**Biomonitoring of Urinary Benzene Metabolite SPMA in the General Population in Central Italy**

**Giovanna Tranfo 1, Daniela Pigini 1, Enrico Paci 1, Lisa Bauleo 2,\*, Francesco Forastiere <sup>2</sup> and Carla Ancona <sup>2</sup>**


Received: 7 June 2018; Accepted: 10 July 2018; Published: 11 July 2018

**Abstract:** Background: Benzene is an important component of cigarette smoke and car exhaust. Products containing benzene in concentrations greater than 0.1% are prohibited in Europe, but 1% of benzene is still allowed in gasoline. The purpose of the study was to assess the levels of urine benzene biomarkers in a sample of the general population not occupationally exposed to benzene, resident in the period 2013–2014 in Central Italy, compared to other groups. Methods: The urinary levels of the benzene metabolites S-phenyl-mercapturic acid (SPMA) and cotinine (nicotine metabolite) were determined by means of HPLC with mass spectrometric detection in 1076 subjects. Results: The median SPMA value in smokers was 1.132 μg/g of creatinine while in non-smokers it was 0.097 μg/g of creatinine, and the 95th percentile results were seven times higher. Conclusion: The main source of benzene exposure in the studied population was active smoking, however, non-smokers were also exposed to airborne benzene concentrations. The concentration ranges found in this study can be used as a background reference for occupational exposure assessment to benzene by means of SPMA biomonitoring.

**Keywords:** human biomonitoring; urine; non-occupational exposure; S-phenyl-mercapturic acid; HPLC-MS/MS

#### **1. Introduction**

Benzene is an important chemical compound used in the manufacturing of polymers, plastics, rubber, dyes, detergents, and other products, and it is a ubiquitous pollutant of indoor and outdoor air. In fact, even though in Europe the sale to or use by consumers of products that contain benzene in concentrations greater than 0.1% by weight is restricted by the European regulation on Registration and Evaluation of Chemicals (REACH), human exposure to this substance can still be due to industrial use, combustions of organics and natural gas, and motor fuels, as 1% in volume of benzene is allowed in gasoline according to Directive 98/70/EC [1].

The Directive 2008/50/EC on ambient air quality and cleaner air for Europe sets objectives for ambient air quality in order to protect human health and the environment as a whole; these objectives relate to sulphur dioxide, nitrogen dioxide, particulate matter, lead, benzene, and carbon monoxide. For benzene, the limit value has been set at 5 μg/m3 as the annual average since 1 January 2010 [2].

Benzene exposure for industrial sectors can be easily assessed if the quantity of material used and the working environment are well defined, while for the general population, it is harder to quantify because individual lifestyles are extremely variable, ambient weather conditions can impact exposure, and living environments are different [3].

The air concentrations for different employment sectors can range from 1 μg/m3 to over 1000 μg/m3 (aviation workers, service station workers, bus drivers, police, urban workers, fishermen, and shoe production workers), but in the last decades, in developed countries, airborne benzene has progressively decreased as a consequence of preventive actions [4]; furthermore, outdoor ambient air concentrations of benzene are dependent on geographical location (i.e., rural versus urban). Benzene is also an important component of cigarette smoke. The contribution of environmental tobacco smoke is a major cause of indoor benzene exposure and depends upon local restrictions on smoking. Other significant sources of indoor benzene exposure are incense burning and traffic emissions.

Benzene is a known carcinogen and causes hematotoxicity at exposure levels below 1 ppm (3.25 mg/m3) [5], which is the occupational limit value in the Recommendation from the Scientific Committee on Occupational Exposure Limits for benzene [6]. Exposure to a time-weighted average (TWA) of 1–5 parts per million (ppm) benzene in ambient air for 40 years is associated with an increased risk of acute myeloid leukemia [7]. The capacity of individuals to metabolize benzene is modulated by genetic factors [8]. Benzene uptake occurs mainly by inhalation in both occupational and non-occupational exposure. Benzene is oxidized to benzene oxide (BO) from cytochrome P450 (CYP) oxidase. A small fraction of BO (about 1% of total benzene uptake at 0.1–10 ppm of exposure) is detoxified through conjugation with glutathione (GSH) by glutathione-S-transferases (GSTs), and excreted in urine as S-phenyl-mercapturic acid (SPMA) [9]. S-phenyl-mercapturic acid (SPMA) is a very specific urinary biomarker of benzene. Its half-life is in a range of 9–13 h, and its accumulation is not probable, therefore, it is a biomarker of recent exposure [3].

Human biomonitoring data integrates all sources of possible exposure to a chemical, but it does not provide information on a single route of exposure. Biomonitoring permits an assessment of the exposure by quantitating a dose biomarker in biological fluids. The knowledge of the concentration of a chemical substances in biological fluids (biomarkers) measured in subjects without occupational exposure, accounts for the biological variability of the examined population and of other factors like residence and lifestyle [3,10]; it is also important for the evaluation of action levels or biological exposure limit (both environmental and occupational).

Cigarette smoking is a major source of exposure to benzene in active smokers [4] and is also able to affect the levels of biological markers of exposure to benzene in non-smokers exposed to passive smoking [11,12].

It has been estimated that smokers receive about 90% of their benzene intake from smoking [7].

An Italian surveillance study showed that during the period from 2014–2017 in the population of 18 and 69-years-olds, non-smokers were the majority (56.4%), ex-smokers (17.6%) were the minority, and 26% of the population were active smokers [13].

Exposure to benzene of the general population has been reduced significantly in Europe both in the outdoors and indoors by lowering the benzene content in gasoline and prohibiting smoking in many public places; therefore, population biomarker values are changing over time, and must be periodically updated.

The objective of the present study was to provide a measure of the environmental benzene exposure, according to smoking status and occupation, during the period from 2013–2014 in a sample of the general population living in Central Italy with no declared occupational exposure to benzene.

#### **2. Materials and Methods**

#### *2.1. Study Population*

The present study is part of a larger biomonitoring study, in which samples were collected between May 2013 and December 2014 from a population randomly selected from the municipality registers of the area of Civitavecchia (Italy) from the about 130,000 inhabitants [14]. The study protocol was approved by the local ethics committee. The subjects who agreed to participate to the study gave a written informed consent and filled in a questionnaire for collecting information on age, lifestyle, and food habits, cigarette, cigar, or tobacco smoking, the starting age for smoking, the end age for smoking for ex-smokers, electronic cigarette smoking, passive smoking, drug use, working activities, hobbies, use of chemical products, and in particular the possible occupational exposure to benzene. Part of this information was collected for the purpose of studying exposure to different chemical pollutants and was not used in the present study. The group studied included 1076 subjects aged 35–69 years. Information about the occupation status (employed, nor employed, housewives, retired) of participants was assessed during the interview. Housewife was considered a separate occupation. The smoking status was assessed using the urinary concentration of cotinine, and the cutoff value for the definition of smoker was set at urinary cotinine ≥100 μg/g of creatinine [15].

#### *2.2. Urine Sample Collection and Preparation*

Fasting subjects collected the first urine of the morning in empty plastic sterile containers on the same day as the medical visit: 30 mL of each sample were transferred into 50 mL Teflon tubes, identified with the subject code, frozen at −20 ◦C, and later transported to the laboratory where they were stored at −20 ◦C until analysis. Urinary creatinine was determined by the method of Jaffè using alkaline picrate test with UV/Vis detection at 490 nm [16]. The samples having a creatinine concentration higher than 3 g/L or lower than 0.3 g/L were discarded and the corresponding volunteers were excluded from the study in accordance with the American Conference of Governmental Industrial Hygienists (ACGIH) recommendation (ACGIH, Cincinnati 2014) [17].

