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Estimated risks of water and saliva contamination by phthalate diffusion from plasticized polyvinyl chloride.

Introduction

In the 20th century, interest in polymers grew rapidly because of the wide range of applications of polymers in the plastic, rubber, fiber, adhesive, paint, and other industries. Plasticization is one of the most commonly used techniques for the manufacture of polymeric materials (Allcock & Lampe, 1990). Plasticization consists of mixing a rigid plastic (thermoplastic) with a low-molecular-weight substance (the plasticizer) to obtain a flexible material (Gachter & Muller, 1993; Peen, 1967). The esters of phthalic acid (phthalates) are some of the plasticizers used worldwide, with dibutyl phthalate, diisobutyl phthalate, dioctyl (also known as di[2-ethylhexyl] phthalate [DEHP]) being among the most frequently used (Gachter and Muller, 1993; Peen, 1967).

DEHP is used in the manufacturing of a variety of consumer products, such as flooring, wall coverings, food containers and wraps, personal care and medical products; as a solvent and plasticizer for cellulose acetate; and in the manufacturing of lacquers, varnishes, and coatings, including the coatings of time-release pharmaceuticals (Duty, Ackerman, Calafat, & Hauser, 2005; Hauser, Meeker, Duty, Silva, & Calafat, 2006).

Routes of Environmental Exposure to DEHP

DEHP can enter the environment in several ways, including outdoor releases from factories that use or manufacture DEHP, diffusion from plastic materials into the soil in landfills and waste disposal sites, and by contamination of the groundwater near the landfills and waste disposal (Agency for Toxic Substances and Disease Registry [ATSDR], 2002; Peijnenburg & Struijs, 2006; Wang, Hu, Cao, Fu & Zhu, 2005). DEHP does not evaporate or break down easily, and only small amounts are released into the air, soil, or water. In the air, DEHP binds to dust particles and is carried down to earth via rain or snow. Thus, only small amounts of DEHP can contaminate plants, fish, and other animals (ATSDR, 2002). Humans residing near landfill and waste disposal sites may be at a higher risk for DEHP exposure than the rest of the population, however (ATSDR, 2002).

DEHP enters the home and work environments through indoor releases of consumer and personal care products (ATSDR, 2002). The highest and most direct exposure to humans is via DEHP diffusion from flexible plastic tubing and blood and intravenous bags from medical devices, which could allow DEHP to enter directly into the bloodstream via blood transfusions, dialysis treatment, parenteral nutrition, or even in the plastic coating of some time-released medications (ATSDR, 2002; Fankhauser-Noti, Biedermann-Brem, & Grob, 2005; Hauser, Meeker, Duty, Silva, & Calafat, 2006; Main et al., 2006; Mikula, Svobodova, & Smutna, 2005; Peijnenburg & Struijs, 2006; Staples, Peterson, Parkerton, & Adams, 1997; Wang, Hu, Cao, Fu, & Zhu, 2005). Infants and toddlers can also be exposed to DEHP through the oral route by placing plastic toys, bottles, or pacifiers in their mouths, and even through breast-feeding, since DEHP can be transmitted through breast milk (Bustamante et al., 2004, 2005; Hauser, Meeker, Duty. Silva, & Calafat, 2006; Marin, Lopez, Sanchez, Vilaplana, & Jimenez, 1998). In the United States however, the use of DEHP in the manufacturing of toys, baby rattles, and teethers has been discontinued and is no longer used in food wrap products (ATSDR, 2002), which may not be the case in other countries.

Potential Toxicity of DEHP to Animals and Humans

The potential toxicity of phthalate esters was overlooked until 1992, when the National Toxicology Program proclaimed that some phthalates had caused endocrine disruption in rats and mice (National Toxicology Program, 1992). The health effects of DEHP have been mostly studied in laboratory animals through the oral route, with very little human data available by any route of exposure. Nevertheless, human studies have also shown endocrine disruption from phthalates in the human male reproductive system (Main et al., 2006; Swan et al., 2005). Furthermore, evidence exists that phthalates used as plasticizing agents produce adverse reproductive and development effects through their metabolites, which can be measured in urine and saliva (Silva et al., 2005, 2006).

Migration and Diffusion Phthalate Studies

Phthalate concentrations in plasticized PVC can reach as high as 60% of the material's weight (Gachter & Muller, 1993; Peen, 1967). Phthalate molecules are very small and are not bound chemically to the plastic; therefore, they can diffuse out of the plastic over a period of time (Aurela, Kulmala, & Soderhjelm, 1999; Castle, Mayo, & Gilbert, 1988, Castle, Mercer, & Startin, 1989; Cohen, Charrier, & Sarfaty, 1991; Page & Lacroix, 1992).

