Dioxin in soil: bioavailability after ingestion by rats and guinea pigs

Bioaccessibility of polychlorinated dioxins and furans in soil from a Superfund site

Bioaccessibilities of PCDD/PCDF congeners contributing to cancer risk were determined in twelve soil samples from the American Creosote Works Superfund site in Florida. Based upon sample locations, congener profiles (i.e., the same dominant congeners), and total (Toxic Equivalent; TEQ) concentrations, each of these samples has PCDD/PCDF contamination reasonably attributable to the site. Bioaccessibility was determined using a 2-phase in vitro extraction method that included both simulated gastric and intestinal conditions of the human GI tract. Measured congener-specific bioaccessibility values ranged from 34.3 to 62.1%. There was no apparent relationship between the extent of chlorination of PCDD/PCDF congeners and their bioaccessibility. TEQ-weighted bioaccessibility values varied among individual soil samples, which is not unexpected based upon the literature. This variability could not be explained by differences in soil pH, composition, or organic carbon content. The average TEQ-weighted bioaccessibility value of 59% for the twelve samples was accepted as representing site-specific bioavailability of PCDD/PCDFs. This value is higher than most dioxin/furan bioaccessibility values reported in the literature and at the upper end of the range of relative oral bioavailability (RBA) values reported for PCDD/PCDFs from in vivo bioavailability studies. This study used a finer fraction of soil particles (<150 microns versus the more typical <250 microns) to better represent soil that is incidentally ingested. This finer fraction would be expected to have a greater surface area available for extraction of PCDD/PCDFs per unit mass, which might account for the greater than expected bioaccessibility.

Gene expression and pathologic alterations in juvenile rainbow trout due to chronic dietary TCDD exposure

The goal of this project was to use functional genomic methods to identify molecular biomarkers as indicators of the impact of TCDD exposure in rainbow trout. Specifically, we investigated the effects of chronic dietary TCDD exposure on whole juvenile rainbow trout global gene expression associated with histopathological analysis. Juvenile rainbow trout were fed Biodiet starter with TCDD added at 0, 0.1, 1, 10 and 100 ppb (ngTCDD/g food), and fish were sampled from each group at 7, 14, 28 and 42 days after initiation of feeding. 100 ppb TCDD caused 100% mortality at 39 days. Fish fed with 100 ppb TCDD food had TCDD accumulation of 47.37 ppb (ngTCDD/g fish) in whole fish at 28 days. Histological analysis from TCDD-treated trout sampled from 28 and 42 days revealed that obvious lesions were found in skin, oropharynx, liver, gas bladder, intestine, pancreas, nose and kidney. In addition, TCDD caused anemia in peripheral blood, decreases in abdominal fat, increases of remodeling of fin rays, edema in pericardium and retrobulbar hemorrhage in the 100 ppb TCDD-treated rainbow trout compared to the control group at 28 days. Dose- and time-dependent global gene expression analyses were performed using the cGRASP 16,000 (16K) cDNA microarray. TCDD-responsive whole body transcripts identified in the microarray experiments have putative functions involved in various biological processes including growth, cell proliferation, metabolic process, and immune system processes. Nine microarray-identified genes were selected for QPCR validation. CYP1A3 and CYP1A1 were common up-regulated genes and HBB1 was a common down-regulated gene among each group based on microarray data, and their QPCR validations are consistent with microarray data for the 10 and 100 ppb TCDD treatment groups after 28 days exposure (p<0.05). In addition, in the 100 ppb group at 28 days, expression of complement component C3-1 and trypsin-1 precursor have a more than 10-fold induction from the microarray experiments, and their QPCR validations are consistent and showed significant induction in the 100 ppb group at 28 days (p<0.05). Overall, lesion in nasal epithelium is a novel and significant result in this study, and TCDD-responsive rainbow trout transcripts identified in the present study may lead to the development of new molecular biomarkers for assessing the potential impacts of environmental TCDD on rainbow trout populations.

Selective soil particle adherence to hands: implications for understanding oral exposure to soil contaminants

Over the last 30 years, there has been extensive research designed to quantify the extent of oral bioavailability and bioaccessibility of organic and inorganic contaminants in soil. One aspect of this research is the soil particle size selected to represent environmental exposures, which may affect study results and comparability across studies. Different research groups have studied soil particle sizes ranging from <45 μm to <2000 μm. This article reviews the historical and technical considerations that pertain to the selection of an appropriate particle size fraction for evaluating the relative oral bioavailability of chemicals from soil, which include (1) how the resultant data will be used in human health risk assessment, (2) soil fractions historically used in oral bioavailability studies, (3) studies of soil adherence to human hands, (4) the distribution of contaminants in soils as a function of particle size, and (5) the effect of differential bioavailability as a function of soil particle size and geochemical matrix. These factors are first discussed from a general perspective, applicable to all contaminants in soil, and then more specifically for polycyclic aromatic hydrocarbons (PAHs) in soil. Based on this review, a specific soil particle size of <150 μm is recommended for future studies on the oral bioavailability and bioaccessibility of PAHs in soil.

A pilot study of oral bioavailability of dioxins and furans from contaminated soils: Impact of differential hepatic enzyme activity and species differences

An in vivo pilot study of the oral bioavailability of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in two soils with distinct congener profiles (one dominated by PCDDs, the other by PCDFs) was conducted in rats and juvenile swine. The pilot study revealed potential confounding of relative bioavailability estimates compared to bioavailability in spiked corn oil gavage for tetrachlorodibenzofuran (TCDF) in the rat study due to differential EROD induction between groups receiving soil and those receiving spiked control PCDDs/PCDFs. A follow-up study in rats with the furan-contaminated soil was then conducted with reductions in the spiked control doses to 20%, 50% and 80% of the soil-feed dose in order to bracket hepatic enzyme induction levels in the soil group. When hepatic enzyme induction was matched between the soil and spiked control groups, the apparent relative bioavailability for TCDF was reduced significantly. Overall, after controlling for hepatic enzyme induction, estimates of relative bioavailability in rats and swine differed for the two soils. In the rat study, the relative bioavailability of the two soils were approximately 37% and 60% compared to corn oil administration for the PCDD- and PCDF- dominated soils, respectively, on a TEQ basis. In swine, both soils demonstrated relative bioavailability between 20% and 25% compared to administration in corn oil. These species differences and experimental design issues, such as controlling for differential enzyme induction between corn oil and soil-feed animals in a bioavailability study, are relevant to risk assessment efforts where relative bioavailability inputs are important for theoretical exposure and risk characterization.

