Toluene in blood as a marker of choice for low-level exposure to toluene

Toluene concentrations in the blood and risk of thyroid cancer among residents living near national industrial complexes in South Korea: A population-based cohort study

BACKGROUND

Toluene is classified as a possible carcinogen, but its role on thyroid cancer is not well established. Vehicle emissions are one of the largest contributed sources of toluene, but no studies evaluating the influence of living near a road on the association between toluene and the incidence of thyroid cancer have been reported. Therefore, we examined potential associations between blood toluene concentrations and incidence risk of thyroid cancer, and an effect modification of living near a road.

METHODS

We conducted a prospective cohort study using data from South Korean "Monitoring Project for Exposure to Environmental Pollutants and Health Effects among Residents Living near Industrial Complexes" survey. Study participants living near national industrial complexes were recruited from January 2003 to 2011. Incidence and mortality cases of thyroid cancer (C73, ICD-10 code) were identified using the National Cancer Registry and Statistics Korea, respectively. Blood toluene concentrations were measured using gas chromatography mass spectrometry. We used Cox proportional hazards regression models to estimate the hazard ratios (HR) and the 95% confidence interval (CI) between blood toluene concentrations and thyroid cancer risk.

RESULTS

During the follow-up (median 8.6 years), 33 cases of thyroid cancer were diagnosed. The geometric mean of the toluene concentration in the blood was 0.56 μg/L for cases and 0.29 μg/L for non-cases. After adjusting for potential confounders, a positive association between blood toluene concentrations and thyroid cancer was found (HR = 2.77, 95% CI = 1.00-7.65 in the highest tertile vs. the lowest tertile, p for trend = 0.044). This positive association was stronger in people living near a road (≤50 m).

CONCLUSIONS

Blood toluene concentrations may be positively associated with the incidence risk of thyroid cancer. Moreover, this association may be stronger among people living near a road.

Estimation of biological occupational exposure limit values for selected organic solvents from logartihm of octarol water partition coefficient

OBJECTIVES

For several organic solvents (solvents in short), biological occupational exposure limits (BOELs) have been established for un-metabolized solvents in urine, based on the solvent exposure-urinary excretion relationship. This study was initiated to investigate the possibiliy of estimating a BOEL from the Pow (the partition coefficient between n-octyl alcohol and water), a physico-chemical parameter.

METHODS

Data were available in the literatures for exposure-excretion relationship with regard to 10 solvents for men and 7 solvents for women.

RESULTS

Statistical analysis revealed that the slopes (after correction for molecular weights and logarithmic conversion) of the exposure-excretion regression lines linearly correlated (p<0.01) with the log Pow values the respective solvents. No significant difference (p>0.05) was observed between men and women, and it was acceptable to combine the data for the two sexes. Thus the log Pow-log slope relation was represented by a single equation for both sexes. Based on the observations, procedures were established to estimate BOEL values from Pow. Successful estimations of BOELs for styrene, tetrahydrofuran and m-xylene (a representative of xylene isomers) were calculated as examples.

CONCLUSIONS

The present study proposed promising procedures for estimation of a BOEL from the Pow.

Biomarkers of toluene exposure in rats: mercapturic acids versus traditional indicators (urinary hippuric acid and o-cresol and blood toluene)

1. Toluene (TOL) is a neurotoxic, ototoxic and reprotoxic solvent which is metabolized via the glutathione pathway, producing benzylmercapturic, o-, m- and p-toluylmercapturic acids (MAs). These metabolites could be useful as biomarkers of TOL exposure. 2. The aims of this study were (1) to provide data on MAs excretion in rat urine following TOL exposure by inhalation, (2) to compare them to data from traditional TOL biomarkers, i.e. TOL in blood (Tol-B), and urinary hippuric acid (HA) and o-cresol (oCre) and (3) to establish a relationship between these different indicators and the airborne TOL concentration (Tol-A). 3. Sprague-Dawley rats were exposed to a range of TOL concentrations. Blood and urine were collected and analyzed to determine biomarker levels. 4. Levels of the four MAs correlate strongly with Tol-A (comparable to the correlation with Tol-B). 5. MAs are thus clearly superior to oCre and HA as potential markers of exposure to TOL.

