Cause specific mortality and cancer incidence among employees exposed to 2,3,7,8-TCDD after a 1953 reactor accident

Application of qualitative and quantitative uncertainty assessment tools in developing ranges of plausible toxicity values for 2,3,7,8-tetrachlorodibenzo-p-dioxin

Increasing interest in characterizing risk assessment uncertainty is highlighted by recent recommendations from the National Academy of Sciences. In this paper we demonstrate the utility of applying qualitative and quantitative methods for assessing uncertainty to enhance risk-based decision-making for 2,3,7,8-tetrachlorodibenzo-p-dioxin. The approach involved deconstructing the reference dose (RfD) via evaluation of the different assumptions, options, models and methods associated with derivation of the value, culminating in the development of a plausible range of potential values based on such areas of uncertainty. The results demonstrate that overall RfD uncertainty was high based on limitations in the process for selection (e.g., compliance with inclusion criteria related to internal validity of the co-critical studies, consistency with other studies), external validity (e.g., generalizing findings of acute, high-dose exposure scenarios to the general population), and selection and classification of the point of departure using data from the individual studies (e.g., lack of statistical and clinical significance). Building on sensitivity analyses conducted by the US Environmental Protection Agency in 2012, the resulting estimates of RfD values that account for the uncertainties ranged from ~1.5 to 179 pg/kg/day. It is anticipated that the range of RfDs presented herein, along with the characterization of uncertainties, will improve risk assessments of dioxins and provide important information to risk managers, because reliance on a single toxicity value limits the information needed for making decisions and gives a false sense of precision and accuracy.

Age-dependent human elimination half-lives of dioxin-like polychlorinated biphenyls derived from biomonitoring data in the general population

Constant elimination half-life is usually used in first-order one-compartment pharmacokinetic models to assess human exposure to dioxin-like polychlorinated biphenyls (PCBs). However, hepatic clearance rates are usually lower for elders than for young people. In this study, levels of 12 dioxin-like PCBs were determined in blood samples collected in 2012 from 305 individuals from the general population. We estimated the historical daily intake of dioxin-like PCBs for individuals with a validated annual exponential decay function. Based on the blood levels and historical daily intake levels, and using the maximum likelihood estimation by running the first-order one-compartment pharmacokinetic model, elimination rates were optimized as exponential functions of age for PCB118, PCB157, PCB189 and PCB126; linear functions of age for PCB114, PCB123, PCB167 and PCB169; and constant functions of age for PCB105 and PCB156. With the exception of PCB105 and PCB156, half-lives generally increased with age for individuals from 24 to 50 years old: from 0.805 to 1.95 years for PCB189, 2.08-4.54 years for PCB157, 3.32-5.58 years for PCB126, 3.52-6.81 years for PCB123, 5.24-12.29 years for PCB169, 6.60-14.40 years for PCB114, 7.50-14.01 years for PCB118, and 9.97-21.97 years for PCB167. The half-lives of PCB105 (5.79 years) and PCB156 (15.1 years) were independent of age. Our research for the first time clarified the effects of age on the elimination rate of dioxin-like PCBs in individuals from the general population, thus reducing uncertainty in future health risk assessments.

Uncertainty analysis in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) cancer dose-response for three occupational cohorts

While the U.S. EPA has issued a draft report with a 1% TCDD effective dose (ED01) of 87.9pg/kg/day based on continuous integration of key scientific evidence, a detailed and comprehensive uncertainty analysis has not been well documented. In this study, a new estimate for ED01 was derived based on uncertainty analysis by quantitatively assessing the potential bias arising from the selection of kinetic models, dose-response models and cohorts. The cumulative serum lipid concentration (CSLC) and cumulative body burden (CBB) were reconstructed as dose metrics using a concentration- and age-dependent pharmacokinetic model (CADM), physiologically based pharmacokinetic model (PBPK), and age-dependent half-life model (FV), and the reconstructed dose metrics based on CADM and PBPK were generally higher than those based on the FV model. Three dose-response curves (linear, multiplicative and power) were used to link dose metrics and cancer risk to estimate ED01, and the linear model resulted in the lowest ED01, followed by the power model and multiplicative model, for the same cohort. Meanwhile, ED01 based on the CADM model was the highest, followed by those based on the PBPK model and first-order model. Finally, the ED01 was estimated to be 17.03±7.83pg/kg/day by statistically analyzing the distribution of ED01 values based on various kinetic models, cohorts and dose-response models. The study presented here strengthens the scientific basis for understanding the potential health implications of TCDD exposure.