#### *2.3. Analytical Method*

The concentration of the benzene and nicotine metabolites, SPMA and cotinine, was determined by isotopic dilution HPLC-MS/MS according to an analytical method previously validated in our laboratories [15]. Briefly, 3 mL of urine were acidified at pH 2, in order to hydrolyze the precursor of SPMA, with the deuterium labeled internal standards. Solid phase extraction (SPE) purification was carried out on Sep-pack C18 cartridges in two steps in order to elute a first fraction containing the acidic metabolite (SPMA) and subsequently a second one at pH 8 containing the cotinine: the two fractions were then injected separately into the API 4000 HPLC-MS/MS system. The HPLC-MS/MS is a Series 200 LC quaternary pump (PerkinElmer, Norwalk, CT, USA) coupled with an AB/Sciex API 4000 triple-quadrupole mass spectrometry detector equipped with a Turbo Ion Spray (TIS) probe. A Sinergi Fusion C18 analytical column (150 × 4.6-mm, 4-μm) was used for the analysis of urine samples and for the calibration standards for SPMA and cotinine. The mobile phase was a linear gradient of acetonitrile and acetic acid 1.0% *v*/*v* in water, flow rate 600 μL/min. The total run time was 10 min for the SPMA and 5 min for the cotinine. The precursor→product ionic transitions monitored were 238.1→109.1 for SPMA and 240.1→109.1 for SPMAd2 (in the negative ion mode) and 177.3→80.10 for cotinine and 180.3→80.10 for cotinine-d3 (in the positive ion mode).

The precision for SPMA at the calibration level of 2 μg/L is 3%, and at the lowest level it is 10%. The limits of detection (LOD) calculated using the approach based on the standard deviation of the response and the slope, and expressed as 3.3 σ/S, were 0.026 μg/L for SPMA and 12.41 μg/L for cotinine. The final concentrations of both analytes were expressed in μg/g of creatinine to normalize values with respect to urine dilution variability. SPMA data below the LOD accounted for 20% of the population, and they have been substituted with the value of <sup>1</sup> <sup>2</sup> LOD in order to calculate the geometric mean (GM) and perform other statistical analysis.

#### *2.4. Statistical Analysis*

Descriptive statistics were carried out, and the SPMA concentration is presented as geometric mean (GM) with its geometric standard deviation (GSD), 5th, 50th, and 95th percentile. The relation among SPMA urinary concentrations and demographic characteristics (gender, age, occupation) in non-smokers was assessed using a linear regression model in which the dependent variable was the log-transformed SPMA. In this case, the measure of risks is expressed in term of Geometric mean ratio (GMR). Confidence interval of 95% were calculated. SAS (SAS Institute Inc., Cary, NC, USA) and STATA version 13 (StataCorp, College Station, TX, USA) software programs were used for the statistical analyses.

#### **3. Results**

Table 1 describes the characteristic of the population sample by smoking status.


**Table 1.** Characteristics of the study population by smoking status.

Active smokers were 27.5% of the sample, females 57.2% of the sample, and 53.5% were employed. In the study sample, active smokers were younger, and there were no subjects with declared occupational exposure to benzene as assessed from the ABC study interview.

Table 2 reports the distribution of SPMA concentrations expressed in μg/g creatinine (Geometric mean and <25th, 25th–50th, 50th–75th, >75th percentile cut-offs) by the main characteristics of the ABC sample according to smoking status, assessed by means of the cotinine concentration of the same urine samples. Values expressed in μg/L are reported in the Supplemental Material (Table S1). Reference values for the biological monitoring of occupational exposures are generally normalized on the basis of creatinine concentration or specific gravity to account for fluctuations in urine dilution. We preferred the use of creatinine as there are many more published results for comparison, and because this is the parameter used by the ACGIH.


**Table 2.** Urinary S-phenyl-mercapturic acid (SPMA) concentrations (μg/g creatinine).

GSD: geometric standard deviation; LOD: Limit of detection, for SPMA 0.026 μg/L.

Among smokers, we found higher SPMA concentrations (μg/g creatinine) in females (GSD (SD) 1.120 (4.612)) and people aged 55–64 years (1.262 (4.392)); housewives showed the highest values ((1.533 (4.827) when compared with the other occupational categories. In non-smokers, the median SPMA concentration was 0.1 μg/g of creatinine, with the 95th percentile equal to 0.7 μg/g of creatinine. No particular differences in gender, age, and occupation categories were observed, although the highest value was found in the oldest /retired subjects (aged ≥ 55), while retired and unemployed showed the highest median.

Table 3 shows results from the multivariate approach; when adjusting for participants' gender, age, and occupation, we found that SPMA concentrations were higher in females compared to males (*p* = 0.053) while no differences were found between age groups and occupational categories.



#### **4. Discussion**

Human biomonitoring is a powerful tool used in national and international surveys to assess the integrated exposure of the population to xenobiotics from different sources. In the population studied, the main source of exposure to benzene was active smoking: the data show that the mean SPMA value in smokers is about ten times that of non-smokers. The highest SPMA level was found in female smokers: women present a higher concentration of metabolites both when normalized for the creatinine and when not (μg/L), even when considering that creatinine concentrations are usually lower than in men.

The assessment of benzene exposure by means of biological monitoring does not permit source apportionment as it is integrated information. In the study sample, there were no subjects with declared occupational exposure to benzene and, therefore, benzene metabolites found in non-smokers could derive from passive smoking, use of incense in the home, or exposure to traffic. The median SPMA concentration of non-smokers was 0.1 μg/g of creatinine, that refers only to 50% of the sample, while the 95th percentile was 0.7 μg/g of creatinine, that is seven times higher than the median. This means that efforts to avoid or reduce benzene exposure should be made at the individual level by subjects having the higher biomarker (exposure) values to reach the lower ones.

Non-smokers' values were further stratified by age and occupational classes to be available as reference or "controls" for studies on subjects with similar characteristics. The highest value was found in the oldest/retired subjects (aged ≥ 65), while unemployed show the highest median. A possible explanation for these differences can be found in life habits (all these subjects have more free time) or in metabolic parameters (slower metabolism, more fat tissue). The same exercise cannot be done for the smokers, as any difference would mainly be due to the number of cigarettes.

We also examined other published data regarding urinary concentrations of SPMA in subjects without occupational exposure, all analytically determined by means of HPLC-MS/MS and published from 2011 to 2015. Only seven papers reported such values, four of which referred to population studies [18–21] and three to controls in studies on occupational exposures [22–24].

Comparison of our results with data from the literature shows that other studies included significantly fewer subjects than this one. Mean and median SPMA urinary concentrations are comparable to those found in the five studies on subjects who were resident in Italy, while the geometric mean found in this study for non-smokers is lower than those reported for African [20] and Chinese subjects [19], where airborne benzene concentrations are apparently higher. Moreover, none of the recent studies explored different age or job classes. Results summarized in Tables are reported as Supplemental Material (Tables S2 and S3).

