Health Assessment, Volume III: Risk Characterization

9.7.1. General Comments

It is clear from the evaluation of the toxicologic literature that dioxin and related compounds have the ability to produce a wide spectrum of responses in animals and, presumably, in humans, if the dose is high enough (Table 9-2). Relatively few chronic effects related to exposure to dioxin-like compounds have been observed in humans. The epidemiologic data are limited due to a number of possible factors: the absence of many, specific individual measurements of dioxin exposure for the general population; a limited number of cross-sectional and prospective studies of more highly exposed populations; the limited ability of epidemiologic studies to detect significant differences between exposed and relatively unexposed populations when the outcomes are relatively rare, the exposures are low, and the population under study is small; and the difficulty in quantifying the impact of all potentially confounding exposures. Evaluation of hazard and risk for dioxin and related compounds must rely on a weight-of-the-evidence approach in which all available data (animal and human) are examined together. This process often requires extrapolation of effects across various animal species as well as to humans.

The reliability of using animal data to estimate human hazard and risk has often been questioned for this class of compounds. Although human data are limited, evidence suggests that animal models are appropriate for estimating human risk if all available data are considered. As discussed in detail in Chapters 2 and 8, humans have a fully functional Ah receptor and both in vivo and in vitro studies demonstrate comparability of biochemical responses in humans and animals (see also Table 8-5). When comparing species and strains for their responses to these compounds, a wide range of sensitivity to TCDD-induced toxicities has been noted. Qualitatively speaking, however, almost every response can be produced in every species if the appropriate dose is administered. Although outliers, i.e., species that are either very sensitive or refractory, can be identified for a particular response, no species is consistently sensitive or refractory for all effects. In addition, sensitivity for a given effect among the majority of species clusters within approximately one order of magnitude (factor of 10). Therefore, despite a range of sensitivities across species, it is reasonable to assume that humans will not be refractory to all effects nor that they will be as sensitive as the most sensitive responder for each effect. Humans are likely, because of interindividual variability in response to a variety of toxic chemicals, which is generally greater than that found in individual species of laboratory animals, to show a wide range of sensitivities for various dioxin-induced toxicities. For purposes of the current assessment, therefore, unless there are data to identify a particular species as being representative of humans for a particular effect, average humans can be reasonably assumed to be of average sensitivity for various effects, recognizing that individuals in the population might vary widely in their sensitivity to individual effects. The uncertainty introduced by this assumption, i.e., that, on average, humans will respond as do average animal models for individual effects of exposure to dioxin-like compounds and that an unknown range of variability exists in the human population for individual effects, should be carefully considered as results of this characterization are applied to individuals or specific subpopulations.

9.7.2. Chloracne

Chloracne and associated dermatologic changes are widely recognized responses to TCDD and other dioxin-like compounds in humans. Chloracne is a severe acne-like condition that develops within months of first exposure to high levels of dioxin. For many individuals, the condition disappears after discontinuation of exposure, despite serum levels of dioxin in the thousands of parts per trillion; for others, it may remain for many years. The duration of persistent chloracne is on the order of 25 years although cases of chloracne persisting over 40 years have been noted. There are very little human data from which to determine definitively the doses at which chloracne is likely to occur. Data from occupational studies suggest that persistent chloracne is more often associated with exposures of high intensity, for long duration, and commencing at an early age. Acute exposures or chronic lower level exposures, if resulting in chloracne, have generally resulted in a condition that resolves itself in a matter of months to a few years. Details of chloracnegenic response in occupationally exposed humans are described in detail in Chapter 7 of the Health Assessment Document.

Induction of chloracne in humans after exposure to dioxin and related compounds is supported by studies in laboratory animals. Rabbits, monkeys, and hairless mice have all proved useful in investigating this response. In addition, cellular systems provide a research tool in elucidating the chloracne response at the cellular level. Keratinocytes, the principal cell type in the epidermis, have been used as an in vitro model for studies of TCDD-induced hyperkeratosis, a feature of chloracne, in human- and animal-derived cell cultures. The response in these systems is analogous to the hyperkeratinization observed in vivo as a part of chloracne.

There is little doubt that chloracne is a human condition often attributable to exposure to dioxin and related compounds. The specific risk factors associated with this response are still obscure. Recognition of chloracne has been associated with high-level exposure to these compounds, and as such, may represent a biomarker of exposure. Because of the wide variability of the chloracnegenic response in humans and its varied persistence, however, the absence of chloracne is not a reliable indicator of low exposure to dioxin and related compounds.

