Dose-response relationships for disposition and hepatic sequestration of polyhalogenated dibenzo-p-dioxins, dibenzofurans, and biphenyls following subchronic treatment in mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters hepatic polyunsaturated fatty acid metabolism and eicosanoid biosynthesis in female Sprague-Dawley rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent aryl hydrocarbon receptor (AhR) agonist that elicits a broad spectrum of dose-dependent hepatic effects including lipid accumulation, inflammation, and fibrosis. To determine the role of inflammatory lipid mediators in TCDD-mediated hepatotoxicity, eicosanoid metabolism was investigated. Female Sprague-Dawley (SD) rats were orally gavaged with sesame oil vehicle or 0.01-10 μg/kg TCDD every 4 days for 28 days. Hepatic RNA-Seq data was integrated with untargeted metabolomics of liver, serum, and urine, revealing dose-dependent changes in linoleic acid (LA) and arachidonic acid (AA) metabolism. TCDD also elicited dose-dependent differential gene expression associated with the cyclooxygenase, lipoxygenase, and cytochrome P450 epoxidation/hydroxylation pathways with corresponding changes in ω-6 (e.g. AA and LA) and ω-3 polyunsaturated fatty acids (PUFAs), as well as associated eicosanoid metabolites. Overall, TCDD increased the ratio of ω-6 to ω-3 PUFAs. Phospholipase A2 (Pla2g12a) was induced consistent with increased AA metabolism, while AA utilization by induced lipoxygenases Alox5 and Alox15 increased leukotrienes (LTs). More specifically, TCDD increased pro-inflammatory eicosanoids including leukotriene LTB4, and LTB3, known to recruit neutrophils to damaged tissue. Dose-response modeling suggests the cytochrome P450 hydroxylase/epoxygenase and lipoxygenase pathways are more sensitive to TCDD than the cyclooxygenase pathway. Hepatic AhR ChIP-Seq analysis found little enrichment within the regulatory regions of differentially expressed genes (DEGs) involved in eicosanoid biosynthesis, suggesting TCDD-elicited dysregulation of eicosanoid metabolism is a downstream effect of AhR activation. Overall, these results suggest alterations in eicosanoid metabolism may play a key role in TCDD-elicited hepatotoxicity associated with the progression of steatosis to steatohepatitis.

Detection of 3,3'-Dichlorobiphenyl in Human Maternal Plasma and Its Effects on Axonal and Dendritic Growth in Primary Rat Neurons

3,3'-Dichlorobiphenyl (PCB 11), a byproduct of pigment production, is increasingly detected in environmental samples. While more highly chlorinated PCB congeners are known developmental neurotoxicants, nothing is known about the potential developmental neurotoxicity of PCB 11. To address this critical data gap, we measured PCB 11 levels in human maternal plasma and quantified the effects of PCB 11 and its major metabolites on morphometric parameters of neuronal connectivity in cultured primary neurons. Mass spectrometry analyses of plasma from 241 pregnant women enrolled in the MARBLES study (University of California, Davis) detected PCB 11 in all samples at concentrations ranging from 0.005 to 1.717 ng/ml. Morphometric analyses of primary neuron-glia co-cultures dissociated from the neocortices or hippocampi of neonatal Sprague Dawley rats exposed to vehicle or concentrations ranging from 1 attamolar (aM) to 1 micromolar (µM) of PCB 11, OH-PCB 11, or PCB 11 sulfate indicated that PCB 11 and both metabolites significantly increased axonal and dendritic growth in cortical and hippocampal pyramidal neurons. PCB 11 significantly altered neuronal morphogenesis at concentrations as low as 1 femtomolar (fM), which is ∼0.22 ng/ml. These data suggest the potential for the developing human brain to be exposed to PCB 11, and demonstrate that environmentally relevant levels of PCB 11 alter axonal and dendritic growth in neuronal cell types critically involved in cognitive and higher-order behaviors. These findings identify PCB 11 as a potential environmental risk factor for adverse neurodevelopmental outcomes in humans.

