Environmental Problems, Their Causes, and Sustainability 1

HormoneEstrogenlike chemicalAntiandrogen chemicalReceptorCellNormal Hormone Process Hormone Mimic Hormone BlockerFigure 19-7 Hormones are molecules that act as messengers in the endocrine system to regulate various bodilyprocesses, including reproduction, growth, and development. Each type of hormone has a unique molecularshape that allows it to attach to specially shaped receptors on the surface of, or inside, cells and to transmit itschemical message (left). Molecules of certain pesticides and other synthetic chemicals have shapes similar tothose of natural hormones and can affect the endocrine system in people and various other animals. Thesemolecules are called hormonally active agents (HAAs). Some HAAs, sometimes called hormone mimics, disruptthe endocrine system by attaching to estrogen receptor molecules (center) and giving too-strong, tooweak,or mistimed signals. Other HAAs, sometimes called hormone blockers, prevent natural hormones suchas androgens from attaching to their receptors (right) so that no signal is given. Some pollutants, called thyroiddisrupters, may disrupt hormones released by thyroid glands and cause growth and weight disorders andbrain and behavioral disorders. Because of the difficulty in determining the harmful effects of long-term exposureto low levels of HAAs, there is uncertainty over their effects on human health.tory animals, and epidemiological studies of humanpopulations. This analysis suggests that a variety ofhuman-made chemicals can act as hormone or endocrinedisrupters, known as hormonally active agents (HAAs).Examples of hormone disrupters are DDT, PCBs, andcertain herbicides.Some, called hormone mimics, are chemicals similarto estrogens (female sex hormones). They can disruptthe endocrine system by attaching to estrogen receptormolecules (Figure 19-7, center). Others, called hormoneblockers, disrupt the endocrine system by preventingnatural hormones such as androgens (male sex hormones)from attaching to their receptors (Figure 19-7,right). Estrogen mimics and hormone blockers aresometimes called gender benders because of their possibleeffects on sexual development and reproduction.There is also growing concern about still anothergroup of HAAs—pollutants that can act as thyroid disruptersand cause growth, weight, brain, and behavioraldisorders.Is long-term exposure to low levels of HAAs athreat to human health? A 1999 study of the possibleeffects of hormonally active agents on humans by aU.S. National Academy of Sciences panel of scientistscame to three major conclusions. First, “adverse reproductiveand developmental effects have been observedin human populations, wildlife, and laboratory animalsas a consequence of exposure to HAAs.” Second,“there have been only a few studies of the effects ofHAAs in humans, but the results of laboratory andwildlife studies suggest that HAAs have the potentialto affect human immune functions.” Third, greatly increasedresearch is needed to come to a more definitiveconclusion about whether low levels of most HAAs inthe environment pose a threat to human health.Bottom line: We do not know whether exposure totrace amounts of various hormonally active chemicalsintroduced into the environment have harmful effectson humans and other animals. Some scientists saythere is no definitive evidence for harm from HAAs tohumans and dismiss it as a minor threat. But otherssay there is enough preliminary evidence to warrantgreatly increased research on their possible effects.This will take decades.Some scientists say we need to wait for the resultsof more research before banning or severely restrictingHAAs. Other scientists believe that as a precaution, weshould sharply reduce the use of potential hormonedisrupters.Why Do We Know So Little about the HarmfulEffects of Chemicals? Establishing Guilt IsDifficultUnder existing laws most chemicals are consideredinnocent until shown to be guilty, and estimatingtheir toxicity to establish guilt is difficult, uncertain,and expensive.According to risk assessment expert Joseph V.Rodricks, “Toxicologists know a great deal about a fewhttp://biology.brookscole.com/miller14417