Endocrine glands, found in all animals with backbones (vertebrates), secrete hormones that travel through the body and trigger responses in receptor cells.  Endocrine systems control important aspects of body function including development, growth, metabolism, and reproduction.  For more information on the endocrine system, go to Tulane University, the American Medical Association or to Columbia Encyclopedia.  It has long been known that environmental contaminants can disrupt the endocrine system.  

Rachael Carson's Silent Spring (1962) brought attention to how the pesticide DDT impaired the reproductivity of some birds resulting in their decline.  Rachael Carson was heavily critized by the pesticide industry, and by some in government, but her assertions about eggshell thinning have been confirmed by many subsequent studies.  DDT alters calcium metabolism in birds, blocks its movement to eggshells during formation, and causes abnormally thin shells that are crushed by the weight of the incubating parent.  Ironically, DDT had been shown to cause acute mortality in robins and other small birds many years before Rachael Carson published Silent Spring.  DDT, sprayed to control beetles that transmitted Dutch elm disease, was concentrated in earthworms to levels that were acutely toxic to birds that fed on them.  DDT was banned in the United States in 1972, based on its hazard to wildlife, but it is still used in other countries to control insects that carry disease or eat foodcrops.  Legitimate debate continues over the risks that DDT poses to wildlife and humans, versus the benefits it can bring by reducing human starvation and diseases like malaria.

EDCs can disrupt the endocrine system in several ways.  They can bind with the hormone receptor and either mimic a hormone, triggering an identical response, or block a hormone from triggering the response.  EDCs also can interfere by increasing or reducing amount of hormones produced by a gland, or by modifying a hormone receptor.  Known and suspected EDCs include natural and pharmaceutical estrogens, phytoestrogens that occur naturally in some vegetables, and manufactured chemicals including polychlorinated biphenyls (PCBs), dioxins, plasticizers, detergents, uranium, perchlorate, tributyltin (TBT) and organochlorine insecticides such as DDT and Lindane. 

EDCs can harm reproductive health, such as by thinned eggshells or sexual disruption of fish, snails, and turtles.  EDCs have impaired the reproductive and immune functions of mammals including seals, polar bears, mink, and rabbits.  It seems as if the jury is still out on the role that EDCs may have played in the decline of amphibian populations, and in causing frog deformations.  EDC health effects in humans are highly controversial.  EDCs are suspected of causing hormonally-sensitive carcinoma (i.e.. breast, cervical, prostate and testicular cancer), deterioration of sperm quality and reduction of male/female birth ratios.  In addition to the harmful effects on wildlife discussed above,  studies have associated DDE with reduced milk production and increased chance of premature birth in women, and with reduced sperm quality in malaria vector-control workers in South Africa where DDT is still used. 

The growth and development of fetuses and embryos are strongly controlled by the endocrine system.  Evidence suggests that they are particularly vulnerable to EDCs delivered by the mother both prenatally and after birth during breast feeding.  Exposure to EDCs in the womb has been suggested as a possible cause of low sperm count, and as a risk factor for testicular cancer, in adult men. 

The University of Ottowa, Tulane University,  U.S. Environmental Protection Agency, European Commission, U.S. Geological Survey, Environment Canada, World Health Organization, and the Japanese Ministry of Environment have informative EDC web pages.  Our Stolen Future, a book about endocrine disruption by Theo Colborn, Dianne Dumanoski and John Peterson Myers (1996) has a website with book chapter summaries and links to news reports and opinions.
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