Dioxins and dibenzofurans in adipose tissue of US Vietnam veterans and controls

Agent Orange and dioxin-induced myeloid leukemia: a weaponized vehicle of leukemogenesis

Agent Orange (AO) was the dominant weaponized herbicide employed by the United States (US) military during the Vietnam war. AO, however, was found to be regularly contaminated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most toxic dioxin known; furthermore, AO was commonly diluted in the field with other aromatic hydrocarbons to assist with delivery mechanisms. Unbeknownst to the US military and the millions exposed, these events have likely contributed to the development of acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) that has affected many veterans. Null studies regarding an association between AO exposure and AML/MDS are limited in their methodology and application. The acknowledgement that the known carcinogen TCDD was a contaminant in AO when paired with a strong biological plausibility for its leukemogenicity and an observed increased risk of AML/MDS in TCDD-exposed individuals should suffice to establish causal association and that veterans to whom this might apply should be awarded appropriate indemnity.

Internal distribution and fate of persistent organic contaminants (PCDD/Fs, DL-PCBs, HBCDs, TBBPA, and PFASs) in a Bos Taurus

While terrestrial organisms such as livestock are consumed regularly, studies of internal distribution and bioaccumulation of persistent organic pollutants (POPs) have been focused more on aquatic organisms. In this study, we have assessed the internal distribution and fate of legacy (PCDD/Fs and PCBs) and emerging POPs (HBCDs and PFASs), and TBBPA in 42 tissues of a Bos Taurus. PCDD/Fs, DL-PCBs, and HBCDs were found 3, 4, and 4-fold higher in the lipid-rich organs (subcutaneous fat, visceral fat, large intestine) compared to the remaining organs and muscles, owing to their hydrophobic properties. The TBBPA concentration in the excrement was 36-fold higher compared to the average tissues, suggesting a short internal half-life of TBBPA. Among PFASs, PFUnDA displayed 98% contribution from all ionic PFASs in the tissues due to its strong binding affinity, high exposure via feed and water, and increasing emergence of PFUnDA and its precursors in the Southeast Asian countries. While our study suggests that, at the moment, there is no significant health risks to the general Korean population, the future changes in environmental exposure as well as the internal dynamics and fate of various POPs species should be kept in mind when consuming various parts of livestock.

Application environmental epidemiology to vehicular air pollution and health effects research

Vehicular pollution is one of the major contributors to the air pollution in urban areas and perhaps and accounts for the major share of anthropogenic green-house gases such as carbon dioxide, carbon monoxide, nitrogen oxides. Knowledge of human health risks related to environmental exposure to vehicular pollution is a current concern. Analyze the range health effects are attributed varied constituents of vehicular air pollution examine evidence for a causal association to specific health effect. In many instances scenario involves exposure to very low doses of putative agents for extended periods, sometimes the period could mean over a lifetime of an individual and yet may result in small increase in health risk that may be imperceptible. Secondary data analysis and literature review. In environmental exposures, traditional epidemiological approaches evaluating mortality and morbidity indicators display many limiting factors such as nonspecificity of biological effects latency time between exposure and magnitude of the effect. Long latency period between exposure and resultant disease, principally for carcinogenic effects and limitation of epidemiological studies for detecting small risk increments. The present paper discusses the methodological challenges in studying vehicular epidemiology and highlights issues that affect the validity of epidemiological studies in vehicular pollution.

Environmental fate and dietary exposures of humans to TCDD as a result of the spraying of Agent Orange in upland forests of Vietnam