#### **5. Conclusions**

Tobacco smoke (active or passive) is confirmed to be the main source of benzene exposure for the general population, and it is the most important confounding factor in the biological monitoring of occupational and environmental exposure to benzene, especially when exposure levels are low and very low. However, benzene exposure can still be high in non-smokers, and individual exposure to benzene can be reduced.

Biological values determined in the general population can be used to understand whether the levels found in workers are indicative of a professional exposure. This is particularly important for substances for which health-based exposure-limit values have not been defined, and it is particularly useful to aid in the interpretation of biological monitoring for genotoxic carcinogens. The comparison should be made taking into account the possible confounding factors, like smoking, that can produce significantly higher levels of urinary SPMA than does the occupational exposure to benzene. The values presented for urinary SPMA in this paper are a sound basis for the definition of occupational exposure to benzene in non-smoking subjects who reside and work in Italy. For smokers, due to the low benzene exposure values reached because of the European legislation on occupational and environmental safety and health, the amount of SPMA produced by smoking could completely mask the airborne benzene

contribution. However, SPMA levels higher than the 95th percentile of the smokers group found in this paper should be further investigated while also considering the cotinine concentration of the subjects.

It must be stressed that environmental levels of airborne benzene should further decrease in time and, therefore, biological values, especially for non-smokers, should be reassessed periodically.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2305-6304/6/3/37/s1, Table S1. Urinary SPMA concentrations (μg/L); Table S2. Summary of the population studies published from year 2011; Table S3. Summary of the occupational exposure studies published from year 2011 (data of controls).

**Author Contributions:** G.T., F.F., L.B., and C.A. conceived and designed the experiments. E.P. performed the experiments. L.B., D.P., and E.P. analyzed the data. L.B., C.A., D.P., and G.T. wrote the paper.

**Funding:** This research received no external funding.

**Acknowledgments:** The analyses described were carried out in the laboratories of the National Institute for Insurance against Accidents at Work (INAIL) Research, Department of Occupational and Environmental Medicine, Epidemiology and Hygiene, Italy, using the research funds of the Institute itself.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Review* **A Review of Biomonitoring of Phthalate Exposures**

**Yu Wang 1, Hongkai Zhu <sup>1</sup> and Kurunthachalam Kannan 1,2,\***


Received: 31 January 2019; Accepted: 29 March 2019; Published: 5 April 2019

**Abstract:** Phthalates (diesters of phthalic acid) are widely used as plasticizers and additives in many consumer products. Laboratory animal studies have reported the endocrine-disrupting and reproductive effects of phthalates, and human exposure to this class of chemicals is a concern. Several phthalates have been recognized as substances of high concern. Human exposure to phthalates occurs mainly via dietary sources, dermal absorption, and air inhalation. Phthalates are excreted as conjugated monoesters in urine, and some phthalates, such as di-2-ethylhexyl phthalate (DEHP), undergo secondary metabolism, including oxidative transformation, prior to urinary excretion. The occurrence of phthalates and their metabolites in urine, serum, breast milk, and semen has been widely reported. Urine has been the preferred matrix in human biomonitoring studies, and concentrations on the order of several tens to hundreds of nanograms per milliliter have been reported for several phthalate metabolites. Metabolites of diethyl phthalate (DEP), dibutyl- (DBP) and diisobutyl- (DiBP) phthalates, and DEHP were the most abundant compounds measured in urine. Temporal trends in phthalate exposures varied among countries. In the United States (US), DEHP exposure has declined since 2005, whereas DiNP exposure has increased. In China, DEHP exposure has increased since 2000. For many phthalates, exposures in children are higher than those in adults. Human epidemiological studies have shown a significant association between phthalate exposures and adverse reproductive outcomes in women and men, type II diabetes and insulin resistance, overweight/obesity, allergy, and asthma. This review compiles biomonitoring studies of phthalates and exposure doses to assess health risks from phthalate exposures in populations across the globe.

**Keywords:** phthalate; DEHP; biomonitoring; human exposure; toxicity; reproductive

#### **1. Introduction**

Phthalates are diesters of phthalic acid (1,2-benzenedicarboxylic acid) and are synthetic organic chemicals used in industries as solvents, plasticizers, and additives in polyvinyl chloride (PVC) plastics or personal care products (PCPs) [1]. More than 25 phthalates are used in commercial applications, with each adding unique qualities to the product into which it is incorporated. Ten commonly used phthalates (Figure 1, Table 1) are dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP), diisobutyl phthalate (DiBP), benzylbutyl phthalate (BzBP), dicyclohexyl phthalate (DCHP), di(2-ethylhexyl) phthalate (DEHP), di-n-octyl phthalate (DnOP), di-isononyl phthalate (DiNP), and di-isodecyl phthalate (DiDP). DEHP, one of the major phthalates in commerce, was first synthesized for use as a plasticizer in 1933 [2]. The application of DEHP as an additive in polyvinyl chloride (PVC) to impart the flexibility of plastic has made phthalates popular around the world. The addition of phthalates to PVC makes it not only flexible but also malleable and durable. PVC products may contain up to 50% (by weight) phthalates [1].

**Figure 1.** Chemical structures of major phthalates and their metabolites studied in the literature.


**Table 1.** Major phthalate diesters and their corresponding metabolites studied in the literature.

Currently, phthalates are used in many types of products, including building materials, automotive parts, medical devices, food packaging, cosmetics, perfumes, toys, teethers, adhesives, paints, floorings, lubricants, hair sprays, shampoos, soaps, nail polishes, and detergents [3–5]. The annual global production of phthalate was 4.7 million metric tons in 2006 [6,7] and ~8 million metric tons in 2015 [8]. In most commercial products, DEHP, DiNP, and BzBP are used as additives, and they easily migrate from those products into the environment through evaporation, leaching, and abrasion [9]. Phthalates have been measured in a range of environmental matrices, including sludge, dust, soil, air, and water [4], and are ubiquitous contaminants in the environment.

Phthalates are reproductive and developmental toxicants [10]. In laboratory animal studies, DEHP has been reported to affect the reproductive system and development [11,12]. Further, changes in hepatic structure and function and kidney function as well as disruption of thyroid signaling, immune function, and metabolic homeostasis were reported [13–16]. The US Environmental Protection Agency (EPA) classified DEHP and BzBP as probable and possible human carcinogens, respectively. European authorities have classified phthalates with three to six carbons in their backbone as Repr 1B Agents (i.e., presumed human reproductive toxicants) (https://echa.europa.eu/substance-information/-/ substanceinfo/100.239.213).

Human exposure to phthalates arises mainly from ingestion, inhalation, and dermal absorption [17,18]. Human biomonitoring studies have measured parent phthalate in serum [19] and their metabolites in human urine [20,21], semen [22,23], and breast milk [24,25]. Studies have demonstrated that phthalate exposure is associated with oxidative stress in humans [26,27]. Some studies have linked phthalate exposure to premature thelarche [28,29], endometriosis [30,31], low semen quality [32], reduced testosterone levels [33], obesity, diabetes, and breast cancer [34,35]. Phthalates are regarded as endocrine-disrupting compounds [36]. One of the most significant effects of phthalates is in terms of fetal development and reproductive anomalies and is referred to as "phthalate syndrome" (e.g., developmental or testicular effects, insulin like factor 3 production) [37,38]. In addition, phthalate exposure might be linked to insulin resistance and obesity in human populations [39,40].