Association between serum concentrations of persistent organic pollutants and smoking in Koreans: A cross-sectional study

Background

Studies on the associations between persistent organic pollutants (POPs) and smoking according to gender and smoking amount (cigarettes/day) are limited, and the results regarding the relationship between POPs and smoking are not completely consistent across studies.

Objectives

The smoking rate in Korea is one of the highest among the Organization for Economic Cooperation and Development (OECD) countries. We investigated the association between serum concentrations of POPs and cigarette smoking in Koreans by smoking status (never-smoker/ever-smoker) and smoking amount (cigarettes/day) according to gender.

Methods

Serum concentrations of 32 polychlorinated biphenyls (PCBs) and 19 organochlorine pesticides (OCPs) were measured in 401 participants (232 men and 169 women) who received health examinations during the Korean Cancer Prevention Study-II. We compared POP levels in ever-smokers and never-smokers and conducted multivariate logistic regression analyses to identify associations between POPs and smoking.

Results

Among women, the concentrations of PCB 156, PCB 167, and PCB 180 were significantly higher in ever-smokers than in never-smokers. After adjustments for age, body mass index, gamma-glutamyl transpeptidase, and alcohol intake, serum PCB 157 concentration was positively associated with male ever-smokers (OR 2.26; 95% CI, 1.01–5.04). In addition, trans-nonachlordane in OCPs as well as PCBs was significantly positively related with female ever-smokers (OR 3.21; 95% CI, 1.04–9.86). We found that subjects who smoked fewer than 15 cigarettes/day had a higher risk of having high POP concentrations than never-smokers.

Conclusions

These results indicate that smoking may be associated with human serum POPs levels.

Evaluation of relative effect potencies (REPs) for dioxin-like compounds to derive systemic or human-specific TEFs to improve human risk assessment

Toxic equivalency factors (TEFs) are generally applied for estimating human risk of dioxins and dioxin-like compounds using systemic (e.g., blood) levels, even though these TEFs are established based on intake doses in rodent studies. This review shows that systemic relative effect potencies (REPs) can deviate substantially from intake REPs, but are similar to in vitro-derived REPs. Interestingly, the in vitro REPs for 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) and 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF) are up to one order of magnitude higher than their in vivo REPs and WHO-TEFs, based on oral intake. In addition, clear species-differences in in vitro REPs were apparent for some congeners. Especially the human-derived REP for polychlorinated biphenyl 126 is one to two orders of magnitude lower than rodent REPs and its current WHO-TEF. Next, suggested adapted systemic or human-specific TEFs for these congeners were applied to calculate changes in systemic TEQ concentrations in studies from the USA, Germany and Japan and compared with either the JECFA TDI or USEPA RfD of TCDD. Overall, the effect of such TEF changes for these three congeners on total TEQ roughly balances each other out in the general population. However, results may be different for situations in which a specific group of congeners dominates. For those congeners that show a distinct deviation between either intake and systemic REPs or between rodent- and human-based in vitro REPs, we propose that especially REPs derived from human-based in vitro models are weighted more heavily in establishing systemic or human-specific TEF values to improve human health risk assessment.

Polybrominated dibenzo-p-dioxins, dibenzofurans, and biphenyls: inclusion in the toxicity equivalency factor concept for dioxin-like compounds

In 2011, a joint World Health Organization (WHO) and United Nations Environment Programme (UNEP) expert consultation took place, during which the possible inclusion of brominated analogues of the dioxin-like compounds in the WHO Toxicity Equivalency Factor (TEF) scheme was evaluated. The expert panel concluded that polybrominated dibenzo-p-dioxins (PBDDs), dibenzofurans (PBDFs), and some dioxin-like biphenyls (dl-PBBs) may contribute significantly in daily human background exposure to the total dioxin toxic equivalencies (TEQs). These compounds are also commonly found in the aquatic environment. Available data for fish toxicity were evaluated for possible inclusion in the WHO-UNEP TEF scheme (van den Berg et al., 1998). Because of the limited database, it was decided not to derive specific WHO-UNEP TEFs for fish, but for ecotoxicological risk assessment, the use of specific relative effect potencies (REPs) from fish embryo assays is recommended. Based on the limited mammalian REP database for these brominated compounds, it was concluded that sufficient differentiation from the present TEF values of the chlorinated analogues (van den Berg et al., 2006) was not possible. However, the REPs for PBDDs, PBDFs, and non-ortho dl-PBBs in mammals closely follow those of the chlorinated analogues, at least within one order of magnitude. Therefore, the use of similar interim TEF values for brominated and chlorinated congeners for human risk assessment is recommended, pending more detailed information in the future.

