Some pesticides pose uncalculated risks

RIVERSIDE, California, January 18, 2005 (ENS): A University of California researcher has found that many widely used pesticides pose previously uncalculated toxic risks due to the differing biological reactions of their isomers in the environment. Isomers are molecules with the same chemical formula but in which the atoms are arranged differently.

Jay Gan, a professor of environmental chemistry at the University of California-Riverside has found that this group of pesticides, although chemically identical, have very different biological and environmental impacts between their two, or more, isomers.

He says this fact may have implications for risk assessment and research and development directions of new products.

Currently about 25 percent of pesticides - including organophosphates and synthetic pyrethroids - fall into this classification and this ratio is expected to increase as new products are being introduced into the market.

Gan and his colleagues at UC Riverside examined five common insecticides, including organophosphates, such as profenofos, and synthetic pyrethroids, such as permethrin.

For all these compounds, one of the isomers 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 isomer lingered longer in the environment than the other, making one form 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 isomer instead of the whole compound, the team concluded.

Gan’s findings add weight to the argument that when assessing risk regulators should consider whether a product is a compound with more than one isomer, and that the chemical industry should pursue the value of producing single isomer products instead of mixed isomer products.

By using pesticides with just the active isomer, farmers will likely achieve the same degree of pest control at a much-reduced rate of chemical use, Gan concludes. This will have environmental benefits as much less chemical is introduced into the environment.

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. 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.