Environmental Exposure to Non-Persistent Endocrine Disrupting Chemicals and Endometriosis: A Systematic Review

Endometriosis is a disease characterized by the presence of the uterine endometrium outside of its normal location. As the etiology of endometriosis is not well known and hormonal imbalance is central to disease pathogenesis, the potential contribution of exposure to endocrine-disrupting chemicals (EDCs) has been hypothesized in endometriosis. A systematic search of the literature was carried out to identify relevant studies using: PubMed, Scopus, Elsevier, Springer; EBSCO, and Web of Science. A total of 22 studies were considered. Most of the studies reviewed in this paper showed an association between exposure to BPA and phthalates and endometriosis. In the case of phthalate exposure, the reviewed studies found an association between the concentration of at least one phthalate metabolite and endometriosis. Only one study was performed to assess the exposure to parabens and a significant relationship with endometriosis was found. Additionally, only one study assessed the relationship of non-persistent pesticide exposure with endometriosis, observing a significant association between endometriosis and the urinary concentration of diazinon, chlorpyrifos, and chlorpyrifos-methyl. Studies struggled to provide a conclusion on the effect of exposure to benzophenones on endometriosis. Despite the numerous limitations of the results, the reviewed studies suggest that exposure to non-persistent endocrine disruptors, especially bisphenol A and phthalates may affect endometriosis. The results of the studies on exposure to parabens, benzophenones, and non-persistent insecticides are inconclusive.


Introduction
Endometriosis is a gynecological disease associated with chronic pelvic pain and infertility. This disease is diagnosed in 5-10% of women of reproductive age [1]. For a woman, the disease is associated with malaise, undergoing therapy, and performing costly operations. Women often have to give up their daily duties because of the bothersome symptoms of the disease. Endometriosis is characterized by the presence of uterine endometrial tissue outside of its normal location. Pathological endometrial tissue occurs on the peritoneum of the pelvis, ovaries, and rectovaginal septum, as well as even in the pericardium, pleura, and brain. The disease causes extensive adhesions and distortions in the pelvis [2]. Endometriosis is diagnosed surgically using the technique of laparoscopy or laparotomy [3]. Another method of endometriosis diagnosis is vaginal ultrasound (TVUS) or magnetic resonance imaging (MRI) [4,5]. The American Society for Reproductive Medicine (ASRM) guidelines are used to assess the severity of the disease. There are four degrees of endometriosis: minimal, mild, moderate, and severe [6]. In addition, there are three types of endometriosis: peritoneal, ovarian, and rectovaginal. The clinical picture differs depending on the patient, and the treatment procedures depend on the symptoms and fertility status [7]. Endometriosis is idiopathic in origin, but there are several theories that could explain its origin. It is assumed that the following factors may be responsible for the development of endometriosis: immune dysfunction, genetic aspects, lifestyle, and environmental pollution [8][9][10]. As the etiology of endometriosis is not well understood and estrogen is central to disease pathogenesis, regulating the key pathological processes in endometriosis including immunological, inflammatory, angiogenic, antiapoptotic, cellular, and molecular mechanisms, the potential contribution of exposure to endocrine-disrupting chemicals (EDCs) has been hypothesized in endometriosis. EDCs are of particular interest as potential contributors to endometriosis because they can alter steroidogenesis and immunologic function, in addition to being the epigenetic causal factors involved in disease progression [11]. According to WHO's definition, an endocrine disruptor is an exogenous substance or mixture that alters the function(s) of the endocrine system and consequently causes adverse health effects in an intact organism, its progeny, or its subpopulations (WHO 1996). EDCs are compounds that have the ability to interact with the endocrine system interfering with its normal functioning. They are widespread in the environment; thus, exposure occurs through contact with these compounds through food, water, air, plastics, or cosmetics. EDCs can be categorized as persistent or non-persistent. Persistent chemicals are those chemicals that tend to endure in the environment for years after their release, whereas, nonpersistent chemicals are exogenous chemicals or mixtures of industrial agents that can interfere with the normal action of hormones with a shorter half-life and lower liposolubility [11].
Numerous studies have been conducted on the influence of exposure to persistent endocrine-disrupting factors (e.g., dioxins, polychlorinated biphenyls, organochlorine pesticides, and some metals) on endometriosis. The obtained results did not confirm the validity of the statement concerning the influence of exposure to dioxins on the risk of developing endometriosis [12,13]. Similarly, numerous studies have been carried out on the influence of polychlorinated biphenyls on the occurrence of endometriosis, and most of them failed to confirm the existence of associations between these chemicals and the occurrence of endometriosis [12,[14][15][16]. In addition, studies on persistent pesticide exposure and endometriosis have been conducted, but the results did not provide a clear indication of a link between pesticide exposure and the occurrence of endometriosis [17][18][19].
Non-persistent compounds are chemicals widely found in the environment. in many everyday products, e.g., plastics, lubricants, solvents, plasticizers, and pesticides. Low levels of exposure may cause endocrine or reproductive disorders [20]. These are substances that may disrupt the functioning of the endocrine system and, consequently, affect the fertility of men and women [21][22][23][24][25][26]. Compared to studies evaluating the link between persistent endocrine-disrupting chemical exposure and endometriosis studies on the effects of exposure to nonpersistent chemicals are rare. As the exposure to non-persistent chemicals is widespread and associated with reproductive and gynecological disorders, as well as poor fertility, this review aims to answer the question regarding the effect of exposure to widespread nonpersistent endocrine-disrupting chemicals on endometriosis, taking into account the limitations and strength of the presented studies.

