Endocrine distrupting chemicals and human health: the plausibility of research results on DDT and reproductive health.

Introduction The publication of Rachel Carson’s Silent Spring1 in 1962 brought in an increased awareness on the effects of chemicals in the environment and that human health is inextricably linked to the health of the environment. Since then there has been growing scientific concern, public debate and media attention over the possible effects in humans and wildlife that may result from exposure to chemicals that have the potential to interfere with the endocrine system. Environmental “Endocrine Disrupting Chemicals” (EDCs) have been described as “exogenous agents that interfere with the synthesis, secretion, transport, binding, action, or elimination of natural hormones in the body that are responsible for the maintenance of homeostasis, reproduction, development, and/or behaviour”2 Increasing number of chemical compounds in the environment have been identified as endocrine disruptors using in vitro and in vivo bioassays. These are most often than not pervasive and widely dispersed in the environment and include pesticides, industrial chemicals, pharmaceuticals and natural hormones acting as ligands for the estrogen-, androgenor arylhydrocarbon receptor or exerting a combined action (e.g. estrogenic and anti-androgenic activity)3 D I C H L O R O D I P H E N Y L T R I C H L O ROETHANE (DDT) DDT the first of the chlorinated organic insecticides, was originally prepared in 1873;4,5 but it was not until 1939 that Paul Muller of Geigy Pharmaceutical in Switzerland discovered the effectiveness of DDT as an insecticide, he was awarded the Nobel Prize in medicine and physiology in 1948 for his discovery. DDT is a synthetic chemical that does not occur naturally in the environment. As a mixture DDT mainly constitutes of p, p’dichlorodiphenyltrichloroethane (p, p’-DDT), (63-77%), o, p’-dichlorodiphenyltrichloroethane (o, p’-DDT) (8-21%), (p, p’-dichlorodiphenyldichloroethylene, (p, p’-DDE) (DDE) (0.3-4%). DDT is rapidly transformed to DDE which is the major metabolite of DDT in biological systems. DDT and DDE have very high octanol-water partition coefficients6 and tend to accumulate in lipid compartments7 in biological systems. DDE has a very long half-life, which to some extent is of toxicological importance.8 The half life of DDT in humans has been estimated to be between 6 and 10 years9 such that short-term exposures are also associated with long-term exposures because both elements are slowly released from body fat10. Initially, DDT was used in the 1940s to control wartime typhus and agricultural pests, and it then was popular on a worldwide basis as a measure to control malaria vectors, until its deleterious effects on wildlife led to a ban on routine DDT use in many countries in the 1970s. Today the production and use of DDT is restricted to the control of public health disease vectors such as malariatransmitting mosquitoes. This was after the Stockholm Convention on Persistent Organic Pollutants in 1995 which considered banning or restricting the production or use of organic pollutants as a result of their toxicity, resistance to breakdown, bioaccumulation potential and potential to be transported over long distances. In some Malaria areas, DDT Indoor Residual Spraying (IRS) is used to decrease the incidence and spread of the disease11,12 its effects not only kill the mosquitoes but also repel them from interior surfaces, thereby further decreasing the odds of infectivity. Only 8 out of the 44 malarious countries were using DDT for indoor residual spraying by 2007.1 Studies have also shown the effectiveness of DDT-impregnated bed nets in malaria control.14,15 DDT use is currently under intense debate in several countries for its use in household spraying for malaria control.