Bioavailability of PCDD/F from contaminated soil in young Goettingen minipigs

For the general population the intake of food of animal origin is the main route of human exposure to polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F). Besides this the ingestion of contaminated soil might be an important exposure path for small children. For risk assessment the knowledge of the bioavailable fraction of soil bound contaminants is important. In a balance study with young Goettingen minipigs the oral bioavailability of PCDD/F from contaminated soil was estimated by determination of the retention of PCDD/F from soil in different organs and tissues. Relative bioavailability was estimated by comparing the retention from soil to the retention of PCDD/F in organs and tissues after oral administration of a PCDD/F mixture extracted from the same soil by solvent. The soil had a PCDD/F-contamination of 5.3 microg I-TEq/kg and originated from a former arable land that had been treated with sludge from the port of Hamburg some years ago. Two groups of each four animals were exposed daily for 28 days via their diet either to 0.5 g soil per kg body weight and day (2.63 ng I-TEq/(kg(bw).d)) or to a daily dose of 1.58 ng I-TEq/(kg(bw).d) given to the diet by solvent. Five unexposed animals were used as a control group. Liver, adipose tissue, muscle, brain and blood were analyzed for their PCDD/F content. Accumulation of PCDD/F from soil or solvent in comparison to control animals was only observed for congeners with 2378-chlorosubstitution and predominantly took place in the liver. Bioavailability of 2378-chlorosubstituted congeners was in the range of 0.64%-21.9% (mean: 10.1%) from soil and 2.8%-59.8% (mean: 31.5%) when administered by solvent. The soil matrix reduced the bioavailability by about 70%. Expressed as I-TEq only 13.8% of the PCDD/F contamination were bioavailable from soil. The relative bioavailability of 2378-chlorosubstituted congeners from soil in relation to administration by solvent was in the range of 2%-42.2% (mean: 28.4%). When not considering the bioavailability, the risk by oral uptake of PCDD/F contaminated soil might be overestimated.

Identifying soil cleanup criteria for dioxins in urban residential soils: how have 20 years of research and risk assessment experience affected the analysis?

This article reviews the scientific evidence and methodologies that have been used to assess the risks posed by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and presents a probabilistic analysis for identifying virtually safe concentrations of TCDD toxicity equivalents (TEQ) in residential soils. Updated data distributions that consider state-of-the-science cancer and noncancer toxicity criteria, child soil ingestion and dermal uptake, bioavailability in soil, and residential exposure duration are incorporated. The probabilistic analysis shows that the most sensitive determinants of dose and risk are childhood soil ingestion, exposure duration, and the selected TCDD cancer potency factor. It also shows that the cancer risk at 1 per 100,000 predicted more conservative (lower) soil criteria values than did the noncancer hazard (e.g., developmental and reproductive effects). In this analysis, acceptable or tolerable soil dioxin concentrations (TCDD TEQ) ranged from 0.4 to 5.5 ppb at the 95th percentile for cancer potency factors from 9600 to 156,000 (mg/kg/d)(-1) with site-specific adjustments not included. Various possible soil guidelines based on cancer and noncancer risks are presented and discussed. In the main, the current toxicology, epidemiology, and exposure assessment data indicate that the historical 1 ppb TEQ soil guidance value remains a reasonable screening value for most residential sites. This analysis provides risk managers with a thorough and transparent methodology, as well as a comprehensive information base, for making informed decisions about selecting soil cleanup values for PCDD/Fs in urban residential settings.

Geophagy and potential contaminant exposure for terrestrial vertebrates

Geophagy, the ingestion of earth or earthlike substances, is a behavior that occurs in a wide range of vertebrate groups. Geophagy can be intentional: to acquire necessary nutrients from soil such as sodium or calcium or to acquire a stomach balm for parasites or toxins. It can also be incidental: acquired while foraging or grooming or from prey that have ingested soil. With the spreading effects of human "development," soils contaminated by anthropogenic products will be more frequently encountered by wildlife. Direct ingestion of such soils may be detrimental to the health of animals. In some cases information on the fraction of ingesta that is soil will enable adequate inferences about exposure and techniques are described to determine that fraction. In other cases, inferences about exposure require information on the daily rate of soil ingestion. The daily rate of soil ingestion can be estimated using the fraction of ingesta that is soil and the projections for food requirements. Also described here are methods to project mean daily food requirements and the mean daily rate of soil ingestion. Contaminant exposure by geophagy is affected by filtration of soil fractions, binding of some elements into compounds not absorbable through the gut wall, and neutralizing of toxicity after absorption. Bioavailability of contaminants in soil may also be related to taxon. Geophagy as an important mechanism of toxic exposure has been clearly demonstrated in several studies.

Oral bioaccessibility of dioxins/furans at low concentrations (50-350 ppt toxicity equivalent) in soil

Animal studies have indicated that the oral bioavailability of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in environmentally contaminated soil could range from 0.5 to 60%. To estimate the oral bioavailability of TCDD, and the 16 other 2,3,7,8-substituted dioxin/furan congeners, this study used a physiologically based extraction test, designed around the anatomic and physiologic characteristics of the human digestive tract. This test measures the fraction of dioxins/furans in soil that would be solubilized in the gastrointestinal tract (i.e., that would be bioaccessible) and therefore available for absorption. Eight soils from Midland, MI, were evaluated in this study and exhibited TCDD concentrations of 1.7-139 pg/g (ppt) and total TEQ concentrations of 6-340 ppt. Bioaccessibility of dioxins/furans from these soils ranged from 19 to 34% averaged across the 17 2,3,7,8-substituted dioxin/furan congeners), with an average of 25%. The total organic carbon in these soils was low--ranging from 1 to 4%--particularly for the soil series from which they were collected. Bioaccessibility of individual congeners did not appear to be correlated with degree of chlorination. Even though these dioxin/furan concentrations are much less than studied previously, these results are consistent with those from animal studies at other sites, which have generally yielded values of 20-60% relative bioavailability for TCDD in soil.