The fate and transport of 2,3,7,8-tetrachloro-p-dibenzodioxin (TCDD) released into the environment of South Vietnam (SVN) as a consequence of the aerial application of the herbicidal defoliant Agent Orange (AO) were simulated for a generic upland forest scenario and followed over a 50-year period (1965, 1968 and 1970 onwards). Modeled concentrations of TCDD in the environment were then used as inputs to a human exposure model, which focused on long-term exposures via the food chain. Intake rates and body burdens of TCDD were estimated for adult males over the course of the simulation period and compared to available biomonitoring data. One of the most important factors determining the magnitude of the simulated human exposure to TCDD was the fraction of the chemical deposited directly to soil (where it was assumed to have a degradation half-life of 10 or 15years) relative to the fraction assumed to remain on/in the forest canopy following the spray application (where it was assumed to have a degradation half-life of ≤48h). The simulated body burdens under the various scenarios considered were broadly consistent with the biomonitoring data from SVN collected in the mid-1980s to late 1990s. Taken together, the modeling results and empirical data suggest that highly elevated exposures to TCDD (i.e., body burdens in the several 100s of pg/g lipid range and greater) were not common among people inhabiting upland forest locations in SVN sprayed with AO and that peak and average body burdens were broadly similar to those of the general population of the U.S. in the 1970s and early 1980s. The model-based assessment is consistent with the 'hot spot' hypothesis i.e., potential exposures to TCDD linked to activities conducted on or near former bases where AO was stored are greater than potential exposures in areas subjected to aerial spraying.

Serum concentrations of TCDD and other dioxin-like compounds in US Air Force veterans of Operation Ranch Hand

We measured serum concentrations of seven dibenzo-p-dioxin congeners (PCDDs), ten dibenzofurans (PCDFs), four non-ortho polychlorinated biphenyls (noPCBs) and six mono-ortho polychlorinated biphenyls (moPCBs) in 1950 veterans of the Vietnam War. The veterans were participants in the Air Force Health Study (AFHS) who attended the final medical examination in 2002. Blood samples were collected from 777 Ranch Hands involved in the aerial spraying of herbicides in Vietnam and a comparison group of 1173 veterans ("Comparisons") who served in Southeast Asia during the same time period. Results for moPCBs were based on a random subsample of 800 veterans. The median 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) concentrations in 2002 were 5.0 pg g(-1) lipid in Ranch Hands and 2.2 pg g(-1) lipid in Comparisons. No substantial differences were found in measured concentrations of other PCDDs, PCDFs, and noPCBs. Similarly, no substantial differences were found for moPCBs in the subsample. The median total dioxin toxic equivalent (TEQ) in Ranch Hands was 18.7 pg g(-1) lipid for PCDDs, 3.4 pg g(-1) lipid for PCDFs, and 3.2 pg g(-1) lipid for noPCBs. Median TEQs in Comparisons were 14.4 pg g(-1) lipid for PCDDs, 3.5 pg g(-1) lipid for PCDFs, and 3.3 pg g(-1) lipid for noPCBs. These TEQs, with the exception of PCDD TEQ in Ranch Hands (primarily due to elevated TCDD), were similar to or lower than those reported for similar age and gender groups in the 2001-2002 National Health and Nutrition Examination Survey (NHANES). These findings support the assumption that the Ranch Hand veterans were not more highly exposed to dioxin-like compounds other than TCDD than were Comparison veterans or the general US population.

Cancer incidence among police officers in a U.S. northeast region: 1976-2006

Police officers are exposed to occupational hazards which may put them at increased risk of cancer We examined the incidence of cancer in a cohort of 2234 white-male police officers in Buffalo, New York. The study population was followed for 31 years (1976-2006). The incidence of cancer, ascertained using a population-based tumor registry, was compared with 9 US regions using the Surveillance Epidemiology and End Results (SEER) program data. Four hundred and six officers (18.2%) developed cancer between 1976 and 2006. The risk of overall cancer among police officers was found to be similar to the general white-male population (standardized incidence ratio [SIR] = 0.94, 95%, confidence interval [CI] = 0.85-1.03). An elevated risk of Hodgkin's lymphoma was observed relative to the general population (SIR = 3.34, 95%, CI = 1.22-7.26). The risk of brain cancer, although only slightly elevated relative to the general population (SIR = 1.61, 95%, CI = 0.73-3.05), was significantly increased with 30 years or more of service (SIR = 2.92, 95%, CI = 1.07-6.36). Incidence ratios were significantly lower than expected for skin and bladder cancer Police officers were at increased risk of Hodgkin's lymphoma overall and of brain cancer after 30 years of service.