In 1999, the European Union (EU) temporarily banned the use of six phthalates in children's toys: DiNP, DEHP, DBP, BzBP, DiDP, and DnOP (http://europa.eu/rapid/press-release\_IP-05-838\_en. htm). Further, in 2009, these phthalates were restricted in toys in Europe (https://eur-lex.europa.eu/ legal-content/EN/TXT/?uri=CELEX%3A32009L0048). The US followed suit in 2008 by passing the Consumer Products Safety Improvement Act, which banned the same six phthalates in children's toys (https://www.cpsc.gov/Regulations-Laws--Standards/Statutes/The-Consumer-Product-Safety-Improvement-Act). Many industries began substituting alternative chemicals for phthalates in their products, and several substitutive phthalate and non-phthalate plasticizers are currently used in many products [41,42]. Although six phthalates are now restricted in children's products in the US and EU, they are unregulated and continue to be used in toys in many other parts of the world, including China and India. In addition, children continue to be exposed to phthalates in cosmetics and PCPs as well as in school supplies made of PVC, including notebooks and binders, art supplies, backpacks, lunchboxes, paperclips, and umbrellas (https://www.sustainableproduction.org/downloads/PhthalateAlternatives-January2011.pdf). Raincoats, boots, handbags, and soft plastic shoes also may contain phthalates.

A search on the basis of Web of Science Core Collection, BIOSIS Previews, Derwent Innovations Index, MEDLINE, and ScieELO Citation Index was carried out to identify studies relevant to biomonitoring and epidemiology on phthalates and phthalate metabolites. Topics of interest included studies on phthalates in urine, serum, and other biofluids. The search terms used were: phthalic acid/phthalates OR phthalate metabolites AND biomonitoring OR epidemiological studies. Publications between 2000 and 2018 were extracted. This review provides a summary of human biomonitoring studies of phthalate diesters and their monoester (primary) and oxidative (secondary) metabolites as well as select epidemiological studies that link phthalate exposure to health outcomes in human populations.

#### **2. Sources of Phthalates**

Owing to their widespread use in consumer products, phthalates are ubiquitous in the environment, and a variety of sources have been reported to contribute to human exposure. For the purpose of exposure analysis, phthalates have often been grouped as lower molecular weight (ester side-chain lengths, one to four carbons; DMP, DEP, and DBP), and higher molecular weight (ester side-chain lengths, five or more carbons; DEHP, DiNP, DiDP, and BzBP) phthalates [43]. The high molecular weight phthalates are used primarily in PVC polymers and plastisol applications, plastics, food packaging, and food processing materials, vinyl toys and vinyl floor coverings, and building products. The low molecular weight phthalates are often used in non-PVC applications, such as personal care products, paints, adhesives, and enteric-coated tablets [44]. BzBP, DEHP, DiNP, DBP, and DiBP are used in toys, bags, gloves, and plastic tubing for improving flexibility and making the polymeric products soft and malleable [4]. DMP and DEP are widely used in cosmetics, such as perfumes, aftershaves, shampoos, makeup, and nail care products [4]. Cosmetics and personal care products are the major sources of human exposure to low molecular weight phthalates. Food packaging plastic film contains phthalates (such as DBP and DEP) at levels of up to 10% by weight. Plasticizer migration occurs when food packaging films come in direct contact with foods, and fatty foods and high temperatures increase

the migration [45]. Diet has been a major source of exposure to high molecular weight phthalates, especially DEHP. In particular, foods packaged in plastic/PVC materials contribute to exposure to DEHP in humans [46].

The major source of exposure to DEP—one of the major phthalates found in human urine—is cosmetics and personal care products [17]. Studies have reported elevated concentrations of phthalates in indoor air and dust [47]. In fact, among various contaminants measured in indoor dust, phthalates, especially DEHP and DEP, are the major contaminants in indoor dust and air [46]. Phthalates also were reported to occur in pharmaceuticals, especially in over-the-counter medications/syrups and in pills with enteric coatings [48,49]. Medical devices that are suspected to contain DEHP include intravenous (IV) storage bags, ventilator tubing, IV infusion sets, endotracheal tubes, IV infusion catheters, nasogastric tubes, blood storage bags, enteral and parenteral nutrition storage bags, blood administration sets, urinary catheters, PVC exam gloves, suction catheters, chest tubes, nasal cannula tubing, hemodialysis tubing, syringes, extracorporeal membrane oxygenation tubing, and cardiopulmonary bypass tubing [50].

Exposure doses to phthalates have been calculated through the ingestion of foods, air inhalation, and dust ingestion for the general population in the US (sampled during 2011–2014) (Table 2) [46]. Dust ingestion is a major source of exposure to phthalates in infants and toddlers, whereas diet is the major source for children and adults. The exposure doses are in the range of a sub to low μg/kg bw/d. Further details of exposure doses calculated through biomonitoring data are provided below.



\* y = years old; "-" means not reported; data source: Tran and Kannan, 2015 [46].

#### **3. Biomonitoring of Phthalates**

Due to the ubiquitous occurrence and widespread exposure of phthalates, their metabolites are one of the most examined environmental chemicals in human biomonitoring studies. The reported half-life of phthalates diesters in blood plasma or urine of humans and rodents was less than 24 h. Several studies have reviewed pharmacokinetics of phthalate esters, and these studies have found rapid hydrolysis of diesters to monoesters in the gastrointestinal tract [1,2]. Binding of DEHP metabolites to blood plasma proteins, existence of biliary excretion, and enterohepatic circulation in humans have been suggested [2]. Nevertheless, urinary excretion has been the major elimination pathway of phthalates [2]. Urinary concentrations of phthalate metabolites are generally 5–20 times higher than that in lipid-rich compartments. For example, urinary concentrations of mono-2-ethylhexyl phthalate (MEHP), mono-isobutyl phthalate (MIBP), mono-ethyl phthalate (MEP), and mono-n-butyl phthalate (MBP) were 20–100 times those in blood or milk [24]. Phthalate metabolites have been measured in various body fluids, including urine [47,51], serum [52,53], semen [32,54], breast milk [55,56], and saliva [57] (Table 3). Phthalates can cross the placental barrier [58] and have been measured in amniotic fluid in human studies [59]. To date, studies that report partitioning of phthalates among various tissues and organs in an organism, at state-state exposure conditions, are not available. It is worth noting that a few earlier reviews have described biomonitoring of phthalates in humans [60]. Biomonitoring studies that report concentrations of phthalates metabolites are presented in Table 3.

**3.**Reported concentrations of major phthalate metabolites in human specimens collected from various countries.

**Table**


**Table 3.** *Cont.*



mean; CR =

Creatinine.

*Toxics* **2019**, *7*, 21

Phthalate diesters (parent compounds) were measured in blood plasma of women with endometriosis in India, and a significant association was found between phthalate exposure and the risk of developing endometriosis [136]. Similarly, studies have determined phthalates in serum samples of couples from Greenland, Poland, and Ukraine that showed that the DEHP levels were associated with reduced time to achieve pregnancy [137]. Phthalate diesters and their metabolites also have been measured in breast milk, serum, and urine from Swedish women [24]. In milk and serum samples, the concentrations of phthalate diesters and their metabolites were below the method limit of detection (0.12–3.0 μg/L). Detectable concentrations of phthalate metabolites, however, were found in urine (0.1–1000 μg/L). Measurements of phthalate diesters in breast milk and serum are prone to false positives due to background contamination. Medical devices, including blood collection devices and plastic containers that are used to collect and store samples, can contain phthalate diesters [49]. If the samples were to be analyzed for phthalate diesters, caution should be taken with the screening devices used to collect and store samples. A comprehensive review of challenges associated with low-level phthalate analysis in biological specimens has been published [17].

