Human Exposure Estimates for Phthalates

Phthalates are important industrial chemicals used in a variety of applications. These chemicals can be ingested, inhaled, or absorbed through the skin, resulting in human exposure and raising significant public health concerns.

Blount et al. (1) provide the first systematic compilation of data that address exposures of the general population to commercially important phthalate diesters. The data result from a study conducted at the Centers for Disease Control and Prevention/ National Center for Environmental Health (CDC/NCEH) that is part of a continuing collaboration with the National Institute of Environmental Health Sciences/National Toxicology Program (NIEHS/NTP). The study was designed to assess human tissue levels of potential or known environmental toxicants using biological samples collected through CDC's NHANES program and state-of-the-art analytical methods. Such data will aid the NTP in identifying chemicals in need of toxicologic evaluation, based on their prevalence in human tissues, as well as providing useful human exposure information on high priority toxic chemicals identified through the NTP or other toxicologic testing activities.

The primary route of human phthalate exposure to the general population has been presumed to be ingestion. Lower molecular weight phthalates can be absorbed percutaneously, and the more volatile congeners can be inhaled. Lipases in the intestinal epithelium, liver, and other tissues hydrolyze the diester to a monoester, which is systemically distributed. Some of the monoester is converted into other chemical species by oxidative metabolism in bodily tissues, and some of it is excreted in the urine as acyl glucuronides. The extent of oxidative metabolism and conjugation would be expected to vary among chemicals, species, and individuals. Blount et al. (1) report the concentrations of seven phthalate monoesters in the urine of 289 people. We present here the results of calculations of the estimated total daily intake of phthalates that would result in the reported urinary concentrations of monoester metabolites. These intake estimates can be used as a measure of total exposure to these phthalates and compared with previous intake estimates determined from levels in environmental media.

The daily exposure can be estimated by using a linear two-compartment model. The normalized integrated rate equations are

[1] FE = 1 - exp(-[k.sub.total]t)

and

[2] FU = [k.sub.u]/[k.sub.total] [1 - exp(-[k.sub.total]t)]

where FE and FU are the total and urinary fractions of the dose eliminated in time t, and [k.sub.total] and [k.sub.u] are the apparent first-order rate constants for total elimination and elimination of urinary monoester, respectively. The two rate constants were calculated from the excreted fractions observed during the 24 hr following a single oral dose of diester, using Equations 1 and 2.

Assuming steady-state intake and metabolic clearance of the diester, the internal exposure rate for an individual is approximated by

[3] intake([micro]g / kg / day)

= ME([micro]g / g) x CE(mg / kg / day)/ f x 1000(mg / g)

x [MW.sub.d]/[MW.sub.m]

where ME is the urinary concentration of monoester per gram creatinine, CE is the creatinine excretion rate normalized by body weight, f is the ratio of urinary excretion to total elimination ([k.sub.u]/[k.sub.total]), and [MW.sub.d] and [MW.sub.m] are the molecular weights of the diesters and monoesters, respectively. We calculated the intake for each individual in the reference population using Equation 3. The presumed parent compound for urinary monoesters generally is the corresponding symmetrical diester. The presumed parent compound of urinary monobenzyl phthalate is n-butyl benzyl phthalate.

Published animal or human data for excretion of metabolites of di-n-butyl phthalate (2,3), n-butyl benzyl phthalate (4,5), di(2-ethylhexyl) phthalate (6,7), and di-n-octyl phthalate (8) allowed calculation of the fractional excretion values. The fraction of the dose of diethyl phthalate appearing as urinary monoethyl phthalate was assumed to be the same as for the di-n-butyl congener. The fraction of n-butyl benzyl phthalate excreted as benzyl phthalate was adjusted using the observation that this monoester accounts for 64% of the urinary metabolites (9). The fraction found for di(2-ethylhexyl) phthalate was used for dicyclohexyl and di-i-nonyl phthalates as well because specific fractions could not be found for these diesters. The fractional excretion values used in our calculations are shown in Table 1. Creatinine excretion was set to 23 and 18 mg/kg/day for men and women, respectively (10).

Table 1. Total fractional excretion (FE) and fractional urinary excretion of monoester (FU) during 24 hr after a single oral dose of diester.

Monoester      Diester              FE         FU

Ethyl          Diethyl            0.94(a)   0.52(a)
n-Butyl        Di-n-butyl         0.94(b)   0.52(c)
Benzyl         n-Butyl benzyl     0.70(d)   0.36(e)
Cyclohexyl     Dicyclohexyl       0.65(f)   0.069(f)
2-Ethylhexyl   Di(2-ethylhexyl)   0.65(g)   0.069(h)
n-Octyl        Di-n-octyl         0.65(f)   0.043(i)
i-Nonyl        Di-i-nonyl         0.65(f)   0.069(f)

(a) Assumed to be the same as-di-n-dibutyl phthalate.

