Endocrine Disruptions and Persistent Organic Pollutants

Ted Shettler, M.D.
Director
Science and Environment Health Network,
Boston, MA

A series of UNEP-led international negotiations aimed at globally eliminating the production and use of twelve toxic long-lived organic chemicals (persistent organic pollutants-POPS) has focused widespread attention on their dangerous properties. These twelve are among the most toxic effects by interfering with hormone function. POPs are particularly troublesome because they persist for many years in the environment. They also tend to bioaccumulate, so that their concentrations build up in organisms high in the food chain, putting humans at particular risk. Moreover, many studies show that the developing organism is particularly susceptible to toxic effects of POPs and other hormone disrupting chemicals, and fetuses and nursing infants may be disproportionately exposed. This presentation will describe the characteristics of these chemicals that justify international concern, review their global distribution, and describe how they exert their toxic effects with particular attention to the developing organism and susceptible life stages. It will explore the controversies surrounding the level of evidence of harm necessary to take health protective measures.

Persistent organic pollutants (POPs) are toxic organic chemicals that are long-lived in the environment and tend to bioaccumulate as they pass up the food chain. Under the leadership of the United Nations Environment Program, a series of international negotiations is underway at eliminating production and use of twelve of the most troublesome POPs. Among these twelve are dioxin, PCBs, DDT, and several other pesticides.

Many POPs exert their toxic effects by interfering with hormone function. A large number of other chemicals in widespread use, not classified as POPs because they do not persist or bioaccumulate, also interfere with hormone function and are under increased scrutiny of medical professionals, regulators, toxicologists, and the general public because of their potential to cause serious harm, particularly to fetuses, infants, and other developing organisms.

Several epidemiological studies show that fetal exposure to PCBs impairs normal brain development such that offspring experience significant developmental delays, certain kinds of learning disabilities, and impaired intellectual function. A large number of animal studies show that fetal exposure to extraordinarily small levels of dioxin causes long-lasting changes in development of the reproductive and immune systems. In animals and humans DDT is metabolized into DDE. In animals, DDE behaves as an anti-androgen, blocking normal function of male hormones and causing demasculinization of males. It is unclear if DDE has a similar effect in humans. DDT has been banned in many countries because of its toxic effect on wildlife reproduction, but it is still widely used in many parts of the world, primarily for mosquito and malaria control. A phase-out of DDT use will depend on replacement with a safer alternative, and several candidates have been identified.

Most POPs tend to be widely spread throughout the world's ecosystems as they enter into global atmospheric transport. Levels of POPs in animal tissue may become alarmingly high. For example, marine mammals tend to accumulate many of these chemicals in their fatty tissue, and indigenous people eating pollutants, are exposed to some of the highest concentrations anywhere on earth. These chemicals also tend to concentrate in breast milk because of its high fat content, and the nursing infant is consequently exposed to larger amounts than adults. This is particularly problematic because the developing infant is likely to be more sensitive to toxic effects than an adult. However, the advantages of breast feeding are clear and unequivocal. The appropriate policy response to chemical contamination of breast milk is not to avoid breast feeding, but rather, to do what is necessary to get the chemicals out of the milk. It is essential that the international community develop the political will to address this important problem and protect the most vulnerable of our communities from the toxic effects of these chemicals.

SOURCE: Health and Environment in Sustainable Development: Five years after the Earth Summit, World Health Organization, 1998