Comparison of different digestive tract models for estimating bioaccessibility of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) from red slag 'Kieselrot'

'Kieselrot' (red slag), a highly PCDD/F-contaminated leaching residue from a copper production process, has been used as surface layer for more than 1,000 sports fields, playgrounds and pavements in Germany and neighbouring countries. Children can ingest this material directly by hand-to-mouth activities or soil-pica behaviour. Furthermore secondary contamination of farm land or kitchen gardens by drift of red slag dust may lead to an enrichment of PCDD/F within the food-chain. PCDD/F can be mobilized from contaminated materials by digestive juices and thus become bioaccessible for intestinal absorption. Two different digestive tract models were used to estimate the bioaccessibility of PCDD/F from red slag and to study the influence of food material on the mobilization of the contaminants. The bioaccessibility of PCDD/F from red slag depends on the charge of red slag material used, the bile content of the intestinal juice and on the presence of lipophilic foodstuffs. A low bioaccessibility of less than 5% was found when using a digestive tract model with a low bile content and in absence of food material. The bioaccessibility was estimated to be more than 60% when using a model with a higher bile content and in the presence of whole milk powder. A low bioaccessibility of PCDD/F from red slag in general--as assumed until now and mentioned in legal provision--was not confirmed by our study. Considering observations for the different homologue groups it is obvious that bioaccessibility is the first of several important steps to estimate human health risks arising from contaminated materials. In case red slag contaminated with PCDD/F their absorption rate in the digestive tract and/or metabolism might be at least just like important.

Reducing uncertainty in the derivation and application of health guidance values in public health practice. Dioxin as a case study

We were requested by the U.S. Environmental Protection Agency (EPA) to clarify the relationships among the minimal risk level (MRL), action level, and environmental media evaluation guide (EMEG) for dioxin established by the Agency for Toxic Substances and Disease Registry (ATSDR). In response we developed a document entitled "Dioxin and Dioxin-Like Compounds in Soil, Part I: ATSDR Interim Policy Guideline"; and a supporting document entitled "Dioxin and Dioxin-Like Compounds in Soil, Part II: Technical Support Document". In these documents, we evaluated the key assumptions underlying the development and use of the ATSDR action level, MRL, and EMEG for dioxin. We described the chronology of events outlining these different health guidance values for dioxin and identified the areas of uncertainty surrounding these values. Four scientific assumptions were found to have had a great impact on this process; these were: (1) the specific uncertainty factors used, (2) the toxicity equivalent (TEQ) approach, (3) the fractional exposure from different pathways, and (4) the use of body burdens in the absence of exposure data. This information was subsequently used to develop a framework for reducing the uncertainties in public health risk assessment associated with exposure to other chemical contaminants in the environment. Within this framework are a number of future directions for reducing uncertainty, including physiologically based pharmacokinetic modeling (PBPK), benchmark dose modeling (BMD), functional toxicology, and the assessment of chemical mixture interactions.

The form and bioavailability of non-ionic organic chemicals in sewage sludge-amended agricultural soils

The application of sewage sludges to agricultural land may increase the concentrations of many toxic organic chemicals in soils which could have adverse effects on wildlife and human health if these compounds enter foodchains. Chlorobenzenes (CBs), polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) are amongst those compounds currently receiving most attention. The "form' in which these, and other organic chemicals, are present in soils and their potential to be lost by various processes including leaching, volatilisation and (bio)degradation is shown to be dependent on the physicochemical characteristics of the soil and sewage sludge, environmental conditions and the properties of the chemicals themselves. The distinction is made between those compounds that are labile, reversibly sorbed and irreversibly sorbed by sewage sludge-amended soils. The implications of the form in which the chemicals are present in soil for their "availability' to transfer from the soil to bacteria, fungi, earthworms, grazing livestock and food crops followed by the potential for further transfers, metabolism or bioaccumulation are discussed. The importance of the timing and method of sewage sludge application to soil on "form' and "availability' are also considered.

The toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and their relevance for toxicity

This article reviews the present state of the art regarding the toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). The absorption, body distribution, and metabolism can vary greatly between species and also may depend on the congener and dose. In biota, the 2,3,7,8-substituted PCDDs and PCDFs are almost exclusively retained in all tissue types, preferably liver and fat. This selective tissue retention and bioaccumulation are caused by a reduced rate of biotransformation and subsequent elimination of congeners with chlorine substitution at the 2,3,7, and 8 positions. 2,3,7,8-Substituted PCDDs and PCDFs also have the greatest toxic and biological activity and affinity for the cytosolic arylhydrocarbon (Ah)-receptor protein. The parent compound is the causal agent for Ah-receptor-mediated toxic and biological effects, with metabolism and subsequent elimination of 2,3,7,8- substituted congeners representing a detoxification process. Congener-specific affinity of PCDDs and PCDFs for the Ah-receptor, the genetic events following receptor binding, and toxicokinetics are factors that contribute to the relative in vivo potency of an individual PCDD or PCDF in a given species. Limited human data indicate that marked species differences exist in the toxicokinetics of these compounds. Thus, human risk assessment for PCDDs and PCDFs needs to consider species-, congener-, and dose-specific toxicokinetic data. In addition, exposure to complex mixtures, including PCBs, has the potential to alter the toxicokinetics of individual compounds. These alterations in toxicokinetics may be involved in some of the nonadditive toxic or biological effects that are observed after exposure to mixtures of PCDDs or PCDFs with PCBs.