Validation of urine density correction in cases of hippuric acid and un-metabolized toluene in urine of workers exposed to toluene

To investigate if it is appropriate to apply urine density correction when a urine sample is dense or dilute. Data on hippuric acid (HA-U), toluene (Tol-U), creatinine (CR) and specific gravity (SG) in end-of-shift urine samples and exposure to air-borne toluene were cited from previous publications. In practice, 837 cases were available, and they were classified into dense, intermediate and dilute groups taking 0.3 and 3.0 g/l of CR and 1.010 and 1.030 of SG as cut-off points. Lines of regression of HA-U and Tol-U (as observed, CR-corrected or SG-corrected) with air-borne toluene were calculated for each density groups, and correlation coefficients (CCs) were compared. The dense groups gave CCs similar to those of the intermediate groups. Dilute versus intermediate group comparison also gave promising results. These conclusions were however based primarily on the findings with observed values, because the numbers of cases in the dilute or dense group were limited when CR- or SG-correction was applied. Literature survey showed that urine density correction does not always improve the correlation between solvents in air and exposure makers in urine. It was concluded that no correction for urine density may be necessary in evaluating HA-U and Tol-U in dense (and probably also dilute) urine samples as markers of occupational toluene exposure. Just in case when correction for urine density is desired for any reason, SG-correction may be recommended.

The effect of workload on biological monitoring of occupational exposure to toluene and n-Hexane: contribution of physiologically based toxicokinetic modeling

A physiologically based toxicokinetic model was used to examine the impact of work load on the relationship between the airborne concentrations and exposure indicator levels of two industrial solvents, toluene and n-Hexane. The authors simulated occupational exposure (8 hr/day, 5 days/week) at different concentrations, notably 20 ppm and 50 ppm, which are the current threshold limit values recommended by ACGIH for toluene and n-hexane, respectively. Different levels of physical activity, namely, rest, 25 W, and 50 W (for 12 hr followed by 12 hr at rest) were simulated to assess the impact of work load on the recommended biological exposure indices: toluene in blood prior to the last shift of the workweek, urinary o-cresol (a metabolite of toluene) at the end of the shift, and free (nonhydrolyzed) 2,5-hexanedione (a metabolite of n-hexane) at the end of the shift at the end of the workweek. In addition, urinary excretion of unchanged toluene was simulated. The predicted biological concentrations were compared with the results of both experimental studies among human volunteers and field studies among workers. The highest predicted increase with physical exercise was noted for toluene in blood (39 microg/L at 50 W vs. 14 microg/L at rest for 20 ppm, i.e., a 2.8-fold increase). The end-of-shift urinary concentrations of o-cresol and toluene were two times higher at 50 W than at rest (for 20 ppm, 0.65 vs. 0.33 mg/L for o-cresol and 43 vs. 21 microg/L for toluene). Urinary 2,5-hexanedione predicted for 50 ppm was 1.07 mg/L at 50 W and 0.92 mg/L at rest (+16%). The simulations that best describe the concentrations among workers exposed to toluene are those corresponding to 25 W or less. In conclusion, toxicokinetic modeling confirms the significant impact of work load on toluene exposure indicators, whereas only a very slight effect is noted on n-hexane kinetics. These results highlight the necessity of taking work load into account in risk assessment relative to toluene exposure.

