feature
Evaluating
Impacts of Hormonally A c t i v e A g e n t s in the Environment
Information about endocrine disrupters is limited, with sparse data, few answers, great uncertainties, and a definite need for further research. CAROL MACZKA, SUSAN PANG, DAVID POLICANSKY, AND ROBERTA WEDGE
1 3 6 A • MARCH 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
report (i) released last year by the National Research Council (NRC) finds that there is large uncertainty about risks from exposures to hormonally active agents (HAAs)— more popularly known as endocrine disr u p t e r s . The NRC C o m m i t t e e on HAAs in the Environment concluded, after examining existing scientific knowledge concerning HAA exposures, that further information is needed to understand the potential for harm to developing organisms. The report stresses that although evidence shows that harmful ecological and health effects can result from exposure to certain chemicals, it remains unclear whether their hormonal properties cause the observed effects, or instead, some other toxicological mechanism is operative.
A
Historically, concern about the threat from HAAs in the environment arose, in part, from the finding that some synthetic chemicals known to cause adverse reproductive and developmental effects in wildlife can mimic the actions of sex hormones. Experience with the pharmaceutical estrogen diethylstilbestrol (DES) focused attention on embryonic development as a target for the potential disruptive effects of chemicals on hormonal activity. DES was used as treatment to prevent miscarriages and was found, many years later, to cause a © 2000 American Chemical Society
rare tumor of the vagina (clear-cell adenocarcinoma) in daughters of women who took DES during pregnancy. These cancers, occurring in the daughters as young adults, illustrate that in utero exposure to a potent estrogen can cause adverse effects not manifested until later in life. Expanding on these findings, Our Stolen Future (2), a popular book published in 1996, cautioned that synthetic chemicals mimicking natural hormones are pervasive in the environment and pose a threat to future generations of people and wildlife by disrupting normal reproductive and developmental processes. Concerns led to action. The 1996 Food Quality and Protection Act and the Safe Drinking Water Act mandated that EPA develop a screening and testing strategy for potential endocrine disrupters, and in 1999, EPA began implementation of its Endocrine Disrupter Screening Program. The EPA, the U.S. Department of the Interior, and the U.S. Congress also requested that the NRC conduct an independent review of this topic, which lead to the NRC report. Despite progress in the regulatory arena, however, there continues to be debate in the scientific community over how great and how widespread the threats are from HAAs. Two controversial issues are how to interpret the endocrine disrupter hypothesis and how to define the agents of concern. The NRC's report, which reviewed a number of HAAs (see box below), purposely chose not to use the term endocrine disrupter, because the term "is fraught with emotional overtones and [is] tantamount to a prejudgment of potential outcomes." The term hormonally active agent was selected as a more neutral mechanistic descriptor and is defined as an agent that has demonstrated hormonelike activity in a test system.
Exposure is widespread Virtually all living organisms are exposed to HAAs directly or indirectly. These exposures may occur by ingesting contaminated foods or water, breathing contaminated air, and through contact with contaminated soils, sediments, or water. Although the presence of naturally occurring estrogenic compounds in plants has received attention in recent years, most HAAs monitored in the environment are produced artificially. These synthetic HAAs are used in a wide variety of industrial applications, ranging from chemical intermediates to textile processing to paper and pulp production. Some of the industrial chemicals (and byproducts) implicated as HAAs— such as dieldrin, polychlorinated biphenyls (PCBs), and toxaphene—are no longer manufactured in the United States. However, several such as bisphenol A and phthalates, are both produced and used here, as well as in
other countries. Dioxins, which are also HAAs, are unique because they have no commercial value but are produced as undesirable byproducts of industrial processes and combustion. Many of the more persistent HAAs have been monitored for years (see Table 1 on next page). Although these studies confirm their presence in all environmental media, levels of some of them, such as PCBs and DDT, are declining in some areas, with concentrations varying according to patterns of use, the presence of microorganisms able to degrade them, ambient air temperatures, and other pertinent physical and chemical factors. Atmospheric concentrations of DDT arc higher near tropical Asia, whereas hexachlorocyclohexanes are higher near the poles. HAAs have also been detected in other relatively pristine environments; for example, PCBs have been detected in lake and river sediments in remote areas of Canada. Typically, urban areas have higher concentrations of industrial HAAs, although use of pesticidal HAAs may result in significant concentrations in some rural areas. The ubiquity of HAAs in the environment is primarily the result of their resistance to chemical and biological degradation, but other factors also influence their distribution. Many HAAs, such as PCBs, are relatively volatile and may be released from soil and water surfaces to be transported long distances in the atmosphere. Others, such as dioxins, are less volatile and bind strongly to organic matter in soils and sediment. This latter behavior is also problematic, as the persistence of HAAs in soils and sediments means that exposures may occur for many years after their release into the environment— perturbations of soil or sediments from tilling, digging, erosion, or flooding may result in recycling of these buried chemicals and a subsequent increased possibility of exposure.