Recent developments on the hazards posed by 2,3,7,8-tetrachlorodibenzo-p-dioxin in soil: implications for setting risk-based cleanup levels at residential and industrial sites

Since the publication of the Times Beach risk assessment in 1984, which suggested that residential soils were of concern when the level of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was in excess of 1 ppb, there has been continued interest in this topic. Studies conducted within the past 5 yr on the environmental and toxicological behavior of TCDD, as well as refinement of parameters regarding human exposure, indicate that previous assessments of the risk to humans posed by TCDD-contaminated soil were overestimated. In this paper, recent information drawn from nearly 100 recently published articles regarding the histopathology interpretation of the Kociba bioassay, environmental fate and half-life of TCDD in soil, and estimates of human exposure via soil ingestion, dermal contact, inhalation, surface runoff, and the consumption of fish were incorporated into a risk assessment. Cleanup levels for TCDD in residential and industrial soils were calculated based on most likely exposure scenarios. Probability distributions of key exposure parameters were incorporated into a Monte Carlo uncertainty analysis to predict the range and probability of TCDD uptake and corresponding cleanup levels in soil. This analysis demonstrated that the most significant route of human exposure to TCDD is through dermal contact with soil, followed by soil ingestion, fish consumption, and inhalation of airborne particulates. At residential sites, soils containing 20 parts per billion (ppb) of TCDD were found to pose a lifetime cancer risk no greater than 1 in 100,000 (10(-5) risk) under typical exposure conditions. Based on the Monte Carlo analysis, soil concentrations for the 75th and 95th percentile person were 12 and 7 ppb (10(-5) risk), respectively. In industrial soils, TCDD concentrations ranged between 131 and 582 ppb (10(-5) risk), depending on the amount of time spent outdoors under typical exposure conditions. Industrial soil concentrations of approximately 93 and 46 ppb (10(-5) risk) were calculated for the 75th and 95th percentile worker, respectively, engaged in outdoor activities. The range of TCDD concentrations in industrial soils was not reduced significantly when the consumption of fish from a neighboring waterway by off-site receptors was considered. While cleanup levels for TCDD should be derived on a site-specific basis, this analysis indicated that soil cleanup standards can be generally higher than those implemented over the past 8 yr.

An assessment and quantitative uncertainty analysis of the health risks to workers exposed to chromium contaminated soils

Millions of tons of chromite-ore processing residue have been used as fill in various locations in Northern New Jersey and elsewhere in the United States. The primary toxicants in the residue are trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)]. The hazard posed by Cr(III) is negligible due to its low acute and chronic toxicity. In contrast, Cr(VI) is considered a inhalation human carcinogen at high concentrations. Approximately 40 commercial and industrial properties in Northern New Jersey have been identified as containing chromite ore processing residue in the soil. One site, a partially-paved trucking terminal, was evaluated in this assessment. The arithmetic mean and geometric mean concentrations of total chromium in soil were 977 and 359 mg/kg, respectively. The data were log-normal distributed. The arithmetic mean and geometric mean concentrations of Cr(VI) in surface soil were 37.6 and 3.1 mg/kg, respectively. The data could not be fit to a standard distribution, likely due to the large number of samples with concentrations below the method detection limit (65%). Dose was calculated for each exposure route using a Monte Carlo statistical simulation. Probability distributions of most exposure parameters were incorporated into the analyses to predict the range and probability of uptake for persons in the exposed population. The exposure parameter distributions included in this assessment are: the concentrations of Cr(VI) and total chromium in air and soil, fraction of the year when suspension of airborne soil particulates is likely to occur due to weather conditions, fraction of Cr(VI) in air which is respirable (less than 10 microns), soil loading rate on skin, occupational tenure, and body weight. The techniques used in this assessment are applicable for evaluating the human health risks posed by most industrial sites having contaminated soil. The estimated average daily dose (ADD) via ingestion and dermal absorption for the individual exposed at the 95th percentile was about 48,000- and 91-fold below the U.S. EPA reference dose (RfD) for Cr(III) and Cr(VI), respectively. Since inhalation of Cr(VI) contaminated dust (but not ingestion or dermal contact) poses a cancer hazard, the lifetime average daily doses (LADDs) associated with exposure at the 50th and 95th percentile were calculated to be 9.8 x 10(-8) and 1.3 x 10(-6), respectively. Based on this analysis, industrial sites having soil concentrations of Cr(VI) below 230 ppm do not pose a significant noncarcinogenic or carcinogenic health hazard following acute or chronic exposure. These risks would be even smaller if the sites were paved.

Human exposures from dioxin in soil--a meeting report

A 1984 risk assessment identified 1 part per billion (ppb) of dioxin in soil as a "level of concern" at Times Beach, Mo. The authors of the assessment had to rely on many assumptions in their analysis, but since that time, a number of investigators have obtained data that bear directly on estimating exposures from substances in soil. Partly because of the assumptions and partly because of the site-specific nature of their analysis, the authors of the Times Beach risk assessment cautioned against the adoption of 1 ppb as a delineator between acceptable and nonacceptable levels of contamination. Those cautions have been more frequently ignored than honored, and 1 ppb has become a de facto standard. In November 1989, the Center for Risk Management at Resources for the Future hosted 50 experts at a workshop that heard and discussed published and new research about exposure estimates and measures. The 1984 assessment identified soil ingestion by toddlers as the single most important source of exposure to dioxin in soil; it assumed that toddlers ingested 10 g soil daily. Research discussed at the workshop shows that the average child ingests about 0.04 g soil daily, but that 1 of 320 studied children ingested 5 g. These findings leave open the risk management decision about whether acceptable exposure levels should be established to protect the average child or the extreme child. Furthermore, the absence of children from commercial and industrial sites led to suggestions that higher concentrations of dioxin are acceptable in soil at such sites. Some workshop participants objected to those suggestions because of the difficulty of assuring that such sites would not revert to residential use in the future. Whether deed restrictions are sufficient to prevent such reversions was seen as an important research topic. Workshop participants repeatedly discussed the importance of site-specific data in estimating exposures: Measured half-lives of dioxin in different soils range from 18 mo to greater than 100 yr; bioavailability from various soils differs by at least 20-fold; and the amounts of soil ingested by grazing cattle can differ 20-fold depending on conditions. Workshop participants agreed upon some suggestions for research and generally favored the development of guidelines for exposure assessment that would allow consideration of site-specific information. Furthermore, they generally agreed that no single concentration should be taken as a level of concern. Instead, levels should be established that consider the planned uses of the sites.

