Hormones are naturally-occurring chemicals that circulate at very low levels in the blood stream of all vertebrate animals including reptiles, amphibians, fish, birds and mammals. In all vertebrate species, hormones act as chemical messengers and as switches, turning on and off bodily systems that control growth, development, learning and behavior. Hormones start affecting every animal shortly after it begins life as a fertilized egg. Hormones control growth and development prior to birth or hatching, and hormones continue to influence behavior throughout life. Hormones tell bears when to hibernate, tell salmon when to return to their spawning grounds, and cause women to menstruate every 28 days or so.
Hormones profoundly affect the nervous system, the reproductive system, and the immune system.
Because of the importance of hormones in the life of all vertebrates, industrial chemicals that can interfere with hormones are exceedingly important from a public health perspective. They also represent major embarrassments and liabilities for the corporations that put such chemicals into common use without adequate safety tests. There is also evidence of suppression of innate and acquired immune responses in seals fed fish from the PCB-contaminated Baltic Sea. Meanwhile between 1000 and 2000 new chemicals are being put into commercial use each year, inadequately tested. Therefore, adequate knowledge of hormone-disrupting chemicals lies many decades in the future, a kind of scientific holy grail.