PCB 126 and other dioxin-like PCBs specifically suppress hepatic PEPCK expression via the aryl hydrocarbon receptor

Dioxins and dioxin-like compounds encompass a group of structurally related heterocyclic compounds that bind to and activate the aryl hydrocarbon receptor (AhR). The prototypical dioxin is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a highly toxic industrial byproduct that incites numerous adverse physiological effects. Global commercial production of the structurally similar polychlorinated biphenyls (PCBs), however, commenced early in the 20(th) century and continued for decades; dioxin-like PCBs therefore contribute significantly to total dioxin-associated toxicity. In this study, PCB 126, the most potent dioxin-like PCB, was evaluated with respect to its direct effects on hepatic glucose metabolism using primary mouse hepatocytes. Overnight treatment with PCB 126 reduced hepatic glycogen stores in a dose-dependent manner. Additionally, PCB 126 suppressed forskolin-stimulated gluconeogenesis from lactate. These effects were independent of acute toxicity, as PCB 126 did not increase lactate dehydrogenase release nor affect lipid metabolism or total intracellular ATP. Interestingly, provision of cells with glycerol instead of lactate as the carbon source completely restored hepatic glucose production, indicating specific impairment in the distal arm of gluconeogenesis. In concordance with this finding, PCB 126 blunted the forskolin-stimulated increase in phosphoenolpyruvate carboxykinase (PEPCK) mRNA levels without affecting glucose-6-phosphatase expression. Myricetin, a putative competitive AhR antagonist, reversed the suppression of PEPCK induction by PCB 126. Furthermore, other dioxin-like PCBs demonstrated similar effects on PEPCK expression in parallel with their ability to activate AhR. It therefore appears that AhR activation mediates the suppression of PEPCK expression by dioxin-like PCBs, suggesting a role for these pollutants as disruptors of energy metabolism.

Non-additive hepatic gene expression elicited by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) co-treatment in C57BL/6 mice

Interactions between environmental contaminants can lead to non-additive effects that may affect the toxicity and risk assessment of a mixture. Comprehensive time course and dose-response studies with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), non-dioxin-like 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) and their mixture were performed in immature, ovariectomized C57BL/6 mice. Mice were gavaged once with 30 μg/kg TCDD, 300 mg/kg PCB153, a mixture of 30 μg/kg TCDD with 300 mg/kg PCB153 (MIX) or sesame oil vehicle for 4,12, 24,72 or 168 h. In the 24h dose-response study, animals were gavaged with TCDD (0.3,1, 3, 6, 10, 15, 30, 45 μg/kg), PCB153 (3,10, 30, 60, 100, 150, 300, 450 mg/kg), MIX (0.3+3, 1+10, 3+30, 6+60, 10+100, 15+150, 30+300, 45 μg/kg TCDD+450 mg/kg PCB153, respectively) or vehicle. All three treatments significantly increased relative liver weights (RLW), with MIX eliciting significantly greater increases compared to TCDD and PCB153 alone. Histologically, MIX induced hepatocellular hypertrophy, vacuolization, inflammation, hyperplasia and necrosis, a combination of TCDD and PCB153 responses. Complementary lipid analyses identified significant increases in hepatic triglycerides in MIX and TCDD samples, while PCB153 had no effect on lipids. Hepatic PCB153 levels were also significantly increased with TCDD co-treatment. Microarray analysis identified 167 TCDD, 185 PCB153 and 388 MIX unique differentially expressed genes. Statistical modeling of quantitative real-time PCR analysis of Pla2g12a, Serpinb6a, Nqo1, Srxn1, and Dysf verified non-additive expression following MIX treatment compared to TCDD and PCB153 alone. In summary, TCDD and PCB153 co-treatment elicited specific non-additive gene expression effects that are consistent with RLW increases, histopathology, and hepatic lipid accumulation.

