endocrine disruptor

endocrine disruptor, any chemical that mimics or interferes with the normal actions of hormones in the body. Endocrine disruptors may be synthetic or natural (e.g., phytoestrogens) in origin and are used in a wide range of products and materials, from cosmetics and plastics to pesticides and industrial solvents. Because hormones are the chemical messengers of the endocrine system—the network of glands that regulates all essential biological processes, including development, metabolism, and reproduction—exposure to endocrine disruptors is a major health issue and an environmental concern in countries worldwide.

Early discoveries of endocrine disruptors

The ability of certain chemicals to interfere with endocrine function was realized in the mid-20th century, and synthetic pesticides were among the first to be studied. The most well-known of those pesticides was DDT (dichlorodiphenyltrichloroethane). In the 1960s DDT was found to accumulate in body tissues and to be harmful especially to birds, in which it caused eggshell thinning that resulted in reduced egg viability and fewer hatchlings. Exposure to the chemical led to the decline of many bird species, including the bald eagle, golden eagle, and brown pelican, and affected the reproduction of some mammalian species, including sea lions, which delivered pups prematurely. The following decade, diethylstilbestrol (DES), a nonsteroidal synthetic estrogen drug, was discovered to cause reproductive tract defects and an otherwise rare reproductive cancer in daughters born to women who had taken the drug while pregnant. DES was known to mimic the actions of estrogen, but it was widely believed that chemicals could not cross the placental barrier, and, hence, its effects on female offspring had not been anticipated.

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By the early 1970s some countries had begun to restrict the use of both DDT and DES. Subsequent research showed that DDT and a number of other chemicals and their metabolites possessed an affinity for hormone receptors and, through their actions at the receptors, whether mimicking natural hormones or blocking receptor activity, effectively disrupted or otherwise altered endocrine function in animals. That realization led, in the early 1990s, to the introduction of the term endocrine disruptor.

Endocrine-disrupting chemicals and their sources

Since the 1960s and ’70s, large numbers of endocrine-disrupting chemicals have been identified, and many of them are widely distributed in the environment. Although a number of them are herbicides, fungicides, or insecticides, a variety of industrial chemicals and some naturally occurring elements have also been discovered to be toxic to the endocrine system. Examples include bisphenol A (BPA), dioxin, lead, mercury, polychlorinated biphenyls (PCBs), and styrenes. Many personal care products, including lotions, perfumes, and shampoos, contain glycol ethers, parabens, and phthalates—chemicals that have been shown to interfere with the function of hormonal pathways in the body. Chemicals such as BPA and phthalates are commonly used as plasticizers and can be found in everyday household items, including plastic food containers, raincoats, and shower curtains. Polybrominated diphenyl ethers (PBDEs), which are synthetic halogenated compounds, are used as flame retardants in a variety of products, including electronics, foams, plastics, and textiles.

Routes of exposure to endocrine disruptors

Exposure to endocrine-disrupting chemicals can occur in various ways. Dioxins, PCBs, and synthetic pesticides that are released into the environment leach into soil and groundwater, potentially contaminating supplies of drinking water. BPA and phthalates tend to leach from plastic containers into the beverages or food they hold. Thus, some chemicals are consumed inadvertently in food or drinks. Exposure can also occur through direct contact with products, particularly in the case of herbicides and pesticides. Cosmetics and certain insect repellents and sunscreens that contain endocrine disruptors are applied to the skin, resulting in direct exposure.

As was discovered with DDT, endocrine disruptors are not readily excreted from the body; rather, they are stored within fat in a process known as bioaccumulation. Moreover, the chemicals work their way up food chains. Endocrine disruptors that leach into the air, soil, or water are taken up by bacteria, algae, and plants. Those organisms are then consumed by higher organisms, including herbivores, which are then consumed by carnivores. As a consequence, many mammals are also likely to transfer chemicals to their developing offspring in the womb. In humans and other placental animals, the developing fetus is exposed to any chemical that crosses the placenta, as well as to chemicals that have been stored in the mother’s fat. Infants also are exposed to endocrine-disrupting chemicals through the mother’s milk supply.