Previous studies have evaluated the migration of phthalates using different methods. Meuling and Rijk (1998) evaluated the migration of di-isononylphthalate (DINP) from infant products into saliva among 20 adult study subjects. The DINP diffusion rate ranged from 82.8 to 146.4 mg/10 [cm.sup.2] per hour at 36[degrees]C. Additionally, 10 subjects were asked to bite and suck an infant toy containing approximately 43% DINP by weight, and saliva was collected and analyzed for DINP content. The mean DINP diffusion rate was 4.3 mg/10 [cm.sup.2] per minute, and the highest DINP diffusion rate recorded was 13.4 mg/10 [cm.sup.2] per minute. Chen (1998) investigated the migration of DINP using mechanical pistons of various sizes to simulate the chewing of different DINP-containing materials in artificial saliva. The in vitro analysis showed a mean phthalate diffusion rate of 3.3mg/10 [cm.sup.2] per minute, and the minimal and maximal values were 2.5 and 4.1 mg/10 [cm.sup.2] per minute, respectively.

Marin and co-authors (1998) determined the quantity of phthalates probably ingested by children chewing materials with phthalate additives. Plasticized products were placed on a 10 [cm.sup.2] surface with artificial saliva and agitated over a 6-hour period at 37[degrees]C. The phthalate diffusion values in the saliva for DEHP and DINP were 0.0055 and 0.039 mg/10 [cm.sup.2] per hour, respectively. The Scientific Committee on Toxicity, Ecotoxicity, and the Environment (1998) has recommended 50 mg/kg per day as the tolerable daily intake value for DEHP and 0.00031 to 0.0072 mg/[cm.sup.2] per hour as the phthalate-diffusion-rate limit for infant toys.

Materials and Methods

A series of plasticized PVC was prepared by mixing solutions of PVC and DEHP in order to obtain materials with homogeneous and controlled phthalate content. The plasticized PVCs were prepared with a high phthalate content (60% by weight) corresponding to the maximal plasticizer concentration determined in a series of PVC toys produced in Mexico. This plastic composition allowed estimating a maximal risk of phthalate loss when the products are in contact with fluids such as water or human saliva. Indeed, a series of plasticized PVC sheets was immersed in three types of media: water, artificial saliva, and human saliva, and the contaminant diffusion rates were compared to values of molecular diffusion in fluids calculated with Fick's equation. All these results allowed estimating the exposure in Mexican infants and children.

PVC powder was donated by Salver Mexican S.A. (Guadalajara, Mexico). DEHP (Chemical Abstracts Service No. 117-81-7, 99% purity; Riedel-deHaen Fine Chemicals, Seelze, Germany) was chosen as the test plasticizer since DEHP is commonly used in PVC compounding

Preparation of Plasticized PVCs

The composition of the sheets was 40% PVC and 60% DEHP by weight and each batch was prepared in triplicate. We chose this composition because 60% DEHP was the maximum phthalate concentration determined in a series of commercial products tested in Mexico. PVC was dissolved in 25 ml of hot acetone (Allied-Signal, Morristown, New Jersey, Chemical Abstracts Service No. 67-64-1, pesticide grade) and agitated, and the dissolved polymer was mixed with DEHP. Through this method, a series of plasticized PVC sheets with a surface area of one side of the sheet of 10 [cm.sup.2] and thickness of 2 mm was obtained.

DEHP Diffusion in Water

A series of six plasticized PVC samples (described in the preceding section) were placed in 5 ml of distilled water, pH 7.0, in tightly closed Erlenmeyer flasks (one sample per flask). The flasks were then introduced into an isothermal water bath at 35[degrees]C [+ or -] 1[degrees]C under static conditions. At three, nine, 24, and 30 hours, the water was removed from the flask and replaced with 5 ml of distilled water. DEHP was extracted from the samples using dichloromethane as the extraction solvent. The DEHP samples were refrigerated at 4[degrees]C to avoid degradation (Ritsema, Cofino, Frintrop, & Brinkman, 1989).

This experiment was then repeated with a second series of plasticized PVC samples but with constant stirring of the flasks to permit evaluation of the effect of dynamic conditions on DEHP migration in water.