Limited validity of o-cresol and benzylmercapturic acid in urine as biomarkers of occupational exposure to toluene at low levels

This study was initiated to evaluate o-cresol and benzylmercapturic acid in urine in comparison with other biomarkers, as tools to estimate the intensity of occupational exposure to toluene at low levels. In total, 108 solvent exposed workers (engaged in tape production) and 17 non-exposed controls (all men) participated in the study. The surveys were conducted in the second half of working weeks. Diffusive sampling was conducted to measure 8-h time-weighted average intensity of occupational exposure to toluene. Blood and urine samples were collected at the end of a working shift. Blood samples were subjected to analysis for toluene (Tol-B), and urine samples were analyzed for benzyl alcohol (BeOH-U), benzylmercapturic acid (BMA-U), o-cresol (o-CR-U), hippuric acid (HA-U) and toluene (Tol-U) by the methods previously described. The toluene concentrations in workplaces were low in general, with a geometric mean (GM) and the maximum concentration of 1.9 ppm and 8.8 ppm, respectively. The statistical analyses of the six biomarkers for correlation with air-borne toluene showed that both Tol-B and Tol-U gave a high correlation coefficient of 0.58 to 0.61 (p<0.01), whereas the coefficients for BeOH-U and BMA-U together with HA-U were all low (up to 0.22, depending on the correction for urine density) and statistically insignificant (p>0.10) in most cases. o-CR-U had an intermediary coefficient of 0.20 (p<0.05). Comparison with previous publications disclosed that BeOH-U, BMA-U and HA-U correlate with toluene in air when the exposure is intense (e.g., 50 ppm or above), but no longer proportional to air-borne toluene when the exposure is low, e.g., 2 ppm. Such appeared to be also the case for o-CR-U. In over-all evaluation, the validity of o-CR-U in monitoring occupational exposure to toluene at low levels (e.g., 2 ppm) appear to be limited, and BMA-U is not an appropriate biomarker. BeOH-U and HA-U are also inappropriate for this purpose. Only Tol-B and Tol-U may be employed to estimate toluene exposure at low levels.

Excretion of unchanged volatile organic compounds (toluene, ethylbenzene, xylene and mesitylene) in urine as result of experimental human volunteer exposure

OBJECTIVES

To investigate elimination of unchanged volatile organic compounds (VOC's) through urine and the use of respective data for occupational exposure assessment, six volunteers were exposed under controlled conditions to toluene (TOL), ethylbenzene (EB), xylene (XYL) and mesitylene (MES) at concentrations ranging from 20 to 200 mg/m(3). The study was to elicit the toxicokinetic data and compare the precision of VOC's exposure assessment based on determining unchanged compounds in blood, urine and their metabolites in urine.

METHODS

During and after exposure blood and urine samples were analysed by gas chromatography using the headspace and SPME headspace technique

RESULTS

The kinetics of VOC's elimination in urine complied with an open two-compartment model. The (half-time) T (1/2 )values varied from 0.45 to 0.88 h for phase I and from 6.7 to 19.2 h for phase II. The precision of the method for unchanged VOC's was similar to that based on unchanged compounds in blood and better than their main metabolites in urine.

CONCLUSION

The obtained result indicate that determining unchanged VOC's in urine can be used as an exposure test even in the ranges of VOC's in the air that are much lower than the current TWA for occupational exposure.

Unmetabolized VOCs in Urine as Biomarkers of Low Level Exposure in Indoor Environments

This study aimed to test the possible use of unmetabolized volatile organic compounds (VOCs) in urine as biomarkers of low-level indoor environmental exposure. Twenty-four subjects in 13 dwellings in a prefecture of Japan participated in this study. Air samples of the breathing zone were collected in the living room and bedroom, along with spot urine samples (before bedtime and first morning voids). Toluene, ethylbenzene, xylene isomers, styrene and p-dichlorobenzene in the air and urine samples were measured by gas chromatography/mass spectrometry. For the 21 subjects without solvent exposure at work, there were significant correlations between the time-weighted average air concentrations in the bedroom and morning urinary concentrations for toluene, o-xylene, total xylene and p-dichlorobenzene (correlation coefficients of 0.54, 0.61, 0.56 and 0.84, respectively). Multiple linear regression analysis showed only air VOCs in the bedroom influenced the morning urinary VOC concentrations. We concluded that unmetabolized VOCs in the urine can provide a reliable biological indicator for air VOC exposures in non-occupational environments.