#### *3.1. Phthalate Metabolites in Urine*

Although microbial degradation of DEHP to MEHP in soils through lipase and esterase enzymes has been shown, environmental degradation/transformation of parent phthalates is slow [25,138]. Because phthalates have a short half-life in human bodies and are excreted quickly in urine as monoester metabolites, the metabolites are suitable biomarkers for human exposure to parent compounds. The half-life of phthalates in human bodies (in plasma and urine) is less than 24 h, and following metabolism, monoesters of phthalates are conjugated with glucuronide or sulfate and excreted in urine [139]. Analysis of metabolites in urine involves enzymatic deconjugation followed by purification. Assessment of human exposure to phthalates is based mainly on the measurement of their urinary monoester metabolites, although several secondary and oxidative metabolites have been reported to occur in human specimens [139]. For instance, DMP, DEP, and DBP undergo degradation/hydrolysis and form their corresponding monoesters, i.e., MMP, MEP and MBP, respectively. Both hydrolysis and oxidation products are formed from the metabolism of DEHP. MEHP, the hydrolysis product of DEHP, is not a major metabolite. The oxidative metabolites, MEOHP, MEHHP, MECPP, and MCMHP, however, are the major metabolites of DEHP and are appropriate biomarkers of exposure to this compound [21]. Some studies suggest, however, that MEHP is more toxic than are other oxidative metabolites. The general metabolic pathways of phthalate esters in humans are shown in Figure 2.

General Population Adults: A large number of studies have reported measurements of phthalate metabolites in human specimens collected from European (Germany, Netherlands, Denmark, Norway, Sweden, Greece, the Czech Republic, Hungary, Slovakia, and Spain) and Asian countries (Japan, China, South Korea, India, Taiwan, Vietnam, Saudi Arabia, Malaysia, and Kuwait) as well as from North American countries. The number of phthalate metabolites measured in urine samples varied considerably; as new analytical standards become made available commercially, more metabolites were added to the list of compounds measured in urine. Although a majority of the recent studies measure close to 20 phthalate metabolites, studies conducted a decade ago measured 10 or fewer metabolites of phthalates.

In general, the concentrations of the sum of 22 phthalate metabolites measured in human urine were on the order of several to hundreds of parts-per-billion (μg/L) [21]. In a majority of the biomonitoring studies, metabolites of DEHP, DEP, and DBP were the major compounds identified in urine, and the profile varied depending on the country. Urine samples collected from 32 men and 53 women (age: 7–64 years) from northern Bavaria (Germany) contained MBP (median: 181 μg/L), MEP (90.2 μg/L), and major DEHP metabolites, such as MEHHP (46.8 μg/L) and MEOHP (36.5 μg/L) [86]. The median concentrations of DEHP metabolites, namely, MEHP, MEOHP, and MEHHP, were 4.5, 28.3, and 35.9 μg/L, respectively, and these three metabolites were highly intercorrelated. The concentration

ratios, MEHHP/MEHP, and MEOHP/MEHP, were calculated to be 8.2, and 5.9, respectively. These ratios suggest that MEHP is further oxidized to form MEHHP and MEOHP [86].

**Figure 2.** Metabolic pathways of phthalate esters in humans.

The urinary concentrations of phthalate metabolites in general populations vary among countries. Some of the highest concentrations of total phthalate metabolites were found in urine collected in 2006–2007 from Kuwaitis, with a maximum value of 19,300 μg/L and a median value for of 1050 μg/L [72]. The occurrence of phthalate metabolites was investigated in urine from Germans, and MBP was found at high concentrations (median: 49 μg/L) [87]. The median concentrations of phthalate metabolites in urine samples from Germany decreased significantly from 2002 to 2008 [41]. Similarly, urinary phthalate metabolites measured in 2015 were significantly lower than those in 2007 in Germany [88].

Biomonitoring studies in other European countries, including France [83], Belgium [65,66], Slovakia [117], and Norway [112], report phthalate metabolite concentrations in the range of 1–100 μg/L in urine from adults. MBP and DEHP metabolites were the predominant compounds found in those studies. Further, a comparative analysis of biomonitoring data in Europe suggested a significant decline in phthalate metabolite concentrations (especially MEP, MBP, MBzP, and DEHP metabolites) from 2011 to 2016 [42]. Several alternative plasticizers, however, are used as replacements for DEHP in European countries. Common alternatives include Hexamoll DINCH (DINCH), acetyl tributyl citrate (ATBC), dioctyl terephthalate (DOTP), 2,2,4-trimethyl 1,3-pentanediol diisobutyrate (TXIB), trioctyl trimellitate (TOTM), and di-(2-ethylhexyl) adipate (DEHA).

In North America, the distribution of phthalate metabolites in urine has been summarized in nationwide monitoring surveys. For example, the US National Health and Nutrition Examination Survey (NHANES) of the Centers for Disease Control and Prevention (CDC) showed that MEP, MEHP, MEHHP, and MEOHP concentrations in urine from adults >20 years of age were 167, 3.99, 18.8, and 12.6 μg/g creatinine (CR), respectively [129]. The NHANES program has measured 15 phthalate metabolites in urine. The weighted geometric mean concentration of 15 phthalate metabolites in the US general population was 125 μg/L for the samples collected in the period of 2007–2008. MEP was the major compound found in urine, accounting for >70% of the total concentrations, which was followed by mono-(2-ethyl-5-hydroxyhexyl) phthalate (MEHHP; ~18% of the total phthalate concentrations). The NHANES data for the US general population in the period of 2005–2006 showed that MCNP, a metabolite of DiDP, was found at a median concentration of 2.70 μg/L [140]. The updated NHANES

report for 2013–2014 are available (https://wwwn.cdc.gov/Nchs/Nhanes/2013-2014/SSPHTE\_H.htm). A 67% decline in DEHP exposure in the US population between 2005/6 and 2011/12 has been reported [141]. Several factors have been shown to affect exposures. The NHANES data showed that several phthalate urinary metabolites were higher in males, Hispanics, and African Americans [142]. The Human Biomonitoring Program of Health Canada measured 11 phthalate metabolites in urine samples of 3236 Canadians and found median MEP and MEHHP concentrations at 49.1 and 23.4 μg/L, respectively [68]. Since 2001, there has been clear evidence of a decline in DEP, DBP, and DEHP exposure in the US [115]. In contrast, urinary DiNP concentrations in the US population increased significantly during the period 2005/6–2011/12 (www.cdc.gov/exposurereport).

Urinary concentrations of phthalate metabolites have been reported for several Asian countries [97]. The measured concentrations in Asian countries were similar to those reported in Europe and North America, although the profiles were distinct. For instance, MBP and MiBP were the major metabolites found in urine from China, and their respective median concentrations were 61.2 and 51.7 μg/L [51]. Similar concentrations of MBP and the sum of DEHP metabolites were reported in urine from Nanjing city (47.1 and 42.0 μg/L) [74] and Taiwan (47.1 and 42.0 μg/L) [121]. In contrast, DEHP metabolites were predominant in urine from Japan, Malaysia, and Vietnam [72]. A nationwide survey of urine samples from 6478 adults during the period of 2012–2014 in Korea showed median urinary concentrations of DEHP metabolites (88.2 μg/L) that were twofold higher than that of MBP (44.2 μg/L) [106,107]. In Israel, phthalate metabolites were found in urine samples collected from 250 adults (ages 20−74), with median concentrations that ranged from 17.1 μg/L (MEOHP) to 37.6 μg/L (MiBP) [97]. DEHP exposure in the Chinese population has increased since 2001 [143].

