Over the past 4 years, a newly recognized environmental threat to health and reproduction has mushroomed into public prominence.
Bearing the clumsy moniker "endocrine disrupters," these pollutants-including PCBs, DDT-breakdown products, dioxins, and certain plasticizers-can mimic or block the action of natural hormones.
By inappropriately turning genes on or off, these compounds can elicit a range of adverse effects. In humans, they may foster cancer in the breast or other reproductive organs. Prenatal exposures appear capable of altering brain development-with impacts on IQ and behavior that persist at least a decade, perhaps for life. Most surprisingly, in some exposed wildlife, creatures whose genes instruct them to be male have matured into individuals that look and act like females.
The newly recognized potential of these pollutants to wreak havoc in so many ways, together with their ubiquity, has given rise to a sense of humility among toxicologists. The compounds have been detected in pesticides, plastics, dental sealants, contraceptives, and dishwashing liquids, and they contaminate water, plants, wildlife, and foods. Explains John A. McLachlan, a pioneer in this field, researchers have traditionally hunted out carcinogens and other environmental toxicants that affect DNA. Such changes tend to "leave well-defined structural alterations," he says, "persistent footprints." Scientists have observed a variety of internal chemical signals by which organisms trigger and modulate normal differentiation of cells, development of organs, immune responses, and neural activity.
"What I think endocrine disrupters have done is show us that they-and presumably other toxicants as well-can exert their damage by mimicking, blocking, or altering these natural signaling pathways," says McLachlan, who heads the Center for Bioenvironmental Research at Tulane and Xavier Universities in New Orleans.
Unfortunately, he notes, they do this without leaving footprints. Hormone disrupters are suspected of causing diverse abnormalities recently observed in wildlife, from extra limbs to altered sexual development, but the case is hard to prove.
McLachlan predicts that perhaps a decade from now, people who today study the immune, nervous, or hormone systems will be collaborating in a search for tests capable of pinpointing agents that can subtly alter communications within and between many-if not all-of these related systems.
These pseudohormones introduce "some profound challenges to toxicology," says Devra Lee Davis, a toxicologist with the World Resources Institute in Washington, D.C. For example, hormone mimics are "forcing us to rethink the notion of dose," she says. The traditional axiom that "the dose makes the poison" has been interpreted to mean that as exposures increase, so does the likelihood that a substance will do damage. However, she observes, several recent laboratory studies suggest that for some hormone mimics, lower doses can cause greater effects than larger ones. …