(b) Data from Tanaka et al.(2).

(c) Data from Foster et al.(3)

(d) Data for the urinary fraction from Nativelle et al. (4); data for the fecal fraction from Eigenberg et al.

(e) Adjusted value from Nativelle et al. (4) for observed urine content (9).

(f) Assumed to be the same as di(2-ethylhexyl) phthalate.

(g) Data for the urinary fraction from Peck and Albro (7); data for the fecal fraction from Kluwe (6).

(h) Data from Peck and Albro (7).

(i) Data from Albro and Moore (8).

The NTP Center for the Evaluation of Risks to Human Reproduction (CERHR) conducts scientific evaluations of the literature on the reproductive and developmental toxicity of selected chemicals to which people are exposed. In Table 2, our intake estimates are compared to general population exposures estimated by the Phthalates Expert Panel of the CERHR based on published data. Because several phthalates have been shown to be developmental toxicants in laboratory studies, exposures of the 97 women of reproductive age (20-40 years) in the total sample and of the remaining individuals were calculated separately. These results are compared in Table 3. Excretion levels of the metabolites that were below the limit of detection (LOD; 1 ng analyte/mL urine) were assumed to be zero.

Table 2. Estimated exposures (mg/kg/day) to the general population based on extrapolated intake from urinary metabolites (Equation 1) in 289 individuals measured by Blount et al. (1).

                                                        95th
Monoester      Diester            Minimum   Median    percentile

Ethyl          Diethyl             <LOD      12           110
n-Butyl        Di-n-butyl          0.084     1.5          7.2
Benzyl         n-Butyl benzyl      0.094     0.88         4.0
Cyclohexyl     Dicyclohexyl        <LOD      0.026        0.25
2-Ethylhexyl   Di(2-ethylhexyl)    <LOD      0.71         3.6
n-Octyl        Di-n-octyl          <LOD      0.0096       0.96
i-Nonyl        Di-i-nonyl          <LOD      <LOD         1.7

Monoester      Diester            Maximum   CERHR(a)

Ethyl          Diethyl              320     NA
n-Butyl        Di-n-butyl           110     2-10(b)
Benzyl         n-Butyl benzyl        29     2(c)
Cyclohexyl     Dicyclohexyl         2.3     NA
2-Ethylhexyl   Di(2-ethylhexyl)      46     3-30
n-Octyl        Di-n-octyl            13     < 3- < 30(d)
i-Nonyl        Di-i-nonyl            22     < 3- < 30(d)

(a) The CERHR Phthalates Expert Panel held its third and final meeting on 12-13 July 2000 in Arlington, Virginia; the CERHR final reports on the seven phthalates evaluated, along with a full description of the center and its activities, are available on the CERHR web site (11).

(b) The upper bound for occupational exposure was estimated as 286 [micro]g/kg/day; the estimate of 2 [micro]g/kg/day is at the 84th percentile of our calculated values.

(c) The CERHR estimate for n-butyl benzyl phthalate is at the 11th percentile of our calculated values.

(d) Di-n-octyl and di-i-nonyl phthalate estimates from the CERHR were reported as less than for di(2-ethylhexyl) phthalate.

Table 3. Comparison of estimated exposures ([micro]g/kg/day) to 97 women aged 20-40 years to the rest of the population (192 individuals)(a) based on extrapolated intake from urinary metabolites (Equation 1) measured by Blount et al. (1).

Monoester      Diester            Minimum     Median

Ethyl          Diethyl            0.90(a)     13(a)
                                  < LOD       11
n-Butyl        Di-n-butyl         0.24(a)    1.7(a)
                                  0.084      1.4
Benzyl         n-Butyl benzyl     0.094(a)   1.2(a)
                                  0.11       0.78
Cyclohexyl     Dicyclohexyl       < LOD      0.051(a)
                                             0.012
2-Ethylhexyl   Di(2-ethylhexyl)   < LOD      0.71(a)
                                             0.71
n-Octyl        Di-n-octyl         < LOD      < LOD(a)
                                             0.015
i-Nonyl        Di-i-nonyl         < LOD      < LOD

                                    95th
Monoester      Diester            percentile   Maximum

Ethyl          Diethyl              90(a)      170(a)
                                   130         320
n-Butyl        Di-n-butyl           32(a)      113(a)
                                   6.5          50
Benzyl         n-Butyl benzyl      4.5(a)      7.8(a)
                                   3.4          29
Cyclohexyl     Dicyclohexyl        0.24(a)     0.45(a)
                                   0.25        2.3
2-Ethylhexyl   Di(2-ethylhexyl)    3.8(a)       10(a)
                                   3.5          46
n-Octyl        Di-n-octyl          0.65(a)     1.5(a)
                                   1.0          13
i-Nonyl        Di-i-nonyl          3.7(a)      7.8(a)
                                   1.4          22

(a) Values for women aged 20-40 years in boldface; remaining values are for the rest of population.