The global distribution of major phthalate metabolites measured in urine from general populations is presented in Figure 3. Urine samples collected from Kuwait during 2006-2007 contained the highest median concentrations of MEP (411 μg/L), MBP (113 μg/L), and DEHP metabolites (180 μg/L), with a sum of median phthalate metabolite concentrations (median) at 1,050 μg/L [72]. This value is the highest among all countries studied. The profiles of phthalate metabolites varied, with MEP as the predominant metabolite in Indian and Kuwaiti urine samples (49% of the total), which were similar to those found in the US. In China (52%), MBP was the major metabolite found in urine. In Korea (46%), Japan (31%), and Vietnam (52%), DEHP metabolites were the dominant ones. MMP accounted for <8% of the total phthalate metabolite concentrations in all Asian countries, except for Japan, where it was 20%. Overall, MEP and DEHP metabolites were the major phthalate metabolites found in urine from most Asian countries, a pattern similar to that found in the US [130]. The reported urinary concentrations of phthalate metabolites among several European countries were similar whereas information for African countries and Australia/Oceanian countries is limited.

Pregnant Women: Phthalates have been widely studied for exposure levels in pregnant women. MEP (222 μg/g CR) was the predominant phthalate metabolite found in urine samples of pregnant women from the Netherlands (Generation R study) [110]. Similar exposure levels were reported for pregnant women from the US [131,132], Canada [69], and Norway [113], with MEP median concentrations exceeding 30 μg/L. In a study of urinary phthalate metabolite concentrations in Spanish pregnant women (*n* = 391), the median concentration of MEP was reported at 246 μg/g CR [118].

Several studies have examined phthalate metabolite concentrations in matched urine samples of newborns and mothers. Maternal urinary concentrations of MEHHP and MEOHP in Korea were 17.7 and 14.7 μg/L, respectively, which were two- to threefold higher than those found in newborns (5.79 and 3.27 μg/L) [144]. Another study, however, showed similar urinary concentrations of phthalate metabolites between 120 mother-and-child pairs [96]. Occurrence of phthalate metabolites in pregnant women suggests potential exposure in the fetus.

**Figure 3.** Urinary concentrations of phthalate metabolites reported in adults (general population) from several countries (MDEHP: Sum of DEHP metabolites; biomonitoring data published after 2000; median concentration is presented).

Children: The NHANES data showed that the concentrations of urinary phthalate metabolites in children 6–11 years old were higher than those in adolescents and adults [142]. Several studies support the CDC's findings that children have higher urinary concentrations than do adults of DBP, BzBP, and DEHP [41,145]. Differences in urinary concentrations of phthalates among infants, children, and adults may reflect different sources and routes of intake. Ingestion is thought to be a primary pathway of exposure to some phthalates, especially those in food packaging [146]. The mouthing behavior of infants and toddlers could potentially increase their exposures to phthalates in toys and other products made with plasticized polymers. The global distribution of reported urinary phthalate metabolite concentrations in children is shown in Figure 4.

**Figure 4.** Urinary concentrations of phthalate metabolites reported in children (general population) from several countries (MDEHP: Sum of DEHP metabolites; biomonitoring data published after 2000, median concentration is presented).

MEP, MBP, and DEHP metabolites were the dominant compounds detected in urine from children. Spot urine samples from 5- to 7-year-old German children contained a median phthalate metabolite concentration (sum of 5 metabolites) of 76.9 μg/L, with DEHP metabolites as major compounds [89]. A similar concentration of DEHP metabolites at 75.7 μg/L was found in urine samples from 8- to 10-year-old German children (*n* = 465) [90]. Several biomonitoring studies reported comparable concentrations of DEHP metabolites and MBP in urine from children in China [75], Korea [108], Canada [70], Brazil [67] and Portugal [114].

In urine samples collected from children in Beijing, China, MBP was the most abundant metabolite (median: 232 μg/L), followed by MiBP (81.3 μg/L), MECPP (79.1 μg/L), and MEP (28.5 μg/L). A significant association between the concentrations of parent phthalate diesters in handwipes and the corresponding monoester metabolites in urine were observed in urine from children, which suggested that dermal absorption is an important exposure pathway for phthalates in children [75]. Mean urinary concentrations of MBP decreased as the children aged [91]. Among children, urinary DBP and DEHP metabolites in boys were higher than those in girls, whereas urinary MEP concentrations were positively correlated with age in both genders [79]. Urinary concentrations of MEP in adolescents were higher than those in children, which was associated with high cosmetic usage among teenagers [79,95].

Urinary phthalate metabolite concentrations have been reported for children and adults from 17 European countries, namely, Belgium, Cyprus, Czech Republic, Denmark, Germany, Hungary, Ireland, Luxembourg, Poland, Portugal, Romania, Slovenia, Slovak Republic, Spain, Sweden, Switzerland, and the United Kingdom (DEMOCOPHES); the geometric mean concentrations of MEP, MBZP, MBP, MiBP, and ΣDEHP metabolites were 34.4, 7.15, 34.8, 45.4, and 47.6 μg/L for children (*n* = 1355) and were 48.2, 4.51, 23.9, 30.1, and 29.2 μg/L for mothers [81], which suggested that children in those countries were more highly exposed to several phthalates than were their mothers. Nevertheless, some studies reported higher urinary MEP concentrations in mothers (45.1–72.0 μg/L) than in children (12.1–16.4 μg/L) [82,92]. The concentrations of DEHP metabolites were reported to be similar between mothers and children [82,109,119]. A significant positive correlation existed in urinary phthalate metabolite concentrations between children and their parents. MECPP, an oxidative metabolite of DEHP, was predominant in urine from children (92.7%) relative to that found in adults (56.7–57.6%). Studies have found that children possess enhanced oxidative metabolism for DEHP [91,109,147]. Another study of urinary phthalate metabolites in 104 paired mothers and school-aged children reported higher concentrations of secondary DEHP metabolites in children than in mothers [93]. A study from Austria showed higher urinary concentrations of phthalate metabolites in children than adults [62]. Overall, these studies suggest higher exposures to phthalates in children than adults.

Highly Exposed Populations: Highly exposed individuals have urinary phthalate metabolite concentrations that often exceed those at the 95th percentile of the general population (https://www. ncbi.nlm.nih.gov/books/NBK215044/). Neonates who receive medical treatments such as transfusions are widely recognized as potentially highlexposed [148]. A study from Slovakia showed that the urinary concentrations of DEHP metabolites, MiBP, and MBP in occupationally exposed individuals from plastic industry were 55.9, 39.2, and 110 μg/L, respectively [115], which were higher than those in urine from women of no known occupational exposures [61]. The median concentrations of MEP, MBP, MiBP, and DEHP metabolites in urine from hairdressing apprentices who attended vocational training schools in Slovakia were 201, 103, 61.4, and 82.7 μg/L, respectively [116]. Some medications contain phthalates in their coatings or delivery systems [49] and may contribute to the high exposures of children, pregnant women, and others who take these medications.