Uncertainties in human health risk assessment of environmental contaminants: A review and perspective

Addressing uncertainties in human health risk assessment is a critical issue when evaluating the effects of contaminants on public health. A range of uncertainties exist through the source-to-outcome continuum, including exposure assessment, hazard and risk characterisation. While various strategies have been applied to characterising uncertainty, classical approaches largely rely on how to maximise the available resources. Expert judgement, defaults and tools for characterising quantitative uncertainty attempt to fill the gap between data and regulation requirements. The experiences of researching 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) illustrated uncertainty sources and how to maximise available information to determine uncertainties, and thereby provide an 'adequate' protection to contaminant exposure. As regulatory requirements and recurring issues increase, the assessment of complex scenarios involving a large number of chemicals requires more sophisticated tools. Recent advances in exposure and toxicology science provide a large data set for environmental contaminants and public health. In particular, biomonitoring information, in vitro data streams and computational toxicology are the crucial factors in the NexGen risk assessment, as well as uncertainties minimisation. Although in this review we cannot yet predict how the exposure science and modern toxicology will develop in the long-term, current techniques from emerging science can be integrated to improve decision-making.

Integrated ecological risk assessment of dioxin compounds

Current ecological risk assessment (ERA) schemes focus mainly on bioaccumulation and toxicity of pollutants in individual organisms. Ecological models are tools mainly used to assess ecological risks of pollutants to ecosystems, communities, and populations. Their main advantage is the relatively direct integration of the species sensitivity to organic pollutants, the fate and mechanism of action in the environment of toxicants, and life-history features of the individual organism of concern. To promote scientific consensus on ERA schemes, this review is intended to provide a guideline on short-term ERA involving dioxin chemicals and to identify key findings for exposure assessment based on policies of different agencies. It also presents possible adverse effects of dioxins on ecosystems, toxicity equivalence methodology, environmental fate and transport modeling, and development of stressor-response profiles for dioxin-like chemicals.

Serum metabolomic pertubations among workers exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been associated with multiple health effects. Mechanistic studies using metabolomics could provide supporting evidence for such associations by identifying relevant biological pathways. In this study, we investigated metabolic perturbations in a cohort of TCDD exposed workers to better understand TCDD related health effects. Eighty one workers who had been exposed to TCDD in the past and 63 nonexposed workers were included in the study. Serum metabolites were detected using ultra high pressure liquid chromatography coupled online to a Q-TOF Premier mass spectrometer with a scan range of 70-1,000 m/z. Current plasma levels of TCDD were determined by high-resolution gas chromatography/isotope dilution high resolution mass spectrometry. TCDD blood levels at the time of last exposure were estimated using a one-compartment first order kinetic model. Differentially expressed metabolites were identified using linear regression models, partial least squares regression (PLSr) and a regression-based Bayesian variable selection approach. Features that were present in all quality control samples and had a coefficient of variation <30% were included in the analyses (n = 421 features). Adjusted linear regression analysis showed several significant perturbations (n = 27; P < 0.05) but these observations did not survive multiple testing correction (q value > 0.05). PLSr analyses and Bayesian variable selection regression analyses revealed no obvious metabolic perturbations associated with TCDD levels. This is the first metabolomic analysis related to TCDD exposure in humans. No significant metabolic features were identified. It is concluded that TCDD exposure at levels present in this study does not lead to significant perturbations of the serum metabolome.