Major homonally active agents reviewed in the NRC report Bisphenol A Butyl benzyl phthalate Chlordecone Dieldrin DDT and its metabolites Methoxychlor PCBs (polychlorinated biphenyls) TCDD (tetrachlorodibenzo-o-dioxin) Toxaphene
MARCH 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 1 3 7 A
TABLE 1
Level of HAAs found in environmental media Elevated concentrations of hormonally active agents (HAAs) have been found in recent years in food, air, water, soils and sediments, and wildlife. Many animals, particularly those at the top of the food chain, have greatly elevated levels of HAAs, such as PCBs and DDT, in their tissues. However, as with environmental media, these levels appear to have been declining since the cessation or reduction in the use and release of these chemicals.
HAA
Environmental media
Level
Phytoestrogen
Soybean
Isoflavones DDT
Soy-based formula Fish-eating birds Eggs offish-eating birds Seawater Great Lakes St. Lawrence River sediment Air around Great Lakes Residential indoor air Lake Michigan Waukegan Harbor sediment Fish tissue, Housatonic River Seals Carnivorous whales Human blood (adults consuming more than 24 pounds of Great Lakes fish per year) Food packaging
84mg/100gdry weight 32-47 ug/mL 400 ug/g 835 ug/g
DDE Nonylphenol
PCBs
Diethylhexyl phthalate
Food f r o m paper packaging
0.001 ng/L 7.8 ug/L 38 ug/g 89-370 pg/m 3 39-^400 ng/m 3 0.47 ng/L >500,000 mg/kg 72,000 ug/kg 75 ug/g 1.4-800 ug/g 19ppb
3000 mg/kg
Exposure to pesticidal HAAs, such as DDT and dieldrin, continues even though the use of those pesticides has been discontinued in the United States for many years. The FDA Total Diet Study (4) indicated that children aged 2 years had the highest exposures to these pesticides on a body weight basis and that most foods with higher lipid content, such as meat and dairy products, contained greater concentrations. Continuing exposure to nonpesticidal HAAs, such as bisphenol A and possibly some phthalates, may occur as a result of their use in food packaging. Alkylphenols, including nonylphenol, have been found in sewage-treatment-plant effluents, and subsequently in drinking water. The NRC committee made several research recommendations concerning exposure to HAAs. The committee recommended that long-term monitoring of known HAA-contaminated media, including aquatic organisms, be conducted as there are limited data on the concentrations, persistence, and recycling of HAAs in and between environmental media. Because organisms are typically exposed to more than one HAA, whether a phytoestrogen or a synthetic HAA, the committee recommended that exposure assessments include all HAAs present in a medium and not focus exclusively on individual chemicals. The committee emphasized that additional research on the intake of phytoestrogens and synthetic HAAs by humans and other biota and a determination of the primary exposure sources are critical to understanding the risk posed by these chemicals in the environment. Because of the ingestion of phytoestrogens as part of the diet and the use of synthetic HAAs as pharmaceuticals and in industrial applications, the committee also recommended that primary routes of exposure, such as diet and drinking water, be studied to establish typical baseline intake levels.