Evaluation of PCDD/F and dioxin-like PCB serum concentration data from the 2001-2002 National Health and Nutrition Examination Survey of the United States population

We analyzed the weighted 2001-2002 National Health and Nutrition Examination Survey data to assess potential differences in mean total 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) TEQ between various groups of individuals and to determine serum reference concentrations for polychlorinated dibenzo-p-dioxins, dibenzofurans (PCDD/Fs) and dioxin-like polychlorinated biphenyls (PCBs) in the general US population. Differences appeared to exist between female smokers and non-smokers and between non-Hispanic groups and all other races. Potential differences were also observed among the four age groups with an increasing trend in mean total TCDD TEQ with increasing age. Both age and gender appeared to confound the association between smoking status and total TCDD TEQ, dictating the need for further analysis. As anticipated, PCBs contributed appreciably to the total TCDD TEQ levels in the referent population and accounted for 38% to 41% of the total TEQ depending on age. Nearly 80% of the mean total TCDD TEQ was attributable to four PCDD/F congeners and three PCB congeners. In this analysis, two methods were used to assess samples where the concentrations were below the limits of detection (LODs), and this did not have significant impact on the mean total TCDD TEQ at the higher percentiles and for older individuals. Comparison of our results to those from a recent PCDD/F biomonitoring study indicates that the mean TCDD TEQ serum concentration of the individuals studied does not appear to be different from typical levels found in the general US population. Additionally, an assessment of data from the National Human Adipose Tissue Survey using our referent statistics shows that levels of these chemicals have been declining in the general population for at least two decades. The reference TEQs presented in this paper provide relevant, current data that can be used to evaluate biomonitoring results of individuals or groups exposed or potentially exposed to PCDD/Fs and PCBs above referent levels.

Association between Agent Orange and birth defects: systematic review and meta-analysis

BACKGROUND

The association between parental exposure to Agent Orange or dioxin and birth defects is controversial, due to inconsistent findings in the literature. The principal aim of this study was to conduct a meta-analysis of relevant epidemiological studies that examined this association and to assess the heterogeneity among studies.

METHODS

Relevant studies were identified through a computerized literature search of Medline and Embase from 1966 to 2002; reviewing the reference list of retrieved articles and conference proceedings; and contacting researchers for unpublished studies. A specified protocol was followed to extract data on study details and outcomes. Both fixed-effects and random-effects models were used to synthesize the results of individual studies. The Cochrane Q test and index of heterogeneity (I2) were used to evaluate heterogeneity, and a funnel plot and Egger's test were used to evaluate publication bias.

RESULTS

In total, 22 studies including 13 Vietnamese and nine non-Vietnamese studies were identified. The summary relative risk (RR) of birth defects associated with exposure to Agent Orange was 1.95 [95% confidence interval (95% CI) 1.59-2.39], with substantial heterogeneity across studies. Vietnamese studies showed a higher summary RR (RR = 3.00; 95% CI 2.19-4.12) than non-Vietnamese studies (RR = 1.29; 95% CI 1.04-1.59). Sub-group analyses found that the magnitude of association tended to increase with greater degrees of exposure to Agent Orange, rated on intensity and duration of exposure and dioxin concentrations measured in affected populations.

CONCLUSION

Parental exposure to Agent Orange appears to be associated with an increased risk of birth defects.

Accident insurance, sickness, and science: New Zealand's no-fault system

This article explores the process of seeking compensation for occupational illness under a no-fault accident insurance scheme. The author uses two case studies--firefighters who attended a fire at a chemical storage depot and timbermill workers who worked with pentachlorophenol--to illustrate how science can be used to deny compensation to sick and dying workers. The results of the studies suggest that a no-fault accident compensation scheme, considered to be a victory for workers, offers no guarantee of just outcomes for working people. And science can be co-opted and used to support business and state interests against workers; this ideological support is increasingly hidden behind the development of "objective" systems of assessing compensation claims.