DEHP Diffusion in Artificial Saliva

As outlined in Table 1, artificial saliva was prepared according to a method previously used by Heftman, 1970. This artificial saliva has been used for a long time as an excellent representation of human saliva.
TABLE 1
Components of Artificial Saliva *

Compound Weight (mg)

Calcium chloride 100

Magnesium chloride 400

Potassium carbonate 810

Sodium chloride 300

Glucose 320

Sodium phosphate 210

Uric acid 20

Urea 220

* Distilled water was asses to these components until the total
volume 1000 ml.


A series of 12 plasticized PVC samples (same dimensions as in the previous experiment) were placed in 5 ml of artificial saliva, pH 6.45, in tightly closed Erlenmeyer flasks (one sample per flask). The flasks were then introduced into an isothermal water bath at 35[degrees]C [+ or -]1[degrees]C under static conditions. At one, three, six, and 12 hours, the artificial saliva was removed from the flask and replaced with 5 ml of artificial saliva. DEHP was extracted from the samples using dichloromethane as the extraction solvent. The DEHP samples were refrigerated at 4[degrees]C to avoid degradation (Ritsema, Cofino, Frintrop, & Brinkman, 1989).

This experiment was then repeated with a second series of plasticized PVC samples but with stirring to permit evaluation of the effect of dynamic conditions on DEHP migration in water.

DEHP Diffusion in Human Saliva in Vivo

Three circular pieces (total surface area of each piece = 10 cm.sup.2]) were cut from a commercial plasticized PVC sample with a DEHP content of 60% by weight. Each piece was chewed for 30 minutes by three different male volunteers, and the saliva produced during the process was retained in the buccal cavity. Five milliliters of saliva were then obtained. DEHP was extracted from the saliva using di-chloromethane as the extraction solvent.

Gas Chromatography

The stored DEHP samples that had been extracted from water and saliva and refrigerated were analyzed in triplicate with a Gas Chromatograph Varian 3300, with a DB-1 column 30 m long, 0.25 mm thick, and a particle size of 0.25 microns, nitrogen vehicle gas (5 ml/min); and flame ionization detection using a Varian 4290 integrator. The column was maintained at 300[degrees]C, the detector at 300[degrees]C, and the injector at 280[degrees]C. One microlitre of sample was then injected, and DEHP was quantified using butylbenzyl phthalate (Aldrich Chemical Co., Toluca, Mexico, Chemical Abstracts Service No. 85-68-7, 98% purity) as the internal standard.

Mathematical Model

In this study, we used a mathematical model to predict the migration of phthalates into water and saliva from plasticized PVC. The model assumed that the diffusion was molecular in nature, that the phthalate concentration was cumulative on time and the media were uniform and non-turbulent. Therefore, for the diffusion under stable conditions of from a PVC sheet with no superficial resistance and in the absence of chemical reactions (Treybal, 1990),it is given:

(1) [[[partial derivative].sub.C]]/[[[partial derivative].sub.t]]=-[D.sub.AB][[[partial derivative].sup.2]C]/[[partial derivative][x.sup.2]]

where

[[partial derivative].sub.C]/[[[partial derivative].sub.t]] = Mass flow by unit area;

[D.sub.AB] = Diffusion coefficient of the component A in B; and

[[partial derivative].sub.C]/[[[partial derivative].sub.x]] = Concentration gradient.

From the previous equation (1), the following equation was obtained for the diffusion from the PVC sheet:

(2) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The diffusivity [D.sub.AB] or diffusion coefficient is a measure of the mobility of diffusion of the solute in the surrounding medium. To calculate the value of the diffusivity, an empiric correlation proposed by Wilkie and Chang was used (Van Krevelen, 1987):

(3) [D.sub.AB]=7.4 X [10.sup.-8]([phi][M.sub.B]).sup.1/2]T/[[eta].sub.AB][[V.sub.A.sup.0.6]

where

[D.sub.AB] = Diffusion coefficient of A in a solution diluted in the solvent B, [cm.sup.2]/s;

[M.sub.B] = Molecular weight of the solvent B, g/mol;

T = Temperature, K;

[[eta].sub.AB] = Viscosity of the solvent B, cP;

[V.sub.A] = Molar volume of the solute A at the normal boiling point, mol/[cm.sup.3];

[phi] = Association factor of the solvent B, adimensional; and

2.26 for water as solvent.

Thus, through this method, and taking into consideration the principles of molecular diffusion (Treybal, 1990) and estimating [D.sub.AB] by the theory of the contribution of the groups (Van Krevelen, 1987) the migration of pathalates from a plasticized PVC sheet placed in water or artificial saliva was determined. In both cases, the initial conditions to solve the equation (2) were t = 0, surface area of the one side of the sheet = 10 [cm.sup.2], [C.sub.0] = 60% weight of DEHP, volume of water or saliva V = 5 mL. The equation (2) was solved using the software Polymath 4.0 (Shacham & Cutlip, 1996). We also compared the theoretical diffusion values calculated using this model with the experimental diffusion values obtained in our experiments.

