Endocrine disruptors are chemicals that disrupt the normal functioning of hormonal systems or the balance of hormones. There are currently no standardized tests to determine whether or not a chemical should be classed as an endocrine disruptor. Chemicals that alter either the levels of estrogens, androgens, or thyroid hormones or the body’s response to them are of special concern because these hormones are very important during development. Laboratory studies show definitively that certain chemicals can have adverse effects on hormone function, which, in turn, produces birth defects, developmental abnormalities, reproductive dysfunction, and cancer. Whether the environmentally ambient concentrations of endocrine-disrupting chemicals create developmental dysfunctions or cancer in humans is still a subject of debate.
There is little debate that wildlife can be affected adversely by endocrine disruptors at ambient concentrations. The classic example is the decline of populations of bald eagles and other birds in the United States. The population decline was reversed after the phase out of DDT. There are many other examples. Recent laboratory studies have found that amphibian gonad development is altered by the herbicide Atrazine at extremely small concentrations, and other studies have shown that sexual development in fish is negatively affected by estrogenic chemicals found in sewage treatment discharges.
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Endocrine disrupting chemicals that act on hormone receptors pose problems for standard toxicology experiments that use high doses and linearly extrapolate the effects to lower doses. The adverse effects to an organism of a hormone or endocrine disruptor are often an inverted-U function; when concentrations are above or below an optimal level, adverse effects occur. Examples are vitamin A (and other retinoids) and thyroid hormone.
A report by the National Research Council concluded that there is evidence that PCBs and the chemicals that accompany them in the environment (dioxins and furans) are correlated with shorter gestation, slower growth, and lower IQ test scores. The panel of scientists could not agree about the source of historical changes in sperm counts, the possible increase in hypospadias (opening of the urethra on the underside of the penis) found in people near toxic waste respositories, and a possible increase in undescended testicles. However, laboratory studies of a number of species of mammals, including monkeys, yield evidence that a variety of endocrine-disrupting chemicals (including DDT, methoxychlor, bisphenolA, PCBs, dioxins, and dibutylpthalate) induce structural and functional reproductive changes. Some of these chemicals also appear to suppress immune function in wildlife and laboratory animals.
The National Research Council committee recommended that epidemiological studies be conducted to examine the effects in humans of endocrine-disrupting chemicals that have been shown to affect laboratory animals. The outcomes to be assessed should depend on the outcomes observed in laboratory animals for the specific chemicals. For different chemicals, the outcomes would be reproductive system structure and function, developmental defects, neurobehavioral functioning and social development, immune system, and different types of cancers. Because of the widespread contamination of the environment by endocrine-disrupting chemicals, the committee also recommended the development of assays that will rapidly screen and identify chemicals with potential endocrine disrupting activity, and that the sensitivity of the fetus to delayed effects of such chemicals be considered in developing such assays.
- Moore, C. F. (2003). Silent scourge: Children, pollution, and why scientists disagree. New York: Oxford University National Research Council. (1999). Hormonally active agents in the environment. Washington, DC: National Academies Press.
- S. Environmental Protection Agency. (n.d.). Endocrine disruptor research initiative. Retrieved from http://www.epa.gov/endocrine/
- Welshons, W. V., Thayer, A., Judy, B. M., Taylor, J. A., Curran, E. M., & vom Saal, F. S. (2003). Large effects from small exposures. I. Mechanisms for endocrine-disrupting chemicals with estrogenic activity. Environmental Health Perspectives, 111(8), 994–1006.