A pharmacokinetic model for estimating exposure of Americans to dioxin-like compounds in the past, present, and future

Empirical evidence suggests that exposure of Americans to dioxin-like compounds was low during the early decades of the 20th century, then increased during the 1940s and 1950s, reaching a peak in the 1960s and 1970s, and progressively decreased to lower levels in the 1980s and 1990s. Such evidence includes dioxin analysis of carbon-dated sediment cores of lakes and rivers, preserved meat samples from different decades of the 20th century, and limited body burden measurements of dioxin-like compounds. Pinsky and Lorber (1998) summarized studies measuring 2,3,7,8-TCDD in blood and adipose tissue, and found a range of 10-20 pg/g (ppt) lipid during the 1970s, and 2-10 ppt lipid during the 1980s. This study reviews body burdens of dioxin toxic equivalents, TEQs, to find a range from approximately 50 to 80 ppt lipid during the 1970s, 30-50 ppt lipid during the 1980s, and 10-20 ppt lipid during the 1990s (TEQs comprised of the 17 dioxin and furan congeners only). Pinsky and Lorber (1998) investigated historical exposure trends for 2,3,7,8-TCDD by using a single-compartment, first-order pharmacokinetic model. The current study extends this prior effort by modeling dioxin TEQs instead of the single compound, 2,3,7,8-TCDD. TEQs are modeled as though they were a single compound, in contrast to an approach where the individual dioxin and furan congeners are modeled separately. It was found that body burdens of TEQs during the 1970s, 1980s and 1990s could be modeled by assuming a historical dose which began the century at low levels of approximately 0.5 pg TEQ/kg/day, rose during the middle decades of the 20th century to over 6 pg TEQ/kg/day, and declined to current levels of approximately 0.5 pg TEQ/kg/day. Trends in individual and population body burdens of TEQs are also investigated using this PK modeling framework. A key uncertainty of this effort - assuming that TEQs behave as though they were a single compound - is discussed and analyzed.

Body weight and risk of soft-tissue sarcoma

The relation between body mass (BMI) and soft-tissue sarcoma (STS) risk was evaluated in a case-control study from Northern Italy based on 217 incident STS and 1297 hospital controls. The risk of STS rose with BMI, with multivariate odds ratios of 3.49 (95% confidence interval (CI) 1.06-11.55) among men and 3.26 (95% CI 1.27-8.35) among women with a BMI > 30 kg m(-2) compared to those with BMI < or = 20 kg m(-2).

Porphyria and chemicals

Porphyria is a genetic family of diseases that is most frequently described as neuropsychiatric or toxogenetic. It is well known to be initiated by drugs, infections, heavy metals, hormones, chemicals and fasting. There are extensive lists of drugs that have been known to cause attacks. Others are thought to be likely to cause attacks on the basis of animal studies or in vitro studies. It has become obvious that lists of chemicals capable of causing illness in porphyrics are sorely lacking. Chemicals that have the same base as drugs that are labeled in the PDR (Physicians Desk Reference) as porphyrogenic have no such labeling in their MSDS (Material Safety Data Sheet). This article is intended to point out why porphyria needs to be considered when illness occurs after chemical exposure. The capability of testing enzymes in the porphyrin pathway allows us to evaluate these patients more thoroughly, for we are now aware that the standard measures for recognizing these diseases are often inadequate. Three examples where illness has occurred after environmental exposure to chemicals will serve as illustrations. One, a documented porphyria, is the Turkish porphyria. The other two are not yet documented as porphyria, but may be some day. One is Agent Orange which caused illness in Vietnam, and the other is exposure to unknown sources of what has been named the Gulf War syndrome.

Declining male fertility and environmental factors

Male reproductive function has recently attracted increasing attention due to reports on time-related decline in semen quality. Analysis of retrospective data indicates that sperm counts may have declined in some parts of the world, but there seem to be regional differences both in the quality of semen and regarding the secular trend. Whereas data on sperm counts may be somewhat difficult to interpret, there is no doubt that incidence of testicular cancer has increased worldwide. In occupational studies, only a few types of exposures were shown to have an impact on male reproductive function; lifestyle and general environmental factors may play a more important role. Prospective studies on secular and geographic trends in male reproductive function are warranted.

