Endocrine disruptor

Endocrine disruptors are exogenous substances that act like hormones in the endocrine system and disrupt the physiologic function of endogenous hormones. Studies have linked endocrine disruptors to adverse biological effects in animals, giving rise to concerns that low-level exposure might cause similar effects in human beings.

Background

Since the publication of Rachel Carson’s Silent Spring (Carson, 1962), there has been increasing awareness that chemicals in the environment can exert profound and deleterious effects on wildlife populations and that human health is inextricably linked to the health of the environment. The last two decades, in particular, have witnessed a growing scientific concern, public debate, and media attention over the possible deleterious effects in humans and wildlife that may result from exposure to chemicals that have the potential to interfere with the endocrine system.

Although researchers had studied the endocrine effects of chemicals in the past, the term endocrine disruptor was coined in 1991 at a conference at the Wingspread Conference Center in Racine, Wisconsin. This conference was chaired by Theo Colburn, then with the World Wildlife Fund and the W. Alton Jones Foundation. The term was introduced into the scientific literature with her 1993 paper. In this paper, she stated that environmental chemicals disrupt the development of the endocrine system, and that effects of exposure during development are permanent.

Endocrine disrupting compounds encompass a variety of chemical classes, including natural and synthetic hormones, plant constituents, pesticides, compounds used in the plastics industry and in consumer products, and other industrial by-products and pollutants. They are often pervasive and widely dispersed in the environment. Some are referred to as persistent organic pollutants (POP's), and can be transported long distances across national boundaries, and have been found in virtually all regions of the world. Others are rapidly degraded in the environment or human body or may be present for only short periods of time but at critical periods of development.

In general, health effects associated with endocrine disrupting compounds include a range of reproductive problems (reduced fertility, male and female reproductive tract abnormalities, and skewed male/female sex ratios, loss of fetus, menstrual problems, changes in hormone levels; early puberty; brain and behavior problems; impaired immune functions; and various cancers.

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 1970's, 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. This was a medical exposure, and not an environmental exposure. The relevance of the DES saga to the risks of exposure to endocrine disruptors is questionable, as the doses involved are much lower for environmental exposures.

The endocrine system

Main article: Endocrine system

Endocrine systems are found in most varieties of animal life. The endocrine system is made up of glands, which secrete hormones, and receptors which detect and react to the hormones.

Hormones are released by glands and travel throughout the body, acting as chemical messengers. Hormones interface with cells that contain matching receptors in or on their surfaces. The hormone binds with the receptor, much like a key would fit into a lock.

The Theory of Endocrine disruption

Altering hormone signals can change how the body works and may lead to health problems. Some people speculate that endocrine disrupting compounds may be linked to many health concerns, such as cancer, diabetes, obesity, learning disabilities and behavioural problems. Solid human evidence linking EDCs with these kinds of effects is hard to determine, but their is increasing evidence from the fields of epidemiology, toxicology, environmental health, and environmental science that suggests there might be something to the claims.

There are now hundreds of studies of cell cultures, laboratory animals, wildlife, and accidentally exposed humans that demonstrate how EDC's cause a wide range of reproductive, developmental, growth, and behavior problems. The most attention has been given to those compounds that produce estrogenic, androgenic, antiandrogenic, and antithyroid actions. Less is known about interactions with other hormones and possible effects on immunity (infections), metabolism, (obesity), the brain (intelligence, behavior), the heart, the lungs, and multiple generations.

The interrelationship between exposures to chemicals and health effects are rather complex and multifactoral and occur along a continuum. It is often hard to definitively link a particular chemical with a specific health effect, and exposed adults may not show any ill effects. But, fetuses and embryos, whose growth and development are highly controlled by the endocrine system, are more vulnerable to exposure and may suffer overt or subtle lifelong health and/or reproductive abnormalities (Bern 1992). Prebirth exposure, in some cases, can lead to permanent alterations and adult diseases.