9.7.3. Carcinogenicity

Since the last EPA review of the human data base relating to the carcinogenicity of TCDD and related compounds in 1988, several new follow-up mortality studies have been completed. Among the most important of these are a study of 5,172 workers by Fingerhut et al. (1991), a study with 1,583 workers by Manz et al. (1991), a smaller study of 247 workers by Zober et al. (1990), and a study of over 18,000 workers by Saracci et al. (1991). Although uncertainty remains in interpreting these studies because not all potential confounders have been ruled out and coincident exposures to other carcinogens is likely, all provide support for an association between exposure to dioxin and related compounds and increased cancer mortality. With the exception of the study by Saracci et al. (1991), these studies have some exposure information that permits an assessment of dose response. These data have in fact served as the basis for fitting the additive and multiplicative risk models in Chapter 8. In addition, more limited results have been presented recently on the Seveso cohort (Bertazzi et al., 1993) and on women exposed to chlorophenoxy herbicides, chlorophenols, and dioxins (Kogevinas et al., 1993). While these two studies have methodologic shortcomings that are described in Chapter 7, they provide findings, particularly for exposure to women, that warrant additional follow-up.

While the data base from epidemiologic studies remains controversial, it is the view of this reassessment that this body of evidence supports the laboratory data indicating that TCDD probably increases cancer mortality of several types. Although not all confounders were ruled out in any one study, positive associations between surrogates of dioxin exposure, either length of occupational exposure or proximity to a known source combined with some information on body burden, and cancer have been reported. These data alone suggest a role for dioxin exposure to contribute to a carcinogenic response but do not confirm a causal relationship between exposure to dioxin and increased cancer incidence. Available human studies alone cannot demonstrate whether a cause and effect relationship between dioxin exposure and increased incidence of cancer exists. Therefore, evaluation of cancer hazard in humans must include an evaluation of all of the available animal and in vitro data as well as the data from exposed human populations.

The Peer Panel that met in September 1993 to review an earlier draft of the cancer epidemiology chapter suggested that the epidemiology data alone were still not adequate to implicate dioxin and related compounds as "known" human carcinogens but that the results from the human studies were largely consistent with observations from laboratory studies of dioxin-induced cancer and, therefore, should not be dismissed or ignored. Other scientists, including those who attended the Peer Panel meeting, felt either more or less strongly about the weight of the evidence from epidemiology studies, representing the range of opinion that still exists on the interpretation of the cancer epidemiology studies.

Many of the earlier epidemiological studies that suggested an association with soft tissue sarcoma were criticized for a variety of reasons. Nonetheless, the incidence of soft tissue sarcoma is elevated in several of the recent studies, supporting the findings from previous studies. The fact that similar results were obtained in independent studies of differing design and evaluating populations exposed to dioxin-like compounds under varying conditions, along with the rarity of this tumor type, weighs in favor of a consistent and real association. On the other hand, arguments regarding selection bias, differential exposure misclassification, confounding, and chance in each individual study have been presented in the scientific literature which increase uncertainty around this association. In addition, excess respiratory cancer was noted by Fingerhut, Zober, and Manz. These results are also supported by significantly increased mortality from lung and liver cancers subsequent to the Japanese rice oil poisoning accident where exposure to PCDFs and PCBs occurred. Again, while smoking as a confounder cannot be totally eliminated as a potential explanation of these results, analyses conducted to date suggest that smoking is not likely to explain the entire increase in lung cancer. The question of confounding exposures, such as asbestos and other chemicals, in addition to smoking, has not been entirely ruled out and must be considered as potentially adding to the observed increases. Although increases of cancer at other sites (e.g., non-Hodgkin’s lymphoma, stomach cancer) have been reported, the data for an association with exposure to dioxin-like chemicals are less compelling.

The comparison of the results of different investigations that examine the outcome of similar exposures must always be evaluated in light of factors that may influence the outcome of the study. A few of these factors include study design, potential confounding factors and exposures (extraneous factors or exposures that relate to both outcome and exposure such as age), biases that affect the selection and participation of the study population, differential exposure misclassification, variation in age of the study population, different conditions of exposure (mode, intensity, duration, and route), and differences in methods used to assess outcomes of interest. Such differences may result in some variation in the results of the compared studies. Given that the studies are well conducted and the variations noted, what is important is the within-study consistency of the results.