Disruption of the gene for CYP1A2, which is expressed primarily in liver, leads to differential regulation of hepatic and pulmonary mouse CYP1A1 expression and augmented human CYP1A1 transcriptional activation in response to 3-methylcholanthrene in vivo

The cytochrome P4501A (CYP1A) enzymes play important roles in the metabolic activation and detoxification of numerous environmental carcinogens, including polycyclic aromatic hydrocarbons (PAHs). In this study, we tested the hypothesis that hepatic CYP1A2 differentially regulates mouse hepatic and pulmonary CYP1A1 expression and suppresses transcriptional activation of human CYP1A1 (hCYP1A1) promoter in response to 3-methylcholanthrene (MC) in vivo. Administration of wild-type (WT) (C57BL/6J) or Cyp1a2-null mice with a single dose of MC (100 μmol/kg i.p.) caused significant increases in hepatic CYP1A1/1A2 activities, apoprotein content, and mRNA levels 1 day after carcinogen withdrawal compared with vehicle-treated controls. The induction persisted in the WT, but not Cyp1a2-null, animals, for up to 15 days. In the lung, MC caused persistent CYP1A1 induction for up to 8 days in both genotypes, with Cyp1a2-null mice displaying a greater extent of CYP1A1 expression. It is noteworthy that MC caused significant augmentation of human CYP1A1 promoter activation in transgenic mice expressing the hCYP1A1 and the reporter luciferase gene on a Cyp1a2-null background, compared with transgenic mice on the WT background. In contrast, the mouse endogenous hepatic, but not pulmonary, persistent CYP1A1 expression was repressed by MC in the hCYP1A1-Cyp1a2-null mice. Liquid chromatography-mass spectrometry experiments showed that CYP1A2 catalyzed the formation of 1-hydroxy-3-MC and/or 2-hydroxy-3-MC, a metabolite that may contribute to the regulation of CYP1A1 expression. In conclusion, the results suggest that CYP1A2 plays a pivotal role in the regulation of hepatic and pulmonary CYP1A1 by PAHs, a phenomenon that potentially has important implications for PAH-mediated carcinogenesis.

Automated dose-response analysis and comparative toxicogenomic evaluation of the hepatic effects elicited by TCDD, TCDF, and PCB126 in C57BL/6 mice

The toxic equivalency factor (TEF) approach recommended by the World Health Organization is used to quantify dioxin-like exposure concentrations for mixtures of polychlorinated dibenzo-dioxins, -furans, and polychlorinated biphenyls (PCBs), including 2,3,7,8-tetrachlorodibenzofuran (TCDF) and 3,3',4,4',5-pentachlorobiphenyl (PCB126) relative to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Whole-genome microarrays were used to evaluate the hepatic gene expression potency of TCDF and PCB126 relative to TCDD with complementary histopathology, tissue level analysis, and ethoxyresorufin-O-deethylase (EROD) assay results. Immature ovariectomized C57BL/6 mice were gavaged with 0.001, 0.01, 0.03, 0.1, 0.3, 1, 3, 10, 30, 100, and 300 μg/kg TCDD and TEF-adjusted doses (TEF for TCDF and PCB126 is 0.1) of TCDF or PCB126 (1, 3, 10, 30, 100, 300, 1000, and 3000 μg/kg of TCDF or PCB126) or sesame oil vehicle and sacrificed 24 h post dose. In general, TCDD, TCDF, and PCB126 tissue levels, as well as histopathological effects, were comparable when comparing TEF-adjusted doses. Automated dose-response modeling (ToxResponse Modeler) of the microarray data identified 210 TCDF and 40 PCB126 genes that exhibited sigmoidal dose-response curves with comparable slopes when compared with TCDD. These similar responses were used to calculate a median TCDF gene expression relative potency (REP) of 0.06 and a median PCB126 gene expression REP of 0.02. REPs of 0.02 were also calculated for EROD induction for both compounds. Collectively, these data suggest that differences in the ability of the liganded aryl hydrocarbon receptor:AhR nuclear translocator complex to elicit differential hepatic gene expression, in addition to pharmacokinetic differences between ligands, influence their potency in immature ovariectomized C57BL/6 mice.

