Many environmental health issues are worsened, or even caused, by our failure to apply basic biological information. For example, we know that predatory insects control populations of plant-eating insects; yet we spray crops with pesticides that kill harmful and beneficial insects indiscriminately and then find their way into bodies of water, where they harm aquatic organisms. We also know that insects can evolve resistance to pesticides rapidly because they have short generation times, yet we continue to use those chemicals in ways that almost guarantee the evolution of resistance (see Chapter 7, page 208). In addition, we understand the effects of hormones on humans and other animals, yet we release endocrine disruptors, chemicals that have hormone-like effects, into the water that we use for drinking, cooking, and bathing.

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Hormones control many elements of the physiology and development of organisms, including embryonic growth and the storage and release of glucose for cellular metabolism. Certain chemicals called endocrine disruptors interfere with these processes (Figure 11.11). They can turn on the signals that hormones carry to cells, turn them off, or change the meaning of the message a hormone is carrying. All these actions can have drastic effects on tissue function. Interference with the development and function of sex organs is one of the most conspicuous effects of endocrine disruptors.

Many endocrine disruptors are by-products of manufacturing processes. For example, a chemical called androstenedione is an anabolic steroid, one of the hormones with masculinizing effects that gained notoriety because many athletes misused it in hopes of enhancing their performance. Problems have arisen where this hormone is released into the environment. For example, androstenedione is released as a manufacturing by-product in the waste from a pulp paper mill on the Fenholloway River in the panhandle of Florida. The mosquitofish upstream from the mill have the expected 50 : 50 ratio of males to females, but downstream from the mill 90% of the females are partially or fully masculinized.

Androstenedione is unusual because it has a masculinizing effect; most endocrine disruptors are estrogen mimics that produce female characteristics. On a daily basis, we are surrounded by estrogen mimics: PCBs (polychlorinated biphenyls; see Chapter 13, page 392) are in the lubricants in our garages or basements, alkylphenols are in the detergents in our laundry rooms and beneath our kitchen sinks, styrenes are in cigarettes and automobile exhaust, and parabens are in cosmetics. Two estrogen mimics are especially pervasive—bisphenol A, which is in some plastics, and the synthetic form of estrogen that is in birth control pills.

Bisphenol A (BPA) keeps polycarbonate plastics flexible and is used in epoxy resins. Infant bottles and plastic water bottles contain BPA, and BPA is present in the epoxy resins that coat the insides of food cans. Most human exposure to BPA comes from these sources. In 2012 the Food and Drug Administration (FDA) did not ban BPA from food containers because their studies showed that the human liver detoxifies BPA, thus concluding that humans have very low levels of BPA in their blood. The same studies also showed, however, that other animals are not able to detoxify BPA, so the FDA’s decision will allow the environmental effects of BPA to continue. Regardless of the decision, great demand among consumers for BPA-free plastic products may lead to a phasing out of its use in plastics manufacturing.

The natural estrogen that women produce is excreted in urine, and the synthetic estrogen found in birth control pills is not completely broken down in sewage treatment plants. Consequently, the sewage effluent that is released into lakes, rivers, or the sea contains mixtures of these estrogens, which have a feminizing effect on aquatic organisms. Feminized male fish have been found in rivers even several kilometers downstream from the discharge pipes of sewage treatment plants.

The list of species that have been feminized by environmental estrogens is long. It includes aquatic species (fish, frogs, salamanders, turtles, and alligators); terrestrial birds; mammals, invertebrates as well as vertebrates; and marine species (harbor seals and harp seals), including freshwater forms. Human males are not immune to the effects of environmental estrogens; the average sperm count of men in Europe and the United States has decreased by about 50% since 1950. Because these environmental estrogens affect so many species, including humans, it’s clear that the hormones and receptors of these organisms must have similar structures.

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