Materials and Methods
The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines was employed in this review.

Search Strategy
A systematic search of the literature was carried out to identify relevant studies published in English from 2003 to February 2021. Relevant studies were also identified through a review of the references cited in all the published studies. The following databases were used: PubMed, Scopus, Elsevier, Web of Science, Springer, and EBSCO. The keywords for our search included a combination of terms referring to exposure to non-persistent chemicals and outcome, i.e., exposure to bisphenols, benzophenones, phthalates, parabens, organophosphate pesticides, synthetic pyrethroids, and endometriosis.
The most recent human studies published in English in peer-reviewed journals since 2003 were included in this review.
The period was chosen to reflect findings over the past 19 years. During that time, animal and in vitro studies provided evidence on the toxicity of several non-persistent environmental EDCs, especially in the case of endometriosis risk.
In total, 482 articles were found as a result of the search, and they were all checked for eligibility. The reference lists of the selected articles were subject to a manual search to identify additional articles.

Selection Criteria
A total of 22 publications on exposure to non-persistent endocrine-disrupting chemicals and endometriosis were selected by two reviewers, with an excellent agreement (k = 0.80). This review included original peer-reviewed studies that looked at exposure to non-persistent endocrine-disrupting chemicals and endometriosis in humans. The majority of the articles (80%) did not meet the inclusion criteria for our study, as they did not address the endometriosis risk. Publications containing duplicate data or published before 2003 were excluded. We also excluded studies that analyzed the impact of environmental EDCs on different endocrinological disorders (uterine leiomyoma, polycystic ovary syndrome, and recurrent miscarriages), as well as occupational exposure studies. Articles focused on animal research, in vitro studies, and review papers as well as articles published in a language other than English were excluded.

Study Selection
Two researchers identified relevant articles and independently assessed those to be included in this review; incongruences were resolved by discussion and the intervention of a third independent author. Articles were displayed by title and abstract. Duplicate and irrelevant items were excluded. The remaining articles were subjected to a full-text review. All full-text articles were thoroughly examined to identify the aims of the studies, statistical methods, and accurate results. The following information was taken into account when selecting the studies: authors and years of publication; the main purpose of the study; results; type of study; accuracy of the results. For the purpose of this review, the following information was abstracted from each study: study population; type of exposure and methods used for its assessment (including biomarkers); type of study, level of exposure to selected endocrine-disrupting factor; results. All articles cited were summarized and discussed.