What emerges from an analysis of the epidemiology data is a view of dioxin-like compounds as potentially multisite carcinogens in more highly exposed human populations that have been studied, consisting primarily of adult males. There are currently very few data for women and children exposed to dioxin-like compounds. Although uncertainty in this view remains, the cancer findings are generally consistent with results from studies of laboratory animals and appear to be plausible given what is known about mechanisms of dioxin action.

While both past and more recent human studies have focused on males, there are some limited data suggesting carcinogenic responses associated with dioxin exposure in females. Because both laboratory animal data and mechanistic inferences suggest that males and females may respond differently to dioxin-like activity, further data will be needed to address this question of differential response.

An extensive data base on the carcinogenicity of dioxin and related compounds in laboratory studies exists and is described in detail in Chapter 6. There is adequate evidence that 2,3,7,8-TCDD is a carcinogen in laboratory animals based on long-term bioassays conducted in both sexes of rats and mice. All studies have produced positive results, leading to the conclusions that TCDD is a multistage carcinogen increasing the incidence of tumors at sites distant from the site of treatment and at doses well below the maximum tolerated dose. Since this issue was last reviewed by the Agency in 1988, TCDD has been shown to be a carcinogen in hamsters, which are relatively resistant to the lethal effects of TCDD. Recent data have also shown TCDD to be a liver carcinogen in the small fish, Medaka (Johnson et al., 1992). Few attempts have been made to demonstrate the carcinogenicity of other dioxin-like compounds. Other than a mixture of two isomers of hexachlorodibenzodioxin (HCDDs), which produced liver tumors in both sexes of rats and mice (NTP, 1980), the more highly chlorinated CDDs and CDFs have not been studied in long-term animal cancer bioassays. However, it is generally recognized that these compounds bioaccumulate and exhibit toxicities similar to TCDD and are, therefore, also likely to be carcinogens (U.S. EPA Science Advisory Board, 1989).

In addition to the demonstration of TCDD as a complete carcinogen in long-term cancer bioassays, a number of dioxin-like PCDDs and PCDFs, as well as several PCBs, have also been demonstrated to be tumor promoters in two-stage (initiation-promotion) protocols in rodent liver and skin. In addition, a recent study has demonstrated the ability of TCDD to neoplastically transform immortalized human cells in culture at very low concentrations of TCDD. While dioxin and related compounds are not generally considered to be "genotoxic" in traditional terms, both empirical data and the results of modeling efforts suggest that they may be functioning indirectly to produce irreversible genetic changes in exposed cells. All of these data add substantially to the weight of the evidence that dioxin and related compounds are likely to be carcinogenic, at least under some circumstances, in humans.

Despite the relatively large number of bioassays on TCDD, the study of Kociba et al. (1978) and those of the NTP (1982), because of their multiple dose groups and wide dose range, continue to be the focus of additional review. Sauer (1990) re-evaluated the female rat liver tumors in the Kociba study using the latest pathology criteria for such lesions. The review confirmed only approximately one-third of the tumors of the previous review (Squire, 1980). While this finding did not change the determination of carcinogenic hazard since TCDD induced tumors in multiple sites in this study, it does have an effect on evaluation of dose-response and on estimates of risk at low doses. These issues will be discussed in a later section of this chapter.

One of the more interesting findings in the Kociba bioassay was reduced tumor incidences of the pituitary, uterus, mammary gland, pancreas, and adrenals. These findings, coupled with the sex specificity of the TCDD-induced liver tumors in rats, emphasize that the carcinogenic actions of TCDD involve a complex interaction of hormonal factors. Moreover, it is hypothesized that cell-specific factors modulate TCDD/hormone actions relevant to cancer. The findings of reduced tumor incidence in certain tissues suggest that dioxin exposure may be exerting an anticarcinogenic effect under certain circumstances or in certain tissues. The complex interplay between dioxin and hormones in terms of both carcinogenic and anticarcinogenic responses will continue to be a matter of hypothesis until specific data to address these issues are obtained.