Health and environmental effects

Endocrine-disrupting chemicals have far-reaching impacts on soil, water, and the health of plants and animals. When present at natural background levels, naturally occurring endocrine-modulating chemicals appear to have little negative impact on the health of environments and animals. Evidence suggests that in humans the consumption of small amounts of some naturally occurring substances, such as phytoestrogens found in certain vegetables, may actually benefit health. However, the vast majority of endocrine disruptors, including some phytoestrogens and especially chemicals that are human-made, pose significant health risks to humans and other animals, even when exposure occurs at only low levels.

Many endocrine disruptors are weakly estrogenic—they possess estrogen-like activity but do not act as strongly as estrogen. Others are androgenic, mimicking the male steroid hormones, and still others interfere with the receptors for progesterone or the thyroid hormones. Though relatively weak, the cumulative effects of endocrine-disrupting chemicals nonetheless have catastrophic impacts on animal development and reproduction. Weakly estrogenic chemicals such as glycol ethers, parabens, and phthalates, for example, can decrease sperm motility and sperm viability, ultimately reducing male fertility. PBDEs and PCBs and their metabolites alter the activity of thyroid hormones, resulting in neurodevelopmental abnormalities. Studies in laboratory animals have shown that PBDE exposure, for example, is associated with increases in oxidative stress and neuronal toxicity in the brain, as well as with aberrations in the formation of synapses (connections) between neurons involved in learning and memory. In humans, prenatal PBDE exposure has been correlated with various neurodevelopmental outcomes by age six, including reductions in fine motor control and attention but gains in coordination and visual perception. Studies have indicated that the effects of some endocrine-disrupting chemicals on gene activity can be transmitted through the germ line (e.g., sperm and eggs) and passed to subsequent generations.

Numerous studies have drawn attention to the detrimental impacts of endocrine-disrupting chemicals on wildlife. Early studies centred on DDT, especially its impact on egg viability in birds. Aquatic species, such as amphibians, fish, and marine mammals, however, are also highly susceptible to chemical exposures in the environment. Among amphibians, the permeability of the skin and their aquatic developmental stages mean that even low concentrations of chemicals can have significant effects. For instance, low-level exposure to atrazine, a widely used herbicide that interferes with androgen signaling, severely impairs normal amphibian gonadal development and, in frogs, has been found to promote the development of intersex individuals (having sex characteristics that are neither clearly male nor clearly female). Among marine species, predators such as swordfish and bluefin tuna have been found to harbour high concentrations of endocrine-disrupting chemicals. Male swordfish in the Mediterranean, for instance, were found to have large amounts of PCBs in their muscle tissue and unusually high levels of typically female-expressed proteins, including vitellogenin. The emergence of intersex among Mediterranean swordfish, in which vitellogenin was found in both intersex individuals and normal males, has been attributed to exposure to weakly estrogenic chemicals.

Mitigating exposure to endocrine-disrupting chemicals

A major means of reducing exposure to endocrine-disrupting chemicals is through increased regulation and control over their manufacture, production, and use. Some countries, such as the United States, have banned the use and manufacture of chemicals such as DDT and PCBs, helping to reduce environmental exposures. The Stockholm Convention on Persistent Organic Pollutants, adopted in 2001, prohibited or eliminated the use and production of toxic carbon-based chemicals that persist and accumulate in the environment, in ratifying countries. Such regulations are not entirely protective, however, since many chemicals known to adversely affect the endocrine system may still escape into the environment through improper handling of wastes or products that already contain the chemicals. Moreover, the use and manufacture of many endocrine-disrupting chemicals are yet to be regulated, and, despite known health risks, a number of chemicals, including DDT, are still used in some areas of the world and thus continue to enter the environment.

Consumers can generally limit their risk of exposure to endocrine disruptors by avoiding the purchase and use of products that contain the chemicals and by avoiding the consumption of fish and other foods derived from animals with high likelihoods of chemical exposure. Granular activated carbon and certain types of water filters can help remove some endocrine-disrupting chemicals, including arsenic and atrazine, from drinking water.