Exposure to Agent Orange and occurrence of soft-tissue sarcomas or non-Hodgkin lymphomas: an ongoing study in Vietnam

Agent Orange was the most common herbicide used in the Second Indochina War in the course of military operations in the former South Vietnam. Agent Orange is contaminated by the carcinogen 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD) in mean concentrations of 2 mg/kg. After much dispute of a causal association between exposure to herbicides containing TCDD and occurrence of soft-tissue sarcoma and non-Hodgkin lymphoma, two simultaneous case-control studies were set up in Vietnam to examine possible relationships. Subject recruitment is ongoing, with target numbers of 150 cases of soft-tissue sarcoma and 150 cases of non-Hodgkin lymphoma and diagnoses at the Cancer Center at Ho Chi Minh City, Vietnam. Two hospital controls are matched to each case. As in other studies of cancer in persons occupationally or otherwise exposed to herbicides and their contaminants, evaluation of past exposure of the recruited subjects is among the most complicated issues. Because accurate records are usually unavailable, surrogate measures of likely exposure are often calculated. As a first approach in our studies we used the Stellman and Stellman exposure index. The index is based on matching subjects' history of residence and the information on times and locations of Agent Orange spraying recorded on HERBS tape by the U.S. Army and taking into account the distance from the spraying as well as environmental and biologic half-life of TCDD. The exposure index is calculated in two centers, New York and Hanoi, with slightly different assumptions. In addition, samples of body tissues from the subjects (20 ml blood, 2 g adipose tissue, and tumor sections in paraffin blocks) are taken and stored. Their future analysis will provide additional source of exposure assessment. Strengths and weaknesses of both exposure measures are discussed in this paper.

2,3,7,8-Tetrachlorodibenzo-p-dioxin mechanistic data and risk assessment: gene regulation, cytotoxicity, enhanced cell proliferation, and tumor promotion

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been found to cause several tumor types in rodents, but TCDD has not been proven to cause cancer in humans, although there have been reported associations. TCDD does not bind to DNA, and indirect tests for DNA damage have been mostly negative. Tumorigenicity by TCDD in rodents has been linked to cellular necrosis, enhanced cell proliferation and tumor promotion. TCDD binds to the Ah receptor, which induces CYP1A1. This binding may be involved in tumorigenicity in rodents; however, additional TCDD-induced toxic changes appear to be required. Biopersistence and organ distribution may play an important role in TCDD dosage extrapolation to humans, but these have not been adequately determined.

Military service in Vietnam and the risk of death from trauma and selected cancers

The postservice mortality of a cohort of 10,716 US Marine veterans who served in Vietnam was compared with that of 9,346 Marine veterans who did not serve in Vietnam. There was a significant excess of death for Vietnam Marines from all causes and all external causes. After adjustments for age and rank in military, overall mortality continued to be statistically significant, with a relative risk of 1.15 (95% confidence interval (CI) = 1.02 to 1.29) for Vietnam Marines compared to non-Vietnam Marines. All external causes was also significant, with a relative risk of 1.21 (95% CI = 1.00 to 1.47). The excess overall mortality was mainly due to excess deaths from external causes. The risks for several site-specific cancers were elevated but not statistically significant. Periodic follow-up of this Marine cohort should continue to determine whether there are statistically significant differences in the mortality patterns of Marine Vietnam and non-Vietnam veterans, especially for cancers.