In summary, publication of additional studies of human populations exposed to dioxin and related compounds since the last EPA assessment (Fingerhut et al., 1991; Manz et al., 1991; Zober et al., 1990; Saracci et al., 1991; Bertazzi et al., 1993; Kogevinas et al., 1993) has strengthened the inference, based on all the evidence from mechanistic, animal, and epidemiologic studies, that these compounds are appropriately characterized as probable human carcinogens. While the data for 2,3,7,8-TCDD are particularly comprehensive, the data on other congeners remain limited. This puts added emphasis on the assumptions and inferences regarding toxicity equivalence in evaluating complex exposures to dioxin and related compounds with regard to carcinogenicity. The evolving understanding of the complex interplay between dioxin-like compounds and hormones and other modulators of cell growth and differentiation continues to complicate more precise determinations of cancer hazard and risk.

9.7.4. Reproductive and Developmental Effects

The potential for dioxins and related compounds to cause reproductive and developmental toxicity in animals has been recognized for many years, and the data base regarding these effects is analyzed in Chapter 5. Recent laboratory studies have suggested that altered development may be among the most sensitive TCDD end points in laboratory animal systems although the likelihood and level of response in humans are much less clear. Although the discussion of these effects in Chapter 5 is divided into developmental toxicity and male and female reproductive toxicity, it is important to recognize the interrelatedness of developmental and reproductive events at all levels of biological complexity. This point is critical for understanding and fully characterizing the hazards and risks of dioxin and related compounds. For example, effects of TCDD on circulating levels of sex hormones and/or on responsiveness to sex hormones in laboratory animals or humans may be translated into reproductive dysfunction if exposure occurs in adulthood as well as abnormal development and/or reproductive dysfunction if exposure occurs prenatally. Therefore, a similar effect of dioxin-like compounds may be manifest as a reproductive end point if exposure occurs to adults or as a developmental and/or a reproductive end point if exposure occurs to the fetus. Likewise, even though effects on organ structure and on growth are considered separate developmental end points that are associated with pre- and postnatal exposure to TCDD in laboratory animals, they are interrelated because effects on prenatal growth can significantly disrupt the structural integrity of an organ system. It is important to note that adverse developmental effects are a complex set of end points, many of which are caused by multiple factors, requiring coincidence of a number of events.

In the current data base, developmental toxicity end points have been observed at lower TCDD exposure levels than male and female reproductive toxicity end points in a number of animal systems. The lowest effective TCDD egg burden for causing developmental toxicity in fish and birds and the lowest effective maternal TCDD body burden for producing a wide range of developmental responses in mammals are summarized in Chapter 5. Of particular interest to the risk assessment process is the fact that a wide variety of developmental events, crossing three vertebrate classes and several species within each class, can be perturbed, suggesting that dioxin has the potential to disrupt a large number of critical developmental events at specific developmental stages. Not only can these changes lead to increases in embryo/fetal mortality, but they can disrupt organ system structure and irreversibly impair organ function.

The laboratory studies demonstrating adverse health effects from prenatal exposures often involved a single dose administered at a discrete time during pregnancy. The doses that produced adverse effects, such as reproductive and developmental toxicity, can be related to longer term body burdens produced by the single dose or to background body burdens. Because the production of prenatal effects often requires exposures to occur during certain critical times during fetal development, the uncertainties in the relationship with steady-state body burdens must be carefully assessed. A single dose may cause a spike in both maternal and fetal blood concentration related to the magnitude of the dose, and the concentrations will fall rapidly as the dioxin-like compounds are redistributed to adipose and other tissues. Application of pharmacokinetic models described earlier in this chapter to estimate blood concentrations at the critical time of development is expected to be a sound method for relating chronic background exposures to the results obtained from single-dose studies.

Because developmental toxicity following exposure to TCDD-like congeners occurs in fish, birds, and mammals, it is likely to occur at some level in humans. It is not currently possible to state exactly how or at what levels humans in the population will respond with adverse impacts on development or reproductive function. Data analyzed in Chapter 5 and Chapter 7 suggest, however, that adverse effects may be occurring at levels lower than originally thought to represent a no observed adverse effect level (NOAEL) in animals. Traditional toxicology studies had led to the conclusion that the NOAEL was in the range of intake values of 1 ng TEQ/kg/day. Current data suggest that the NOAEL in animals should be lower. This issue will be discussed further in the dose-response section of this chapter.