Exposure Assessment: The concentrations of phthalate metabolites measured in urine can be used to assess the amount of parent phthalate to which humans are exposed, when the fraction of the metabolite excreted in urine is known, as presented in the equation below [147]:

$$\text{Estimated parentphthate concentration} = \frac{\text{Metabolic concentration}}{\text{Excretion fraction}} \tag{1}$$

The estimated daily intake (*EDI*) of parent phthalates is then calculated by taking the average weight of an individual with the average urinary excretion rate, as shown in the equation below:

$$\text{Estimated daily intake (EDI)} = \frac{\text{Estimated parent platform} \times \text{Daily zinc excretion volume}}{\text{Average body weight}} \tag{2}$$

Several studies have estimated exposure doses to phthalates in populations, which allowed for comparison against a reference dose (*RfD*), the maximum acceptable oral dose of a toxic substance, of the US EPA. The estimated mean daily exposure doses to DEP and DBP in Asian countries and the US were one to two orders of magnitude below the EPA *RfD* (DEP = 800, DBP = 100, and DEHP = 20 μg/kg body weight (bw)/day). The estimated daily exposure doses to DEHP in Kuwait and India, however, were close to the RfD of the US EPA [72]. Similarly, high concentrations of DEHP metabolites (mean concentration = 338 μg/L) were reported in urine from the Saudi population [26].

The calculated *EDI*max values for DEHP and DBP were 8 and 0.08 μg/kg bw/day, respectively, for the population in Taiwan, which were one to two orders of magnitude lower than the tolerable daily intake (*TDI)* values (the daily intake amount of a chemical that has been assessed to be safe for human being on a long-term basis) suggested for DEHP (50 μg/kg bw) and DBP (10 μg/kg bw) by the European Food Safety Authority (EFSA) [147].

The 95th percentile for DEHP exposure doses calculated for the general population (*n* = 85) and children (*n* = 254) from Germany were 21 and 25 μg/kg bw/day, respectively, which exceeded the RfD (20 μg/kg bw/day) and the TDI (20–48 μg/kg bw/day) [149]. Further, elevated exposure to phthalates, especially DEHP, in neonates admitted to intensive care units was reported (median: 42 μg/kg bw/day; 95th percentile: 1780 μg/kg bw/day) [149], and the exposure dose was higher than the RfD.

Some studies defined "Biomonitoring Equivalents (BEs)" as the concentration or range of concentrations of a chemical or its metabolite in a biological medium (blood, urine, or other medium) that is consistent with an existing health-based exposure guideline (e.g., RfD and TDI) [150,151]. BE values for MBP, MBzP, and MEP were reported at 18000, 3800 and 2700 μg/L, respectively [150], and the BE values range from 1500 to 3600 μg/L for MiNP [151]. These values may be used as screening tools for evaluation of biomonitoring data for phthalate metabolites in the context of existing risk assessments and for prioritization of the potential need for additional risk assessment efforts for each of these compounds relative to other chemicals [150,151].

Although current exposure doses in the general population are below the tolerance limits reported by environmental agencies, certainly population groups, especially children, are exposed to high levels of phthalates. Studies of the effects of phthalates from early life stage exposures are warranted.

#### *3.2. Phthalate Metabolites in Serum*

The biomonitoring studies of human phthalate exposure have been based on urinary concentrations of phthalate metabolites. However, when only serum was available for analysis, MEP and MiBP representing low molecular weight phthalates, and MECPP and MCiOP representing high molecular weight phthalates, have been used as indicators of phthalate exposure [77]. A study reported the correlations of phthalate metabolite concentrations among urine, serum, and seminal plasma of young Danish men [77]. The mean concentrations of MEP, MBP, and DEHP metabolites were one to two orders of magnitude lower in serum (MEP: 4.2, MBP: 0.4, and DEHP: 7.6 μg/L) and seminal plasma (1.0, 0.8, and 0.6 μg/L) than in urine (326, 42.5, and 115 μg/L). Another study, however, showed that the distribution pattern of monoester metabolites in serum was similar to that of urine [152], especially for MEHP (the metabolite of DEHP) [152]. Nevertheless, MEHHP, MEOHP, MECPP, and MCMHP were found at much higher concentrations in urine than in serum [153]. The presence of MEHP in serum was more likely related to contamination that arises from sampling devices.

Whole blood and cord blood samples from 128 healthy pregnant women and their newborns were analyzed for phthalate metabolites. Median concentrations of MEHHP and MEOHP were 0.31 and <LOD μg/L in maternal blood and 0.32 and <LOD μg/L in cord blood, respectively. MEHHP and

MEOHP also were reported to occur in the placenta at concentrations of 0.09 and <LOD ng/g [144]. MBP, MEHP, MEP, and MiBP were detected in blood serum at median concentrations of 0.54 0.49, 0.50, and 0.5 μg/L, respectively [24], and these concentrations were at least an order of magnitude lower than those measured in urine.

In the serum of patients who were undergoing dialysis [53,154–156], phthalate acid (PA) was found as a metabolite of phthalates at remarkably high concentrations of 5.22 ± 3.94 mg/L [155]. Another study also reported the occurrence of PA in serum (0.205 ± 0.067 mg/L) of patients who were undergoing dialysis [154]. Accumulation of PA in patients who are undergoing dialysis has been suggested [156]. Serum concentrations of MEHP and DEHP were reported in autistic children [157].

#### *3.3. Phthalate Metabolites in Amniotic Fluid, Breast Milk, Semen, and Saliva*

MBP was found in >93% of amniotic fluid samples collected from the US [59] at concentrations two- to threefold lower than those of serum and four- to sevenfold lower than those of urine [59]. Studies have reported the occurrence of phthalates in breast milk [158]; the reported concentrations in breast milk were much lower than those in urine but similar to those in amniotic fluid. Monoester metabolites of phthalates were measured in breast milk from 33 lactating mothers in North Carolina. MCPP (0.2 μg/L) and MEOHP (0.3 μg/L), MECPP (0.1–0.4 μg/L), and MEHHP (0.2–0.3 μg/L) were detected in some samples [131]. MiNP was the major metabolite found in breast milk collected from mothers in Denmark (101 μg/L) and Finland (89 μg/L) [25,159]. Median concentrations of MBP, MBzP, and MEHP in breast milk were 0.54, 0.50, and 0.49 μg/L, respectively [24].

Human saliva samples (*n* = 39) also contained phthalate metabolites [57,160]. Salivary concentrations of phthalate metabolites in 39 adult volunteers were in the ranges of <1 to 10.6 μg/L for PA, 91.4 μg/L for MEP, 65.8 μg/L for MBP, and 354 μg/L for MBzP. MBP was the most (85%) frequently detected compound in saliva [57]. Two phthalate metabolites (2.2 μg/L MCPP and 2.3 μg/L MECPP) were detected in a saliva sample from a US woman [131].