Uncertainties in the values of the parameters in Equations 1 and 2 are potential sources of error in our estimates of exposure. Creatinine excretion is known with 10% accuracy (10). FE is generally accurate to approximately 50% (4,5). FU can vary 15-fold among species, with humans in the middle of the range (12). This variability is an order of magnitude higher than the reproducibility of the same measurement among laboratories (4-6,12). Owing to the lack of human excretion data, fractional excretion values for the rat were used for some congeners. Therefore, our exposure estimates are probably reliable within an order of magnitude.

Exposures for the general population, estimated by the CERHR Phthalates Expert Panel from published data, are in good agreement with our calculated human daily intake estimates based on CDC median values and presented in Table 2. However, the maximal values of excreted monoesters (1) indicate that some individual exposures are substantially higher than previously estimated for the general population.

Women of reproductive age appear to be exposed to higher levels of di-n-butyl phthalate than are the remainder of the study population. This is particularly evident in the 95th percentile column for n-butyl phthalate in Table 2, where the estimated exposure values for women 20-40 years of age are approximately 5 times greater than the corresponding values for the other 192 individuals in the study.

The data reported by Blount et al. (1) will certainly lead to further efforts to derive accurate estimates of human exposures based on urinary metabolite levels. In addition, their data lead to several questions that should be addressed in the immediate future; for example:

* What are the sources and circumstances of exposure that result in a higher urinary level of diethyl phthalate metabolites than of the other six phthalates studied?

* What is the evidence for reproductive and developmental toxicity of diethyl phthalate?

* What are the sources and circumstances of exposure that result in some women of reproductive age having higher urinary levels of n-butyl phthalate than the remainder of the study population?

* At what levels are humans exposed to other phthalates not included in this study?

It is important that answers to these and related questions be pursued by public health agencies including the NIEHS/NTP.

REFERENCES AND NOTES

(1.) Blount BC, Silva MJ, Caudill SP, Needham LL, Pirkle JL, Sampson EJ, Lucier GW, Jackson RJ, Brock JW. Levels of seven urinary phthalate metabolites in a human reference population. Environ Health Perspect 108:979-982 (2000).

(2.) Tanaka A, Matsumoto A, Yamaha T. Biochemical studies on phthalic esters. III. Metabolism of dibutyl phthalate (DBP) in animals. Toxicology 9:109-123 (1978).

(3.) Foster PMD, Cook MW, Thomas LV, Walters DC, Gangolli SD. Differences in urinary metabolic profile from di-n-butyl phthalate-treated rats and hamsters. A possible explanation for species differences in susceptibility to testicular atrophy. Drug Metab. Dispos 11:59-61 (1983).

(4.) Nativelle C, Picard K, Valentin I, Lhugenot JC, Chagnon MC. Metabolism of n-butyl benzyl phthalate in the female Wistar rat. Identification of new metabolites. Food Chem Toxicol 37:905-917 (1999).

(5.) Eigenberg DA, Bozigian HP, Carter DE. Distribution, excretion, and metabolism of butylbenzyl phthalate in the rat. J Toxicol Environ Health 17:445-456 (1986).

(6.) Kluwe WM. Overview of phthalate ester pharmacokinetics in mammalian species. Environ Health Perspect 45:3-9 (1982).

(7.) Peck CC, Albro PW. Toxic potential of the plasticizer di(2-ethylhexyl) phthalate in the context of its disposition and metabolism in primates and man. Environ Health Perspect 45:11-17 (1982).

(8.) Albro PW, Moore B. Identification of the metabolites of simple phthalate diesters in rat urine. J Chromatog 94:209-218 (1974).

(9.) Castle L. Personal communication.

(10.) Harper HA, Rodwell VW, Mayes PA. Review of Physiological Chemistry. Los Altos, CA:Lange Medical Publications, 1977.

(11.) The National Toxicology Program (NTP) Center for the Evaluation of Risks to Human Reproduction (CERHR). Available: http://cerhr.niehs.nih.gov [cited 14 September 2000].

(12.) Albro PW, Corbett JT, Schroeder JL, Jordan S, Matthews HB. Pharmacokinetics, interactions with macromolecules and species differences in metabolism of DEHP. Environ Health Perspect 45:19-25 (1982).

Michael C. Kohn
Frederick Parham
Scott A. Masten
Christopher J. Portier
Michael D. Shelby
Environmental Toxicology Program
National Institute of Environmental
Health Sciences
Research Triangle Park, North Carolina
E-mail: kohn@niehs.nih.gov

John W. Brock
Larry L. Needham
National Center for Environmental Health
Centers for Disease Control and Prevention
Atlanta, Georgia

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