Exposure to Phthalates and Endometriosis
Phthalates are synthetic chemicals with a wide range of applications. They are used in the production of plastics, e.g., paints, adhesives, floors, rubber materials, medicines, and even packages intended for contact with food [27][28][29][30]. Due to their wide application, they are present in the environment. Exposure to phthalates occurs through food, skin, and air [31,32]. Some phthalates can cause allergies and may disrupt the functioning of the human endocrine system [33][34][35]. Moreover, it has been found that exposure to phthalates during fetal life may affect the DNA methylation of genes responsible for the androgenic, estrogenic, and spermatogenetic responses [36]. Research has been conducted on the effects of phthalates on children's health, reproductive disorders, and premature puberty among girls [37]. The studies showed an adverse effect of phthalates on the level of reproductive hormones such as luteinizing hormone, free testosterone, and sex hormone-binding globulin, as well as thyroid function [38,39].
In the human body, phthalates have a short half-life of about 12 h [40]. Phthalates can be divided into short branched and long-branched phthalates. Short-branched phthalates are hydrolyzed to monoester phthalates and then excreted in the urine. In contrast, long-branched phthalates undergo several biotransformations and are then excreted in the urine or feces.
In five studies, no association was found between the concentration of phthalates and the risk of endometriosis [42,44,46,49,52].
In conclusion, seven of the presented studies found an association between the concentration of at least one phthalate metabolite and endometriosis. In five studies, no association was noted between phthalate concentration and endometriosis.

Exposure to Bisphenol A and Endometriosis
Bisphenol A (BPA) is a monomer found in many plastics and epoxy resins. Bisphenol A is a common chemical in everyday items. Approximately eight billion pounds of BPA are produced annually, of which up to over 200 thousand pounds per year may be released into the environment [53]. BPA is used in the production of toys, containers for drinks and food, sports equipment, medical equipment, and cables. Human exposure occurs through the diet, through inhalation of house dust, and through the skin [54]. Bisphenol A has been shown to disrupt hormonal balance. This compound shows estrogenic activity because it binds to estrogen receptors [55]. Moreover, it has been shown that BPA can bind to androgens, blocking their endogenous action, and may it act on the secretion of thyroid hormones [56]. Exposure of the fetus and newborn to bisphenol A may have negative developmental effects such as reduced maturation cycle, prostate changes, altered development of mammary glands, changes in body weight, and changes in the brain. On the other hand, exposure to bisphenol A in adulthood may lead to sperm damage, a decrease in estradiol, miscarriage or premature birth, development of diabetes mellitus [57], and reproductive system impairment in both women and men [58]. Low doses of BPA may also affect the functioning of the human endocrine system [59].
In conclusion, the majority of the studies showed an association between BPA exposure and endometriosis, with only three studies observing no association.

Exposure to Parabens and Endometriosis
Parabens are commonly used as preservatives in food, cosmetic, and pharmaceutical products. These compounds are easily absorbed by the human body. Industrially, parabens are produced by esterification of PHBA (4-hydoxybenzoic acid) with appropriate alcohol in the presence of a catalyst. High efficiency and ease of reaction have contributed to the popularity of using parabens as a preservative [67]. Parabens may disrupt the hormonal balance by acting on enzymes responsible for the synthesis of estrogens or by modifying them to a free, unconjugated form [68]. In addition, parabens can also lead to hormonal disruptions in men. Exposure to parabens can result in androgen antagonistic activity, inhibition of sulfotransferase enzymes, and genotoxic activity [69]. Parabens are associated with increased levels of estradiol in healthy premenopausal women which can lead to irregular menstrual cycles. However, no relationship was observed between the concentration of paraben metabolites and the occurrence of polycystic ovary syndrome (PCOS) [70]. Parabens are components of feminine hygiene products, which may additionally result in increased exposure to this group of chemical compounds [71].
Only one study investigated the relationship between the development of endometriosis and exposure to parabens [72] (Table 3). Peinado et al. (2021) conducted a case-control study among women aged 20 to 54 years from the EndEA (Endometriosis y Exposicion Ambiental) study, involving two hospitals in Spain. A concentration of parabens was detected in urine samples. The women completed a questionnaire about lifestyle and the cosmetic products used. Endometriosis was confirmed by laparoscopy. A significant relationship was identified between the occurrence of endometriosis and the concentration of MeP (methylparaben) (OR: 5.63, p < 0.001). For the remaining examined parabens-EtP (ethylparaben), PrP (propylparaben), and BuP (buthylparaben)-no association was found.
As this was the first study to examine the relationship between exposure to parabens and endometriosis, it is difficult to draw conclusions. More studies should be performed in this direction to establish recommendations.