MBP and MBzP were found in semen from US men [32,54]. High concentrations of DEHP and its metabolites (-40.6 μg/L) were found in semen from German men [161]. Studies have also indicated that semen quality can be affected by environmentally relevant phthalate exposures [121]. Further, DEHP (4.20 μg/L) and DBP (2.06 μg/L) were reported at high concentrations in male seminal plasma from men in the US. The metabolites of DEHP (-0.98 μg/L) and MBP (2.97 μg/L) also were present in considerable concentrations in seminal plasma in the same study [162]. These results suggested that phthalate metabolites can partition in seminal plasma. Similarly, DEHP (2.09 μg/L) and DBP (1.75 μg/L), as well as their metabolites, were found as the predominant phthalates/phthalate metabolites in seminal plasma from male partners who were planning for pregnancy. This study showed adverse associations between seminal phthalate metabolite concentrations and semen quality [163].

Phthalate metabolites were measured in nail samples from Belgium, and the total concentrations ranged between <12 and 7980 ng/g. It should be noted, however, that some phthalates, especially DBP, are used in nail polishes and that care should be exercised in interpreting such measurements. MEHP, MBP, and MEP were the major metabolites detected in every nail sample, with a median concentration of 138, 74, and 64 ng/g, respectively [135]. Another study of nail samples from Oslo, Norway, showed the presence of monoesters, such as MMP (geometric mean 89.7 ng/g), MEP (104.8 ng/g), and MBP (89.3 ng/g) [112]. The utility of other biologic matrices, such as blood, breast milk, semen, and nails, for assessing human exposure to phthalates remains largely unknown due to the limited data.

#### **4. Select Epidemiological Studies Linking Phthalate Exposure and Health Outcomes**

Controlled laboratory animal studies on the toxic effects of phthalates have enabled understanding of biological plausibility and potential mechanisms of actions of this class of chemicals. Thus far, the majority of the laboratory animal exposure/toxicity studies have focused on DEHP and DBP/DiBP, with limited studies examining the toxicities of other phthalates [164–191]. The reproductive and developmental effects of phthalates are among the most studied and well-described toxic endpoints in

those studies. The toxic endpoints determined in animal studies, following phthalate exposure, include retention of nipples, anogenital distance, pathological changes in testes and male reproductive accessory glands, hypospadias, cryptorchidism, and semen parameters. Phthalates have well-documented anti-androgenic activity in rodent studies that result in reduced circulating testosterone. Several reviews have been published on the toxicity of phthalates [10,14,168–170]. As a class of well-studied endocrine disrupting chemicals, exposure to phthalates has been linked to sex anomalies, endometriosis, altered reproductive development, early puberty and fertility, breast and skin cancer, allergy and asthma, overweight and obesity, insulin resistance, and type II diabetes.

#### *4.1. Diabetes*

Diabetes is a metabolic disease that results in elevated blood glucose levels. Epidemiological studies in the US [192,193] reported that women with higher urinary concentrations of MBP, MiBP, MBzP, and MCPP and those of DEHP metabolites showed increased risk of diagnosis for diabetes in comparison with those who had lower concentrations of phthalates. Phthalate exposures have been shown to result in insulin resistance [166,194].

#### *4.2. Overweight and Obesity*

Overweight and obesity can be associated with many chronic diseases, including diabetes. Phthalate exposure was associated with increased body mass and waist circumference [195]. Some phthalate metabolites (MEP, MBP, and MiBP) were associated with obesity in children, whereas MEHP, MECPP, MEHHP, MEOHP, MBzP, and MCNP were associated with obesity in adults. Further, DEHP metabolites were found to be significantly associated with obesity in adult females and older males [196]. Urinary concentration of MBP was associated with fat deposition in boys in China [197].

Several studies have shown a significant association between obesity and phthalate exposure [193,196,198]. MEP, MEHP, MBzP, MEHHP, and MEOHP were associated with obesity in the US population [198]. MBzP, MEHHP, MEOHP, and MEP were associated with increased waist circumference and BMI [193] In contrast, higher concentrations of MEP and DEHP were found in the serum and urine of individuals who were undergoing weight loss [199]. Food intake is the main source of phthalate exposure (for high molecular weight phthalates). Therefore, overweight population with high food intake might have high phthalate exposures.

#### *4.3. Allergy and Asthma*

Exposure to high molecular weight phthalates are is associated with allergies and asthma [200,201]. Studies indicated that children are prone to exposure to DEHP, BzBP, DBP, and DEP and that exposure was associated with allergic rhinitis, atopic dermatitis, and conjunctivitis [202]. DEHP and BzBP and their monoesters are regarded as allergens, and exposure to them has been associated with asthma and wheezing in adults [200,201]. Exposure of DEHP, BzBP, DBP, and DEP during gestation has been associated with allergic responses in infants and toddlers [200]. Urinary MEHP concentrations are correlated with asthma in children [203]. Prenatal exposure to DEHP metabolites and BzBP has been associated with the risk of developing asthma at the age of 7 years and older [204].

#### *4.4. Reproductive Health*

Urinary MEP and MBP and the metabolites of DEHP and DiNP are associated with anomalies in pubertal development in girls [205]. A significant association between urinary concentrations of MBzP, MEHP, and MEP and increased risk of endometriosis was found in women [206]. Exposure to MEP, MiBP, and MBP pose an increased risk of pregnancy loss in Chinese women.

Poor semen quality was associated with exposure to phthalate metabolites. MBP and MBzP were strongly associated with spermatotoxicity and subfertility in males [32,54]. Significantly higher concentrations of DEHP (4.66 μg/mL) and MEHP (3.19 μg/mL) were found in the urine of 40 Turkish boys with gynecomastia as compared to that of control groups [207]. Several reviews have appeared on the reproductive and developmental toxicities of phthalates [208]. Whereas some inconsistencies exist across phthalates for specific health outcomes associated with exposures, moderate to strong evidence of male reproductive effects have been demonstrated in the literature [208]. Because humans are exposed to thousands of harmful chemicals, establishing the link between exposure to a single substance class and adverse health outcomes is fraught with uncertainties.

#### **5. Conclusions and Perspectives**

Human biomonitoring studies are useful in elucidating exposures and body burdens of phthalates at a population level. Although the sources of exposure to phthalates are well described, several questions about cumulative exposures to phthalates throughout the life span, relative contributions of various sources to cumulative exposures, and mixed exposures that may include phthalates or other chemicals that may elicit common adverse outcomes remain unanswered. Biomonitoring studies clearly demonstrate that human exposures are almost ubiquitous, and, in most cases, children have higher exposures than do adults. The existing studies indicate that the observed associations between phthalate exposure and disease outcomes are exploratory and preliminary, the health effects of phthalate exposure warrant further study. Robust analytical methods exist to measure more than 20 phthalate metabolites in urine, a preferred matrix of choice for biomonitoring studies. Although studies have reported the occurrence of phthalate metabolites in other human specimens, including serum, seminal plasma, and amniotic fluid, the relevance of these matrices in understanding toxic effects needs further investigation. Although biomonitoring studies select major biomarkers/metabolites of phthalates, several other intermediate and transformation products of phthalates appear to exist in human specimens. These intermediates may have more pronounced effects on health. Lack of analytical standards hinders the identification of those intermediate biological transformation products of phthalates. Further, the interaction of phthalate metabolites with other contaminants should be considered in future investigations.

There is a lack of biomonitoring data on phthalate exposures in developing countries in Africa and South America. Studies are needed in those regions with regard to exposures and associated health outcomes in populations. Further, epigenetic effects of phthalate exposures warrant further investigation.

**Funding:** Research reported in this publication was supported, in part, by the National Institute of Environmental Health Sciences of the National Institutes of Health under Award No. U2CES026542-01. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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