Hodgkin's disease and Vietnam service

Earlier studies that showed an association between exposure to phenoxy herbicides and the risk of malignant lymphomas have sparked concerns among Vietnam veterans over Agent Orange exposure. The Department of Veterans Affairs (VA) undertook a hospital-based case-control study to examine the association between military service in Vietnam and several histologic types of malignant lymphomas. This is a report of 283 Vietnam-era veteran patients who were treated in one of 172 VA hospitals from 1969 to 1985 with a diagnosis of Hodgkin's Disease (HD). Four hundred and four Vietnam-era veteran patients with diagnosis other than malignant lymphoma served as a comparison group. Military service in Vietnam was not associated with any significant increase in the risk of HD (adjusted odds ratio = 1.28; 95% confidence interval = 0.94, 1.76). Surrogate measures of potential Agent Orange exposure such as service in a specific military branch, in a certain region within Vietnam, in a combat role, or extended Vietnam service time were not associated with any significant increased risk of HD.

Dioxins and dibenzofurans in adipose tissue of the general US population and selected subpopulations

OBJECTIVES

The Environmental Protection Agency's National Human Adipose Tissue Survey (NHATS) was conducted in fiscal year (FY) 1987 to (1) estimate average concentrations of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in the adipose tissue of humans in the US population, (2) identify differences in average concentrations among subpopulations, and (3) compare average concentrations with those from the FY 1982 NHATS:

METHODS

Population estimates of the average levels of PCDDs and PCDFs were established on the basis of 865 human adipose tissue specimens collected in FY 1987. Average levels among subpopulations were compared.

RESULTS

The average concentration of 2,3,7,8-tetrachlorodibenzo-p-dioxin in the adipose tissue of the US population was 5.38 pg/g, increasing from 1.98 pg/g in children under 14 years of age to 9.40 pg/g in adults over 45. The effect of age was significant for nine compounds. Regional differences in the levels of 2,3,4,7,8-pentachlorinated dibenzofurans were statistically significant, but there were no significant differences associated with sex or race.

CONCLUSIONS

The survey provides a baseline of average levels of PCDDs and PCDFs in the adipose tissue of humans in the US population.

Risk of testicular cancer associated with surrogate measures of Agent Orange exposure among Vietnam veterans on the Agent Orange Registry

A case-control analysis was undertaken to examine the association between various surrogate measures of Agent Orange exposure and testicular cancer among Vietnam veterans. Study subjects were selected from the Department of Veterans Affairs Agent Orange Registry. The case patients consisted of 97 veterans with a diagnosis of testicular cancer, and 311 veterans without any clinical diagnosis served as a comparison group. The surrogate measures were branch of service, type of duty, corps area, and location of the individual's unit in relation to recorded Agent Orange spray tracts. Only Navy veterans had a statistically significant increased risk of testicular cancer (odds ratio (OR) = 2.60; 95% confidence interval (CI), 1.08 to 6.24). Risk of testicular cancer was not significantly increased for ground troops (OR = 0.46; 95% CI, 0.25 to 0.86), for combat duty (OR = 0.91; 95% CI, 0.52 to 1.58), for service in the III Corps area (OR = 1.10; 95% CI, 0.66 to 1.84), and for being close to spray tracts within 90 days/8 km (OR = 0.99; 95% CI, 0.54 to 1.84) or 3 days/2 km (OR = 1.39; 95% CI, 0.50 to 3.80). The study results are not consistent with the hypothesis that Agent Orange may be a risk factor for testicular cancer among Vietnam veterans.

Measurement issues in environmental epidemiology

This paper deals with the area of environmental epidemiology involving measurement of exposure and dose, health outcomes, and important confounding and modifying variables (including genotype and psychosocial factors). Using examples, we illustrate strategies for increasing the accuracy of exposure and dose measurement that include dosimetry algorithms, pharmacokinetic models, biologic markers, and use of multiple measures. Some limitations of these methods are described and suggestions are made about where formal evaluation might be helpful. We go on to discuss methods for assessing the inaccuracies in exposure or dose measurements, including sensitivity analysis and validation studies. In relation to measurement of health outcomes, we discuss some definitional issues and cover, among other topics, biologic effect markers and other early indicators of disease. Because measurement error in covariates is also important, we consider the problems in measurement of common confounders and effect modifiers. Finally, we cite some general methodologic research needs.

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.