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Endocrine-Disrupting Compounds in the Environment

One of the most recent concerns associated with environmental contaminants is that some of these toxic chemicals may interfere with animal hormones. These compounds are referred to as endocrine disruptors because they disrupt normal functioning of the endocrine system. Many of the chemicals that may pose this threat have been in the environment for a long time (such as DDT), but only recently has this toxic mode of action been identified. The number of observations of aberrant endocrine functions in both human and wildlife populations that could be explained by contaminants working as endocrine disruptors is especially disconcerting (Colborn et al. 1993).

   

Some of the effects that have been attributed to endocrine disruptors have been dramatic enough to be described in the news media, including male American alligators with underdeveloped sex organs, vitellogenin (an egg and yolk protein normally found only in females) in male animals, and male Florida panthers with undescended testes and abnormal sex hormone ratios (Begley and Glick 1994; Raloff 1994). These examples typify estrogenic effects of endocrine disruptors, xenobiotic chemicals that, because of their chemical structures, can mimic estrogen, a female sex hormone. When the body's hormone receptors recognize the contaminant as estrogen, they respond as they would to the hormone, and the result is feminization of the exposed organism. Some of these effects may be more subtle than physical abnormalities and may manifest themselves as behavioral changes (Fox et al. 1978), such as aberrant behavior of birds during nesting, which can have significant effects on their nesting success.

   

Endocrine disruptors also may work in ways other than as estrogen mimics. Some compounds are thought to alter the activity of other sex hormones (androgens and progesterone), thyroxine, glucocorticoids, and possibly others. Endocrine disruptors may act not only by mimicking a hormone, as in the case of the estrogens previously described, but may also act as inhibitors (Kelce et al. 1995). Much current evidence points to early development (embryo, fetus, juvenile) as the most sensitive stages for exposure, although the effects on exposed young are often not apparent until an organism reaches sexual maturity, complicating the demonstration of cause and effect (Colborn et al. 1993).

   

Although some startling observations have been made in wildlife populations, epidemiological data from humans heighten concern about the endocrine disruption theory. There is evidence of a significant decrease in sperm counts in men over the last 50 years (Carlsen et al. 1992), with the exposure to endocrine-disrupting compounds a possible cause. Also, increasingly frequent human cancers over the last 20 years include those of the breast, ovaries, testes, and prostate (Reis et al. 1994), all tissues that are sensitive to sex hormones. One of the confounding factors in the theory of endocrine-disrupting contaminants as responsible for some of these observations is that naturally occurring compounds in some fruits and vegetables can also have estrogenic activity. Some scientists believe the amount of estrogenic activity an individual would receive from these naturally occurring chemicals in the diet is far greater than that from environmental contaminants.

   

These observations, and the implications of involvement of manufactured chemicals as a cause, prompted a national workshop sponsored by the U.S. Environmental Protection Agency in April 1995. This workshop brought together experts from human and ecological health research to discuss what was known about the potential for endocrine disruption due to environmental exposure, and to suggest what research is most urgently needed. While it appears that there is not enough scientific evidence to adequately address the health risks of endocrine disruptors, a dedicated research effort needs to be undertaken because of the magnitude of the potential impacts and the plausibility of this toxic mode of action.

   
  Author
Michael J. Mac
USGS National Center
Mailstop 300
12201 Sunrise Valley Drive
Reston, Virginia 20192

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