some of the more studied associations, such as between phthalates and semen quality, the data across studies are not consistent. This may be due to small study sizes and lack of statistical power or differences in study design, study populations, exposure assessment strategies, exposure levels, exposure sources, exposure routes, multiple/competing physiologic mechanisms, analytical approaches and potential confounding variables considered in the statistical analysis (e.g. age, BMI, season). The limited human data, and in certain instances inconsistent data across studies, highlight the need for further epidemiological research on these classes of chemicals. Most studies to date have been cross-sectional in nature. Future longitudinal studies are needed to explore the temporal relationship between exposure to plastic additives and adverse reproductive and developmental outcomes to provide more information on whether these relationships may be causal in nature. Owing to the complex nature of the endocrine system, studies should evaluate not only individual hormone levels but also the ratios between relevant hormones (e.g. LH : testosterone ratio in males as a marker for Leydig cell function) that may help provide clues to the biological mechanisms of xenobiotic activity in humans.

Researchers face a number of challenges that need to be addressed to further our understanding of the relationship between plastic additives and adverse human health effects. One future challenge includes the shifts in exposure levels among populations over time caused by the ever-changing patterns of production and use of these compounds. Another challenge is to understand how simultaneous co-exposures to these chemicals may affect endocrine function. It is well known that humans are exposed to all these compounds simultaneously, and to many other chemicals. However, most studies to date have only addressed single chemicals or classes of chemicals, and there are limited data on the interactions between chemicals within a class or across classes. Chemicals may interact additively, multiplicatively or antagonistically in what is commonly referred to as the ‘cocktail effect'. The human health risks of exposure to chemical mixtures are much understudied. Despite these challenges, evolving and innovative technologies designed to improve the assessment of human exposure and intermediate biological markers of effect should provide enhanced opportunities for improving our understanding of the relationship between these environmental chemicals and reproductive and developmental health. Innovations include improved biomarkers of exposure, more sophisticated statistical methods that deal with multiple exposures simultaneously and sensitive new measures of intermediate alterations in human endocrine function, reproductive health and foetal/child development.

More information is required on biological mechanisms of plastic additives in humans as well as the clinical and public health consequences of changes of intermediate markers of effect observed in human studies. For example, to date, in most studies that have reported statistically significant hormone alterations attributed to environmental and occupational exposures, the actual degree of hormone alteration has been considered subclinical. However, much remains unknown as to whether hormone changes currently considered subclinical may be associated with increased risk of adverse systemic effects in the long term. Furthermore, although seemingly subtle, small changes in hormone levels resulting from exposure may be of public health importance when considering the prevalence of exposure to plastic additives and EDCs among entire populations. Finally, human research is needed on potential latent and transgenerational effects (e.g. epigenetic modifications) of exposure to plastic additives and other EDCs, including the possibility of environmentally linked foetal origins of adult diseases, as well as genetic, metabolic, demographic or environmental characteristics resulting in increased individual susceptibility to adverse health effects following exposure.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2873014/

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