Increasingly, study is being focused on prenatal exposure in the first trimester as the most significant point of contamination.

For example, testicular cancer in young men takes root before birth during fetal development. Certain cancers and uterine abnormalities in women are associated with exposure to DES in the womb. In another case, phthalates in pregnant women’s urine was liked to subtle, but specific, genital changes in their male infants – a shorter, more female-like anal-genital distance and associated incomplete descent of testes and a smaller scrotum and penis. The ubiquitous compounds, found in plastics and cosmetics, are known to block the androgens that control development of the normally longer male anal-genital distance.

Scientific Objection to the Theory

One major objection to the theory of endocrine disruptors is the dosage effect. There is a large gap between high exposures seen in a laboratory experiment versus the relatively low levels found in the environment.

The theory has been applied inappropriately, leading to delays in recognizing environmental problems with other causes. For example, a worldwide decline in amphibians has been attributed to endocrine disruptors, but ise now believed to be largely due to infectious diseases.)

The dosage objection could be overcome if low concentrations of different endocrine disruptors were synergistic, which was asserted in a paper by Arnold. This paper was published in Science in June of 1996, and was one reason for the passage of the Food Quality Protection Act of 1996. His results could not be confirmed with the same and alternative methodologies, and the original paper was retracted. Its author was prosecuted for scientific misconduct.

Thre are philosophical objections to the term endocrine disruptor based on the logical principals of begging the question and fallacies of definition. It is not proven that these chemicals disrupt the endocrine system, at least in the concentrations in which they are found in the environment. For that reason, some scientists prefer the less emotionally laden term HAA (for Hormonally Active Agent).

Types of Endocrine Disruptors

Most people are exposed to chemicals with estrogenic effects in their everyday life, because endocrine disrupting chemicals are found in low doses in literally thousands of consumer products. Among the most commonly encountered are Bisphenol A, Polybrominated diphenyl ethers (PBDE's), and a variety of Phthalates. There is a great deal of dispute in the scientific community surrounding the question. Many believe that there is little evidence that the degree of exposure in humans is enough to warrant concern, , while many others believe there is ample evidence that these chemicals pose at least some risk to human health.

As awareness of the problem has increased, more and more attention is being focused on the health risks to children potentially posed by many common endocrine disrupting chemicals. Bisphenol A has come under a great deal of scrutiny as it is a common component of plastic baby bottles. In March of 2007, a class action lawsuit was filed in California charging that manufacturers and retailers of plastic baby bottles failed to warn consumers that their products contained Bisphenol A, a chemical that they allege poses developmental and health risks to infants and children.

Bisphenol A: Where found: in some plastic water and baby bottles, plastic food containers, dental materials, linings of metal food cans. Major health concern: another endocrine disruptor, linked to reduced sperm counts. May contribute to miscarriages, obesity and some cancers.

Polybrominated diphenyl ethers: Where found: fire retardants used in plastic cases of televisions and computers, electronics, carpets, lighting, bedding, clothing, car components, foam cushions and other textiles. Major health concern: PBDE's are structurally very similar to Polychlorinated biphenyls (PCBs), and have similar neurotoxic effects. Have the potential to disrupt thyroid hormone balance and contribute to a variety of neurological and developmental deficits, including low intelligence and learning disabilities. Many of the most common PBDE's were banned in the European Union in 2006.

Phthalates: Where found: some soft toys, flooring, medical equipment, cosmetics and air freshener. Major health concern: known to disrupt the endocrine system and implicated in feminized genitals of baby boys. Banned in California and Europe.

Some of the other most well-known examples of EDCs are 17-alpha ethinylestradiol (the contraceptive pill), Dioxins, PCBs, PAHs, furans, phenols and several pesticides (most prominent DDT and its derivatives). Substances with estrogenic side effects include the xenoestrogens. There is a long list of substances which may disrupt the endocrine system but have not yet been scientifically proven to do so.

