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From January 14, 2005

New Pesticide Isomer Study Shows New Breakdown and Toxicity Risks
(Beyond Pesticides, January 14, 2005)
Researchers at the University of California, Riverside have demonstrated that isomers – or the mirror-image structures – of some pesticides, although chemically identical, have very different biological and environmental impacts between the two sides. This may have significant implications for regulation of these pesticides.

The environmental risks of pesticides have been traditionally evaluated on the basis of their specific chemical structure, according to Jay Gan, a UCR professor of environmental chemistry. He found, however, that this group, known as chiral pesticides, including many widely used organophosphates and synthetic pyrethroids, pose previously uncalculated toxic risks due to the differing biological reactions of the isomers in the environment.

A characteristic of chiral compounds is that they occur as isomers with two (or more) identical but mirror-image structures that, as Gan’s research indicates, while chemically identical, may behave biologically differently. These mirror-image molecules are known as enantiomers. Currently about 25 percent of pesticides fall into this classification and this ratio is expected to increase as new products are being introduced into the market.

The findings were published in a paper titled “Enantioselectivity in Environmental Safety of Current Chiral Insecticides” in last week’s online edition of the Proceedings of the National Academy of Sciences. Dr. Gan published the paper in cooperation with a team of UCR colleagues including Daniel Schlenk, professor of aquatic ecotoxicology; Soil Physics Professor, William A. Jury; and visiting professor Weiping Liu.

Dr. Gan and his colleagues at UC Riverside examined chiral insecticides that are widely used today. They examined five common insecticides, including organophosphates, such as profenofos, and synthetic pyrethroids, such as permethrin. For all these compounds, one of the optical isomers, or enantiomers, was consistently over 10 times more toxic than the other to Ceriodaphia, a small crustacean often used to assess water toxicity.

The researchers also found that a specific enantiomer lingered longer in the environment than the other enantiomers, making one enantiomer of permethrin almost twice as prevalent in sediment or runoff water. This means that the environmental impact of these pesticides may depend on the behavior of a particular enantiomer instead of the whole compound, the team concluded.

Dr. Gan also believes that knowing about such selectivity would also be valuable for the chemical industry. For instance, if only one enantiomer is known to contribute to the pest control efficacy, it would be advantageous to manufactured products containing just the active component. The rate of use may be cut in half. “The difference in terms of pesticide regulation and future R&D directions could be pretty drastic for chiral pesticides,” said Dr. Gan.

TAKE ACTION: Let EPA Administrator Michael Leavitt know that you want the Environmental Protection Agency to take this new information regarding pesticide isomers into account and error on the side of caution in regulating pesticides.