Biomonitoring-based environmental public health indicators

This chapter discusses the use ofbiomonitoring-based indicators of exposure to environmental pollutants in environmental health information systems. Matrices for biomonitoring, organization and standardization of surveillance programs, the use of intake and body burden data, and the interpretation of surveillance data are discussed. The concept of environmental public health indicators is demonstrated using the "Persistent organic pollutants in human milk" indicator implemented in the Environment and Health Information System (ENHIS) of the WHO Regional Office for Europe. This indicator is based on the data from the WHO-coordinated surveillance of persistent organic pollutants in human milk as well as data from selected national studies. The WHO survey data demonstrate a steady decline in breast milk concentrations of dioxins across Europe. The data from biomonitoring surveys in Sweden also show a steady decline of breast milk concentrations of most persistent organic pollutants since 1970s with the exception of polybrominated diphenyl ethers (PBDEs) which increased rapidly until the late 1990s and then started to decline after the implementation of policy measures aiming at reducing exposures. The application of human biomonitoring data in support of environmental public health policy actions requires carefully designed standardized and sustainable surveillance, comprehensive interpretation of the data, and an effective communication strategy based on credible information presented in the form of indicator factsheets.

Interpreting PCB levels in breast milk using a physiologically based pharmacokinetic model to reconstruct the dynamic exposure of Italian women

Polychlorinated biphenyls (PCBs) are persistent contaminants suspected to cause adverse health effects in humans. As PCBs levels in food have not been monitored frequently in the past, modeling approaches based on environmental data have been proposed to predict the human dietary intake. In this work, we propose to improve these approaches by taking into account internal levels of PCBs in humans. This methodology is based on the analysis of biomonitoring data using exposure and physiologically based pharmacokinetic (PBPK) modeling to determine the most probable scenario of exposure. Breast milk concentrations were measured in Italian women for PCB-138, PCB-153 and PCB-180. For each congener, three exposure scenarios were derived and a PBPK model was used to relate the lifetime exposure to the breast milk levels. For the three PCBs, we determined the most probable scenario of exposure. Our results support the adequacy of the exposure and the PBPK models for PCB-180 and PCB-153, whereas we observed discrepancies between the models and the biomonitoring data for PCB-138. Our intake estimates are in good agreement with previous exposure assessments based solely on food contamination demonstrating the relevance of our approach to reconstruct accurately the exposure and to fill in data gaps on exposure.

Is the fear of dioxin cancer more harmful than dioxin?

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a proven animal carcinogen. Occupational cohorts with the highest exposures imply that there is a small risk of all cancers combined, but it is difficult to pinpoint the confounding effect of the main chemicals. Studies after major accidents do not unequivocally confirm this risk. The risks to populations at the current dioxin levels seem trivial if present at all. There is increasing evidence that the aryl hydrocarbon receptor (AhR), i.e. the so called "dioxin receptor", is a physiological transcription factor exerting important functions in the body. Consequently a certain level of AhR activation may be beneficial rather than harmful. This challenges the wisdom of excessive regulation of dioxin levels in certain foods and nutrients. This could pose indirect nutritional risks, in fact being more harmful than even the worst case predictions of the putative cancer risks attributable to dioxins.

The role of exposure reconstruction in occupational human health risk assessment: current methods and a recommended framework