Biological monitoring of exposure: trends and key developments

The concept of biological monitoring (BM) has gained the special interest of individual scientists and international organizations. Today, when analytical problems have almost ceased due to new laboratory techniques and quality assurance systems, the methods for interpretation of results have become the most important issue. There are important discrepancies regarding the role of biological monitoring of occupational exposure between Europe and the United States. BM has been an important tool of medical health surveillance in the European countries. In the United States it belongs rather to the field of occupational hygiene. It seems that both the approaches can be accepted. More attention should be paid to the development of the truly health-based biomarkers of exposure based on the dose-effect and dose-response relationships. New areas of application of BM of occupational exposure include determination of DNA and protein adducts, unchanged volatile organic compounds in urine, monitoring of exposure to pesticides, antineoplastic drugs, hard metals, and polycyclic aromatic hydrocarbons. In the general environment BM is the most valuable tool for acquiring knowledge of current levels of internal exposure to xenobiotics, identifying the hot spots and developments in trends of exposure. BM can provide policy makers with more accurate information on the control measures undertaken. At present, the main areas include heavy metals, persistent organic pollutants and pesticides. BM of chemical exposure has become increasingly important in the assessment of the health risk in occupational and environmental medicine. Therefore it would be worthwhile to include BM in the curricula for the training of occupational hygienists.

A comparison of 1H8- and 2H8-toluene toxicokinetics in men

1. To examine the bioequivalence of an isotope-labelled tracer to study toxicant disposition, we conducted 33 controlled human exposures to a mixture of 50 ppm 1H8-toluene and 50 ppm 2H8-toluene for 2 h, and measured concentrations in blood and breath, and metabolite levels in urine for 100 h post-exposure. 2. A physiologically based kinetic (PBK) model found that compared with 1H8-toluene, 2H8-toluene had a 6.4+/-13% (mean+/-SD) lower AUC, a 6.5+/-13% higher systemic clearance (1.46+/-0.27 versus 1.38+/-0.25 l/h-kg), a 17+/-22% larger terminal volume of distribution (66.4+/-14 versus 57.2+/-10 l/kg) and a 9.7+/-26% longer terminal half-life (38+/-12 versus 34+/-10 h) (p < 0.05 for all comparisons). 3. The higher 2H8-toluene clearance may have been due to an increased rate of ring oxidation, consistent with the 17% higher observed fraction of 2H5- versus 1H5-cresol metabolites in urine. 4. The larger terminal volume and half-lives for 2H8-toluene suggested a higher adipose tissue/blood partition coefficient. 5. Observed isotope differences were small compared with interindividual differences in 1H8-toluene kinetics from previous studies. 6. The PBK model allowed us to ascribe observed isotope differences in solvent toxicokinetics to underlying physiologic mechanisms.

The urinary excretion of solvents and gases for the biological monitoring of occupational exposure: a review

'In the field' application of the measurement of urinary excretion of unmodified solvent for the biological monitoring of exposed workers has been investigated in many recent papers. The results obtained for several solvents are reviewed. The values of correlation coefficients (r) and regression lines obtained for benzene, toluene, xylene, styrene, n-hexane, cyclohexane, 2- and 3-methylpentane, methyl chloride, tetrachloroethylene, carbon tetrachloride, methyl chloroform, p-dichlorobenzene, nitrous oxide, halothane, isoflurane, enflurane, acetone, methyl ethyl ketone and methyl isobutyl ketone are presented. The correlations observed were generally good: r values range from 0.50-0.97, and the majority are between 0.84 and 0.90. The regression lines reported for the same solvent in different studies present some variability: this is possibly due to an inadequate control of factors influencing the relationship between external dose and absorption, such as differences in body burden, work load, individual characteristics, etc. These factors are discussed. As a whole, results reported in the literature show that measuring of urinary excretion of unmodified solvents provides a highly sensitive and specific exposure index, and can also be applied for the biological monitoring of occupational exposure to low levels of solvents or to solvent mixtures. Nevertheless, for an adequate assessment of biological limit values, further studies evaluating the reproducibility of regression lines are needed, given that the aspects influencing the correlation between external dose and urinary excretion are fully controlled. Another crucial aspect is the correlation with early effects: even though this has yet to be evaluated for several solvents, for others such as styrene and perchloroethylene a good correlation was obtained, further supporting the usefulness of the measurement of urinary excretion of solvent for the biological monitoring of occupational exposure.

