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During recent years numerous newspaper and magazine articles have suggested that humans may be at risk because small amounts of well known environmental contaminants such as dioxin PCBs and DDT can affect hormone levels Hormones are produced by the endocrine system as regulators of biological function in target organs Because hormones play a critical role in early development toxicological effects on the endocrine system often have an impact on the reproductive system The term endocrine disruptor is used to describe chemicals that can mimic hormones and may either enhance or counteract their effects It has been suggested that these hormone changes can in turn lead to a variety of health problems including cancer decreased fertility and abnormalities in newborns Evidence provided to support these claims of human and wildlife harm is largely from laboratory studies in which large doses are fed to test animals usually rats or mice and field studies of wildlife species that have been exposed to the chemicals mentioned above In laboratory studies high doses are required to give weak hormone activity These doses are not likely to be encountered in the environment However the process of bioaccumulation can result in top-level predators such as humans to have contaminants at levels many million times greater than the environmental background levels Guilette 1994 In field studies toxicity caused by endocrine disruption has been associated with the presence of certain pollutants Findings from such studies include reproductive disruption in starfish due to PCBs bird eggshell thinning due to DDT reproductive failure in mink small penises in alligators due to DDT and dicofol Guillette 1994 Colburn et al 1996 In addition a variety of reproductive problems in many other species are claimed to be associated with environmental contamination although the specific causative agents have not been determined One recent discovery that complicates the situation is that there are many naturally occurring phytoestrogens or chemicals of plant origin that exhibit weak estrogenic properties
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