In 1978, during a routine ecological assessment of several British waterways, wildlife biologists discovered an unusually high number of abnormal fish living downstream of two sewage-treatment plants. The fish were considered intersexual because their gonads contained both ovarian and testicular tissue. Nearly 2 decades later, after the development of more-sensitive analytical techniques, researchers provided an explanation. They traced the animals’ reproductive problems to low concentrations of estrogens, known as the female-sex hormones, that had entered the environment in waters released by the sewage plants.
Researchers had found that concentrations as low as a few parts per trillion lead to reproductive abnormalities in fish. “This changed the whole thinking about chemicals in the environment,” says John P. Sumpter, an ecotoxicologist at Brunel University in Uxbridge, England. Having generally focused on chemical pollution at higher concentrations, researchers began to consider that perhaps biologically active chemicals at low concentrations “are the things we should be more concerned about,” he says.