0.1-25 mg/kg
Determining exposures to synthetic HAAs, as well as to phytoestrogens, is confounded by the fact that HAAs may be ingested as part of a normal diet and that some HAAs are deliberately used as therapeutic agents (see Table 2 on next page). It has been estimated that "more than 95% of the U.S. population has detectable levels of PCBs, regardless of age, sex, race, or geographic location," (3) with concentrations increasing with age. Infants and children may be at risk from HAAs consumed in breast milk. In addition, the use of soy-based formula for infants has raised concerns about its long-term health effects, as these infants may be receiving a dose of isoflavones, a form of phytoestrogen, "6 to 11-fold higher than concentrations known to cause hormonal effects in adults" (i). Along with children, other subpopulations may have elevated exposures to HAAs. These groups include subsistence fishermen and aboriginal populations, such as the North American Inuits, who consume a diet high in contaminated marine mammals and fish. 1 3 8 A • MARCH 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
Evaluating health effects Developmental Effects and Reproduction: The NRC committee undertook a critical analysis of the literature on the link between HAA exposure and reproductive and developmental effects observed in laboratory studies and reported in human and wildlife populations. At issue was whether exposure to environmental HAAs affects animals and humans similarly and the extent of harm caused by exposure to these compounds at prevalent environmental concentrations. Only a few, primarily estrogenic, HAAs were evaluated—those that were both extensively studied and reasonably suspected of having hormonal activity. Emphasis was placed on data linking reproductive and developmental effects to critical periods of exposure. Analysis of data revealed that adverse reproductive and developmental effects have been observed in human populations, wildlife, and laboratory animals as a consequence of exposure to HAAs. The few humanexposure studies available provide convincing evidence that prenatal exposure to PCBs from maternal consumption of contaminated fish or other food products can cause lower birth weight, shorter gestation, IQ and memory deficits, and delayed neuromuscular de-
velopment. Prenatal and postnatal exposure to PCBs and polychlorodibenzofurans (PCDFs) from accidental contamination of rice oil in Yusho, Japan, and YuCheng, Taiwan, clearly resulted in various developmental deficits in children. There have been reports of increases in the incidence of certain male reproductive disorders, such as hypospadias (urethra opening found at the bottom rather than the tip of the penis), cryptorchidism (undescended testicles), and testicular cancer, but the committee found that these disorders could not be linked with exposures to environmental HAAs. Sperm concentration is another male reproductive end point that was considered by the committee. In its evaluation of retrospective analyses of trends in sperm concentration over the past 50 years, the committee found evidence of substantial regional variation, but found that the data were inadequate for making a determination of whether sperm concentrations have declined. It suggested that prospective and cross-sectional studies are needed to clarify this issue. As demonstrated by laboratory experiments of specific HAAs found in effluents, polluted water, and other contaminated areas, there is convincing evidence that some effects observed in the wild are a consequence of HAA exposures. For example, "defects seen in alligators from Lake Apopka in Florida (the site of a chemical spill containing dicofol and DDT), including small penis size and abnormal testes in males, and abnormal ovaries in females, are consistent with structural and functional reproductive abnormalities that occur following perinatal exposure of laboratory rodents to estrogenic and anti-androgenic chemicals." Laboratory experiments have also replicated reproductive and developmental abnormalities—gonadal abnormalities, abnormal breeding behavior—observed in several populations offish and among some North American gull populations. Extrapolation of laboratory effects and effects seen in wildlife to humans is difficult, yet the degree to which such observations apply to species other than those in which they were made is crucial to any judgment about the seriousness of the risk to humans. Although endocrine systems are remarkably well conserved among vertebrate groups, there are significant differences in their operation. The committee found that there was insufficient information available to extrapolate reliably across species and recommended that wildlife and human populations continue to be monitored for adverse reproductive effects. Although for the most part, data are unavailable to link reproductive and developmental effects to critical periods of exposure and more particularly to concentrations ordinarily found in the environment, laboratory studies using a variety of species have shown that exposure during development to certain HAAs can produce structural and functional abnormalities of the reproductive tract. Studies involving the maternal consumption of contaminated fish and the accidental contamination of rice oil in Yusho, Japan, and Yu-Cheng, Taiwan, strongly suggest the vulnerability of the human fetus during critical stages of development.
TABLE 2
Estimated daily exposure to HAAs In general, the greatest exposure of humans and most animals to hormonally active agents (HAAs) is through their diet.
