Concentrations in the synapses between nerves is regulated by the enzyme acetocholinesterase. The landmark publication of Rachel Carson's Silent Spring in 1967 sounded the alarm for harmful effects of chemicals on wildlife. The 1996 publication of Our Stolen Future by Theo Colborn, Dianne Dumanoski, and John Peterson Myers was a clarion call for recognition of similar effects on humans. The authors of Our Stolen Future revealed how many of the findings in animals correlate with disturbances in normal reproductive and developmental processes in humans, citing data from decades of research on wildlife that traced birth defects, sexual abnormalities, and reproductive failures to synthetic chemicals that mimic natural hormones. There is increasing evidence that environmental hormones may be linked to increases in hormone-related cancers, endometriosis, certain behavioral aberrations, and reproductive effects such as an alarming apparent drop in male sperm counts worldwide in recent years.
Many environmental chemicals exhibit estrogenic activity.
Estrogenic compounds, both environmental and endogenous, may interact with estrogen, estrogen receptors, or other hormones and transcription factors in the biochemical pathway of hormone activity. Numerous examples of reproductive abnormalities in wildlife caused by environmental contaminants that have estrogenic properties have been described. In experiments by McLachlan and colleagues, reported in the 7June 1996 issue of Science, a combination of the weakly estrogenic pesticides endosulfan, dieldrin, toxaphene, and chlordane, measured by yeast cell ER binding assays, produced estrogenic activity 150-1,600 times greater than for any of the chemicals alone. Synergistic responses were also observed in mammalian cell culture experiments. The group's results indicated that estrogenic effects are additive rather than synergistic, and therefore mixtures of two compounds can be expected to be weakly estrogenic and will not have a dramatic adverse environmental and human health impact.