Legal approach

The multitude of possible endocrine disruptors are technically regulated in the United States by many laws, including: the Toxic Substances Control Act, the Federal Insecticide, Fungicide, and Rodenticide Act, the Food, Drug and Cosmetic Act, the Clean Water, the Safe Drinking Water Act, and the Clean Air Act.

The Congress of the United States has improved the evaluation and regulation process of drugs and other chemicals. The Food Quality Protection Act of 1996 and the Safe Drinking Water Act of 1996 simultaneously provided the first legislative direction requiring the EPA to address endocrine disruption through establishment of a program for screening and testing of chemical substances.

In 1998 the EPA announced the Endocrine Disruptor Screening Program by establishment of a framework for priority setting, screening and testing more than 85,000 chemicals in commerce. The basic concept behind the program is that prioritization will be based on existing information about chemical uses, production volume, structure-activity and toxicity. Screening is done by use of in vitro test systems (by examining, for instance, if an agent interacts with the estrogen receptor or the androgen receptor) and via the use of in animal models, such as development of tadpoles and uterine growth in prepubertal rodents. Full scale testing will examine effects not only in mammals (rats) but also in a number of other species (frogs, fish, birds and invertebrates).

After failing to meet several deadlines to begin testing, the EPA finally announced that they were ready to begin the process of testing dozens of high production volume chemicals that are known endocrine disruptors early in 2007, eleven years after the program was announced. Unfortunately, when the final structure of the tests was announced there was widespread dismay at their design. Critics have charged that the entire process has been compromised by chemical company interference. . In 2005, the EPA appointed a panel of experts to conduct an open peer-rewiew of the program and its orientation. Their results found that "the long-term goals and science questions in the EDC program are appropriate"., however this study was conducted over a year before the EPA announced the final structure of the screening program.

Among the charges leveled are that the EPA:

• Allowed lab tests, using rodents, that are so badly designed, they're almost certain to miss harmful chemicals. For instance, the EPA favors using a breed of rat that is relatively insensitive to several known hormone-disrupting chemicals. And the EPA plans to allow those rats to be fed chow that could mask the effect of some chemicals.

• Failed to guarantee that tests will be conducted on prenatal exposure to chemicals. In late May of 2007 a group of 200 scientists signed a declaration warning that exposure to chemicals in the womb may make babies more likely to develop diabetes, obesity, attention deficit disorder and infertility. The group urged action from governments around the world.

• Demanded the wrong dosage range, also raising the odds that harmful effects will be missed.

• Said it might allow chemical companies to tailor certain aspects of the tests.

One of the most prominent researchers in the field of endocrine disruption, Frederick Vom Saal, a developmental biologist at the University of Missouri was quoted as saying: "If your objective is not to find anything, that's the perfect way to do it"

See also

• Endocrine system
• Hormone
• Xenoestrogen
• Theo Colborn
• Precautionary principle

External links

• List of suspected or possible endocrine disruptors
• Global assessment of the state-of-the-science of endocrine disruptors published in 2002 by the World Health Organisation (WHO).
  • Scientific Facts on Endocrine Disruptors, a peer-reviewed summary of the above WHO assessment, by GreenFacts.
• The NSF sponsored Environmental Signaling Network's website
• The EU strategy on EDCs
• The US EPA (strategy) on EDCs
• Synopses of new scientific findings about endocrine disruption
• Journalist's report on endocrine disruptors (OnEarth Magazine, Winter 2006)
• PBS Frontline Interview of Dr. Theo Colborn by Doug Hamilton, 1998.
• Hormonal Chaos: The Scientific and Social Origins of the Environmental Endocrine Hypothesis by Sheldon Krimsky
• "Sex-change chemicals in Potomac", BBC News, last updated 18 January 2007.

References

Colborn, Theo; Dianne Dumanoski; and John Peterson Myers. Our stolen future : are we threatening our fertility, intelligence, and survival? : a scientific detective story. New York : Dutton, 1996. 306 p. ISBN 0452274141