Scientific research in human epidemiology, laboratory animals, and fish and wildlife suggests that environmental pollutants can disrupt the endocrine system, leading to negative health consequences.

It is important to gain a better understanding of what concentrations of chemicals found in the environment can have a negative effect. Different types of scientific studies (eg, epidemiology, mammalian toxicology, and ecological toxicology) are needed to resolve uncertainty surrounding endocrine disruption. Many such studies are underway by government agencies, industry and academia.

Disturbances of the endocrine system can occur in different ways. Some chemicals mimic a natural hormone, tricking the body into overreacting to a stimulus (eg, a growth hormone that results in increased muscle mass), or reacting at inappropriate times (eg, producing insulin when it’s not needed). Other endocrine disruptors block the effects of a hormone from certain receptors (eg, growth hormones needed for normal development). Still others directly stimulate or inhibit the endocrine system, causing overproduction or underproduction of hormones (eg, an overactive or underactive thyroid gland).

Certain drugs are used to intentionally cause some of these effects, such as birth control pills. However, in many situations involving environmental chemicals, an endocrine effect is not desirable.

An example of the devastating consequences of exposing developing animals, including humans, to endocrine disruptors is the case of the potent drug diethylstilbestrol (DES), a synthetic estrogen. Before its ban in the early 1970s, doctors mistakenly prescribed DES to as many as five million pregnant women to block miscarriage and promote fetal growth. It was discovered after the children went through puberty that DES affected the development of the reproductive system and caused vaginal cancer.

Since then, Congress has improved the evaluation and regulation of drugs and other chemicals. The legal requirement to establish a screening program for hormone-disrupting substances is a very important step.

Growing scientific evidence shows that humans, livestock, and fish and animal species have exhibited adverse health consequences from exposure to environmental chemicals that interact with the endocrine system. To date, such problems have been detected in domestic or wild animal species with relatively high exposure to:

• organochlorine compounds (eg 1,1,1-trichloro-2,2-bis(p-chlorophenyl);
• ethane (DDT) and its metabolite dichlorodiphenyldichloroethylene (DDE);
• polychlorinated biphenyls (PCBs) and dioxins; and
• some naturally occurring phytoestrogens.

Effects from exposure to low levels of endocrine disruptors have also been observed (eg, parts-per-billion levels of tributyltin have caused masculinization in female marine molluscs such as dog snails and ivory shells). Adverse effects have been reported for humans exposed to relatively high concentrations of certain pollutants. Whether such effects occur in the human population as a whole at concentrations present in the ambient environment, drinking water and food remains unclear.

Several conflicting reports have been published on declines in the quality and quantity of sperm production in humans over the past four decades, and increases in certain cancers (eg, breast, prostate, testicular) have been reported. Such effects may have an endocrine-related basis, which has led to speculation about the possibility that these endocrine effects may have environmental causes. However, there is still considerable scientific uncertainty about the actual causes of such effects.

Nevertheless, there is little doubt that small disruptions in endocrine function, especially during certain highly sensitive stages of the life cycle (eg, development, pregnancy, lactation) can lead to profound and lasting effects.