Long-term carcinogenesis studies on 2,3,7,8-tetrachlorodibenzo-p-dioxin and hexachlorodibenzo-p-dioxins

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 1,2,3,6,7,8- and 1,2,3,7,8,9-hexachlorodibenzo-p-dioxins (HCDDs) are among the most toxic and carcinogenic of "man-made" chemicals. These "dioxins," as well as many of the other polychlorinated dibenzodioxins (PCDDs) and dibenzofuran (PCDFs) derivatives, are chlorinated aromatic compounds which are chemically stable, insoluble in water, and highly soluble in fats and oils. TCDD acts as a complete carcinogen in several species, causing both common and uncommon tumors at multiple sites. It is a highly potent chemical carcinogen in chronic animal studies, producing carcinogenic effects in laboratory animals with doses as low as 0.001 micrograms/kg/day. In rats, TCDD induces neoplasms in the lung, oral/nasal cavities, thyroid and adrenal glands, and liver. In mice, TCDD induces neoplasms in the liver and subcutaneous tissue, thyroid gland, and thymic lymphomas. In hamsters, it induces squamous cell carcinomas of the facial skin. Tumors of the integumentary system are reported after oral (mice and rats), intraperitoneal (hamsters), and dermal (mice) administration. A mixture of HCDDS (defined as the mixture of the 1,2,3,6,7,8- and 1,2,3,7,8,9 isomers used in the NTP experiments) are potent liver carcinogens in mice and rats. Pharmacokinetic studies in laboratory animals indicate that 50-90% of dietary TCDD is absorbed. It concentrates in adipose tissue and the liver. In mammals, the TCDD present in the liver is slowly redistributed and stored in fatty tissue. Elimination of TCDD occurs via excretion of metabolites in the bile and urine and passively through the gut wall. Metabolism is slow: the biological half-life of TCDD varies from weeks (rodents) to years (humans), and is strongly dependent upon the rate of TCDD metabolism. Many of the toxic effects of TCDD, including teratogenicity, may arise by receptor-mediated mechanisms. The induction of cytochrome P-448 and related enzymes by TCDD occurs by such a mechanism, and is related to the binding of TCDD to the Ah receptor. The specific mechanism(s) by which TCDD exerts its carcinogenic effects is unclear: receptor-binding may be part of the story. The role of the Ah receptor has been indicated in a skin promotion assay. The evidence for mutagenicity is inconclusive. TCDD did not induce lethal mutations, chromosomal aberrations, micronuclei or sister chromatid exchanges in rodents treated in vivo, nor was it mutagenic to bacteria, but it did enhance transformation of mouse C3H 10T1/2 cells by N-methyl-N'-nitro-N-nitrosoguanidine and was mutagenic to mouse lymphoma cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Correlation between acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and total body fat content in mammals

Single oral 30-day LD50s of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were correlated with total body fat (TBF) content in various species and strains of laboratory mammals. LD50 values and TBF contents were either obtained from the literature or determined by experiments. A log (LD50) vs. log (TBF) plot yielded a highly significant linear regression equation (r2 = 0.834, P less than 0.001, n = 20). It is suggested that this correlation exists for at least two reasons: (1) increasing TBF content in organisms represents an enhanced capacity to remove TCDD from the systemic circulation and (2) different TBF content reflects a differential role and regulation of fat metabolism for various organisms. Extrapolation of this correlation to man suggests that adult humans are among the less sensitive species to the acute toxicity of TCDD.

How toxic is 2,3,7,8-tetrachlorodibenzodioxin to humans?

The tissue levels of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) measured in different species are reviewed. Based on the correlation of tissue levels and a toxic response in different species, humans are less or no more susceptible to the toxic effects of TCDD than most of the laboratory animals that have been studied. Thus, the present exposure of the general population to environmental levels of TCDD are related compounds should not be of concern.

Pharmacokinetics of [125I]-2-iodo-3,7,8-trichlorodibenzo-p-dioxin in mice: analysis with a physiological modeling approach

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent inducer of hepatic microsomal enzymes. The influence of an inducing dose of TCDD on tissue distribution and other pharmacokinetic behavior of a TCDD analog in the mice was examined by employing a high specific activity radioligand. [125I]-2-iodo-3,7,8-trichlorodibenzo-p-dioxin (ITCDD). Female C57BL/6J mice were pretreated with 0.1 mumol/kg of TCDD or the vehicle only, followed by 0.1 nmol ITCDD/kg 3 days later. The control animals had the highest concentration of ITCDD-derived radioactivity in the fat, but the TCDD-pretreated animals had the highest concentration in their livers. Whole-body elimination of ITCDD approximated first-order behavior, and induction by pretreatment with the inducing dose of TCDD almost doubled the rate of excretion (control mice, t1/2 = 14.2 days; pretreated mice, t1/2 = 8.0 days). All disposition results in naive and pretreated mice were satisfactorily described by a consistent physiologically based pharmacokinetic model (Leung et al., 1988a) in which induction increased the amount of microsomal ITCDD-binding protein from 1.75 to 20 nmol/liver and increased the rate constant for metabolism of free ITCDD from 1 to 3/hr/kg liver. The binding affinity of the microsomal ITCDD-binding protein was the same (20 nM) in both induced and noninduced mice. Model simulations indicated a time delay in the elimination of nonparent ITCDD metabolites from the body and a more rapid absorption of the parent ligand in the pretreated mice. Consistent with previous physiological modeling with TCDD in different mouse strains, the primary factor influencing the liver/fat concentration ratio appears to be the affinity and capacity of the microsomal TCDD-binding proteins, which are altered by induction. These dose-dependent pharmacokinetic differences with ITCDD are important considerations for TCDD risk assessment in which data from high dose rodent experiments are extrapolated to predict behavior at much lower environmental concentrations in exposed humans.