PCB153-elicited hepatic responses in the immature, ovariectomized C57BL/6 mice: comparative toxicogenomic effects of dioxin and non-dioxin-like ligands

Polychlorinated biphenyls (PCBs) are ubiquitous contaminants found as complex mixtures of coplanar and non-coplanar congeners. The hepatic temporal and dose-dependent effects of the most abundant non-dioxin-like congener, 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153), were examined in immature, ovariectomized C57BL/6 mice, and compared to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the prototypical aryl hydrocarbon receptor (AhR) ligand. Animals were gavaged once with 300 mg/kg PCB153 or sesame oil vehicle and sacrificed 4, 12, 24, 72 or 168 h post dose. In the dose-response study, mice were gavaged with 1, 3, 10, 30, 100 or 300 mg/kg PCB153 or sesame oil for 24 h. Significant increases in relative liver weights were induced with 300 mg/kg PCB153 between 24 and 168 h, accompanied by slight vacuolization and hepatocellular hypertrophy. The hepatic differential expression of 186 and 177 genes was detected using Agilent 4 x 44 K microarrays in the time course (|fold change|> or =1.5, P1(t)> or =0.999) and dose-response (|fold change|> or =1.5, P1(t)> or =0.985) studies, respectively. Comparative analysis with TCDD suggests that the differential gene expression elicited by PCB153 was not mediated by the AhR. Furthermore, constitutive androstane and pregnane X receptor (CAR/PXR) regulated genes including Cyp2b10, Cyp3a11, Ces2, Insig2 and Abcc3 were dose-dependently induced by PCB153. Collectively, these results suggest that the hepatocellular effects elicited by PCB153 are qualitatively and quantitatively different from TCDD and suggestive of CAR/PXR regulation.

The 2005 World Health Organization reevaluation of human and Mammalian toxic equivalency factors for dioxins and dioxin-like compounds

In June 2005, a World Health Organization (WHO)-International Programme on Chemical Safety expert meeting was held in Geneva during which the toxic equivalency factors (TEFs) for dioxin-like compounds, including some polychlorinated biphenyls (PCBs), were reevaluated. For this reevaluation process, the refined TEF database recently published by Haws et al. (2006, Toxicol. Sci. 89, 4-30) was used as a starting point. Decisions about a TEF value were made based on a combination of unweighted relative effect potency (REP) distributions from this database, expert judgment, and point estimates. Previous TEFs were assigned in increments of 0.01, 0.05, 0.1, etc., but for this reevaluation, it was decided to use half order of magnitude increments on a logarithmic scale of 0.03, 0.1, 0.3, etc. Changes were decided by the expert panel for 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.3), 1,2,3,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.03), octachlorodibenzo-p-dioxin and octachlorodibenzofuran (TEFs = 0.0003), 3,4,4',5-tetrachlorbiphenyl (PCB 81) (TEF = 0.0003), 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169) (TEF = 0.03), and a single TEF value (0.00003) for all relevant mono-ortho-substituted PCBs. Additivity, an important prerequisite of the TEF concept was again confirmed by results from recent in vivo mixture studies. Some experimental evidence shows that non-dioxin-like aryl hydrocarbon receptor agonists/antagonists are able to impact the overall toxic potency of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds, and this needs to be investigated further. Certain individual and groups of compounds were identified for possible future inclusion in the TEF concept, including 3,4,4'-TCB (PCB 37), polybrominated dibenzo-p-dioxins and dibenzofurans, mixed polyhalogenated dibenzo-p-dioxins and dibenzofurans, polyhalogenated naphthalenes, and polybrominated biphenyls. Concern was expressed about direct application of the TEF/total toxic equivalency (TEQ) approach to abiotic matrices, such as soil, sediment, etc., for direct application in human risk assessment. This is problematic as the present TEF scheme and TEQ methodology are primarily intended for estimating exposure and risks via oral ingestion (e.g., by dietary intake). A number of future approaches to determine alternative or additional TEFs were also identified. These included the use of a probabilistic methodology to determine TEFs that better describe the associated levels of uncertainty and "systemic" TEFs for blood and adipose tissue and TEQ for body burden.

