Overview of Endocrine Disruption

In the last two decades there has been a growing awareness of the possible adverse effects in humans a nd wildlife from exposure to chemicals t hat can interfere with the endocrine system. These effects can include:

• developmental malformations;
• interference with reproduction;
• increased cancer risk; and
• disturbances in the immune and nervous system function.

Clear evidence exists that some chemicals cause these effects in wildlife, but limited evidence exists for the potential of chemicals to cause these effects in humans at environmental exposure levels. Very few chemicals have been tested for their potential to interfere with the endocrine system. Current standard test methods do not provide adequate data to identify potential endocrine disruptors (EDs) or to assess their risks to humans and wildlife.

How Chemicals Can Disrupt the Endocrine System

Scientific research on human epidemiology, laboratory animals, and fish and wildlife suggests that environmental contaminants can disrupt the endocrine system, leading to adverse health consequences.

It is important to gain a better understanding of what concentrations of chemicals found in the environment may cause an adverse effect. Various types of scientific studies (e.g., epidemiology, mammalian toxicology, and ecological toxicology) are necessary to resolve uncertainty surrounding endocrine disruption. Many such studies are underway by government agencies, industry and academia.

Disruption of the endocrine system can occur in various ways. Some chemicals mimic a natural hormone, fooling the body into over-responding to the stimulus (e.g., a growth hormone that results in increased muscle mass), or responding at inappropriate times (e.g., producing insulin when it is not needed). Other endocrine disruptors block the effects of a hormone from certain receptors (e.g., growth hormones required for normal development). Still others directly stimulate or inhibit the endocrine system and cause overproduction or underproduction of hormones (e.g. an over or underactive thyroid).

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

Examples of Endocrine Disruption

One example of the devastating consequences of the exposure of developing animals, including humans, to endocrine disruptors is the case of the potent drug diethylstilbestrol (DES), a synthetic estrogen. Prior to its ban in the early 1970s, doctors mistakenly prescribed DES to as many as five million pregnant women to block spontaneous abortion 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 statutory requirement to establish an endocrine disruptor screening program is a highly significant step.

Growing scientific evidence shows that humans, domestic animals, and fish and wildlife 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 wildlife species with relatively high exposure to:

• organochlorine compounds (e.g., 1,1,1- trichloro-2,2-bis(p-chlorophenyl);
• ethane (DDT) and its metabolite dichorodiphenyldichloroethylene (DDE);
• polychlorinated biphenyls (PCBs), and dioxins); and
• some naturally occurring plant estrogens.

Effects from exposure to low levels of endocrine disruptors have been observed as well (e.g., parts-per-trillion levels of tributyl tin have caused masculinization of female marine molluscs such as the dog whelk and ivory shell). Adverse effects have been reported for humans exposed to relatively high concentrations of certain contaminants. However, whether such effects are occurring in the human population at large at concentrations present in the ambient environment, drinking water, and food remains unclear.

Several conflicting reports have been published concerning declines in the quality and quantity of sperm production in humans over the last four decades, and there are reported increases in certain cancers (e.g., breast, prostate, testicular). 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, considerable scientific uncertainty remains regarding the actual causes of such effects.

Nevertheless, there is little doubt that small disturbances in endocrine function, particularly during certain highly sensitive stages of the lifecycle (e.g., development, pregnancy, lactation) can lead to profound and lasting effects.