While human data on potential developmental effects of dioxin-like compounds are limited, developmental effects in human infants exposed to a complex mixture of PCBs, CDFs, and PCQs in the Yusho and Yu-Cheng poisoning episodes were probably caused by the combined exposure to those PCB and CDF congeners that are Ah-receptor agonists. However, it should be noted that not all effects that are seen are attributable only to dioxin-like compounds. Similarity of the effects observed in human infants prenatally exposed to this complex mixture with those reported in adult monkeys exposed only to TCDD increases the probability that at least some of the effects in the Yusho and Yu-Cheng children are due to the TCDD-like congeners in the contaminated rice oil ingested by the mothers of these children. Most significant is a clustering of effects in organs derived from the ectodermal germ layer, a syndrome referred to as ectodermal dysplasia. Included in this syndrome are effects on the skin, nails, and meibomian glands that occur in both adult monkeys exposed to TCDD and in Yusho and Yu-Cheng infants exposed transplacentally to PCB, CDF, and PCQ contaminated rice oils. In addition, accelerated tooth eruption has been reported both in human infants affected by the Yusho and Yu-Cheng exposures and in neonatal mice exposed to TCDD. Yu-Cheng children exposed transplacentally to PCB, CDF, and PCQ contaminated rice oil have also exhibited developmental and psychomotor delay during developmental and cognitive tests (Chen et al., 1992). Some investigators believe that, because these effects do not correlate with TEQ, the effects are exclusively due to nondioxin-like PCBs or a combination of all congeners. However, monkeys pre- and postnatally exposed to TCDD are also affected by a deficit in cognitive function. Recent studies presented at Dioxin '93 (Hsu et al., 1993; Lai et al., 1993) have demonstrated that these effects persist throughout childhood, as does the growth retardation (Guo et al., 1994). The concept that the ectodermal dysplasia syndrome in Yusho and Yu-Cheng infants may be caused by the combination of PCB and CDF congeners in the rice oil that are Ah receptor agonists but are less potent than TCDD is consistent with structure-activity results for various developmental end points in different species of fish, birds, and mammals.

In mammals, postnatal functional alterations involving learning behavior and the developing reproductive system appear to be the developmental events most sensitive to prenatal dioxin exposure. The developing immune system may also be highly sensitive. Alterations in structures and diminished prenatal viability and growth begin to predominate at maternal TCDD body burdens and/or daily TCDD doses during gestation that are above 100 ng/kg in virtually every species tested. These doses of TCDD are not maternally toxic. Higher dose levels can be demonstrated to result in prenatal mortality. A general finding in fish, bird, and mammalian species is that the embryo or fetus is more sensitive to TCDD-induced mortality than the adult. Thus, the timing of TCDD exposure during the life history of an animal can greatly influence its susceptibility to overt dioxin toxicity.

With respect to male and female reproductive end points, there are clear effects following dioxin exposure of the adult animal. Such reproductive effects generally occur at TCDD body burdens that are higher than those required to cause the more sensitive developmental end points. For example, TCDD exposure of the adult male rodent causes reduced testis and accessory sex organ weights, abnormal testis structure, decreased spermatogenesis, reduced fertility, decreased testicular testosterone synthesis, reduced plasma androgen concentrations, and altered regulation of pituitary LH secretion. However, in laboratory animal studies, these effects are detectable only at TCDD exposure levels that are overtly toxic to the animal. In the more limited studies focusing on female reproduction, the primary effects include decreased fertility, inability to maintain pregnancy, and in the rat, decreased litter size. Signs of ovarian dysfunction and alterations in hormone levels have also been reported.

Exposure of female mice and rats to TCDD has an antiestrogenic effect on the uterus. The dose of TCDD required to produce this response is generally higher than that needed to cause the most sensitive signs of developmental toxicity in these species. More specifically, hydronephrosis and cleft palate in mice and reductions in spermatogenesis in rats occur at maternal doses of TCDD that are far less than those needed to exert a demonstrable antiestrogenic effect when adult female mice and rats are exposed to dioxin. The precise mechanism of TCDD's antiestrogenic effect is not fully understood. It may be caused by both a decrease in available estrogen receptor number and/or by an increase in cytochrome P-4501A-mediated estrogen metabolism within the target cell.