Exposure reconstruction for substances of interest to human health is a process that has been used, with various levels of sophistication, as far back as the 1930s. The importance of robust and high-quality exposure reconstruction has been recognized by many researchers. It has been noted that misclassification of reconstructed exposures is relatively common and can result in potentially significant effects on the conclusions of a human health risk assessment or epidemiology study. In this analysis, a review of the key exposure reconstruction approaches described in over 400 papers in the peer-reviewed literature is presented. These approaches have been critically evaluated and classified according to quantitative, semiquantitative, and qualitative approaches. Our analysis indicates that much can still be done to improve the overall quality and consistency of exposure reconstructions and that a systematic framework would help to standardize the exposure reconstruction process in the future. The seven recommended steps in the exposure reconstruction process include identifying the goals of the reconstruction, organizing and ranking the available data, identifying key data gaps, selecting the best information sources and methodology for the reconstruction, incorporating probabilistic methods into the reconstruction, conducting an uncertainty analysis, and validating the results of the reconstruction. Influential emerging techniques, such as Bayesian data analysis, are highlighted. Important issues that will likely influence the conduct of exposure reconstruction into the future include improving statistical analysis methods, addressing the issue of chemical mixtures, evaluating aggregate exposures, and ensuring transparency with respect to variability and uncertainty in the reconstruction effort.

TCDD exposure estimation for workers at a New Zealand 2,4,5-T manufacturing facility based on serum sampling data

Employment in the manufacture of the herbicide 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) is associated with potential exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and elevated serum lipid TCDD concentrations can be measured in workers for decades after terminated occupational exposure. As part of an epidemiological study of 1599 workers employed at a facility in New Plymouth, New Zealand that manufactured 2,4,5-T, serum TCDD concentrations measured in blood samples from 346 workers were used with work history records and a simple pharmacokinetic model in a linear regression to estimate dose rates associated with specific job exposure groups at the facility. The model was used to estimate serum TCDD concentration profiles over time for each individual in the full study group and accounted for 30% of the observed variance in TCDD concentrations in the serum donor subgroup. The model underestimated measured concentrations substantially for eleven individuals in the study group; examination of questionnaire data revealed a variety of activities apart from routine employment at the facility that may have contributed to the measured serum TCDD concentrations. Estimated serum TCDD concentrations were below 300 p.p.t. for all individuals in the cohort over the entire study time period, lower than estimates for other 2,4,5-T worker populations. This finding is consistent with occupational medicine records, which indicated that no cases of chloracne were ever diagnosed among workers employed on the site. The modeled exposures will be used in an evaluation of mortality patterns of workers at this facility.

Interpretation of studies on the developmental reproductive toxicology of 2,3,7,8-tetrachlorodibenzo-p-dioxin in male offspring

There have been several studies on the maternal administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and effects in the reproductive tract of male offspring, subsequent to risk assessments undertaken in 2001. This review compares the methodology and results to examine key methodological features, and consistency in reported outcomes. Maternal dosing at >0.8 microg TCDD/kg causes lethality and weight loss, and it is difficult to distinguish between direct and indirect effects of TCDD at these dose levels. Statistically significant effects of maternal doses of <1 microg TCDD/kg (i.e. the dose levels relevant for risk assessment) on prostate weight or epididymal sperm counts in offspring were reported in the minority of studies. The pharmacokinetics of TCDD differs considerably between acute and chronic dosing, and with dose level of TCDD. On the basis of body burden, TCDD had different potency at inducing adverse effects in the only comparison study between acute and chronic dosing. Understanding of the pharmacokinetics of TCDD and relationship to adverse effects in offspring is required. These analyses identify key features of TCDD developmental toxicity in male offspring, and identify data needs for future risk assessment.

Cancer incidence in the population exposed to dioxin after the "Seveso accident": twenty years of follow-up

BACKGROUND

The Seveso, Italy accident in 1976 caused the contamination of a large population by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Possible long-term effects have been examined through mortality and cancer incidence studies. We have updated the cancer incidence study which now covers the period 1977-96.

METHODS

The study population includes subjects resident at the time of the accident in three contaminated zones with decreasing TCDD soil levels (zone A, very high; zone B, high; zone R, low) and in a surrounding non-contaminated reference territory. Gender-, age-, and period-adjusted rate ratios (RR) and 95% confidence intervals (95% CI) were calculated by using Poisson regression for subjects aged 0-74 years.

