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Open Access Publications from the University of California

Detecting Endocrine Disruption in the Sacramento- San Joaquin Delta


In 2001, the EPA began restricting the use of organophosphate pesticides in bug sprays and strips to protect the health of humans and other mammals. Their main replacement has been another class of highly toxic pesticides known as pyrethroids.

The pyrethroid molecule is basically a chemically stabilized version of the natural pyrethrin molecule found in mums (yes, the flowers). In its modified form, it is more persistent in the environment, more hydrophobic (it will stick to sediment) and more toxic.

Vertebrates, including humans, have enzymes that can break down low doses of pyrethroids but they tend to have endocrine- disrupting effects. Both the pesticides and the compounds produced during their metabolism can potently mimic sex hormones.

Fishes are particularly sensitive to endocrine disrupting compounds (EDCs), due to their size and continuous exposure via contaminated water. Fish exposed to EDCs may display a variety of physiological and behavioral abnormalities, including poor sperm quality, skewed sex ratios and possibly population declines. The advent of new technologies has made it possible to document endocrine-disrupting effects at the parts-per-trillion range in sensitive aquatic species. Some studies have shown that low concentrations of EDCs can sometimes be more potent than higher ones.

Besides the obvious implications for aquatic ecosystems, EDCs also pose significant human health concerns. According to scientists, the endocrine system is highly conserved evolutionarily among vertebrates. Pesticides, plasticizers and other EDCs that cause problems for fish will likely be harmful to people at high doses or over longer exposure periods.

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