Exposure to Benzophenones and Endometriosis
Benzophenones (BP) are filters for ultraviolet light. BP absorbs mainly UV-B light. Due to their properties, they are used in the production of creams and products to protect human skin against the harmful effects of ultraviolet radiation [73]. As benzophenones are frequently used, many studies examined the relationship between exposure to benzophenones and reproductive and gynecological disorders. Studies found a correlation between exposure to benzophenones and fetal growth [74]. Fetal exposure to benzophenones may cause delayed growth, and the effects of exposure are more pronounced in female fetuses [74]. It was found that BP may affect reproductive function by interfering with estrogen receptors [75].
In two studies, the link between exposure to benzophenones and the occurrence of endometriosis was analyzed [72,76] (Table 4). In a case-control study performed by Peinado et al. (2021) [72] among women aged 20 to 54 years from the EndEA (Endometriosis y Exposicion Ambiental) study involving two hospitals in Spain, the concentrations of benzophenone-1 (BP-1), benzo-phenone-3(BP-3), and 4-hydroxibenzophenone (4-OH-BP) were determined in urine samples. There was a significant correlation between BP-1 (OR:5.12, p = 0.011) and BP-3 (OR: 4.98, p = 0.008) and the occurrence of endometriosis. A matched cohort study by Kunisue et al. (2012) [76] was performed among women aged 18-54 years participating in the ENDO project (Endometriosis, Natural History, Diagnosis, and Outcomes). The women were divided into two groups; women who had undergone laparoscopy or laparotomy (operative cohort), and women who were diagnosed with magnetic resonance imaging (population cohort). The research showed no relationship between the concentrations of 2-hydroxy-4-methoxybenzophenone (2OH-4MeO-BP), 2,4dihydroxybenzophenone (2,4OH-BP), and 4-hydroxybenzophenone (4OH-BP) and the occurrence of endometriosis. As only two studies were performed on the effect of exposure to benzophenones on endometriosis, it is difficult to provide a conclusion.

Exposure to Non-Persistent Pesticides and Endometriosis
Organophosphorus (OP) and pyrethroid (PYR) pesticides are non-persistent endocrine disruptors [77,78]. OP is widely used in agriculture and horticulture to control plant pests. Organophosphorus pesticides exhibit toxicological effects by inhibiting the enzyme acetylcholinesterase [79]; moreover, OP can cause chronic neuropsychiatric disorders [80]. The relationship between exposure to pesticides and fetal death due to congenital abnormalities was investigated, revealing that exposure to pesticides between 3 and 8 weeks of pregnancy could lead to the assumed hypothesis [81]. It has been shown that the use of sprayers with OP as a component may have a negative impact on the quality of sperm in men [82]. Furthermore, organophosphorus pesticides could impair the functioning of the sexual endocrine system [83]. Pyrethroid pesticides are also used in agriculture and horticulture as insecticides. It is suspected that some may be carcinogenic, and it is also assumed that they may have a negative effect on the endocrine system [84]. A study was conducted showing that PYR exposure may have an effect on birth weight [85]. The ef-fect of pyrethroids on the estrogenic and anti-progestogenic pathways was investigated, concluding that some pyrethroids may contribute to reproductive dysfunction [86].