HAA
Use
Estradiol Bisphenol A
Oral contraceptive Plasticizer in food cans Contaminant in food and water Food packaging
PCB Diethylhexyl phthalate Nonylphenol DDT
Isoflavones
Contaminant in drinking water Food contaminant (diet high in marine mammals) Soy-based formula
Daily exposure for adults (ug/day)
20-50 6.3 0.0002 100-800 0.7 24.2 (Inuit women)
4.5-8.0 mg/kg (4-month-old infant)
Neurological Effects: Epidemiology studies reveal associations between prenatal exposure to PCBs and delayed cognitive and behavioral development. For example, Jacobson and Jacobson (5) conducted a series of studies in Michigan that tracked, from infancy through childhood, the cognitive development of children exposed to PCBs in utero. The children studied were exposed from the diet of their mothers, who regularly consumed PCB-contaminated fish during pregnancy and throughout lactation. The investigators found consistent impairments on cognitive development, including deficits in short-term memory and visual discrimination, as well as lower IQ scores. Effects were linked to prenatal exposure but not to postnatal exposures. Gladen and Rogan (6) have also reported delays in mental and psychomotor development in a cohort of children in North Carolina who were also exposed to PCBs in utero. These developmental delays were observed for up to 24 months of age but not thereafter. Laboratory studies with experimental animals support the epidemiology findings that prenatal exposure to PCBs can affect early neurological development. However, because most of the available data only concern PCBs, the NRC committee recommended that neurological end points be tracked in epidemiology studies of human populations exposed to other types of HAAs. Immunological Effects: There is evidence of immune suppression from exposure to organochlorines (predominantly PCBs) in wildlife populations, such as the tern and herring gull populations of the Great Lakes (7). Experimental studies with marine mammals have shown that innate and acquired immune responses are impaired in seals fed fish from organochlorine-contaminated waters (8, 9). Such immunosuppression is believed to be the reason for increased incidences of bacterial and viral inMARCH 1, 2000/ ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 1 3 9 A
fections in seal populations found in contaminated waters. Studies with laboratory animals have shown that HAAs can affect diverse elements of the immune system. These analyses suggest that HAAs have the potential to affect human immunity, but there are few studies of the immunological effects of hum a n exposure to HAAs. The committee recommended mat epidemiology studies of populations exposed to HAAs track immunological end points, such as autoimmune problems. Carcinogenicity: It has been hypothesized that environmental exposure to HAAs results in an elevated risk of hormone-related cancers in humans. Human and experimental animal data link these cancers to exposure to endogenous hormones and DES. The committee evaluated human and animal data relating environmental HAAs to cancers of the female and male reproductive systems and endocrine organs. The committee limited its review to cancer sites that are known from ancillary data to have some hormonal dependence and where activity should be most evident. However, the committee recognized that some of the compounds discussed have been known to cause cancer in other organ systems and in a variety of species. An evaluation of animal data on 15 substances, including several pesticides, butyl benzyl phthalate, PCBs, and dioxin, failed to produce compelling evidence that HAAs induce cancers of the female and male reproductive systems and endocrine organs, although some HAAs—toxaphene, DDE, DDT, tetrachlorodibenzo-p-dioxin (TCDD), and endrin—have been shown to induce tumors of the thyroid, pituitary, or renal glands in particular species and strains of laboratory animals. In reaching this conclusion, the committee cautioned that "not all HAAs have yet been tested for carcinogenicity, including some phthalates and alkylphenols." More important, the committee emphasized that "with the exception of DDT, no data are available on the carcinogenic effects of perinatal exposure to environmental HAAs to the F, (first filial generation) or succeeding generations." Lack of data led the committee to recommend research in appropriate laboratory animals on the role of perinatal exposure to suspected chemicals in inducing cancers later in life or in subsequent generations. The committee recommended that initial studies should focus on HAAs that have been shown to induce cancer of the thyroid, pituitary, and adrenal glands in some laboratory animals. Data for evaluating the relationship between HAAs and human cancers were essentially limited to studies involving exposure to a few HAAs (DDT and its metabolite DDE, TCDD, and various PCBs). Other compounds with potential hormonal activity have received little attention. The committee further limited its investigation to known targets for endogenous and exogenous sex steroid hormones—breast and endometrial cancers in women and prostate and testicular cancers in men. The majority of these studies focused on the breast. The committee concluded that the current literature does not support associations between adult 1 4 0 A • MARCH 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