Results

DEHP Diffusion in Water

Figure 1 shows the theoretical aqueous concentration of DEHP diffused from a sheet of plasticized PVC with 60% DEHP by weight at 36[degrees] calculated using equation 2. The model indicated that phthalate diffuses very poorly in water. From hour 4 onward, however, the theoretical diffusion continuously increased until the DEHP saturation point in water was reached, after 12 hours of immersion. Once the DEHP concentration reached the saturation value, molecular diffusion stopped.

[FIGURE 1 OMITTED]

Figure 1 also shows the experimental aqueous DEHP concentrations. At exposure times shorter than 4 hours, the theoretical and experimental diffusion values were similar: the molecular diffusion of DEHP from the plastic to the aqueous phase was very slow. At longer exposure times, however, the experimental migration of the plasticizer to the water remained low while the theoretical values increased. The experimental DEHP saturation values was not reached even after 14 hours of sample immersion in water . We speculated that the observed low values of DEHP diffusion might due to the static conditions of the aqueous medium. The stirring performed in the second set of experiments, however, had no effect on DEHP diffusion in water (Figure 1).

DEHP Diffusion in Artificial Saliva

The organic compounds present in the salvia were expected to affect DEHP diffusion since these compounds can be made soluble by the plasticizer, thereby accelerating the diffusion rate. The chromatographic analysis of the PVC samples immersed in artificial saliva for different periods revealed low DEHP values and very weak phthalate diffusion into the continuous medium (Figure 2). As with the experiments conducted with water, good agreement occurred between the theoretical and experimental data for short exposure periods, and strong differences existed for long exposure periods.

[FIGURE 2 OMITTED]

DEHP Diffusion in Human Saliva (In Vivo)

Chromatographic analysis of DEHP diffusion in saliva in vivo for 30 minutes indicated a diffusion value of 1.51 [mu]g/c[m.sup.3]. Thus, even when DEHP was extracted from PVC by chewing, the phthalate migration still occurred very slowly.

Predicted DEHP Exposures for Infants and Children in Mexico

In Mexico, infants and children who use plastic products (e.g., pacifiers, chewable toys, and bottles) are orally exposed to these products an average of 29 minutes per day (Bustamante, 2004). This value of time (t = 29 min), surface (=10 [cm.sup.2]), and the DEHP diffusion values estimated in our study were employed to calculate with equation (2) daily DEHP doses in an infant with a weight of 8 kg and an oral cavity volume of 20 c[m.sup.3] (Table 2).
TABLE 2

Accumulative Diffusion Rates and Daily Received Doses of DEHP in Water,
Artificial Saliva, and Human Saliva *

System Accumulative Diffusion Rate Daily Received
 ([micro]g/[cm.sup.2]/h) Dose ([micro]g/kg)

Distilled water 0.36 1.08

Artificial saliva 4.10 12.30

Human saliva (in vivo) 6.04 18.12
n
* Diffusion from a sheet of plasticized PVC with composition of
40% PVC and 60% DEHP by weight; temperature, 36 [degrees] C.


Discussion

The strong differences observed in our study between the theoretical and experimental data at longer exposure times can be explained in terms of one of the main assumptions of our model. We assumed that the phthalate concentrations on the PVC sheet submerged in water would remain constant. This would require that each time a phthalate molecule left the PVC sheet it was immediately replaced by another phthalate molecule. As observed during the PVC-water experiments, however, the phthalate molecules were mostly drained. That is, once the phthalate molecules left the PVC, they were not replaced. The disappearance of DEHP at this critical point prevented increase in the diffusion. For the short exposure times representative of actual exposure times in humans, however, strong agreement occurred between the theoretical and experimental data.

Previous studies have evaluated the migration of phthalates using different methods. Meuling and Rijk (1998) evaluated the migration of DINP from infant products into saliva among 20 adult study subjects. The DINP diffusion rate ranged from 82.8 to 146.4 mg/10 [cm.sup.2] per hour at 36[degree]C. Additionally, 10 subjects were asked to bite and suck an infant toy containing approximately 43% DINP by weight, and saliva was collected and analyzed for DINP content. The mean DINP diffusion rate was 4.3 mg/10 [cm.sup.2] per minute, and the highest DINP diffusion rate recorded was 13.4 mg/10 [cm.sup.2] per minute. We found a much lower diffusion rate of DEHP, which could be indicative of differential diffusion rates for specific phthalates, in addition to more aggressive conditions (biting products) used by Meuling and Rijk.