A mixture of dioxins, furans, and non-ortho PCBs based upon consensus toxic equivalency factors produces dioxin-like reproductive effects

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD; dioxin) and related polyhalogenated aromatic hydrocarbons (PHAHs) alter the reproductive development of laboratory animals. Therefore, we exposed animals to a mixture of dioxins, furans, and polychlorinated biphenyls (PCBs) that included TCDD, 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,3,7,8-pentachlorodibenzofuran (1-PeCDF), 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF), octachlorodibenzofuran (OCDF), 3,3',4,4'-tetrachlorobiphenyl (PCB77), 3,3',4,4',5-pentachlorobiphenyl (PCB126), and 3,3',4,4',5,5'-hexachlorobiphenyl (PCB169). The mixture composition approximated the relative abundance of these compounds in foodstuff (L. S. Birnbaum and M. J. DeVito, 1995, Toxicology Vol. 105, pp. 391-401). Following the work of Gray et al. with TCDD (1997, Toxicology and Applied Pharmacology Vol. 146, pp. 11-20), we exposed time-pregnant dams on gestation day (GD) 15 at doses up to 1.0 microgram TCDD toxic equivalency (TEQ)/kg and the development of offspring was monitored. This mixture significantly increased the time to puberty in both male and female offspring. At postnatal day (PND) 32 seminal vesicle weights were decreased; however, only ventral prostate weight was affected at PND 49 and no effects were seen at PND 63. In female offspring, the mixture caused dose-dependent increases in the incidence of vaginal thread. Ethoxyresorufin-O-deethylase (EROD) activity was higher than with TCDD the comparable TEQ exposure. Based on the slightly lowered responsiveness to the mixture, we used 2.0 microgram TEQ/kg to examine reproductive effects. This dose elicited the responses observed with 1.0 microgram TCDD/kg. Results indicate that the mixture causes a similar spectrum of effects seen with TCDD and the slightly lowered degree of response based on administered dose appears to be due to decreased transfer of mixture components to the offspring. Thus, the use of the WHO consensus TEFs (M. Van den Berg et al., 1998, Environ. Health Perspec. 106, 775-792) reasonably predicts the developmental toxicity of this mixture of dioxin-like PHAHs.

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

OBJECTIVE

To evaluate the long term health consequences of past occupational exposure to 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD).

METHODS

Cancer incidence and cause specific mortality were examined up to and including 1992 in a group of 243 men with external comparisons and internal dose-response analyses. Model based estimates of TCDD dose (expressed in micrograms/kg body weight) were developed for all cohort members with an approach that incorporated detailed accounts of each employee's work activities, analyses of TCDD in blood lipid of 138 employees, and internally derived estimates of elimination rates of TCDD.

RESULTS

The estimated dose of TCDD for 135 men was > or = 0.1 microgram/kg body weight and for 69 men > or = 1 microgram/kg body weight. Increased cancer risk ratios were found with higher doses of TCDD and longer interval since first exposure for all sites combined and digestive and respiratory cancers in particular. Within the high dose group (> or = 1 microgram/kg body weight), total cancer mortality was increased > or = 20 years after first exposure (13 cases, standardised mortality ratio (SMR) 1.97, 95% confidence interval (95% CI) 1.05-3.36) as was respiratory cancer (six cases, SMR 3.06; 95% CI 1.12-6.66). Among current cigarette smokers, 12 cancer deaths occurred in the high dose group (SMR 3.42, 95% CI 1.77-5.97) compared with seven deaths at lower doses of TCDD (SMR 1.29, 95% CI 0.52-2.66). Regression analyses based on the Cox's proportional hazards model provided further evidence of a relation between cumulative dose of TCDD and occurrence of both overall and digestive cancer. No evidence of an effect of TCDD on overall mortality or deaths due to circulatory disease was found and no cases of non-Hodgkin's lymphoma or soft tissue sarcoma have been found to date.

CONCLUSIONS

Our findings are consistent with a carcinogenic effect induced by TCDD at doses > or = 1 microgram/kg body weight. With such a small cohort, the risk estimates are not very stable and could be affected by selection and confounding.