A physiological pharmacokinetic description of the tissue distribution and enzyme-inducing properties of 2,3,7,8-tetrachlorodibenzo-p-dioxin in the rat

A five-compartment physiologically based pharmacokinetic (PB-PK) model was developed to describe the tissue disposition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the Sprague-Dawley rat. This description included blood, liver, fat, muscle/skin, and visceral tissue groups. On the basis of other literature, the liver compartment was modeled to include two TCDD-binding sites, corresponding to a cytosolic receptor and a microsomal binding protein. A pharmacodynamic description was developed in which microsomal enzyme induction, both of arylhydrocarbon hydroxylase activity and of the amount of the microsomal TCDD-binding protein, was linked to fractional occupancy of the cytosolic receptor. This description was then used to analyze previously published data on TCDD disposition. The dissociation constant of the cytosolic Ah receptor (KB1) in vivo was estimated to be 15 pM by fitting enzyme induction data from McConnell et al. (1984). The ratio of liver to fat concentration of TCDD (about 4:1) was found to be primarily determined by the dissociation constant of the microsomal binding protein (7 nM) and the basal and induced concentration of this protein in the liver (25 and 200 nmol/liver, respectively). With these parameter values, the tissue distribution of TCDD in fat and liver, the two primary sites of accumulation, was accurately described following either single or repeated dosing with TCDD in the rat. The pharmacokinetic behavior described by the model was extremely sensitive to binding affinities, and only moderately sensitive to binding capacities in the dose range studied. Induction of microsomal TCDD-binding proteins was necessary in order to account for the differences in disposition at low (0.01 microgram/kg) and high (1.0 microgram/kg) daily doses of TCDD. Since the tumorigenicity of TCDD in rats is believed to be correlated with the biological responses of the Ah-TCDD complex, the present physiological pharmacokinetic description, which contains information on receptor occupancy at various dose levels, provides a plausible mechanistic connection for devising pharmacodynamic models which predict the cancer risk of TCDD in the rat.

Evaluation of potential transmission of 2,3,7,8-tetrachlorodibenzo-p-dioxin-contaminated incinerator emissions to humans via foods

Interest in the potential sources of human exposure to TCDD (dioxins, TCDD and equivalents, or 2,3,7,8-tetrachlorodibenzo-p-dioxin) via foods has recently shifted from phenoxy herbicides to products of combustion and waste disposal. Proposals to locate municipal waste combustors in rural areas have raised concerns that emissions, which could contain TCDD, could contaminate animal feeds and such human foods as milk, meat, and vegetables. Important factors that can affect the results of an assessment of incinerator emissions include (1) the emission and deposition rates of TCDD from the source, (2) the fractional retention and half-life of fly ash on plants, (3) the environmental half-life of TCDD, (4) the animal feeding and management systems, (5) the bioavailability of TCDD and related compounds, (6) the metabolism and pharmacokinetics of TCDD in farm animals, (7) food consumption levels, (8) the half-life of TCDD in humans, and (9) the model selected to estimate cancer risk. For persons living in the area of highest deposition near an incinerator, a possible uptake of TCDD from foods of animal origin was estimated to be about 10-40 fg/kg.d, which was much greater than the 1-5 fg/kg.d uptake estimated for foods of plant origin. The total uptake of TCDD from foods by the maximally exposed population will usually be about 500- to 1000-fold greater than that due to inhalation. Although milk was assumed to be the most important food pathway in several previous assessments that evaluated the hazards of airborne emissions, we determined that the deposition-forage-cattle-beef pathway was the more important route of exposure. The previous assessments appear to have used inappropriate pharmacokinetic models for TCDD and to have overestimated pasture use for dairy cows. The amount of TCDD accumulated in soil from airborne emissions was found to be less important than the amount deposited in forage, a finding that is the opposite of the usual conclusions drawn for other routes of TCDD introduction into agricultural environments. Based on the assumption and parameters used in this assessment, the potential human health risks due to TCDD emissions from incinerators are insignificant compared to other background sources of TCDD. It would be desirable to measure TCDD in soil and crops around existing facilities to better evaluate this assessment, but it is likely that concentrations would be too low to reliably quantitate.

Improving toxicology testing protocols using computer simulations

Computer simulation can be used to integrate existing toxicity information within a biologically realistic framework. Simulation models calculate relevant measures of target tissue dose based on physiological, biochemical and physicochemical properties and readily support the dose, route, species and interchemical extrapolations necessary for human risk assessment. Because these models require very specific information, much of which can be obtained in vitro, they are much less dependent on extensive animal experiments than conventional risk assessment methods. With continuing development, simulation modeling will become an invaluable tool for improving experimental designs, for interpreting animal toxicity tests, and for estimating the importance of the animal toxicity observations for people.

Important recent advances in the practice of health risk assessment: implications for the 1990s

Health risk assessments have been so widely adopted in the United States that their conclusions are a major factor in many environmental decisions. The procedure by which these assessments are conducted is one which has evolved over the past 10-15 years and a number of short-comings have been widely recognized. Unfortunately, improvements in the process have often occurred more slowly than advancements in technology or scientific knowledge. Recent significant advances for more accurately estimating the risks posed by environmental chemicals are likely to have a dramatic effect on the regulation of many substances. Each of the four portions of risk assessment (hazard identification, dose-response assessment, exposure assessment, and risk characterization) has undergone significant refinement since 1985. This paper reviews some of the specific changes and explains the likely benefits as well as the implications. Emphasis is placed on the improved techniques for (a) identifying those chemicals which may pose a human cancer or developmental hazard, (b) using statistical approaches which account for the distribution of interindividual biological differences, (c) using lognormal statistics when interpreting environmental data, (d) using physiologically based pharmacokinetic models for estimating delivered dose and for scaling up rodent data, (e) using biologically based cancer models to account for the seven or more apparently different mechanisms of chemical carcinogenesis, (f) describing the severity of the public health risks by considering those portions of the population exposed to various concentrations of a contaminant, and (g) reviewing how criteria for acceptable risk have been influenced by the number of exposed persons. The net benefit of these improvements should be a reduction in the uncertainty inherent in current estimates of the health risks posed by low level exposure to carcinogens and developmental toxicants.

Dioxins and the Ah receptor

Despite continuing controversies related to public policy, information on the molecular biology of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has advanced significantly over the past decade. Current understanding of the biological mechanisms of TCDD action is based upon the interactions of TCDD with a genetically expressed cytosolic macromolecule that functions as a receptor in many cells across many species. The Ah receptor recognizes TCDD and structurally similar molecules and serves as the transducing step whereby TCDD alters gene expression through the association of the TCDD:receptor complex with specific TCDD-responsive elements on the genome. Understanding these molecular events and their relevance to the organ-level manifestations of TCDD toxicity may be critical to formulating scientifically based assessments of the risk of TCDD exposure.