Chen (1998) investigated the migration of DINP using mechanical pistons of various sizes to simulate the chewing of different DINP-containing materials in artificial saliva. The in vitro analysis showed a mean phthalate diffusion rate of 3.3 mg/10 [cm.sup.2] per minute, and the minimal and maximal values were 2.5 and 4.1 mg/10 [cm.sup.2] per minute, respectrvely

The diffusion rate we found is similar to that reported by Marin and co-authors (1998), 5.5 [micro]g vs. 4.10 [micro]g/10 [cm.sup.2] per hour. The small observed difference may be the result of the different agitation times used (35[degrees]C for 12 hours under static conditions in our study vs. a six-hour period at 37[degree]C in Marin's). The phthalate diffusion values in the saliva for DEHP and DINP were 0.0055 and 0.039 mg/10 [cm.sup.2] per hour, respectively.

In this research a series of plasticized PVC was prepared by mixing solutions of PVC and DEHP in order to obtain materials with homogeneous and controlled phthalate content. The plasticized PVCs were prepared with a high phthalate content (60% by weight) corresponding to the maximal plasticizer concentration determined in a series of PVC toys produced in Mexico. This plastic composition allowed estimating a maximal risk of phthalate loss when the products are in contact with fluids such as water or human saliva. A series of plasticized PVC sheets were immersed in three types of media: water, artificial saliva, and human saliva, and the contaminant diffusion rates were compared to values of molecular diffusion. These last ones were calculated with Fick's equation.

When we determined diffusion in vivo, the mean value of DEHP migration was 6.04 [micro]g/[cm.sup.2] per hour, and the received dose was 18.12 [micro]g/ kg per day. These values are considerably lower than the phthalate-diffusion-rate limit for infant toys of 0.00031 to 0.0072 mg/m[c.sup.2] per hour and the tolerable daily intake of 50 mg/kg per day as determined by the Scientific Committee on Toxicity, Ecotoxicity, and Environment (1998).

The values of phthalate diffusion obtained in our experiments can be useful when estimating the risk of phthalate exposure in infants and children. In Mexico, infants and children who use plastic products (e.g., pacifiers, chewable toys, and bottles) are orally exposed to these products and average of 29 minutes per day (Bustamante et al., 2004).

Conclusion

Our findings indicate the phthalate daily dose from the oral use of soft plastic products such as chewable toys, pacifiers, and bottles may be fairly low. The DHEP diffusion rates in water, artificial saliva, and human saliva were even slower than those predicted by Fick's equation for molecular diffusion. It means that the phthalate molecules are trapped in the PVC matrix and they can not be released under the normal conditions of use of PVC products (toys, pacifiers, bottles). Given the observed slow and brief phthalate diffusion, oral exposure to these contaminants in Mexican infants and children may be considerably lower than the tolerable daily intake of 50 mg/kg per day as recommended by the Scientific Committee on Toxicity, Ecotoxicity, and the Environment.

Acknowledgements: This project was financed by Consejo Nacional de Ciencia y Tecnologia 2003-C01-017 and 2245/2006 Universidad Autonoma del Estado de Mexico.

Corresponding Author: L. Patricia Bustamante Montes, Professor, Universidad Autdnoma del Estado de Mexico, Facultad de Medicina, Paseo Tollocan esquina Colon, Toluca, Mexico 50180. E-mail: patriciiab@yahoo.com.mx.

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Although most of the information presented in the Journal refers to situations within the United States, environmental health and protection know no boundaries. The Journal periodically runs International Perspectives to ensure that issues relevant to our international constituency, representing over 60 countries worldwide, are addressed. Our goal is to raise diverse issues of interest to all our readers, irrespective of origin.

Kira S. Corea-Tellez, M.S. Patricia Bustamante-Montes, M.D., Dr.P.H. Magdalena Garcia-Fabila, MS. Maria A. Hernandez-Valero, Dr.P.H. Flavio Vazquez-Moreno, Ph.D.
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Title Annotation:INTERNATIONAL PERSPECTIVES
Author:Corea-Tellez, Kira S.; Bustamante-Montes, Patricia; Garcia-Fabila, Magdalena; Hernandez-Valero, Mari
Publication:Journal of Environmental Health
Article Type:Report
Geographic Code:1USA
Date:Oct 1, 2008
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