These studies indicate that while there is variability between species in the profile of developmental responses elicited by TCDD, essentially all dioxin-like PCB, CDD, and CDF congeners that have Ah receptor affinity and intrinsic activity produce the same pattern of developmental effects within a given vertebrate species if a sufficiently high dose of the congener is given. Data to support these conclusions regarding reproductive and developmental hazards of dioxin and related compounds continue to accumulate, but the weight of the evidence is still a subject of much scientific debate.

9.7.5. Immunotoxicity

Concern over the potential toxic effects of chemicals on the immune system arises from the critical role that the immune system plays in maintaining health. It is well recognized that suppressed immunological function can result in increased incidence and severity of infectious diseases as well as some types of cancer. Conversely, the inappropriate enhancement of immune function or the generation of misdirected immune responses may precipitate or exacerbate the development of allergic and autoimmune diseases. Thus, suppression as well as enhancement of immune function are considered to represent potential immunotoxic effects of chemicals.

Extensive evidence has accumulated over the past 20 years to demonstrate that the immune system is a target for toxicity of TCDD and structurally related compounds, including PCDDs, PCDFs, PCBs, and PBBs. This evidence is described in detail in Chapter 4. The evidence has derived from numerous studies in various animal species, primarily rodents, but also guinea pigs, rabbits, monkeys, marmosets, and cattle. Epidemiological studies also provide some evidence for the immunotoxicity of dioxin and related compounds in humans. In animal studies, relatively high doses of HAH produce lymphoid tissue depletion, except in the thymus where cellular depletion occurs at lower doses. Alterations in specific immune effector functions and increased susceptibility to infectious disease have been identified at doses of TCDD well below those that cause lymphoid tissue depletion. Both cell-mediated and humoral immune responses are suppressed following TCDD exposure, suggesting that there are multiple cellular targets within the immune system that are altered by TCDD. Evidence also suggests that the immune system is indirectly targeted by TCDD-induced changes in nonlymphoid tissues. In addition, in parallel with increased understanding of the cellular and molecular mechanisms involved in immunity, studies on TCDD are beginning to establish biochemical and molecular mechanisms of TCDD immunotoxicity.

The ability of an animal to resist and/or control viral, bacterial, parasitic, and neoplastic diseases is determined by both nonspecific and specific immunological functions. Decreased functional activity in any immunological compartment may result in increased susceptibility to infectious and neoplastic diseases. In terms of risk assessment, host resistance is often accorded the "bottom line" in terms of relevant immunotoxic end points. Animal host resistance models that mimic human disease are available and have been used to assess the effect of TCDD on altered host resistance. Results from host resistance studies provide evidence that exposure to TCDD results in increased susceptibility to bacterial, viral, parasitic, and neoplastic diseases. These effects are observed at relatively low doses and likely result from TCDD-induced suppression of immunological function. The specific immunological functions targeted by TCDD in each of the host resistance models remain to be fully defined.

Despite considerable investigation, the cells that are altered by TCDD exposure, leading to suppressed immune function, have not been unequivocally identified. Direct in vitro effects of TCDD on purified B cell activity have been reported, while direct effects on macrophages and T cells in vitro have not been described. The in vitro effects of TCDD on lymphocytes, however, appear to be influenced by cell culture conditions, which may explain the discrepancies in effects observed in different laboratories. Although the direct effects of TCDD on T cells in vitro have not been demonstrated, it is clear that functional T cell responses generated in vivo are compromised following in vivo exposure. TCDD may alter immune function by indirect mechanisms. One potentially important indirect mechanism is via effects on the endocrine system. Several endocrine hormones have been shown to regulate immune responses, including glucocorticoids, sex steroids, thyroxine, growth hormone, and prolactin. Importantly, TCDD and other related compounds have been shown to alter the activity of these hormones.

It is important to consider that if an acute exposure to TCDD even temporarily raises the TCDD body burden at the time when an immune response is initiated, there may be a risk of adverse impacts even though the total body burden may indicate a relatively low average TCDD level. Furthermore, because TCDD alters the normal differentiation of immune system cells, the human embryo may be very susceptible to long-term impairment of immune function from in utero effects of TCDD on developing immune tissue. There are currently no data to directly support this hypothesis. Concern arises as a consequence of inferences derived from an understanding of dioxin action and observations in humans and laboratory animals.