Bioavailability and cytochrome P-450 induction from 2,3,7,8-tetrachlorodibenzo-P-dioxin contaminated soils from Times Beach, Missouri, and Newark, New Jersey

Bioavailability of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) from contaminated soils from Times Beach, Missouri and Newark, New Jersey, was examined using liver concentrations and toxicity in guinea pigs observed up to 60 days following a single oral administration, and induction of cytochrome P-450 in rats sacrificed 24 hours after a single oral dose as endpoints. Both soils are contaminated with several chlorinated dioxins and numerous other compounds. Times Beach soil resulted in greater TCDD concentration in liver and TCDD was considerably more bioavailable from Times Beach soil than from Newark soil. However, both soils induced cytochrome P-450 activity to approximately the same extent. Moreover, similar banding patterns of microsomal proteins were seen on polyacrylamide electrophoretic gels. The many other compounds present in the soils, particularly in Newark, may account for the similar protein bands and levels of cytochrome P-450 observed.

The influence of aging on intestinal absorption of TCDD in rats

The effects of age on intestinal absorption of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were studied using adult male Fischer-344 rats of 3 different age groups: 13 weeks old (young), 13 months old (mature), and 26 months old (senescent). Absorption was measured with an in situ intestinal recirculation perfusion procedure. Absorption expressed in terms of ng TCDD absorbed/g intestinal dry weight/h was 166, 149 and 143 ng/g/h in the young, mature and senescent groups, respectively. When absorption was calculated in terms of ng TCDD absorbed/g mucosal dry weight/h, the decrease between the senescent rats and the 2 younger age groups, from 544 ng/g/h (young) to 351 ng/g/h (senescent), was not statistically significant (P less than 0.05). It was demonstrated that absorption of TCDD was unaffected by the presence of 2,4,5,2',4',5'-hexachlorobiphenyl (HCB) in the perfusate, but that HCB absorption was (P less than 0.01) enhanced by the presence of TCDD.

A critical examination of assumptions used in risk assessments of dioxin contaminated soil

Environmental standards for 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin, TCDD) are currently being considered by regulatory agencies worldwide. Among these are limits for tap water, soil at industrial sites, residential soil, fish, ambient air, and fly ash. Thus far, in the United States, no standards have been promulgated but a few have been suggested. This paper critically evaluates several aspects of previously proposed approaches to setting limits for TCDD in residential soil and soil within industrial sites. Factors and assumptions which significantly affect the predicted degree of hazard associated with exposure to soil contaminated with low levels of dioxin are discussed. This paper shows how different, more justifiable assumptions than those used by the Centers for Disease Control (CDC) regarding the quantities of soil typically consumed by children, TCDD's nongenotoxicity, dermal exposure to soil, the concentration of airborne soil particles, dioxin's bioavailability in soil, and extrapolation of the dose response curve can profoundly affect the results of the risk assessment and, subsequently, the magnitude of the recommended limits. Two case studies which quantitatively illustrate the effect of these assumptions on the risk estimates are presented. Non-U.S. regulatory agencies have considered TCDD's nongenotoxicity in estimating that the virtually safe dose (VSD) or acceptable daily dose for dioxin is approximately 10 pg/kg/day (10,000 fg/kg/day). These approaches are compared and contrasted with the method used by the United States EPA whose risk estimates are higher and whose VSD is approximately 1,000-fold lower. Alternative approaches to interpreting the cancer data indicate that a VSD of 130 pg/kg/day is more scientifically justified than risks estimated using standard approaches. This assessment indicates that a soil concentration of TCDD considerably in excess of 1 ppb should be acceptable for residential and nonresidential areas.

Bioavailability of dioxin in soil from a 2,4,5-T manufacturing site

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD) is a highly toxic contaminant produced in the manufacture of phenoxy herbicides. Despite its high TCDD content, soil from a contaminated area associated with a 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) manufacturing site in Newark, New Jersey, did not induce acute toxicity when administered to guinea pigs (the most sensitive species) by gavage. Analysis of liver samples demonstrated low bioavailability of TCDD from this soil. A comparative analysis of soils showed that Soxhlet extraction was necessary for the determination of TCDD on Newark soil, whereas solvent extraction was sufficient for soil from Times Beach, Missouri. The difference in the bioavailability of TCDD from these soils is correlated with TCDD extractability and may be related to the different compositions of the soils.

Uptake of 2,3,7,8-tetrachlorodibenzo-p-dioxin from plasma lipoproteins by cultured human fibroblasts

Tritiated 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) added to human plasma in vitro associated with the plasma lipoproteins. The effects of plasma and lipoproteins on cellular uptake of dioxin were studied using normal human skin fibroblasts and mutant fibroblasts from a patient with homozygous familial hypercholesterolemia. The latter cells lack the normal cell membrane receptor for low density lipoprotein (LDL). The time- and temperature-dependent cellular uptake of [3H]dioxin was greatest from LDL, intermediate from high density lipoprotein (HDL) and least from serum. A significantly greater uptake from LDL by the normal cells compared to the mutant cells indicated the involvement of the LDL receptor-mediated pathway. Concentration-dependent studies indicated that the cellular uptake at 37 degrees C of [3H]dioxin varied linearly with dioxin concentration at constant LDL concentration. Thin-layer chromatography (TLC) showed that conversion to more polar compounds may have occurred after 24-h incubation with cells. [3H]Dioxin could be removed from cells efficiently by incubation with 20% serum greater than HDL greater than LDL. Since the vehicle of delivery may influence subsequent location and metabolism of this compound in cells, it is concluded that the physiologic vehicles (either serum- or LDL-associated dioxin), rather than organic solvents, should be used in experiments with cultured cells or perfused organs.