RESULTS

All cancer incidence did not differ from expectations in any of the contaminated zones. An excess of lymphatic and hematopoietic tissue neoplasms was observed in zones A (four cases; RR, 1.39; 95% CI, 0.52-3.71) and B (29 cases; RR, 1.56; 95% CI, 1.07-2.27) consistent with the findings of the concurrent mortality study. An increased risk of breast cancer was detected in zone A females after 15 years since the accident (five cases, RR, 2.57; 95% CI, 1.07-6.20). No cases of soft tissue sarcomas occurred in the most exposed zones (A and B, 1.17 expected). No cancer cases were observed among subjects diagnosed with chloracne early after the accident.

CONCLUSION

The extension of the Seveso cancer incidence study confirmed an excess risk of lymphatic and hematopoietic tissue neoplasms in the most exposed zones. No clear pattern by time since the accident and zones was evident partly because of the low number of cases. The elevated risk of breast cancer in zone A females after 15 years since the accident deserves further and thorough investigation. The follow-up is continuing in order to cover the long time period (even decades) usually elapsing from exposure to carcinogenic chemicals and disease occurrence.

Diabetes and cancer in veterans of Operation Ranch Hand after adjustment for calendar period, days of spraying, and time spent in Southeast Asia

BACKGROUND

The Air Force Health Study was launched in 1980 as part of a Federal effort to resolve the Agent Orange issue.

OBJECTIVES

To study diabetes and cancer with additional adjustment for days of spraying, calendar period of service, and time spent in Southeast Asia (SEA).

METHODS

This was a longitudinal study of veterans of Operation Ranch Hand, the unit responsible for spraying Agent Orange and other 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-contaminated herbicides in Vietnam from 1962 to 1971.

RESULTS

Associations between TCDD and diabetes and between TCDD and cancer in Ranch Hand veterans are strengthened after adjustment for calendar period of service, days of spraying, and, for cancer, time spent in SEA.

CONCLUSIONS

Calendar period of service, days of spraying, and time spent in SEA are important confounders in the Air Force Health Study.

Concentration and age-dependent elimination kinetics of polychlorinated dibenzofurans in Yucheng and Yusho patients

Half-life estimates of three polychlorodibenzofurans (PCDFs) were calculated using serial blood samples collected over a 15 to 19-year period. Blood fat PCDFs were modeled in eight individuals who were exposed to contaminated rice oil in Japan (Yusho, n = 5) and in Taiwan (Yucheng, n = 3). The elimination kinetics of PCDFs were concentration-dependent, with faster rates observed at higher concentrations and the apparent transition to slower rates occurring at about 1-3 ppb. Average half-lives of 1.1, 2.3, and 1.5 years above the transition concentration and 7.2, 5.7, and 3.5 years below it were estimated for 2,3,4,7,8-pentaCDF, 1,2,3,4,7,8-hexaCDF, and 1,2,3,4,6,7,8-heptaCDF, respectively. A positive linear correlation of half-life with age was observed for the combined group, with a rate of increase of 0.19, 0.12, and 0.05-year half-life per year of increase in age for penta-, hexa-, and hepta-CDF, respectively. The distinctly younger Yucheng patients exhibited far lower variability in half-lives and age-related trends that were quite consistent with the corresponding data on 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for younger persons exposed in the Seveso incident. These age- and concentration-dependent half-lives for PCDFs may have important risk assessment implications for estimating body burdens. The current study provides limited additional evidence that PCDFs, like TCDD, are more rapidly eliminated in younger individuals.