exposure to DDT, DDE, TCDD, and PCBs and breast cancer. Current literature also does not support associations between those HAAs and other hormonally sensitive cancers (testicular, prostate, or endometrial cancer). However, few of those studies measured concentrations of those compounds in adults in relation to cancer. In addition, similar to the animal studies, no data were available on exposures to environmental HAAs and their possible effects during susceptible periods such as fetal life or pregnancy and transgenerational effects. With respect to breast cancer, at least seven studies were published before 1995 that contained data on some organochlorine concentrations in tissue or sera (10). These studies do not support an association between DDT metabolites or PCBs and risk of breast cancer. Additional studies have since been published; several included large numbers of women and employed internal dose measurements of exposure. These studies also do not support an association of DDE or PCBs with cancer in humans. However, because of the limited number of compounds that have been studied, the committee called for "case-control and retrospective cohort studies to document the presence or absence of associations between HAAs and various cancers in humans. If such associations are found, the possibility of causality must be investigated. Such studies should take into account the latency period between exposure and disease, the timing of likely exposure windows with respect to cancer, indicators of susceptibility, and the role of potential confounders." The committee further recommended that markers of exposure be used to provide an accurate assessment of internal dose and cautioned that the "biologic potency of HAAs must be related to that of endogenous hormones, pharmacologic estrogens, and phytoestrogens since large segments of the population are exposed to these compounds." Ecological Effects: Ecological studies indicate that environmental HAAs probably have contributed to declines in some wildlife populations, such as fish and birds of the Great Lakes and alligators of Lake Apopka in Florida, and possibly to diseases and deformities in mink in the United States, river otters in Europe, and marine mammals in European waters. Such contaminants, along with inbreeding, may have contributed to the low birth rate of the endangered Florida panther. It is difficult, however, to determine a clear causal relationship between these changes and exposure to HAAs given all of the other environmental factors involved. Thus, the committee recommended long-term studies of wildlife populations exposed to HAAs to assess the effects of these chemicals on population size, age structure, sex ratios, and other ecological factors. Such research should examine links between chemical exposures and alterations in key life stages to understand how they affect populations in the long term.
Mechanisms of action As part of its charge, the committee was asked to identify known and suspected toxicological mechanisms of action for putative HAAs. The concerns
about HAAs have been expressed as the idea that some environmental chemicals can act analogously to specific hormones—the chemicals either overmodulate or undermodulate activities of hormones, producing effects that are hormonally mediated. The early focus of the literature on compounds with gonadal steroid activity and the need to limit the scope of the study prompted the committee to emphasize these activities and their consequences. However, it was recognized that interference by HAAs with other hormonal and nonhormonal physiological systems may have major impacts on development and other functions; literature on these functions was not abundant. For most associations reported between HAAs and various biological outcomes, the specific mechanism of action is not well understood. The committee concluded that "lack of knowledge about a mechanism does not mean that a reported effect is unconfirmed or unimportant, nor does demonstration of a mechanism document that the resulting effects are unique to that mechanism or are pervasive in natural systems." It does suggest that additional studies are warranted. The committee recommended that a battery of shortterm assays be developed for rapid and inexpensive screening of putative HAAs. These assays should detect diverse responses that depend on hormone receptors, should detect other indirect responses, and should be readily adapted for use in multiple laboratories.
AMERICAN CHEMICAL SOCIETY
Finally, we draw attention to the committee's discussions of scientific issues that make judgments about the effects of chemicals in the environment more difficult. The issues apply especially to chemicals with more than one possible mechanism of action, chemicals that occur in mixtures, and chemicals whose effects might be noticed only long after the exposures occur.
References (1) National Research Council. Hormonally Active Agents in the Environment; National Academy Press: Washington, DC, 1999. (2) Colborn, T.; Dumanoski, D.; Myers, J. P. Our Stolen Future; Plume/Penguin: New York, 1996. (3) Robinson, E E.; Mack, G. A.; Remmers, J.; Levy, R.; Mohadjer, L. Environ. Res. 1990, 52 (2), 175-192. (4) Gunderson, E. L. J. AOACInt. 1995, 78 (6), 1353-1363. (5) Jacobson, J. L.; Jacobson, S. W. N. Engl. J. Med. 1996, 335 (11), 783-789. (6) Gladen, B. C; Rogan, W. J. /. Pediatr. (St. Louis) 1991, 119 (1, Part 1), 58-63. (7) Grasman, A.; Fox, G.; Scanlon, P.; Ludwig, J. Environ. Health Perspect. 1996, 104 (Suppl. 4), 829-842. (8) de Swart, R. L. et al. Ambio 1994, 23 (2), 155-159. (9) de Swart, R. L. et al. Infect. Agents Dis. 1996, 4 (3), 125130. (10) Adami, H. O. et al. Cancer Causes Controls, Pap. Symp. 1995, 6 (6), 551-556. Carol Maczka is senior program director, Susan Pang is program officer, David Policansky is associate director, and Roberta Wedge is program director on the Board on Environmental Studies and Toxicology of the National Research Council in Washington, D.C.
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