In summary, evidence has accumulated to demonstrate that the immune system is a target for toxicity of TCDD and structurally related compounds. The evidence has derived from numerous studies in various animal species. Animal studies suggest that some immunotoxic responses may be evoked at very low levels of dioxin exposure. Epidemiological studies also provide conflicting evidence for the immunotoxicity of these compounds in humans. Few changes in the immune system in humans associated with dioxin body burdens have been detected when exposed humans have been studied. Both direct and indirect (e.g., hormonally mediated) impacts on the immune system have been hypothesized to be the basis of dioxin immunotoxicity. While there is speculation that the developing immune system may be particularly sensitive to the effects of exposure to dioxin and related compounds, additional research will be needed to support this hypothesis.

9.7.6. Other Effects

A number of other effects of dioxin and related compounds have been discussed in some detail throughout the chapters in this assessment. While they illustrate the wide range of effects produced by this class of compounds, some may be specific to the species in which they are measured and may have limited relevance to the human situation. On the other hand, they may be indicative of the fundamental level at which dioxin produces its biological impact and may represent a continuum of response expected from these fundamental changes. While all may not be adverse effects (some may be adaptive and of neutral consequence), several effects have been noted in human studies or in primates that deserve special mention.

9.7.6.1. Circulating Reproductive Hormones

Two cross-sectional epidemiologic studies have detected an association between levels of male reproductive hormones and exposure to TCDD. Decreased testosterone levels were detected in two of the three studies where testosterone was evaluated and luteinizing hormone (LH) was increased in one of the two studies evaluating that end point. The fact that the results are based on a single sample rather than on the currently preferred series of three samples adds to the uncertainty of these findings. Animal data are available to support the plausibility of these findings. The mechanism(s) responsible for this effect are largely unknown, but changes in receptor level or function and hormone metabolism and homeostasis need to be investigated. If these data continue to hold up in future observations, their clinical significance will need to be further evaluated. Follow-up studies are currently under way.

9.7.6.2. Diabetes and Fasting Serum Glucose Levels

Epidemiologic evidence has been presented to suggest an increased risk of diabetes and for an elevated prevalence of abnormal fasting serum glucose levels with dioxin exposure. Three studies found that individuals with elevated serum levels of TCDD had a slight but statistically significant or borderline significant increased risk for developing diabetes or having elevated fasting serum glucose. There are virtually no animal data to corroborate these findings although some data have indicated effects of TCDD on glucose metabolism and insulin function. While the findings of a greater prevalence of elevated fasting glucose may presage the development of diabetes, in the NIOSH study of chemical workers, the traditional risk factors for diabetes (age, body mass index or weight, and family history of diabetes) appear substantially more influential than TCDD exposure in the development of the disease.

9.7.6.3. Enzyme Induction

One of the best characterized effects of exposure to dioxin-like compounds is the induction of cytochrome P-450 1A1 (CYP1A1). CYP1A1 is one of a family of proteins involved in the activation and detoxification of both endogenous and exogenous chemicals. Dioxin also increases the activity of a number of other enzymes involved in biotransformation reactions. Increased activity of these enzymes has been implicated mechanistically in the toxic responses seen in animals in response to dioxin-like compounds. For example, it has been hypothesized that increases in UDP-glucuronyltransferases leads to elimination of thyroxine and may lead indirectly to increased thyroid-stimulating hormone synthesis by the pituitary and subsequent hyperplastic and hypertrophic responses by the thyroid. There is speculation that such prolonged stimulation may lead to the thyroid tumors seen in both rats and mice exposed to TCDD. Therefore, while changes in enzyme activity in response to dioxin and related compounds may result in detoxification of certain chemicals, examples exist in experimental animals of changed metabolism leading directly or indirectly to adverse effects, some as severe as cancer. Data to confirm this effect of dioxin and related compounds in humans are not available.

9.7.6.4. Gamma Glutamyl Transferase (GGT) Activity

GGT is one of the many hepatic enzymes that are measured in human serum to evaluate liver toxicity. Of these, GGT is the only hepatic enzyme found in a number of human studies to be chronically elevated in adults exposed to high levels of TCDD. The consistency of the findings in a number of studies suggests that the finding may reflect a true effect of exposure but for which the clinical significance is unclear. Long term, pathologic consequences of elevated GGT have not been illustrated by excess mortality from liver disorders or cancer or in excess morbidity in the available cross-sectional studies. There are few animal data to support these findings.

9.7.6.5. Endometriosis