Reverse dosimetry: interpreting trihalomethanes biomonitoring data using physiologically based pharmacokinetic modeling

Biomonitoring data provide evidence of exposure of environmental chemicals but are not, by themselves, direct measures of exposure. To use biomonitoring data in understanding exposure, physiologically based pharmacokinetic (PBPK) modeling can be used in a reverse dosimetry approach to assess a distribution of exposures possibly associated with specific blood or urine levels of compounds. Reverse dosimetry integrates PBPK modeling with exposure pattern characterization, Monte Carlo analysis, and statistical tools to estimate a distribution of exposures that are consistent with biomonitoring data in a population. The present study used an existing PBPK model for chloroform as a generic framework to develop PBPK models for other trihalomethanes (THMs). Using Monte Carlo sampling techniques, probabilistic information about pharmacokinetics and exposure patterns was included to estimate distributions of THMs concentrations in blood in relation to various exposure patterns in a diverse population. In addition, the possibility of inhibition of hepatic metabolism among THMs was evaluated under the scenarios of household exposure. These studies demonstrated how PBPK modeling can be used as a tool to estimate a population distribution of exposures that could have resulted in particular biomonitoring results. When toxicity level is known, this tool can also be used to estimate proportion of population above levels associated with health risk.

On the translation of uncertainty from toxicokinetic to toxicodynamic models--the TCDD example

When estimating human health risks from exposure to TCDD using toxicokinetic and toxicodynamic models, it is important to understand how model choice and assumptions necessary for modeling add to the uncertainty of risk estimates. Several toxicokinetic models have been proposed for the risk assessment of dioxins, in particular the elimination kinetics in humans has been a matter of constant debate. For a long time, a simple linear elimination kinetics has been common choice. Thus, it was used for the statistical analysis of the largest occupationally exposed cohort, the German Boehringer cohort. We challenge this assumption by considering, amongst others, a nonlinear modified Michaelis-Menten-type elimination kinetics, the so-called Carrier kinetics. Using the area under the lipid TCDD concentration time curve as dose metrics, we model the time to cancer-related death using the Cox proportional hazards model as toxicodynamic model. This risk assessment set-up was simulated in order to quantify uncertainty of both the dose (TCDD body burden) and the risk estimates, depending on the use of the kinetic model, variations of carcinogenic effect of TCDD and variations of latency period (lag time). If past exposure is estimated assuming a linear elimination kinetics although a Carrier kinetics actually holds, then high exposures in reality will be underestimated through statistical analysis and low exposures will be overestimated, respectively. This bias will carry over on the estimated individual concentration-time curves and the therefrom derived TCDD dose metric values. Using biased dose values when estimating a dose-response relationship will finally lead to biased risk estimates. The extent of bias and the decrease of precision are quantified in selected scenarios through this simulation approach. Our findings are in concordance with recent results in the field of dioxin risk assessment. They also reinforce the general demand for the scheduled uncertainty assessments in risk analyses.

TCDD exposure-response analysis and risk assessment

We examined the relation between cancer mortality and time-dependent cumulative exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) estimated from a concentration- and age-dependent kinetic model of elimination, and we estimated incremental cancer risks at age 75. Data from the National Institute for Occupational Safety and Health study of 3,538 workers with occupational exposure to TCDD were analyzed using standardized mortality ratios and Cox regression procedures. Analyses adjusted for potential confounding by age, year of birth, and race and considered exposure lag periods of 0, 10, or 15 years. Other potential confounders including smoking and other occupational exposures were evaluated indirectly. To explore the influence of extreme values of cumulative TCDD ppt-years, we restricted the analysis to observations with exposure below the 95th percentile or used logarithmic (ln) transformed exposure values. We applied penalized smoothing splines to examine variation in the exposure-response relation across the exposure range. TCDD was not statistically significantly associated with cancer mortality using the full data set, regardless of the lag period. When we restricted the analysis to observations with exposure below the 95th percentile, TCDD was associated positively with cancer mortality, particularly when a 15-year lag was applied (untransformed exposure data: regression coefficient , standard error (s.e.) = 1.4 x 10(-6), p < 0.05; ln-transformed exposure data: , s.e. = 2.9 x 10(-2), p < 0.05). The estimated incremental lifetime risk of mortality at age 75 from all cancers was about 6 to more than 10 times lower than previous estimates derived from this cohort using exposure models that did not consider the age and concentration dependence of TCDD elimination.