Discussion
Most of the studies reviewed in this paper showed an association between exposure biomarkers and non-persistent EDCs, with endometriosis, involving at least one metabolite of these compounds ( Table 6). Five of the reviewed studies showed an association between the concentration of BPA and endometriosis, while four observed no association. Table 6. Exposure to non-persistent endocrine disrupting chemicals exposure and endometriosis.

Chemical Compound Endometriosis
Phthalates

Chemical Compound Endometriosis
Benophenones In the case of phthalates exposure, the seven reviewed studies found an association between the concentration of at least one phthalate metabolite and endometriosis whereas most studies on single compounds indicated no significant association. Additionally, several studies included up to 10 metabolites, increasing the risk of random associations. Five studies found no association between phthalate concentration and endometriosis. Only one study was performed to assess the link between exposure to parabens and endometriosis, finding a significant relationship between the concentration of MeP and endometriosis. For the remaining examined parabens-EtP, PrP, and BuP-no association was found. Additionally, only one study assessed the effect of exposure to non-persistent pesticide exposure on endometriosis, observing a significant association between endometriosis and the urinary concentration of diazinon (the parent compound of IMPY) as well as chlorpyrifos and chlorpyrifos-methyl (parent compounds of TCPY).
Only two studies were performed on the effect of exposure to benzophenones on endometriosis. Their results were inconclusive, making it difficult to provide a conclusion on this effect. A comparison of the reviewed studies is presented in Table 6.
The inconsistencies in the results may have been due to many limitations of the presented studies, such as differences in the confounding factors used in the statistical models, study design, study population, biomarkers of exposure, biological fluids used for assessment, creatinine or specific gravity adjustment, time of exposure, and outcome assessment (diagnosis versus questionnaire data).
In most of the presented studies, a case-control study design was used, which is often used to identify factors that may contribute to a medical condition by comparing subjects who have that condition/disease (cases) with those who do not but are otherwise similar (controls). On the other hand, case-control studies also have some limitations, whereby associations measured may or may not represent causal relationships. It can be hard to establish if there is true temporality (i.e., if the exposure preceded the outcome; or vice versa). Furthermore, cases and controls may have different recollections of exposure, leading to a unique source of bias. The study populations were mostly recruited mostly from fertility and gynecology centers. Non-persistent endocrine-disrupting chemicals were analyzed in urine in most of the reviewed studies. Additionally, the authors did not state the number of analyzed urine samples collected from each patient. As nonpersistent endocrine disruptors are metabolized in 24-48 h, a single urine sample may not reliably define exposure and its association with endometriosis. However, as people do not change their lifestyle very often, exposure is typically habitual. Thus, as reported by Meeker et al. (2005) [88], a single sample can adequately predict longer-term average exposure. The outcomes (endometriosis) in most presented studies were assessed by surgery or magnetic resonance imaging. However, in two studies, the diagnosis was based on questionnaire data. In most studies, similar confounding factors were used in the statistical models, e.g., age, smoking status, body mass, age at menarche, education level, and pregnancy status.
In the case of studies investigating phthalate exposure, the divergence of the results may have arisen from the various confounding factors identified in the studies, such as differences in creatinine adjustment, sample size, study design, phthalate metabolites assessed, and biological fluids in which the concentrations of phthalates were measured. In studies of the effect of BPA exposure on endometriosis, the inconsistent results may have been due to the differences in the selection of study groups. In benzophenone studies, the use of diverse biomarkers (different benzophenones) and various confounding factors may have affected the results.

Conclusions
In conclusion, despite the numerous limitations of the results, the reviewed studies suggest that exposure to non-persistent endocrine disruptors, especially in the case of bisphenol A and phthalates is associated with endometriosis. The results of the studies on parabens, benzophenones, and non-persistent insecticides were inconclusive.
The studies were mostly well-designed epidemiological studies, using biomarkers of exposure, where the outcome (endometriosis) was based on a confirmed diagnosis. Additionally, the statistical models were adjusted for potential confounding factors.
Due to the insufficient evidence, further epidemiological studies are needed to confirm these findings.