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.