The central nervous system and exposure to toluene: a risk characterization

The principal health outcome of exposure to toluene is dysfunction of the central nervous system. Effects range from fatalities and severe neurological disorders in toluene abuse situations to deficits in neurobehavioral function in occupational populations. An Inhalation Reference Concentration (RfC) of 0.4 mg toluene/m3 or 0.1 ppm was developed by the U.S. EPA to protect general populations chronically exposed to toluene. The RfC was derived from results of an occupational study involving Asian workers who developed neurobehavioral deficits at a mean toluene exposure level at the time of the study of 88 ppm. The derivation incorporated several uncertainty factors, one of which was a factor of 10 to account for sensitive subpopulations. Recent evidence indicates that some Japanese and possibly other Asian populations harbor a defective gene for aldehyde dehydrogenase, and thus exhibit a decreased rate of toluene metabolism. Although it is not known if reduced metabolism by aldehyde dehydrogenase also was a factor in the occupational study, preshift blood levels of toluene were considerably higher than preshift levels from non-Asian workers exposed to similar air levels of toluene. The elevated blood levels are consistent with defective metabolism but remain to be confirmed. Inasmuch as air levels of toluene in urban environments are about 10-fold lower than the RfC, an adequate measure of protection is afforded by the RfC with or without an uncertainty factor for sensitive subgroups. However, the uncertainty factor for sensitive subgroups should be retained because there is no information regarding toluene metabolism in children.

Simultaneous spectrophotometric determination of o-cresol and m-cresol in urine by use of the kinetic wavelength-pair method

The kinetic wavelength-pair method was applied to the simultaneous determination of o-cresol and m-cresol, based on their oxidative coupling with aniline in the presence of hypochlorite as oxidant and nitroprusside as catalyst. The benzoquinoneanils produced exhibited severe spectral overlap. Resolution of this isomer mixture by using discrete wavelengths was subject to a high degree of error for the determination of m-cresol since the ratios vo-cresol/vm-cresol (where v denotes initial rate for the oxidative coupling reaction) and epsilon o-cresol/epsilon m-cresol (where epsilon is the absorptivity of reaction products) were greater than unity at any given wavelength. Selectivity in the resolution was achieved by measuring the initial rate difference at the wavelength pair 666-566 nm where the contribution of o-cresol was removed, o-Cresol and m-cresol were simultaneously determined at mass ratios between 5:1 and 1:5 at concentrations from 1 to 5 micrograms ml-1, with relative standard deviations of less than 3%. The proposed method was applied to the determination of o-cresol and m-cresol in urine samples, with analytical recoveries ranging between 95 and 105%.

Toluene itself as the best urinary marker of toluene exposure

Head-space gas chromatography (GC) and high-performance liquid chromatography (HPLC) (with fluorescence detectors) methods were developed for toluene (TOL-U) and o-cresol (CR-U) in urine, respectively. In order to identify the most sensitive urinary indicator of occupational exposure to toluene vapor (TOL-A) among TOL-U, CR-U, and hippuric acid in urine (HA-U), the two methods together with an HPLC (with untraviolet detectors) method for determination of HA-U were applied in the analysis of end-of-shift urine samples from 115 solvent-exposed workers (exposed to toluene at 4 ppm as geometric mean). Regression analysis showed that TOL-U correlated with TOL-A with a significantly higher correlation coefficient than did HA-U or CR-U. With regard to the TOL-A concentrations at which the exposed subjects could be separated from the nonexposed by the analyte, TOL-U achieved separation at < 10 ppm TOL-A, whereas both HA-U and CR-U did so only when TOL-A was 30 ppm or even higher. The ratio of the analyte concentrations at 50 ppm TOL-A to those at 0 ppm TOL-A was also highest for TOL-U. Overall, the results suggest that TOL-U is a better marker of exposure to toluene vapor than HA-U or CR-U.