[The liver and environmental poisons]

For almost a century now numerous examples of acute and subacute hepatic injury from exposure to toxic agents in the occupational or non-occupational environment have been extensively studied and are well documented, but such events are comparatively rare. In contrast, epidemiological data associating exposure to environmental chemicals with chronic liver disease or primary hepatic malignancies in the human is scarce as compared with the vast body of literature concerning chronic pulmonary disease as a consequence of exposure at the workplace. Large-scale industrial production of many newly synthesized organic chemicals began during the period 1930-1940 but it was not until the 1960s that the output increased exponentially. Consequently, the spectrum of environmental influences is gaining increasing complexity since simultaneous or sequential exposure to a variety of pollutants is becoming the rule rather than the exception. Possible interaction or synergism of environmental agents--even of those which in themselves or for their low dosage level may be considered "harmless" - and particularly latency periods of more than one decade further complicate preventive strategies. The liver, as the central site for the biotransformation of xenobiotics, deserves special attention when new chemicals which are to be introduced into the environment are being tested for their potential toxicity, especially since many hepatotoxic agents have been shown to undergo bioactivation in the liver. Currently available information on hepatic injury due to environmental agents is briefly reviewed and comprises solvents and degreasing agents, pesticides, polyhalogenated biphenyls, dioxins and dibenzofuranes, epoxy resin hardeners, vinyl chloride, naturally occurring hepatotoxins in plants and fungi, herbal medicines and traditional remedies and a side-light on the Reye syndrome and the Spanish "toxic oil syndrome".

Teratogenic potency of TCDD, TCDF and TCDD-TCDF combinations in C57BL/6N mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-tetrachlorodibenzofuran (TCDF) cause the same spectrum of fetal anomalies in C57BL/6N mice. Pregnant dams were treated with TCDD, TCDF and combinations of the 2 compounds on gestation day 10, and examined for maternal and fetal effects on day 18. The fetal kidneys were the most sensitive target for teratogenicity. The dose response for cleft palate induction fit the probit model for both compounds, suggesting that TCDD was approximately 30 times more potent than TCDF. The interaction between these 2 compounds was consistent with a model for additive toxicity.

Toxic interaction of specific polychlorinated biphenyls and 2,3,7,8-tetrachlorodibenzo-p-dioxin: increased incidence of cleft palate in mice

The induction of cleft palate in C57BL/6N mice is an extremely reproducible and sensitive indicator of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity. This endpoint was used to look for potential interactions between two polychlorinated biphenyl (PCB) congeners and TCDD. Both 2,3,4,5,3',4'-hexachlorobiphenyl (HCB) and 2,4,5,2',4',5'-HCB are of relatively low toxic potency, but their biological properties differ. Pregnant mice were treated with TCDD and either HCB on gestation Days 10 through 13, and the fetuses examined for the presence of cleft palate and renal abnormalities on gestation Day 18. At a dose of TCDD which caused a low level of cleft palate, moderate hydronephrosis was observed. No renal or palatal anomalies were detected after 2,4,5,2',4',5'-HCB treatment, and the combination of this isomer with TCDD had no effect on the incidence of TCDD-induced cleft palate. 2,3,4,5,3',4'-HCB caused mild renal toxicity, but no cleft palate. However, treatment of pregnant mice with a combination of TCDD and 2,3,4,5,3',4'-HCB resulted in a 10-fold increase in the incidence of cleft palate. Thus, the toxicity of compounds such as TCDD may be enhanced by compounds of relatively low acute toxicity such as selected PCBs. The widespread environmental occurrence of such combinations suggests a need for further evaluation of the mechanism of this interaction.

Bioavailability of soil-borne polybrominated biphenyls ingested by farm animals

Concentrations of polybrominated biphenyl (PBB) were measured in the fat of livestock on several farms on which soil-borne PBB in confinement areas was the only source of residue. Ratios of concentrations in fat to concentrations in soil were 0.37 for dairy heifers, 0.27 for primaparous dairy cows, 0.10 for multiparous dairy cows, 0.27 for beef cows, 0.39 for beef calves, 0.37 for ewes, and 1.86 for swine. Multiparous dairy cows had lower ratios because of the excretion of PBB in milk during long-term lactation, and swine had higher ratios because they ingest greater amounts of soil than other species. Diets containing 5% PBB-contaminated soil, or 5% contaminated soil amended with activated carbon, were fed to lambs for 56 d. Accumulation of soil-borne PBB in fat, when adjusted for intake, did not differ significantly from accumulation of PBB from a diet in which PBB was added to cornmeal. Amending soil with activated carbon had no effect on residue accumulation. About 70% of PBB in a control diet with PBB added to cornmeal was absorbed, as measured by using titanium as an unabsorbed marker. Absorption of soil-borne PBB was 65% from unamended soil, 57% from soil amended with 0.3% activated carbon, and 56% from soil amended with 0.6% activated carbon. The differences were not great enough to be of practical importance. These results with PBB may be useful in assessing and managing risks of other soil-borne contaminants that have chemical characteristics similar to those of PBB.

Characteristics of an extraction and purification procedure for chlorinated dibenzo-p-dioxins and dibenzofurans in soil and liver

Liver is extracted with chloroform-methanol to give essentially quantitative transfer of endogenous chlorinated dibenzo-p-dioxins (CDDs) and dibenzofurans (CDFs) into the organic phase. A new procedure involving LH-20 Sephadex is used to remove most of the lipids from the extract. Soil is extracted by a simple, rapid and economical procedure giving very high recoveries of CDDs and CDFs from sandy soil, various types of clay, and humus-rich loam. Subsequent cleanup on basic and acidic alumina complete the preparation for gas chromatography-mass spectrometric analysis. The use of propylene glycol as a "keeper" and of 2,3,7-trichlorodibenzo-p-dioxin as a carrier minimizes losses during evaporation of solvents and on glass surfaces. Interactions of 2,3,7,8-CDD with organic material in loam slightly reduce recovery but there is no indication of high affinity binding sites, the losses being apparently associated with simple distribution coefficients. Special precautions needed to avoid losses of CDFs on alumina chromatography are described, and the effect of "aging" spiked soil is discussed.

Health implications of 2,3,7,8-tetrachlorodibenzodioxin (TCDD) contamination of residential soil

Extrapolations from animal toxicity experiments (including carcinogenicity and reproductive effects) to possible human heath effects can be used to estimate a reasonable level of risk for 2,3,7,8-tetrachlorodibenzodioxin (2,3,7,8-TCDD). Extrapolations are derived from: (1) review of published studies, (2) a complex set of assumptions related to human exposure to contaminated soil, and (3) estimates of (a) a dose response curve, (b) appropriate margins of safety, and/or (c) applicable mechanisms of action. One ppb of 2,3,7,8-TCDD in soil is a reasonable level at which to begin consideration of action to limit human exposure for contaminated soil.